Mühleberg Nuclear Power Plant
|Mühleberg Nuclear Power Plant|
Mühleberg Nuclear Power Plant
|Construction began||1967 – 1971|
|Commission date||6 November 1972|
|Operator(s)||BKW FMB Energie AG|
|Nuclear power station|
|Reactor supplier||General Electric|
|Units operational||1 x 355 MW|
The Mühleberg Nuclear Power Plant (in German Kernkraftwerk Mühleberg, abbreviated to KKM) is located in the Mühleberg municipality in the (Canton of Berne, Switzerland) north of the village of Mühleberg and near a hydroelectric plant. It is operated by the BKW FMB Energie AG.
- 1 History
- 2 Technical specifications
- 3 Cracks in the core shroud
- 4 Nuclear events
- 5 See also
- 6 References
- 7 External links
In parallel with the planning of Beznau 1, the then Bernische Kraftwerke AG decided to build a second nuclear power plant in the canton of Berne. Mühleberg was identified as possible location and the Swiss Federal Office of Energy (SFOE) approved this choice on 21 July 1965. Two years later, on 21 March 1967, a first partial construction permit was issued, followed on 7 March 1968 by the final one.
The reactor entered criticality in March 1971 but due to a fire in a turbine housing the plant had to be shut down for repairs. It eventually started commercial operation on 6 November 1972.
KKM was the only Swiss nuclear power plant that did not have an unlimited operating license. Due to the problems noticed during the commissioning and to some ongoing tests on the emergency cooling systems performed in the United States, until 1980 it received a series of six-month license extensions, then increased to one year. They were followed by a 5 year, a 7 year, and a 10 year license extension. These extensions were justified by technical improvements the operator was instructed to perform. The operating license was extended again in 1998 and was due to expire on 31 December 2012, however, an unlimited license was granted on December 21, 2009.
The plant had requested a limitation removal in 1990 and again in 1996, but they were both rejected by the Swiss Federal Council for political and technical reasons. A third request pending since 2005 at the SFOE was accepted on December 21, 2009.
Acceptance of the power plant by the local population has varied. When KKM was built there was little opposition to nuclear energy, but the situation rapidly changed after the Chernobyl disaster. In 1992 a public referendum in the canton of Berne rejected granting an unlimited operating license to the power plant, with 51% voting against. In 2003, 68% of the population rejected the initiative named Strom ohne Atom, that proposed that the plant should be shut down in 2005 to be replaced by non-nuclear power generation. A similar proposal at cantonal level had already been rejected in 2000 by 64% of voters. In 2006 the Bernese government put forward a medium-term energy strategy including the renunciation to nuclear power. Swiss parliament, however, noted this and withheld their approval, delegating any decision on an eventual shutdown to the federal authorities. The same organ in 2007 urged the Bernese government to work towards granting an unlimited operating license.
No decision has been made about a definitive shutdown date.
The operators Axpo and BKW announced through their subsidiary company Resun AG on 4 December 2008 that they had submitted to federal authorities a framework permit application to build two new nuclear reactors, one of which would be located in Mühleberg. Although precise technical specifications were not defined, the reactor of choice should be of 3rd generation light water type with a net electric power between 1200 and 1600 MW. Cooling should be ensured by an hybrid tower. Following the Fukushima Daiichi nuclear disaster the permit application was suspended indefinitely.
Reactor and generators
The KKM consists of a single boiling water reactor (BWR) of type 4 provided by General Electric Technical Services Company (GETSCO), a then subsidiary of General Electric specialized in nuclear power plants. All the 240 assemblies contain uranium oxide rods. The pressurized vessel has a diameter of 4.04 m and is 19 m high, has a wall thickness of 10.2 cm and operates at 288 °C and 72 bar.
The thermal power of 1097 MW is converted into 355 MW of net electrical power by two Brown Boveri steam turbine generators connected to the 220 kV and 50 kV grids. The power output was increased in 1990 from the original 320 MW.
The system is cooled with water pumped from the Aar. This in average increases the river temperature by 1.3 °C. To prevent damage to the fish population, the power plant's activity must be reduced when the river temperature exceeds 18 °C.
|Unit||Type||Net electrical capacity||Gross electrical capacity||Construction start||Critical state||Connected to electricity grid||Commercial operation||Shutdown|
|Mühleberg||BWR||355 MW||372 MW||Mar. 1967||Mar. 1971||Jul. 1971||Nov. 1972||-|
As all Swiss nuclear power plants, the KKM is subjected to a periodic complete safety assessment. The most recent one took place in 2007.
The reactor is hosted in a five floor concrete-steel building. The walls have a thickness of 60 cm. In its lowest, partially underground part is located the 1.7–1.8 m thick steel and concrete pressure suppression drywell. The structure is able to withstand an earthquake. In case of a plane crash, however, the building could be damaged. The reactor would nevertheless remain intact due to the five 30–50 cm thick insoles and the massive drywell.
The reactor is fed with water through two loops, each provided with its own pump. A third pump serves as reserve.
The emergency cooling of the nucleus is entrusted to three independent systems. A doubly redundant low pressure core spray with a 2·100% heat removal capacity is activated in case of leaks any size in the coolant system. Small leaks may be compensated by the redundant high pressure reactor core isolation cooling system (RCIC) or the redundant alternate low pressure system (ALPS).
By loss of cooling agent in the drywell, the excessive steam will spread into a torus-shaped pressure-suppression pool, where it will condense decreasing the pressure. The 4000 m3 big torus is filled with 2000 m3 water and is in turn cooled by a redundant torus cooling system and a redundant shutdown and torus cooling system with a total of 4·100% residual heat removal capacity. In the extreme case of a meltdown a further system is entrusted with the integrity of the primary containment by spraying and flooding the drywell.
Emergency power is provided through two connections with the nearby Mühleberg Hydroelectric Power Plant and three diesel generators.
In 1989 the SUSAN (Selbstständiges, unabhängiges System zur Abfuhr der Nachzerfallswärme) emergency building started operation. Bunkerized and partially underground, its function is to ensure an emergency shutdown of the reactor even in case of extreme external agents like plane crashes or simultaneous downfall of the dams upstream the plant with resulting submergence of the structures. Amongst others it contains the RCIC, the ALPS, the TCS, some systems for pressure reduction and two of the three emergency diesel generators.
The KKM yearly produces around 35–38 m3 of radioactive waste. They are compressed in situ and transferred to the Central Interim Storage Facility (ZZL) in Würenlingen for conditioning. After this procedure they are returned to the interim storage facility of the Mühleberg plant. KKM estimates its storage capabilities to last at least until 2022. Due to the 2006 moratorium on the recycling of spent fuel, the entire stock of exhausted rods is now transferred to the ZZL for storage.
Cracks in the core shroud
Major concerns about the safety of the reactor arise from the formation of cracks in the so-called core shroud inside the reactor vessel since the 1990s . This is due to steel corrosion by the coolant. Notwithstanding the countermeasures (reinforcements of the shell, chemical additives in the coolant), the cracks are increasing in length year after year. The longest crack is one third of the limit allowed by the safety specifications and it is expected that by 2012 it is still less than half this value. The situation is continuously monitored and other chemical measures for its stabilization are being tested.
In 1986, due to the presence of similar cracks, some components of the cooling water circulation loop had to be exchanged. A substitution of the core shroud or the pressure vessel is however not considered necessary by the Swiss Federal Nuclear Safety Inspectorate (ENSI) that confirms that the power plant is able to operate safely at least until 2012, when the license will expire provisionally. Further technical improvements are needed for an extension until 2032, as desired by the operator.
As of February 2009 no operative nuclear event (INES level 2 or above) ever occurred. Since 1995 one anomaly took place.
Level 1 events
- During a routine check of the reactor protection system in June 1998, a technician opened a steam relief valve in error. The steam that escaped condensed within the torus and heated the water it contained. After 14 minutes the raised temperature within the torus triggered an emergency shutdown of the reactor. The opening of the incorrect valve and the delay before the control room staff reacted appropriately led the event to be classified as INES level 1 (Swiss scale level B).
Significant events before 1995
- In September 1986 a faulty filter in the reconditioning unit for low and middle-active waste leaked a small quantity of radioactive aerosol. As result the surrounding area now shows an anthropogenic average dose rate of 0.0051 mSv/y (status 2007). For comparison the corresponding Swiss legal limit for population exposure is 0.3 mSv/y and the natural dose in the Mühleberg region achieves 1 mSv/y
- In July 1971, during systems tests prior to full commercial operation, some turbine oil ignited. The fire caused heavy damage to the turbine's housing, including cables connected to safety systems. Sufficient safety system connections remained to allow the reactor to be shut down. The nuclear part of the plant was not affected, but the power generation systems needed extensive repairs. The incident caused the start of commercial operation to be significantly delayed.
March 2012: court-order to scrap the reactor
After the request by an anti-nuclear group the Federal Administrative Court of Switzerland ruled in March 2012 that BKW FMB Energy should shut down the reactor by mid-2013. In its rulings the court cited the incomplete repair of cracks in the structures inside the reactor. Besides this the verdict mentioned the safety concerns related to the plant's earthquake resistance. Nevertheless, the Swiss NPPs -Mühleberg included - are judged to be robust against natural hazards like earthquakes by expert organisations like the ENSI and recently as well in the international peer review by ENSREG. The mentioned scarp decision was only ordered by the court under the circumstance that save operation can't be demonstrated until mid-2013
|Wikimedia Commons has media related to Mühleberg Nuclear Power Plant.|
- Swiss Federal Office of Energy (2006-01-23), Authorizations for nuclear installations, retrieved 2009-02-11. In German.
- Swiss Federal Council (1992-12-14), Decision of the Federal Council on the request for an unlimited operating license, retrieved 2009-02-11. In German.
- Swiss Federal Council (1998-10-28), Decision of the Federal Council on the request for an unlimited operating license, retrieved 2009-02-11. In German.
- "Public consultation on the request of the Mühleberg Nuclear Power Plant for an unlimited operating license" (Press release). Swiss Federal Office of Energy. 2008-05-06. Retrieved 2009-02-11. In German.
- "Vote results in canton of Berne on the initiative Strom ohne Atom". Retrieved 2009-02-11. In German.
- "Vote results on the Bernese cantonal initiative for the shutdown of the Mühleberg Nuclear Power Plant". Retrieved 2009-02-11. In German.
- Executive Council of Berne (2006-06-05), Energy policy 2006, retrieved 2009-02-11. In German.
- Grand Council of Berne (2006-11-29), Daily journal of the November 2006 session, pp. 1007–1023, retrieved 2009-02-11[dead link]. In German.
- Grand Council of Berne (2007-11-29), Daily journal of the November 2007 session, pp. 1182–1187, retrieved 2009-02-11[dead link]. In German.
- "Axpo and BKW submit framework permit applications for replacement nuclear power plants in Beznau and Mühleberg" (Press release). Resun AG. 2008-12-04. Retrieved 2009-02-11.
- "Technical specifications of Mühleberg 2". Resun AG. Retrieved 2011-07-26.
- "News: Suspension of general licence procedure". Resun AG. Retrieved 2011-04-11.
- BKW FMB Energie AG, Technical specifications of the KKM, retrieved 2009-02-11[dead link]. In German.
- Swiss Federal Office of Energy (2008-06-01), Swiss electricity statistic 2007, retrieved 2009-02-11. In German.
- Daniel Bernet (November 2000), Problematic points for the waters of the High Aar and its fish population, retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate (2007-12-12), Safety-Related Statement of the HSK on the Periodic Security Inspection of the Mühleberg Nuclear Power Plant, retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate (2003-04-03), Position of the Swiss Federal Nuclear Safety Inspectorate regarding the safety of the Swiss nuclear power plants in the event of an intentional aircraft crash, retrieved 2009-02-11. In German.
- International Atomic Energy Agency (1979-04-27), Decontamination of operational nuclear power plants, retrieved 2009-02-11.
- Swiss Federal Nuclear Safety Inspectorate and TÜV Energie Consult GbR (1999-11-01), Position of the HSK and the TÜV Energie Consulting on the cracks in the KKM containment vessel, retrieved 2009-02-11. In German.
- "Nuclear events". Swiss Federal Nuclear Safety Inspectorate. Retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate (1999-04-01), ENSI Annual Report 1998, retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate (2008-05-08), ENSI Annual Report 2007, retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate (2003-05-01), ENSI directive R-11, retrieved 2009-02-11. In German.
- Swiss Federal Nuclear Safety Inspectorate. "Dose rate baseline map of Switzerland - Muhleberg region". Retrieved 2009-02-11. In German.
- Fokus Anti-Atom. "Short History of the KKM". Retrieved 2009-02-11. In German.
- NHK-world (9 March 2012) Swiss court orders nuclear shutdown
- News-release from "Nuklearforum"
- Official website. In German and French.
- Power Reactor Information System - Mühleberg
- International Nuclear Safety Center - Mühleberg