Fukushima nuclear accident

"Fukushima nuclear accidents" redirects here. For the incidents at Fukushima II (Fukushima Daini), see Fukushima II Nuclear Power Plant.

Fukushima nuclear accident

Satellite image on 16 March of the four damaged reactor buildings
Date	11 March 2011 (2011-03-11)–ongoing
Time	14:46 JST (05:46 UTC)
Location	Ōkuma, Fukushima, Japan
Coordinates	37°25′17″N 141°1′57″E
Outcome	INES Level 7 (ratings by Japanese authorities as of 11 April)
Deaths	None
Non-fatal injuries	37 with physical injuries, 2 workers taken to hospital for radiation exposure
File:Japan Nuclear power plants map.gif

The Fukushima I nuclear accidents (福島第一原子力発電所事故, Fukushima Dai-ichi () genshiryoku hatsudensho jiko) are a series of ongoing equipment failures and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the 9.0 magnitude Tōhoku earthquake and tsunami on 11 March 2011. The plant comprises six separate boiling water reactors maintained by the Tokyo Electric Power Company (TEPCO). Experts consider it to be the second largest nuclear accident after the Chernobyl disaster, but more complex as all reactors are involved.

At the time of the quake, reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, and emergency generators started up to run the control electronics and water pumps needed to cool them. The plant was protected by a seawall designed to withstand a 5.7 metres (19 ft) tsunami, but not the 14-metre (46 ft) wave which arrived 15 minutes after the earthquake. The entire plant was flooded, including low-lying generators and electrical switchgear in reactor basements, and its connection to the electical grid was broken. All power for cooling was lost and reactors started to overheat, despite shutdown, due to natural decay of the fission products created before shutdown. The flooding and earthquake damage hindered external assistance.

Evidence soon arose of partial core meltdown in reactors 1, 2, and 3; hydrogen explosions destroyed the upper cladding of the buildings housing reactors 1, 3, and 4; an explosion damaged the containment inside reactor 2; and multiple fires broke out at reactor 4. In addition, spent fuel rods stored in spent fuel pools of units 1–4 began to overheat as water levels in the pools dropped. Fears of radiation leaks led to a 20-kilometre (12 mi) radius evacuation around the plant while workers suffered radiation exposure and were temporarily evacuated at various times. One generator at unit 6 was restarted on 17 March allowing some cooling at units 5 and 6 which were least damaged. Grid power was restored to parts of the plant from 20 March, but machinery for reactors 1–4 damaged by floods, fires and explosions remained inoperable. Japanese officials initially assessed the accident as level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The INES level was raised successively to 5 and then the maximum 7.

Measurements taken by the Japanese science ministry and education ministry in areas of northern Japan 30–50 km from the plant showed radioactive caesium levels high enough to cause concern. Food grown in the area was banned from sale. It was suggested that worldwide measurements of iodine-131 and caesium-137 indicate that the releases from Fukushima are of the same order of magnitude as the releases of those isotopes from the Chernobyl disaster in 1986; Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants. Plutonium contamination has been detected in the soil at two sites in the plant.

The International Atomic Energy Agency (IAEA) announced on 27 March that workers hospitalized as a precaution on 25 March had been exposed to between 2 and 6 Sv of radiation at their ankles when standing in water in unit 3. The international reaction to the accidents was also concerned. The Japanese government and TEPCO have been criticized for poor communication with the public and improvised cleanup efforts. Experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over.

Fukushima I Nuclear Power Plant

Simplified cross-section sketch of a typical BWR Mark I containment, as used in units 1 to 5. Key: DW, dry well enclosing reactor pressure vessel; WW, Torus-shaped wetwell all around the base enclosing steam suppression pool. Excess steam from the dry well enters the wetwell water pool via downcomer pipes; SFP, spent fuel pool area; RPV, Reactor Pressure Vessel; SCSW, Secondary Concrete Shield Wall.

Main article: Fukushima I Nuclear Power Plant

The Fukushima I Nuclear Power Plant consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).

Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971. It was designed for a peak ground acceleration of 0.18 g (1.74 m/s²) and a response spectrum based on the 1952 Kern County earthquake. Units 2 and 3 are both 784 MWe type BWR-4 reactors, unit 2 commenced operating in July 1974 and unit 3 in March 1976. The design basis for all units ranged from 0.42 g (4.12 m/s²) to 0.46 g (4.52 m/s²). All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s²) for 30 seconds, but no damage to the critical parts of the reactor was discovered.

Units 1–5 have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure. From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.

At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:

Location	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6	Central Storage
Reactor Fuel Assemblies	400	548	548	0	548	764	0
Spent Fuel Assemblies	292	587	514	1331	946	876	6375
New Fuel Assemblies	100	28	52	204	48	64	n/a

Cooling requirements

Diagrammatic representation of the cooling systems of a BWR.

See also: Decay heat - Power reactors in shutdown and Nuclear safety systems

Cooling is needed to remove decay heat from the reactor core even when a plant has been shut down. Nuclear fuel releases a small quantity of heat under all conditions, but the chain reaction when a reactor is operating creates short lived fission products which continue to release heat despite shutdown. Immediately after shutdown, this decay heat amounts to approximately 6% of full thermal heat production of the reactor. The decay heat in the reactor core decreases over several days before reaching cold shutdown levels. Nuclear fuel rods that have reached cold shutdown temperatures typically require another several years of water cooling in a spent fuel pool before decay heat production reduces to the point that they can be safely transferred to dry storage casks.

In order to safely remove this decay heat, reactor operators must continue to circulate cooling water over fuel rods in the reactor core and spent fuel pond. In the reactor core, circulation is accomplished by use of high pressure systems that pump water through the reactor pressure vessel and into heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, taking away the heat which is pumped out to the sea or site cooling towers.

To ensure that cooling water can continue to be circulated, reactors typically have redundant electrical supplies to operate pumps when the reactor is shut down, including electric generators, electrical supplies from the grid, and batteries. In addition, boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam still being produced after a reactor shutdown, which can inject water directly into the reactor. Steam turbines results in less dependence on emergency generators, but steam turbines only operate so long as the reactor is producing steam, and some electrical power is still needed to operate the valves and monitoring systems.

If the water in the unit 4 spent fuel pool had been heated to boiling temperature, the decay heat has the capacity to boil off about 70 tonnes of water per day (12 gallons per minute), which puts the requirement for cooling water in context.

Beginning and cause (2011 Tōhoku earthquake and tsunami)

Further information: Timeline of the Fukushima I nuclear accidents and 2011 Tōhoku earthquake and tsunami

Map of Japan's electricity distribution network, showing incompatible systems between regions.

The 9.0 M_W Tōhoku earthquake occurred at 14:46 JST on Friday, 11 March 2011 with epicentre near the island of Honshu. It resulted in maximum ground accelerations of 0.56, 0.52, 0.56 g (5.50, 5.07 and 5.48 m/s²) at units 2, 3 and 5 respectively, above their designed tolerances of 0.45, 0.45 and 0.46 g (4.38, 4.41 and 4.52 m/s²), but values within the design tolerances at units 1, 4 and 6. The Fukushima I facility had not initially been designed for a tsunami of the size that struck the plant, nor had the reactors been modified when later concerns were raised in Japan and by the IAEA. When the earthquake occurred, reactor units 1, 2, and 3 were operating, but units 4, 5, and 6 had already been shut down for periodic inspection. When the earthquake was detected, units 1, 2 and 3 underwent an automatic shutdown (called scram).

After the reactors shut down, the plant's own electricity generation ceased, eliminating the normal source of electricity used to power cooling and control systems. Electricity provided by other power plants could have been used, but the earthquake damaged the power grid, preventing its delivery. Consequently, 13 on-site emergency diesel generators began powering the plant's cooling systems. However a 15 metres (49 ft) tsunami wave topped the plant's 5.7 metres (19 ft) seawall, flooded the low-lying generator building and disabled 12 of the 13 generators at approximately 15:41. At this point, TEPCO notified authorities, as required by law, of a "First Level Emergency."

After the diesel generators failed, emergency power for control systems was supplied by batteries that were designed to last about eight hours. Batteries and mobile generators were dispatched to the site, but were delayed by poor road conditions. The alternative power sources arrived approximately 13 hours later, the first arriving at 21:00 JST 11 March. However, work to connect portable generating equipment to power water pumps was still continuing as of 15:04 JST on 12 March because the normal connection point in a basement was flooded. TEPCO switched its efforts to provide power to the facility to re-stringing high voltage cables to the grid. Power was first returned to units 5 and 6, on 20 March, leaving the facility without power for approximately 9 days.

Reactor unit 1

Cooling problems at unit 1 and first radioactivity release

Aerial view of the plant area before the accidents. When this photograph was taken in 1975, unit 6 was under construction. North is at top in the image.

On 11 March at 14:46 JST, unit 1 scrammed in response to an earthquake seconds earlier. Thereafter, off-site power was lost due to "malfunction of one out of two off-site power systems," leading to a startup of emergency diesel generators. Thirteen power emergency diesel generators were available to power safety and cooling systems at the 6 units, an unknown number began providing power to the facility and unit 1. Then at approximately, 14:52 JST, the plant was hit by a tsunami, leading to failure of 12 of the 13 diesel generators at approximately 15:41 JST. The tsunami also destroyed "all of the No. 1 plant's pumps, which were also not located within a sealed structure." The failure of the emergency diesel generators "resulted in the complete loss of alternating current" for unit 1. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems." The alert was temporarily cleared when water level monitoring was restored for unit 1; it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.

After the loss of site power, unit 1 initially continued cooling using the isolation condenser system; by midnight water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases. In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for unit 1 and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Chief Cabinet Secretary Yukio Edano stated later in the morning the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST, the pressure inside reactor 1 was reported to be 2.1 times the "design capacity", 820 kPa. Isolation cooling ceased to operate between midnight and 11:00 JST, at which point TEPCO started relieving pressure and injecting water. One employee working inside unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.

Rising heat within the containment area led to increasing pressure. Electricity was needed for both the cooling water pumps and ventilation fans used to drive gases through heat exchangers within the containment. Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off; this also means cooling water is being lost and must be replaced. If there was no damage to the fuel elements, water inside the reactor should be only slightly radioactive.

In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." Dose rates recorded on the main gate rose from 69 nGy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 385.5 μSv/h at 10:30 JST. At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near reactor 1, which indicated some of the core's fuel had been damaged. Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred. Radiation levels at the site boundary exceeded the regulatory limits.

On 14 March, radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 751 μSv/h on one location and at 02:40 an intensity of 650 μSv/h at another location on the premises. On 16 March, the maximum readings peaked at 10850 μSv/h.

Explosion of reactor 1 building

At 15:36 JST on 12 March, there was an explosion at unit 1. Four workers were injured. The side walls of the upper level of the reactor building were blown away, leaving in place only the vertical steel framed gridworks. The black roof was falling down and covering the level and some machinery on the south side. The lower walls were intact compared to what would happen later to reactors 3 and 4. A video of the explosion shows that it was primarily directed sideways.

The roof of the building was designed to provide ordinary weather protection for the areas inside, not to withstand the high pressure of an explosion or to act as containment for the reactor. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel and containing spent fuel ponds.

Experts soon agreed the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels, and this hydrogen then leaked into the containment building. Exposed zircaloy cladded fuel rods became very hot and reacted with steam, oxidising the alloy, and releasing hydrogen. Safety devices normally burn the venting hydrogen before it reaches explosive concentrations. These systems failed, though, possibly due to the shortage of electrical power.

Officials indicated the container of the reactor had remained intact and there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. ABC News (Australia) reported that according to the Fukushima prefectural government, radiation dose rates at the plant reached 1015 microsievert per hour (1015 µSv/h). The IAEA stated on 13 March that four workers had been injured by the explosion at the unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but that fell below their guidance for emergency situations.

Two independent nuclear experts cited design differences between the Chernobyl Nuclear Power Plant and the Fukushima I Nuclear Power Plant, one of them saying he did not believe that a Chernobyl-style disaster would occur.

Seawater used for cooling

At 20:05 JST on 12 March, the Japanese government ordered seawater to be injected into unit 1 in a new effort to cool the reactor core. The strategy had been held as a last resort because contamination by salt and other impurities will force TEPCO to decommission the reactor. TEPCO started seawater cooling at 20:20, adding boric acid as a neutron absorber to prevent a criticality accident. The water was to take five to ten hours to fill the reactor core, after which the reactor would cool down in around ten days. The injection of seawater into the reactor pressure vessel was performed by means of mobile fire trucks of the fire department. At 01:10 on 14 March, injection of seawater was halted for two hours because all available water in the plant pools had run out (similarly, feed to unit 3 was halted). NISA news reports stated 70% of the fuel rods had been damaged when uncovered.

On 18 March, a new electrical distribution panel was installed in an office adjacent to unit 1 to supply power via unit 2 when it was reconnected to the transmission grid two days later. On 21 March, injection of seawater continued, as did repairs to the control instrumentation. On 23 March, it became possible to inject water into the reactor using the feed water system rather than the fire trucks, raising the flow rate from 2 to 18 m³/h (later reduced to 11m³/h), and on 24 March, lighting was restored to the central operating room.

As of 24 March, the spent fuel pool was "thought to be fully or partially exposed", according to CNN. Pressure in the reactor increased due to the seawater injection, resulting in steam discharges, later alleviated by reducing the water flow. Temperature increases were also reportedly temporary. TEPCO attributed some of the steam to water in the spent fuel pool.

Reactor Stabilization

On 25 March, fresh water became available again to be added to the reactor instead of salt water, while work continued to repair the unit's cooling systems. A volume of 1,890 cubic metres (500,000 US gal) of fresh water was brought to the plant by a barge provided by US Navy. On 29 March, the fire trucks which had been used to inject water into the reactor were replaced by electrically-driven pumps.

On 28 March, pumping began to remove water contaminated with radioactive ¹³⁷Cs and ¹³¹I from basement areas, storing it in the condenser system. By 29 March, pumping was halted because condensate reservoirs were almost full and plans were being considered to transfer water to the suppression pool surge tanks.

On 7 April, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further explosions, but increase the reactor temperature. The injection was expected to take 6 days. If the injection was successful, TEPCO expected to repeat the operation on the other units at Fukushima. Later on 7 April, but prior to a large aftershock, temperatures in the reactor core unexpectedly "surged in temperature to 260 C," the cause was unknown, but the temperature dropped 246 degrees by 8 April.

Possibility of criticality at unit 1

Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 March to 16 March raised the possibility that nuclear fission could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi. 16 March reports that fuel rods in the spent fuel pool at unit 4 could have been exposed appeared to indicate that fission may have occurred in that fuel pool. Later reports of exceptionally high iodine-134 levels appeared to confirm this theory because iodine-134 would be indicative of fission reactions. High measurements of chlorine-37 and chlorine-38 were also found in the now retracted TEPCO report. The high chlorine levels caused some nuclear experts to calculate that fission may instead be occurring in unit 1. Despite the accepted erroneous nature of the iodine-134 report, the IAEA appeared to accept the chlorine-based analysis as a possible theory suggesting fission by stating at a press conference that "melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions". Later isotope analysis released by TEPCO appeared to indicate no testing for chlorine but did show high levels of other possible fission products.

Reactor unit 2

Unit two was operating at the time of the earthquake and experienced the same controlled initial shutdown as the other units. Diesel generators and other systems failed when the tsunami overran the plant. Reactor core isolation cooling system (RCIC) initially operated to cool the core but by midnight the status of the reactor was unclear; some monitoring equipment was still operating on temporary power. Coolant level was stable and preparations were underway to reduce pressure in the reactor containment vessel should this become necessary, though TEPCO did not state in press releases what these were, and the government had been advised that this might happen. RCIC was reported by TEPCO shut down around 19:00 JST on 12 March, but again reported to be operating as of 09:00 JST 13 March. Pressure reduction of the reactor containment vessel commenced before midnight on 12 March though IAEA reported as of 13:15 JST 14 March that according to information supplied to them, no venting had taken place at the plant. A report in the New York Times suggested that plant officials initially concentrated efforts on a damaged fuel storage pool at unit 2, distracting attention from problems arising at the other reactors, but this incident is not reported in official press releases. The IAEA reported that on 14 March at 09:30 RCIC was still operating and that power was being provided by a mobile generator.

Cooling problems at unit 2

On 14 March, TEPCO reported the failure of the RCIC system. Fuel rods had been fully exposed for 140 minutes and there was a risk of a core meltdown. Reactor water level indicators were reported to be showing minimum values at 19:30 JST on 14 March.

At 22:29 JST, workers had succeeded in refilling half the reactor with water but parts of the rods were still exposed, and technicians could not rule out the possibility that some had melted. It was hoped that holes blown in the walls of reactor building 2 by the earlier blast from unit 3 would allow the escape of hydrogen vented from the reactor and prevent a similar explosion. At 21:37 JST, the measured dose rates at the gate of the plant reached a maximum of 3.13 millisievert per hour, which was enough to reach the annual limit for non-nuclear workers in twenty minutes, but had fallen back to 0.326 millisieverts per hour by 22:35.

It was believed that around 23:00 JST, the 4 m long fuel rods in the reactor were fully exposed for the second time. At 00:30 JST on 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at unit 3 might have caused a glitch in the cooling system of unit 2: Four out of five water pumps being used to cool unit 2 reactor had failed after the explosion at unit 3. In addition, the last pump had briefly stopped working when fuel ran out. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods. As of 04:11 JST on 15 March, water was being pumped into the reactor of unit 2 again.

Explosion in reactor 2 building

An explosion was heard after 06:14 JST on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel. The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate all non-essential workers from the plant. Only a minimum crew of 50 men, also referred to as the Fukushima 50, was left at the site. Soon after, radiation equivalent dose rates had risen to 8.2 mSv/h around two hours after the explosion and again down to 2.4 mSv/h, shortly after. Three hours after the explosion, the rates had risen to 11.9 mSv/h.

While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes. In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at unit 2 of "less than five percent". The Japan's Nuclear and Industrial Safety Agency (NISA) stated 33% of the fuel rods were damaged, in news reports the morning of 16 March. On 30 March, NISA reiterated concerns about a possible unit 2 breach at either the suppression pool, or the reactor vessel. NHK World reported the NISA's concerns as "air may be leaking," very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels."

Continuing radiation

By midday on 19 March grid power had been connected to the existing transformer at unit 2 and work continued to connect the transformer to the new distribution panel installed in a nearby building. Outside electricity became available at 15:46 JST on 20 March, but equipment still had to be repaired and reconnected.

On 20 March, 40 tons of seawater were added to the spent fuel pool. The temperature in the spent fuel pool was 53 °C as of 22 March 11:00 JST. The Japan Atomic Industrial Forum (JAIF) reported "high radiation readings" in the area.

Unit 2 was considered the most likely unit to have a damaged reactor containment vessel, as of 24 March. On 27 March, TEPCO reported measurements of very high radiation levels, over 1000 mSv/h, in the basement of the unit 2 turbine building, which officials reported was 10 million times higher than what would be found in the water of a normally functioning reactor. Hours into the media frenzy, the company retracted its report and stated that the figures were not credible. "because the level was so high the worker taking the reading had to evacuate before confirming it with a second reading." Shortly following the ensuing wave of media retractions that discredited the report worldwide, TEPCO clarified its initial retraction; the radiation from the pool surface in the basement of the unit 2 turbine building was found to be "more than 1,000 millisieverts per hour," as originally reported, but the concentration of radioactive substances was 100,000 times higher than usual, not 10 million. On 28 March, the Nuclear Safety Commission announced its suspicion that radioactive materials had leaked from unit 2 into water in trenches connecting unit 2's buildings, leading TEPCO to reduce the amount of water pumped into the reactor because of fears that the water could leak into the sea. The reduction in water pumping could have raised reactor temperatures.

On 27 March, the IAEA reported temperatures at the bottom of the Reactor Pressure Vessel (RPV) at unit 2 fell to 97 °C from 100 °C on Saturday. Operators attempted to pump water from the turbine hall basement to the condenser. However, "both condensers turned out to be full." Therefore, condenser water was first attempted to be pumped to storage tanks, freeing condenser storage for water currently in the basement of unit 2. The pumps now being used can move 10 to 25 tons of water per hour. On 19 April 2011, TEPCO began transferring excess, radioactive cooling water from the reactor's basement and maintenance tunnels to a waste processing facility.

On 29 March, Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core could have possibly melted through the reactor vessel onto a concrete floor, raising concerns of a major release of radioactive material.

Reactor unit 3

Unlike the other five reactor units, reactor 3 runs on mixed uranium and plutonium oxide, or MOX fuel, making it potentially more dangerous in an incident due to the neutronic effects of plutonium on the reactor, the long persistence of plutonium toxicity, and the plutonium carcinogenic effects in the event of release to the environment. Units 3 and 4 have a shared control room.

Cooling problems at unit 3

Early on 13 March an official of the Japan Nuclear and Industrial Safety Agency (NISA) told at a news conference that the emergency cooling system of unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core. At 05:38 there was no means of adding coolant to the reactor due to loss of power. Work to restore power and to vent excessive pressure continued. At one point, the top three metres of mixed oxide (MOX) fuel rods were not covered by coolant.

At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health" and manual venting took place at 08:41 and 09:20. At 09:25 JST on 13 March, operators began injecting water containing boric acid into the primary containment vessel (PCV) via the pump of a fire truck. When water levels continued to fall and pressure to rise, the injected water was switched to seawater at 13:12. By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased. A rise was eventually recorded but the level stuck at 2 m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.

Injection of seawater into the primary containment vessel (PCV) was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed. On the morning of 15 March, Secretary Edano announced that according to TEPCO, at one location near reactor units 3 and 4, radiation at an equivalent dose rate of 400 mSv/h was detected. This might have been due to debris from the explosion in unit 4.

Explosion of reactor 3 building

At 12:33 JST on 13 March, the chief spokesman of the Japanese government, Yukio Edano said hydrogen gas was building up inside the outer building of unit 3 just as had occurred in unit 1, threatening the same kind of explosion. At 11:15 JST on 14 March, the envisaged explosion of the building surrounding reactor 3 of Fukushima 1 occurred, due to the ignition of built up hydrogen gas. The Nuclear and Industrial Safety Agency of Japan (NISA) reported, as with unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in unit 1 and felt 40 kilometers away. Pressure readings within the reactor remained steady at around 380 kPa at 11:13 and 360 kPa at 11:55 compared to nominal levels of 400 kPa and a maximum recorded of 840 kPa. Water injection continued. Dose rates of 0.05 mSv/h were recorded in the service hall and of 0.02 mSv/h at the plant entrance. It was reported that day that eleven people were injured in the blast. Six soldiers from the Japanese Central Nuclear Biological Chemical Weapon Defence Unit are reported to have been killed in the explosion.

Spent fuel pool

Around 10:00 JST on 16 March, NHK helicopters flying 30 km away videotaped white fumes rising from the Fukushima I facility. Officials suggested that the reactor 3 building was the most likely source, and said that its containment systems may have been breached. The control room for reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST. At 16:12 JST, Self Defence Force (SDF) Chinook helicopters were preparing to pour water on unit 3, where white fumes rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50 mSv. At 21:06 pm JST, the government reported that major damage to reactor 3 was unlikely but that it nonetheless remained their highest priority.

Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter and four helicopter drops of seawater took place around 10:00 JST. The riot police used a water cannon to spray water onto the top of the reactor building and then were replaced by members of the SDF with spray vehicles. On 18 March a crew of firemen took over the task with six fire engines each spraying 6 tons of water in 40 minutes. 30 further hyper rescue vehicles were involved in spraying operations. Spraying continued each day to 23 March because of concerns the explosion in unit 3 may have damaged the pool (total 3,742 tonnes of water sprayed up to 22 March) with changing crews to minimise radiation exposure. Lighting in the control room was restored on 22 March after a connection was made to a new grid power supply and by 24 March it was possible to add 35 tonnes of seawater to the spent fuel pool using the cooling and purification system. Grey smoke was reported to be rising from the southeast corner of unit 3 on 21 March. The spent fuel pool is located at this part of the building. Workers were evacuated from the area. TEPCO claimed no significant change in radiation levels and the smoke subsided later the same day.

On 23 March, black smoke billowed from unit 3, prompting another evacuation of workers from the plant, though Tokyo Electric Power Co. officials said there had been no corresponding spike in radiation at the plant. "We don't know the reason for the smoke", Hidehiko Nishiyama of the Nuclear Safety Agency said.

Possible breach

On 25 March, officials announced the reactor vessel might be breached and leaking radioactive material. High radiation levels from contaminated water prevented work. Japan Nuclear and Industrial Safety Agency (NISA) reiterated concerns about a unit 3 breach on 30 March. NHK World reported the NISA's concerns as "air may be leaking," very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted," but that "there is no indication of large cracks or holes in the reactor vessels." As with the other reactors, water was transferred from condenser reservoirs to the suppression pool surge tanks so that condensers could be used to hold radioactive water pumped from the basement.

Reactor unit 4

Reactor unit 3 (right) and unit 4 (left) on 16 March.

At the time of the earthquake unit 4 had been shut down for shroud replacement and refueling since 29 November 2010. All 548 fuel rods had been transferred in December 2010 from the reactor to the spent fuel pool on an upper floor of the reactor building where they were held in racks containing boron to damp down any nuclear reaction. The pool is used to store rods for some time after removal from the reactor and now contains 1,479 rods. Recently active fuel rods produce more decay heat than older ones. At 04:00 JST on Monday 14 March, water in the pool had reached a temperature of 84 °C compared to a normal value of 40–50 °C. The IAEA was advised that the temperature value remained 84 °C at 19:00 JST on 15 March, but as of 18 March, no further information was reported. On 11 April, a fire broke out at unit 4.

Explosion of reactor 4 building

At approximately 06:00 JST on 15 March, an explosion—thought to have been caused by hydrogen accumulating near the spent fuel pond—damaged the 4th floor rooftop area of the unit 4 reactor as well as part of the adjacent unit 3. Later reports from the US Nuclear Regulatory Commission speculated that fuel could have been ejected from the unit 4 spent fuel pond during this event. Later on the morning of 15 March, at 09:40, the unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there. TEPCO said workers extinguished the fire by 12:00. As radiation levels rose, some of the employees still at the plant were evacuated. On the morning of 15 March, Secretary Edano announced that according to the TEPCO, radiation dose equivalent rates measured from the unit 4 reached 100 mSv/h. Edano said there was no continued release of "high radiation".

Japan's nuclear safety agency NISA reported two holes, each 8 meters square, or 64 square metres (690 sq ft), in a wall of the outer building of unit 4 after the explosion. At 17:48 it was reported that water in the spent fuel pool might be boiling. By 21:13 on 15 March, radiation inside the unit 4 control room prevented workers from staying there permanently. Seventy staff remained at the plant, while 800 had been evacuated. By 22:30, TEPCO was reportedly unable to pour water into the spent fuel pool. By 22:50, the company was considering using helicopters to drop water, but this was postponed because of concerns over safety and effectiveness, and the use of high-pressure fire hoses was considered instead.

A fire was discovered at 05:45 JST on 16 March in the northwest corner of the reactor building by a worker taking batteries to the central control room of unit 4. This was reported to the authorities, but on further inspection at 06:15 no fire was found. Other reports stated that the fire was under control. At 11:57, TEPCO released a photograph showing "a large portion of the building's outer wall has collapsed." Technicians considered spraying boric acid on the building from a helicopter.

Spent Fuel Pool

On 16 March, the chairman of United States Nuclear Regulatory Commission (NRC), Gregory Jaczko, said in Congressional testimony that the NRC believed all of the water in the spent fuel pool had boiled dry. Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan. At 13:00 TEPCO claimed that helicopter observation indicated that the pool had not boiled off. The French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) agreed, stating that helicopter crews diverted planned water dumps to unit 3 on the basis of their visual inspection of unit 4.

At approximately 14:30 on 16 March, TEPCO announced that the storage pool, located outside the unit 4 containment area, might be boiling. Around 20:00 JST it was then planned to use a police water cannon to spray water on unit 4.

On 18 March, it was reported that water sprayed into the spent fuel pool was disappearing faster than evaporation could explain, suggesting leakage. SDF military trucks sprayed water onto the building to try to replenish the pool on 20 March. On 22 March, the Australian military flew in Bechtel-owned robotic equipment for remote spraying and viewing of the pool. The Australian reported this would give the first clear view of the pool in the "most dangerous" of the reactor buildings.

The IAEA reported, "From 22 March to 25 March 130 to 150 tonnes of seawater were poured into the spent fuel pool each day using a concrete pump equiped with a long articulated arm. Seawater was also poured in through spent fuel cooling system from 21:05 UTC 24 March to 01:20 25 March. White smoke was still being observed coming from the reactor building as of 23:00 UTC 25 March." On 29 March, the seawater was changed to fresh water.

Analysis of spent fuel pool water collected on 12 April suggests that while some of the 1535 fuel assemblies stored there may have been damaged, the majority of the stored fuel assemblies are intact based on measured radiation levels. TEPCO further stated that "the fuel rods in the Unit 4 pool had released caesium-134 and -137 in the process of being damaged," and that TEPCO would "need to continue monitoring it." On 13 April, TEPCO reported that the temperature of the spent fuel pool had increased to 90 degrees, and that the radiation level 6 meters above the pool had reached 84 mSv/h. The spike was later attributed to a failure to properly keep the SFP covered in water.

Possibility of criticality in the spent fuel pool

TEPCO claimed that there was a small but non-zero probablity that the exposed fuel assemblies could reach criticality. The BBC commented that criticality would never mean a nuclear explosion, but could cause a sustained release of radioactive materials. Criticality is usually considered highly unlikely due to the low enrichment level used in light water reactors.

Reactor units 5 and 6

Both reactors were offline at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010), although they were still fueled, unlike reactor 4 where the fuel rods had been removed prior to the earthquake.

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well. At 09:16 JST, the removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape. At 21:00 on 15 March, water levels in unit 5 were reported to be 2 m above fuel rods, but had fallen 40cm in 5 hours. Published water temperatures on 18 March showed 182°C inside reactor 5 and 161°C in reactor 6.

On 17 March, unit 6 was reported to have operational diesel-generated power and this was to be used to power pumps in unit 5 to run the Make-up Water Condensate System (MUWC) to supply more water. Preparations were made to inject water into the reactor pressure vessel once external power could be restored to the plant, as water levels in the reactors were considered to be declining. It was estimated that grid power might be restored on 20 March through cables laid from a new temporary supply being constructed at units 1 and 2.

Information provided to the IAEA indicated that storage pool temperatures at both units 5 and 6 remained steady around 60–68 °C between 19:00 JST 14 March and 21:00 JST 18 March, though rising slowly. On 18 March reactor water levels remained around 2 m above the top of fuel rods. It was confirmed that panels had been removed from the roofs of units 5 and 6 to allow any hydrogen gas to escape. At 04:22 on 19 March, the second unit of emergency generator A for unit 6 was restarted which allowed operation of pump C of the residual heat removal system (RHR) in unit 5 to cool the spent fuel storage pool. Later in the day, pump B in unit 6 was also restarted to allow cooling of the spent fuel pool there. Temperature at unit 5 pool decreased to 48 °C on 19 March 18:00 JST, and 37 °C on 20 March when unit 6 pool temperature had fallen to 41 °C. On 20 March, NISA announced that both reactors had been returned to a condition of cold shutdown. External power was partially restored to unit 5 via transformers at unit 6 connected to the Yonoromi power transmission line on 21 March.

On 23 March, it was reported that the cooling pump at reactor No 5 stopped working when it was transferred from backup power to the grid supply. This was repaired and the cooling restarted approximately 24 hours later. RHR cooling in unit 6 was switched to the permanent power supply on 25 March.

Radiation levels and radioactive contamination

Map of contaminated areas around the plant (22 March-03 April).

Comparison of radiation levels for different nuclear events.

Main article: Radiation effects from Fukushima I nuclear accidents

Radioactive material was released from containment on several occasions after the tsunami struck, the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Using Japanese Nuclear Safety Commission numbers, Asahi Shimbun reported that by 24 March the accident might have emitted 30,000 to 110,000 TBq of iodine-131. The highest reported radiation dose rate outside was 1000 mSv/h on 16 March, prompting a temporary evacuation of plant workers. On 29 March, at times near unit 2, radiation monitoring was hampered by a belief that some radiation levels may be higher than 1000 mSv/h, but that "1,000 millisieverts is the upper limit of their measuring devices." The maximum permissible dose for Japanese nuclear workers was increased to 250 mSv/year, for emergency situations after the accidents. TEPCO has been criticized in providing safety equipment for its workers, including accusations of a lack of monitoring equipment and giving the most dangerous work to subcontractors.

The Japanese Ministry of Health, Labour and Welfare announced that levels of radioactivity exceeding legal limits had been detected in milk produced in the Fukushima area and in certain vegetables in Ibaraki. Measurements made by Japan in a number of locations have shown the presence of radionuclides on the ground. On 23 March, Tokyo drinking water exceeded the safe level for infants, prompting the government to distribute bottled water to families with infants. Seawater near the discharge of the plant elevated levels of iodine-131 were found on 22 March, which had increased to 3,355 times the legal limit on 29 March. Also concentrations far beyond the legal limit were measured for caesium-134 and caesium-137 were more than 100 times above the limit.

Contamination of basements, wiring trenches, and pipe tunnels

Side view of the Fukushima trenches and tunnels. 1: Reactor building, 2: Turbine generator and associated condenser.

Leakage route of highly radioactive water through a gravel layer.
1: Reactor building, 2: Turbine building, 3: Injection of sodium silicate.

As illustrated in the diagram to the right, the Fukushima I nuclear plant has a number of trenches and pipe tunnels that stretch from each unit's reactor (diagram #1), to the unit's turbine building (diagram #2), to the sea (to the right of diagram #6). In some locations these connections are open trenches, while in other locations the connections are pipe tunnels.

During work to restore power to unit 2 on 27 March, TEPCO reported very high levels of radiation in water in the basement of the unit 2 turbine building. While first reported radiation levels of more than 10 million times usual appeared later to be erroneous, the radiation measurements were more than 100,000 times higher than usual. On 28 March, the Nuclear Safety Commission announced its suspicion that "radioactive substances from temporarily melted fuel rods at the No. 2 reactor had made their way into water in the reactor containment vessel and then leaked out through an unknown route". Highly radioactive water was later found in trenches at three of the units. These trenches stretch toward, but do not directly connect to, the sea (see diagram #6). On 30 March, the units 2 and 3 trenches were 1 m below the level at which they would overflow into the sea. In comparison, the unit 1 trench was 10 cm from overflowing.

The high levels of water in the trenches combined with their potential to overflow to the sea complicated the cooling efforts because the water required to cool the reactor was believed to also be filling the trenches. Hence, cooling unit 2 with large quantities of fresh water was expected to cause the trenches, leading to the sea, to fill and overflow—worsening the radioactivity release. Consequently, TEPCO reduced the amount of water injected into unit 2 from 16 to 7 ton per hour. TEPCO used two approaches to prevent the highly radioactive water from leaking into the sea.

The first approach to prevent tunnel water from leaking into the sea was to pump the tunnels dry. Beginning on 27 March, operators attempted to pump water from the turbine hall basement (see the tunnel below diagram #2) to the condenser (the large black vessel). By pumping water out of the basement, TEPCO expected to lower the trench water level, and reduce the likelihood of overspill to the sea. However, "both condensers turned out to be full," which prevented pumping. Therefore, pumps able to shift 10 to 25 tons of water per hour were used to move condenser water to storage tanks, freeing condenser storage for water that was in the basement of unit 2. However, since both the storage tanks and the condensers were nearly full, TEPCO also considered using tankers or a "mega float" as a temporary storage location for the radioactive water. Regardless of the availability of offshore storage for radioactive-contaminated water, TEPCO decided to pump its least contaminated water, approximately 100 times the legal limit, from a wastewater treatment plant, out to sea on 5 April to free storage space. The wastewater facility had 11,500 tons of water stored (by 10 April 8900 tons had been pumped into the sea). At the same time, on 5 April, TEPCO began pumping water from the units condensers of units 1-3 to their respective condensation storage tanks to free room for the trench water.

The second approach used by TEPCO to limit overflow into the sea was to plug a leak in a pit which was located near the unit 2 reactor basement and above the trench system. The crack in the pit was believed to be the primary source of water to the trench system. Reports indicated that the leak was from a crack 20 cm in size, and that it may have been leaking since the magnitude 9 earthquake shook the plant on 11 March. However, radiation levels above the pit exceeded 1,000 mSv/h (1 Sv/h, 100 Rem/h), hampering technicians to safely work. Regardless, TEPCO attempted to use sandbags and concrete to plug the leak. However, by 2 April, TEPCO acknowledged the water was still leaking into the trenches and to the sea. On 2 April, TEPCO said that it had again attempted to plug the hole, now using 2,000 liters of a synthetic resin. TEPCO attempted to inject a polymeric water absorbent, used for diapers, into pipes leading to the pit; this absorbent was also coupled with sawdust and shredded newspapers. However, on 3 April and 4 April, this approach appeared to have failed to slow the leak, leading TEPCO to use a colored dye to confirm the location and size of the leak. The dye indicated the leak was from a cracked pipe and seeping through gravel into the pit. On 5 April, TEPCO began using liquid glass to attempt to stop the leak. Finally, on 6 April, TEPCO drilled a hole into the pit near unit 2 and injected water glass (sodium silicate) into the pit. The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture. Shortly afterward, TEPCO announced that water had stopped leaking from the pit.

In an attempt to prevent future leaks, TEPCO installed seven steel plates at unit 2 that would prevent water from flowing out the plant's water intakes (see diagram #6). Additional plates were expected to be added at the other Fukushima units. However, these plates were later suspected of "stirring up" radioactive debris, and to have significantly increased radiation measured in the sea. With the leak plugged, at least temporarily, on 10 April TEPCO returned to the work began on 27 March, removing water from the tunnel system so repairs could be made to the plant's original cooling system. Removal is considered essential because the water is so radioactive, in excess to the 1000 mSv/h measuring equipment's range, that repair work cannot be safely conducted without removing the water.

By 13 April, TEPCO had pumped approximately "250 tonnes of highly radioactive water from the trench into the unit's turbine condenser," lowering the trench water-level by 4 cm. The water was approximately 99 cm deep originally. TEPCO estimated that pumping would take "about 40 hours to move some 700 tonnes of water from the trench." Water would eventually have to be removed from the unit 2 basement, as well as from the trenches and basements of units 1 and 3. By 15 April, TEPCO estimated that 660 tons of 60,000 tons of the highly radioactive water had been pumped from the trenches. The water level was believed to have fallen by 8 cm. and TEPCO announced it expected to start storing some of the trench water in storage spaces freed up by dumping 9,100 tons of slightly contaminated water from a wastewater treatment plant from 4 April to 10 April. However, shortly after announcing the reduction in level, the water level began increasing again, a 2.5 cm increase on 16 April, and a 3 cm increase on 17 April were believed to have been caused by the earlier efforts to patch leaks to the ocean.

Long term, TEPCO is "also considering pouring adhesive concrete into the suppression chamber of reactor 2 to patch the hole that is believed to be causing radioactive water to leak into the turbine building and the trench."

Central fuel storage areas

Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond. This contains 6375 fuel assemblies and was reported "secured" with a temperature of 55 °C. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities. On 21 March temperatures in the fuel pond had risen a little to 61 °C and water was sprayed over the pool. Power was restored to cooling systems on 24 March and by 28 March temperatures were reported down to 35 °C.

Accident rating

Radiation releases during the initial hydrogen explosions.

The severity of the nuclear accident is provisionally rated 7 on the International Nuclear Event Scale (INES). This scale runs from 0, indicating an abnormal situation with no safety consequences, to 7, indicating an accident causing widespread contamination with serious health and environmental effects. Prior to Fukushima, the Chernobyl disaster was the only level 7 accident on record, while the Three Mile Island accident was a level 5 accident.

The Japan Atomic Energy Agency initially rated the situation at unit 1 below both of these previous accidents; on 13 March it announced it was classifying the event at level 4, an "accident with local consequences". On 18 March it raised its rating on units 1, 2 and 3 to level 5, an "accident with wider consequences". It classified the situation at unit 4 as a level 3 "serious incident".

Several parties disputed the Japanese classifications, arguing that the situation was more severe than they were admitting at the time. On 14 March, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6. One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6. as of 18 March, the French nuclear authority—and as of 15 March, the Finnish nuclear safety authority—estimated the accidents at Fukushima to be at Level 6 on the INES. On 24 March, a scientific consultant for noted anti-nuclear environmental group Greenpeace, working with data from the Austrian ZAMG and French IRSN, prepared an analysis in which he rated the total Fukushima accident at INES level 7.

The Asahi Shimbun newspaper reported on 26 March that the accident might warrant level 6, based on its calculations. The Wall Street Journal stated that Japan's NISA would make any decision on raising the level. INES level 6, or "serious accident," had only been applied to the Kyshtym disaster (Soviet Union, 1957), while the only level 7 was Chernobyl (Soviet Union, 1986). Previous level 5 accidents included the Windscale fire (United Kingdom, 1957); the Lucens reactor (Switzerland, 1969); Three Mile Island (United States, 1979); and the Goiânia accident (Brazil, 1987).

Assessing "seriousness" as partial or full meltdown at a civilian plant, the New York Times reported on 3 April that based on remote sensing, computer "simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown." The Times counted three previous civilian meltdowns, from World Nuclear Association information: Three Mile Island; Saint-Laurent Nuclear Power Plant (France, 1980, INES level 4); and Chernobyl.

On 11 April, the Japanese Nuclear and Industrial Safety Agency (NISA) temporarily raised the disaster at Fukushima Daiichi to Level 7 on the INES scale, by considering the whole event and not the single incidents to the reactors (rated between 3 and 5). This would make Fukushima the second Level 7 "major accident" in the history of the nuclear industry; however, up to now, only about a tenth of the radiations released in Chernobyl went out of damaged Fukushima I plant.

Radiation in other countries

The Fukushima accident has led to "trace" amounts of radiation, including iodine-131 and caesium-134/137, being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria).

Radiation from direct fallout

The levels detected by air filters in countries outside Japan are extremely low. Health Canada stated that the increase measured in Canada was less than the natural day-to-day variation in the existing background levels; the presence of fallout could only be detected by analysing the isotopes present, and there was no significant increase in the total level of radiation .

Health Canada put the extra radiation in Canada's air due to Fukushima for March 18 at 0.0005 µSv, compared to background levels from 0.02 to 1.20 µSv depending on the region .

Monitoring of levels in the US by the EPA and independent university teams also showed that levels were low. .

Radiation in rainwater and food

Radioisotopes can be concentrated by precipitation or by bioaccumulation (where plants/animals, including ultimately humans, selectively take up and concentrate particular elements). The caesium radioisotopes are potentially more dangerous than iodine-131 in the long term, because they have longer half-lives (two years for Cs-134, 30 years for Cs-137) than I-131 (half-life of 8 days), so the risk of persistence in the environment and of long-term accumulation in organisms is greater.

I-131 can be concentrated by leafy vegetables and in milk/cheese. CRIIRAD, a French NGO, warned on 7 April that children and pregnant women in Europe should limit consumption of these, in addition to avoiding rainwater as a primary drinking source, as a precautionary measure, although it put the risk as "quite low". Gerry Thomas, a molecular pathologist and director of the Chernobyl tissue bank at Imperial College London, dismissed the warning as "absolutely ridiculous" .

Low levels of caesium radioisotopes were detected in China, and in CA, USA. Detectable levels of radioactive isotopes in milk were present in 6 of the cities tested by the EPA in the USA, with the maximum levels reported in the city of Hilo, HI; the levels were 24 pCi/l (890 mBq/l), 19 pCi/l (700 mBq/l) and 18 pCi/l (670 mBq/l) for caesium-134, caesium-137 and iodine-131 respectively.

A university of Berkeley team observed a peak I-131 level of 540 picocuries per liter in rainwater. This greatly exceeded the EPA's 3 picocurie per liter standard for radioactive iodine in drinking water, although that is based on consumption of the water every day for 70 years.

It is important to note that the allowed level set by regulatory agencies can vary; the levels mandated by the FDA for milk are thousands of times higher than those mandated by the EPA for water. This is partly due to different assumptions about how long the product will be consumed, and partly due to different thresholds of risk.

The levels of contamination observed are much smaller than the level of background radiation or radiation exposure from activities like flying. Robert Alvarez of the Institute for Policy Studies criticised such direct comparisons to external sources, saying they do not take into account the risk of accumulation (although he agreed that the risks are extremely small).

As of April 12, 2011, no serious contamination had been observed of food and water produced outside Japan.

Radioactive debris

A marine oceanographer at the International Pacific Research Center expressed concern that current models are not adequate to predict how contaminated debris swept out of the reactor will behave .

Reactions

Reaction in Japan and evacuation measures

Main article: Evacuations and Japanese reaction to the Fukushima I nuclear accidents

A Tokyo Metropolitan Police Department Kidotai (riot police) water cannon; this type was used at Fukushima to cool the plant.

J-village, Naraha, where the TEPCO "base camp" has been established.

A nuclear emergency was declared by the Government at 19:03 on 11 March. Later Prime Minister Naoto Kan issued instructions that people within a 20 km (12 mile) zone around the plant must leave, and urged that those living between 20 km and 30 km from the site to stay indoors. The latter zone was later subject to voluntary evacuation. The Prime Minister visited the plant for a briefing on 12 March. He called for calm and against exaggerating the danger.

On 30 March, the IAEA discovered 20 MBq/m² of iodine-131 samples taken from 18 to 26 March in Iitate, Fukushima, 40 km northwest of the Fukushima I reactor. The IAEA recommended expanding the evacuation area, based on its criteria of 10 MBq/m². Secretary Edano stated the government would wait to see if the high radiation continued. On 31 March, the IAEA announced a new value of 7 MBq/m², in samples taken from 19 to 29 March in Iitate. The material decays at 8% to 9% each day.

Major news source reporting at least 2 TEPCO employees confirmed dead from "disaster conditions" following the earthquake. "Their bodies had to be decontaminated," suggesting death might have taken place due to explosion.

TEPCO established a "base camp" at J-Village, a sport training centre located in Naraha and Hirono, some 20 km South of the plant.

One month after the tsunami hit, engineers are no closer to regaining control of the six reactors, and Japan plans to extend the evacuation zone around the Fukushima I plant because of high radiation levels.

Six weeks after the crisis began, plans were announced for a large-scale study of the environmental and health effects of radioactive contamination from the nuclear plant. Academics and researchers from across Japan will work with the Fukushima Prefectural Government starting in May.

International reaction

Evacuation flight departs Misawa.

Main article: International reaction to Fukushima I nuclear accidents

The international reaction to the nuclear accidents has been a humanitarian response to the 2011 Tōhoku earthquake and tsunami, also to those people affected by the events at Fukushima I. The response has also included the expression of concern over the developments at the reactors and the risk of escalation. The accidents have furthermore prompted re-evaluation of existing and planned national nuclear energy programs, with some commentators questioning the future of the nuclear renaissance.

USA, Australia and Sweden have instructed their citizens to evacuate a radius of minimum 80 km. Spain has advised their citizens to leave an area of 120 km, Germany and Australia have advised their citizens to leave even the metropolitan area of Tokyo, and South Korea advises to leave farther than 80 km and to have plans to evacuate by all possible means. Travel to Japan is very low, but additional flights have been chartered by some countries to assist those who wish to leave. In mid-March, several nations had begun official efforts to evacuate their citizens from Japan.

The World Health Organization announced its intention to conduct continuing public health studies over the next 20 years.

Reactor status summary

Legend

No immediate concern

Concern

Severe Condition

Status of Fukushima I at 15 April 10:00 JST	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6
Rated Electrical Power output (MWe)	460	784	784	784	784	1,100
Rated Thermal Power output (MWt)	1,380	2,381	2,381	2,381	2,381	3,293
Type of reactor	BWR-3	BWR-4	BWR-4	BWR-4	BWR-4	BWR-5
Containment type	Mark I	Mark I	Mark I	Mark I	Mark I	Mark II
Core fuel assemblies	400	548	548	0	548	764
Spent fuel assemblies	292	587	514	1,331	946	876
Spent fuel residual decay heat	60 kW	400 kW	200 kW	2,000 kW	700 kW	600 kW
Fuel type	Low-enriched uranium	Low-enriched uranium	Mixed-oxide (MOX) and low-enriched uranium	Low-enriched uranium	Low-enriched uranium	Low-enriched uranium
Status at earthquake	In service	In service	In service	Outage (scheduled)	Outage (scheduled)	Outage (scheduled)
Fuel integrity	Damaged (70% estimated)	Damaged (30% estimated)	Damaged (25% estimated)	Some spent fuel damaged	Not damaged	Not damaged
Reactor pressure vessel integrity	Unknown	Damage and leakage suspected	Damage and leakage suspected	Not damaged (defueled)	Not damaged	Not damaged
Containment integrity	Not damaged (estimation)	Damage and leakage suspected	Not damaged (estimation)	Not damaged	Not damaged	Not damaged
Core cooling system 1 (ECCS/RHR)	Not functional	Not functional	Not functional	Not necessary (defueled)	Functional	Functional
Core cooling system 2 (RCIC/MUWC)	Not functional	Not functional	Not functional	Not necessary (defueled)	Functional (in cold shutdown)	Functional (in cold shutdown)
Building integrity	Severely damaged due to hydrogen explosion	Slightly damaged, also panel removed to prevent hydrogen explosion	Severely damaged due to hydrogen explosion	Severely damaged due to hydrogen explosion	Panel removed to prevent hydrogen explosion	Panel removed to prevent hydrogen explosion
Pressure vessel, water level	Fuel exposed partially or fully	Fuel exposed partially or fully	Fuel exposed partially or fully	Safe (defueled)	Safe (in cold shutdown)	Safe (in cold shutdown)
Pressure vessel, pressure (Two instrument trains)	Stable at 0.423 MPa (Train A) / 0.948 MPa (Train B – suspected faulty) at 14 April 12:00 JST	Stable at -0.016 MPa (Train A) / -0.021 MPa (Train B) at 14 April 12:00 JST	Stable at -0.017 MPa (Train A) / -0.085 MPa (Train B) at 14 April 12:00 JST	Safe (defueled)	Safe (in cold shutdown)	Safe (in cold shutdown)
Pressure vessel, temperature	Decreased to 180°C on 18 April	Decreased to 141°C on 18 April	Unchanged at 91°C on 18 April	Safe (defueled)	Safe (in cold shutdown)	Safe (in cold shutdown)
Containment pressure	Stable at 0.190 MPa (absolute) at 11 April 06:00 JST	Stable at atmospheric pressure on 10 April	Stable at atmospheric pressure on 10 April	Safe	Safe	Safe
Water injection into core	Seawater injection started 12 March, with freshwater injection started 25 March and continuing	Seawater injection started 14 March, with freshwater injection started 26 March and continuing	Seawater injection started 13 March, with freshwater injection started 25 March and continuing	Not necessary (defueled)	Not necessary	Not necessary
Seawater injection into containment vessel	Not required	Not required	Not required	Not necessary	Not necessary	Not necessary
Containment venting	Temporarily stopped	Temporarily stopped	Temporarily stopped	Not necessary	Not necessary	Not necessary
INES (individual)	Level 5	Level 5	Level 5	Level 3	–	–
INES (combined)	Level 7
Environmental effect	Radiation levels: South side of office building: decreased to 550 µSv/hour at 14 April 21:00 JST Main gate: decreased to 68 µSv/hour at 14 April 21:00 JST West gate: decreased to 29 µSv/hour at 14 April 21:00 JST Radiation levels higher than 1000 mSv were detected at the surface of water accumulating in the basement of Unit 2 and the piping tunnel outside the Unit 2 building on 27 March Plutonium was detected in small amounts in soil samples near the station on 21, 22, 25, and 28 March Radioactive iodine, caesium, ruthenium, and tellurium have been detected in seawater samples in the vicinity of the station since 21 March Levels of radioactive I-131 4,385 times higher than regulatory limits were detected in seawater samples on 30 March Levels of radioactive I-131 7.5 million times higher than regulatory limits were detected in seawater samples near the Unit 2 water intake on 2 April On 2 April, highly radioactive (over 1000 mSv/hour) water was discovered leaking from a concrete structure housing electrical cables into the sea through cracks in the concrete wall; as of 6 April, this leakage has been stopped On 4 April, the release of 10,000 tons of low-level radioactive wastewater began in order to make room for the highly radioactive water discovered on 2 April. Regarding the influence of the low level radioactive waste release, TEPCO evaluated that eating fish and seaweed caught near the plant every day for a year would add some 25% of the dose that the general public receive from the environment for a year. TEPCO and MEXT has expanded the monitoring for the surrounding sea area since 4 April. Radioactive material was detected in underground water sampled near the turbine buildings on 30 March Radioactive material has been detected in milk and agricultural products from Fukushima and neighboring prefectures, prompting government limits on shipments (21 March) and intake (23 March) for some products Radioactive iodine was found in tap water in some prefectures between 21 and 27 March, prompting government warnings not to drink the water in those regions; as of 2 April, these warnings have been lifted Levels of radioactive caesium above regulatory limits were detected in small fish caught off the coast of Ibaraki on 4 April
Evacuation radius	20 km from Nuclear Power Station (NPS), but 30 km should consider leaving as of 25 March 11:30 JST
General status from all sources regarding reactor cores	Stabilized by injecting sea water and boron	Stabilized by injecting sea water and boron	Stabilized by injecting sea water and boron	Defueled	Cold shutdown on 20 March 14:30 JST	Cold shutdown on 20 March 19:27 JST
General status from all sources regarding Spent Fuel Pools (SFP)	Sprayed freshwater injection started, 26°C on 12 April 07:50	Freshwater injection continues, 71°C on 14 April 12:00 JST	Sprayed freshwater injection continues, 59°C on 12 April 07:50	Sprayed freshwater injection continues after hydrogen explosion from pool on 15 March, 37°C on on 12 April 07:50	Cooling system restored, 35.5°C on 14 April 13:00 JST	Cooling system restored, 32.0°C on 4 April 13:00 JST
Information sources

Cleanup

Initially, TEPCO did not put forward a strategy to regain control of the situation in the reactors. Helmut Hirsch, a German physicist and nuclear expert, says "they are improvising with tools that were not intended for this type of situation". However, on 17 April, TEPCO appeared to put forward the broad basis of a plan which includes: (1) reaching "cold shutdown in about six to nine months;" (2) "restoring stable cooling to the reactors and spent fuel pools in about three months;" (3) putting "special covers" on units 1, 3, and 4 starting in June; ; (4) installing "additional storage containers for the radioactive water that has been pooling in the turbine basements and outside trenches;" (5) using radio-controlled equipment to clean up the site; and (6) using silt fences to limit ocean contamination. Previously, TEPCO publicly committed to installing new emergency generators 20m above sea level, twice the height of the generators destroyed by the 11 March tsunami. Toshiba and Hitachi had both proposed plans for shuttering the facility.

Critics were "not fully convinced TEPCO could meet the timetable it has set for itself to achieve a cold shutdown" because the "scale and complexity of the challenge is unprecedented." Long term plans for units 5 and 6 have not been announced, "but they too may need to be decommissioned."

Attempted

Legend

Effective

Partially effective

Not effective

Not applicable or unknown

Solution attempted	General effectiveness	Specific effectiveness
		Reactor cores							Spent fuel pools
		1	2	3	4	5	6		1	2	3	4	5	6
Backup diesel generators The built-in backup diesel generators operated initially.	All generators failed when the 14 meter tsunami overtopped tsunami walls designed for a 5.7 m tsunami. One generator repaired 17 March at units 5 and 6 to cool spent fuel pools. A second on 19 March powered reactor cooling and reactors 5 and 6 were brought back to cold shutdown.
Backup batteries The built-in backup batteries maintained some control functions and limited cooling for 8 hours after the generators failed.	Effective but only designed to work for 8 hours. The operators were unable to connect portable generators before the 8 hours ran out. Note, limited cooling was not designed to stabilise reactors indefinitely, even if batteries were recharged.
Mobile power units After the power loss due to the earthquake and tsunami, mobile power units were sent to the plant.	Flooding of plant prevented mobile generators being connected and sufficiently large units were not available	Some central monitoring systems
Repair power lines to provide electricity 1 km of new grid cable and replacement switchgear was installed to provide power to plant.	Power was connected to the distribution panels of Units 2 and 5 on Sunday 20 March and power is now available to Units 1, 2, 5 and 6 but only equipment in units 5 and 6 is sufficiently repaired to function. Units 3 and 4 are scheduled to have electricity connected to their distribution panels on 22 March.	Central control room
Emergency cooling systems The built-in Emergency cooling systems (ECCS) include: High Pressure Coolant Injection System (HPCI), Reactor Core Isolation Cooling System (RCIC), Automatic Depressurization System (ADS), Low Pressure Core Spray System (LPCS), Low Pressure Coolant Injection System (LPCI), Depressurization Valve System (DPVS), Passive Containment Cooling System (PCCS), and Gravity Driven Cooling System (GDCS).	The RCIC operated initially and the HPCI worked until the toruses overheated. Cooling systems were restored to Units 5 and 6 on 20 March. BBC news reported on 17 March, that some of the original water pumps might be inoperable due to damage from injected sea water, the earthquake, tsunami or explosions.
Cooling core containment areas by adding sea water Sea water with neutron absorbing boric acid is being manually injected into the Reactor Pressure Vessel of units 1, 2 and 3 via fire extinguisher system line.	Sea water cooling has been partially effective for the cooling reactor core but fuel damage has taken place. At one point available water in site pools ran out. White smoke or steam was reported rising from Unit 3.
Cooling core containment with fresh water and boron By 28 March, all core cooling was using fresh water rather than sea water, which is corrosive. In unit 2, neutron-absorbing boric acid was added to the water.	Unknown.
Spraying water into spent fuel pools with water cannon and fire engines Police, military and firemen working in shifts to reduce radiation exposure used water cannons and fire engines to spray water onto the roofs and into the spent fuel pools of reactor unit Number 3 although it is unclear if any water reached the spent fuel pool. Unit 4 was sprayed also.	SFP 1: As of Saturday 19 March, SFP 1's decrease in water level suggests it may have a leak and spraying may be attempted. SFP was 60 °C on 20 March according to an infrared helicopter measurement. SFP 2: On 20 March, 40 tons of sea water was sprayed into unit 2. SFP was 40 °C on 20 March according to an infrared helicopter measurement. Spraying appeared to have been effective even though the building is nearly intact and there were few holes to spray water into. SFP 3: On Thursday 17 March, steam emanated from the roof of unit 3 after spraying which suggested that spraying was at least partially successful in reaching overheated spent fuel rods. Radiation levels dropped slightly after the steam had dissipated suggesting the cooling may have been successful. SFP 3 may have a leak and reduced water holding capacity due to a previous explosion. SFP 3 was sprayed for 13 hours on 20 March using unmanned vehicles and spraying is continuing. SFP was 60 °C on 20 March according to an infrared helicopter measurement. Water in SFP 4 was disappearing faster than evaporation could explain and some suspected that a hole was leaking water from that pool. Inspection during helicopter water drops indicated there was water in the pool of Unit 4; so, spraying on Friday 18 March focused on Unit 3. SFP 4 stopped boiling although it is releasing a large amount of heat due to the large amount of fuel stored there. On Sunday 20 March, SFP 4 was sprayed with 100 tons of water using unmanned vehicles. SFP was 40 °C on 20 March according to an infrared helicopter measurement.
Helicopter dumping of sea water into spent fuel pools Helicopters used for forest fire suppression dumped sea water onto the reactors.	Strong winds prevented effective targeting of the dumped water since high radiation levels prevented the helicopters from flying low. Video appeared to show much of the water dispersing in mid-air weakening the intensity of the water to cool the overheating reactor.

Proposed

Boric acid is airlifted to Fukushima for addition to cooling water.

Boron

Officials have considered insertion or targeted aerial dropping of boric acid, boronated plastic beads or boron carbide pellets into the spent fuel pools to absorb neutrons. France flew 95 tonnes of boron to Japan on 17 March 2011 and the US has provided 9 tons. Neutron absorbing boric acid has been injected into the reactor cores, but is unclear if boron was included with the spraying of spent fuel pools (SFP)s.

Sarcophagus

On 18 March, Reuters reported that Hidehiko Nishiyama, Japan's nuclear agency spokesman when asked about burying the reactors in sand and concrete, said: "That solution is in the back of our minds, but we are focused on cooling the reactors down." Considered a last-ditch effort since it would not provide cooling, such a plan would require massive reinforcement under the floor, as for the Chernobyl Nuclear Power Plant sarcophagus.

Fabric cover

An effort has been undertaken to fit the three damaged reactor buildings with fabric covers and filters to limit radiation release. The cost of building structures around units 1 - 4 and wrapping them with the sheets are estimated to reach 80 billion yen. On 6 April, sources told Kyodo News that a major construction firm was studying the idea, and that construction wouldn't "start until June." The plan has been criticized for potential only having "limited effects in blocking the release of radioactive substances into the environment."

Scope of cleanup

International experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. John Price, a former member of the Safety Policy Unit at the UK's National Nuclear Corporation, has said that it "might be 100 years before melting fuel rods can be safely removed from Japan's Fukushima nuclear plant". Edward Morse, a professor of nuclear engineering at the University of California, Berkeley, has said:

... there would be at least six months of emergency stabilisation, about two years of temporary remediation and up to 30 years of full-scale clean-up. Furthermore, the high levels of ground contamination at the site are raising concerns about the viability of individuals to work at the site in coming decades.

However, according to BBC News, Japanese reactor maker Toshiba said it could decommission the earthquake-damaged Fukushima nuclear power plant in about 10 years, a third quicker than the American Three Mile Island plant. As a comparison, it took 11 years after the accident before the vessel for the partially melted core at Three Mile Island was first opened, with cleanup taking several more years.

TEPCO announced on 17 April 2011 that it expected to have the automated cooling systems restored in the damaged reactors in about three months and have the reactors put into cold shutdown status in six months.

Debris

On 10 April 2011, TEPCO began using remote-controlled, unmanned heavy equipment to remove debris from around reactors 1–4. The debris and rubble, caused by hydrogen explosions at reactors 1 and 3, was impeding recovery operations both by being in the way and emitting high radioactivity. The debris will be placed into containers and kept at the plant.

Water

The Japanese government has requested that Russia send the floating water decontamination plant Landysh to assist in processing radioactive water from the damaged reactors. Landysh was built by Russia with funding from Japan to process liquid wastes produced during the decommissioning of nuclear submarines.

Administrative issues

Safety record

The Fukushima Daiichi nuclear power complex was central to a falsified-records scandal that led to the departure of a number of senior executives of TEPCO. It also led to disclosures of previously unreported problems at the plant. In 2002, TEPCO admitted it had falsified safety records at the No. 1 reactor at Fukushima Daiichi. As a result of the scandal and a fuel leak at Fukushima, the company had to shut down all of its 17 nuclear reactors to take responsibility.

In addition to concerns from within Japan, the International Atomic Energy Agency (IAEA) has also expressed concern about the ability of Japan's nuclear plants to withstand seismic activity. At a meeting of the G8's Nuclear Safety and Security Group, held in Tokyo in 2008, an IAEA expert warned that a strong earthquake with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.

In March 2006 the Japanese government opposed a court order to close a nuclear plant in the west part of the country over doubts about its ability to withstand an earthquake. Japan's Nuclear and Industrial Safety Agency believed it was "safe" and that "all safety analyses were appropriately conducted."

Regulatory relationship with nuclear industry

In 2010, Toru Ishida, the former director general of the Ministry of Economy, Trade and Industry (METI), which has responsibilities that include regulating nuclear industry, left the agency and joined TEPCO a few months later to become a senior adviser. He followed Susumu Shirakawa, another METI veteran who was a board member and executive vice president at TEPCO until retiring in June 2010.

Regulatory capture may have contributed to the cascade of failures which were revealed after the tsunami receded. Regulatory capture may have also contributed to the current situation. Critics argue that the government shares blame with regulatory agency for not heeding warnings, for not ensuring the independence of the nuclear industry's oversight while encouraging the expansion of nuclear energy domestically and internationally.

Future of TEPCO

On 30 March 2011, the president of TEPCO, Masataka Shimizu, was hospitalised with symptoms of dizziness and high blood pressure in the wake of an increasingly serious outlook for the Fukushima plant and increasing levels of radiation from the stricken plant, as well as media reports of TEPCO's imminent nationalization or bankruptcy triggered by the situation at the Fukushima plant.

Specific government plans to "dismantle" TEPCO were circulating by April 16, expected to include state involvement culminating in a bankruptcy and reorganization process.

See also

Background genpatsu-shinsai, an event predicted by professor Katsuhiko Ishibashi in 1997 International Nuclear Event Scale Kyshtym disaster (1957, INES 6) Three Mile Island accident (1979, INES 5) Chernobyl disaster (1986, INES 7) Orders of magnitude (radiation) Nuclear safety Hydrogen production at high temperature Fukushima I Nuclear Power Plant

Ongoing Timeline of the Fukushima I nuclear accidents Fukushima 50 2011 Japanese nuclear accidents 2011 Tōhoku earthquake and tsunami International reaction to Fukushima I nuclear accidents Evacuations and Japanese reaction to the Fukushima I nuclear accidents Radiation effects from Fukushima I nuclear accidents

Lists List of civilian nuclear accidents List of civilian nuclear incidents List of civilian radiation accidents Lists of nuclear disasters and radioactive incidents Japanese nuclear incidents

Major earthquakes of Japan 1923 Great Kantō earthquake Tokai earthquakes

External links

• Webcam Fukushima nuclear power plant I
• Chronology of images from a webcam Template:Ref-ru
• TEPCO News Releases, Tokyo Electric Power Company
• NISA Information update, Nuclear and Industrial Safety Agency, the nuclear safety authority of Japan
• JAIF Information update, Japan Atomic International Forum
• JAEA Information update, Japan Atomic Energy Agency
• IAEA Update on Japan Earthquake, International Atomic Energy Agency
• Second Explosion at Reactor No. 3 on YouTube
• Nature Journal – Specials: Japan earthquake and nuclear crisis
• Anatomy of a meltdown, Nature
• Weather forecast for the area
• FLEXPART analysis showing particle transport forecast for the northern hemisphere
• Weather Maps: Precipitation
• Radiation Dose Chart
• Extensive collection of sources about the Fukushima nuclear accidents (expert links and research sources from Journalismus.com) (pdf) (German)