A reactor shutoff rod failure, combined with several operator errors, led to a major power excursion of more than double the reactor's rated output at AECL's NRXreactor. The operators purged the reactor's heavy water moderator, and the reaction stopped in under 30 seconds. A cover gas system failure led to hydrogen explosions, which severely damaged the reactor core. The fission products from approximately 30 kg (66 lb) of uranium were released through the reactor stack. Contaminated light-water coolant leaked from the damaged coolant circuit into the reactor building; some 4,000 m3 (140,000 cu ft) were pumped via pipeline to a disposal area to avoid contamination of the Ottawa River. Subsequent monitoring of surrounding water sources revealed no contamination. After the incident, approximately 1202 people were involved in the two year cleanup. No immediate fatalities or injuries resulted from the incident; a 1982 follow-up study of exposed workers showed no long-term health effects, though Atomic Energy of Canada Limited (AECL) dosimetry files were lost in a 1956 fire. Future U.S. PresidentJimmy Carter, an engineer and then a Lieutenant in the US Navy, was among the cleanup crew.
Due to inadequate cooling a damaged uranium fuel rod caught fire and was torn in two as it was being removed from the core at the NRU reactor. The fire was extinguished, but not before radioactive combustion products contaminated the interior of the reactor building and, to a lesser degree, an area surrounding the laboratory site. Approximately 679 people were employed in the clean-up.  A Corporal named Bjarnie Hannibal Paulson who was at the cleanup did not die from his exposure, but developed unusual skin cancers. Paulson had to testify at many hearings before he was awarded compensation for his radiation injuries.
25 October 1958 - INES Level needed - Vinča, Serbia (then Yugoslavia) - Criticality excursion, irradiation of personnel
During a subcritical counting experiment a power buildup went undetected at the Vinca Nuclear Institute's zero-power natural uranium heavy water moderated research reactor. Saturation of radiation detection chambers gave the researchers false readings and the level of moderator in the reactor tank was raised triggering a criticality excursion which a researcher detected from the smell of ozone. Six scientists received radiation doses of 2–4 sieverts (200–400 rem)  (p. 96). An experimental bone marrow transplant treatment was performed on all of them in France and five survived, despite the ultimate rejection of the marrow in all cases. A single woman among them later had a child without apparent complications. This was one of the first nuclear incidents investigated by then newly formed IAEA.
A partial core meltdown took place when the Sodium Reactor Experiment (SRE) experienced a power excursion that caused severe overheating of the reactor core, resulting in the melting of one-third of the nuclear fuel and significant releases of radioactive gases. Regarded by some[who?] as the worst nuclear accident in U.S. history, the amount of radiation released is variously reported as 240 to 260 times worse than Three Mile Island. Over the succeeding years, the site was cleaned up and all buildings and contamination removed. The soil was removed and other soil brought in and now forms a portion of an area near the Simi Valley Adventist Hospital.
A core melt accident occurred at the Westinghouse Waltz Mill test reactor. From what information remains of the event, one fuel element melted, resulting in the disposition of 2 million gallons of contaminated water generated during the accident. At least a portion of the water was retained on site in lagoons, a condition which eventually led to detectable Sr-90 in ground water plus contaminated soil. The site is currently undergoing cleanup.
An error by a worker at a United Nuclear Corporation fuel facility led to an accidental criticality. Robert Peabody, believing he was using a diluted uranium solution, accidentally put concentrated solution into an agitation tank containing sodium carbonate. Peabody was exposed to 100 Gy (10,000 rad) of radiation and died two days later. Ninety minutes after the criticality, a plant manager and another administrator returned to the building and were exposed to 1 Gy (100 rad), but suffered no ill effects.
5 October 1966 — INES Level needed - Monroe, Michigan, United States - Partial meltdown
A sodium cooling system malfunction caused a partial meltdown at the Enrico Fermi demonstration nuclear breeder reactor (Enrico Fermi-1 fast breeder reactor). The accident was attributed to a zirconium fragment that obstructed a flow-guide in the sodium cooling system. Two of the 105 fuel assemblies melted during the incident, but no contamination was recorded outside the containment vessel.
Winter 1966-1967 (date unknown) – INES Level needed – location unknown – loss of coolant accident
The SovieticebreakerLenin, the USSR’s first nuclear-powered surface ship, suffered a major accident (possibly a meltdown — exactly what happened remains a matter of controversy in the West) in one of its three reactors. To find the leak the crew broke through the concrete and steel radiation shield with sledgehammers, causing irreparable damage. It was rumored that around 30 of the crew were killed. The ship was abandoned for a year to allow radiation levels to drop before the three reactors were removed, to be dumped into the Tsivolko Fjord on the Kara Sea, along with 60% of the fuel elements packed in a separate container. The reactors were replaced with two new ones, and the ship re-entered service in 1970, serving until 1989.
Graphite debris partially blocked a fuel channel causing a fuel element to melt and catch fire at the Chapelcross nuclear power station. Contamination was confined to the reactor core. The core was repaired and restarted in 1969, operating until the plant's shutdown in 2004.
A total loss of coolant led to a power excursion and explosion of an experimental nuclear reactor in a large cave at Lucens. The underground location of this reactor acted like a containment building and prevented any outside contamination. The cavern was heavily contaminated and was sealed. No injuries or fatalities resulted.
De-fuelling and partial dismantling occurred from 1969 to 1973. In 1988, the lowest caverns were filled with concrete, and a regulatory permit was issued in December 1990. Currently, the archives of the Canton of Vaud are located in the caverns.
A fire in a cable duct after a short-circuit disabled the electrical power supply for all feedwater and emergency core cooling pumps. A power supply was improvised by the operating personnel after several hours.
Operators neglected to remove moisture-absorbing materials from a fuel rod assembly before loading it into the KS 150 reactor at power plant A-1. The accident resulted in damaged fuel integrity, extensive corrosion damage of fuel cladding and release of radioactivity into the plant area. The affected reactor was decommissioned following this accident.
Equipment failures and worker mistakes contributed to a loss of coolant and a partial core meltdown at the Three Mile Island Nuclear Generating Station 15 km (9.3 mi) southeast of Harrisburg. While the reactor was extensively damaged, on-site radiation exposure was under 100 millirems (less than annual exposure due to natural sources). Area residents received a smaller exposure of 1 millirem (10 µSv), or about 1/3 the dose from eating a banana per day for one year. There were no fatalities. Follow-up radiological studies predict between zero and one long-term cancer fatality.
A brief power excursion in Reactor A2 led to a rupture of fuel bundles and a minor 80 GBq (2,200 mCi) release of nuclear materials at the Saint-Laurent Nuclear Power Plant. The reactor was repaired and continued operation until its decommissioning in 1992.
March 1981 — INES Level 2 - Tsuruga, Japan - Radioactive materials released into Sea of Japan + Overexposure of workers
An operator error during a fuel plate reconfiguration in an experimental test reactor led to an excursion of 3×1017 fissions at the RA-2 facility. The operator absorbed 20 Gy of gamma and 17 Gy of neutron radiation which killed him two days later. Another 17 people outside of the reactor room absorbed doses ranging from 350 mGy to less than 10 mGy. pg103
26 April 1986 — INES Level 7 - Prypiat, Ukraine (then USSR) - Power excursion, explosion, complete meltdown
An inadequate reactor safety system led to an uncontrolled power excursion, causing a severe steam explosion, meltdown and release of radioactive material at the Chernobyl nuclear power plant located approximately 100 kilometers north-northwest of Kiev. Approximately fifty fatalities (mostly cleanup personnel) resulted from the accident and the immediate aftermath. An additional nine fatal cases of thyroid cancer in children in the Chernobyl area have been attributed to the accident. The explosion and combustion of the graphite reactor core spread radioactive material over much of Europe. 100,000 people were evacuated from the areas immediately surrounding Chernobyl in addition to 300,000 from the areas of heavy fallout in Ukraine, Belarus and Russia. An "Exclusion Zone" was created surrounding the site encompassing approximately 3,000 km2 (1,200 sq mi) and deemed off-limits for human habitation for an indefinite period. Several studies by governments, UN agencies and environmental groups have estimated the consequences and eventual number of casualties. Their findings are subject to controversy.
Spherical fuel pebbles became lodged in the pipe used to deliver fuel elements to the reactor at an experimental 300-megawatt THTR-300HTGR. Attempts by an operator to dislodge the fuel pebble damaged the pipe, releasing activated coolant gas which was detectable up to two kilometers from the reactor.
6 April 1993 — INES Level 4 - Tomsk, Russia - Explosion
A pressure buildup led to an explosive mechanical failure in a 34 m3 (1,200 cu ft) stainless steel reaction vessel buried in a concrete bunker under building 201 of the radiochemical works at the Tomsk-7 Siberian Chemical Enterprise plutonium reprocessing facility. The vessel contained a mixture of concentrated nitric acid, 8,757 kg (19,306 lb) uranium, 449 g (15.8 oz) plutonium along with a mixture of radioactive and organic waste from a prior extraction cycle. The explosion dislodged the concrete lid of the bunker and blew a large hole in the roof of the building, releasing approximately 6 GBq (160 mCi) of Pu 239 and 30 TBq (810 Ci) of other radionuclides into the environment. The contamination plume extended 28 km (17 mi) NE of building 201, 20 km (12 mi) beyond the facility property. The small village of Georgievka (pop. 200) was at the end of the fallout plume, but no fatalities, illnesses or injuries were reported. The accident exposed 160 on-site workers and almost two thousand cleanup workers to total doses of up to 50 mSv (the threshold limit for radiation workers is 20 mSv/yr).
Operators attempting to insert one control rod during an inspection neglected procedure and instead withdrew three causing a 15 minute uncontrolled sustained reaction at the number 1 reactor of Shika Nuclear Power Plant. The Hokuriku Electric Power Company who owned the reactor did not report this incident and falsified records, covering it up until March, 2007.
Inadequately trained part-time workers prepared a uranyl nitrate solution containing about 16.6 kg (37 lb) of uranium, which exceeded the critical mass, into a precipitation tank at a uranium reprocessing facility in Tokai-mura northeast of Tokyo, Japan. The tank was not designed to dissolve this type of solution and was not configured to prevent eventual criticality. Three workers were exposed to (neutron) radiation doses in excess of allowable limits. Two of these workers died. 116 other workers received lesser doses of 1 mSv or greater though not in excess of the allowable limit.
Partially spent fuel rods undergoing cleaning in a tank of heavy water ruptured and spilled fuel pellets at Paks Nuclear Power Plant. It is suspected that inadequate cooling of the rods during the cleaning process combined with a sudden influx of cold water thermally shocked fuel rods causing them to split. Boric acid was added to the tank to prevent the loose fuel pellets from achieving criticality. Ammonia and hydrazine were also added to absorb iodine-131.
20 t (20 long tons; 22 short tons) of uranium and 160 kg (350 lb) of plutonium dissolved in 83 kl (2,900 cu ft) of nitric acid leaked over several months from a cracked pipe into a stainless steel sump chamber at the Thorp nuclear fuel reprocessing plant. The partially processed spent fuel was drained into holding tanks outside the plant.
Tritium contamination of groundwater was discovered at Exelon's Braidwood station. Groundwater off site remains within safe drinking standards though the NRC is requiring the plant to correct any problems related to the release.
35 l (7.7 imp gal; 9.2 US gal) of a highly enriched uranium solution leaked during transfer into a lab at Nuclear Fuel Services Erwin Plant. The incident caused a seven-month shutdown. A required public hearing on the licensing of the plant was not held due to the absence of public notification.
After the 2011 Tōhoku earthquake and tsunami of March 11, the emergency power supply of the Fukushima-Daiichi nuclear power plant failed. This was followed by deliberate releases of radioactive gas from reactors 1 and 2 to relieve pressure.
On March 12, triggered by falling water levels and exposed fuel rods, a hydrogen explosion occurred at reactor 1, resulting in the collapse of the concrete outer structure. Although the reactor containment itself was confirmed to be intact, the hourly radiation from the plant reached 1.015 Sv (101.5 rem) - an amount equivalent to that allowable for ordinary people in one year.
Residents of the Fukushima area were advised to stay inside, close doors and windows, turn off air conditioning, and to cover their mouths with masks, towels or handkerchiefs as well as not to drink tap water. By the evening of March 12, the exclusion zone had been extended to 20 kilometres (12 mi) around the plant and 70,000 to 80,000 people had been evacuated from homes in northern Japan.
On March 14, a second, hydrogen explosion (nearly identical to the first explosion in Unit 1) occurred in the reactor building for Unit 3, with similar effects.
On March 15, a third explosion occurs in the “pressure suppression room” of Unit 2 and is initially said not to have breached the reactor’s inner steel containment vessel, but later reports indicated that the explosion damaged the steel containment structure of Unit 2 and much larger releases of radiation were expected than previously.
On March 15, a fourth explosion damages the 4th floor area above the reactor and spent fuel pool of the Unit 4 reactor. Contrary to the TEPCO press release, aerial photos show that most of the outer building was actually destroyed. The fuel rods (both new and spent fuel) of reactor Unit 4, stored in the now exposed spent fuel pool, were reportedly exposed to air – this would have risked the melting of the nuclear fuel. However later research found the fuel rods been covered by water all the time.
On 16 March TEPCO estimated that 70% of the fuel in Unit 1 had melted, and 33% in Unit 2, further suspecting that Unit 3's core might also be damaged. In November 2011 TEPCO released the report of the Modular Accident Analysis Program (MAAP). The report showed that the reactor pressure vessel (RPV) in Unit 1 (commonly known as the reactor core) had been damaged during the disaster, and that significant amounts of fuel had fallen into the bottom of the primary containment vessel (PCV) – the erosion of the concrete of the PCV by the molten fuel immediately after the disaster was estimated to have been stopped in approx. 0.7 metres (2 ft 4 in) depth, with the thickness of the containment being 7.6 metres (25 ft). Gas sampling done before the report detected no signs of an ongoing reaction of the fuel with the concrete of the PCV and all the fuel in Unit 1 was estimated to be "well cooled down, including the fuel dropped on the bottom of the reactor". MAAP further showed that fuel in Unit 2 and Unit 3 had melted, however less than Unit 1, and fuel was presumed to be still in the RPV, with no significant amounts of fuel fallen to the bottom of the PCV. The report further suggested that "there is a range in the evaluation results" from "most fuel in the RPV (some fuel in PCV)" in Unit 2 and Unit 3, to "all fuel in the RPV (none fuel fallen to the PCV)". For Unit 2 and Unit 3 it was estimated that the "fuel is cooled sufficiently". The larger damage in Unit 1 was according to the report due to long time that cooling water was no injected in Unit 1, letting much more decay heat accumulate – for about 1 day there was no water injection for Unit 1, while Unit 2 and Unit 3 had only a quarter of a day without water injection. As of December 2013, it was reported that TEPCO estimated for Unit 1 that "the decay heat must have decreased enough, the molten fuel can be assumed to remain in PCV (Primary container vessel)".
^[Begründung zur atomrechtlichen Anordnung vom 3. Juni 1986 des Ministers für Wirtschaft, Mittelstand und Technologie, declaration given in the Landtag (parliament) of the state Northrhine-Westphalia on 4th June 1986]