The Green environment minister Jürgen Trittin has claimed that the reliability of the Brunsbüttel plant has been "called into question" because full details of an incident on 14 December, 2001, were not reported to the federal authorities until February. The incident involved the rupture of a head spray cooling system pipe.
The Federal Ministry of Environment and Nuclear Safety (BMU), which is headed by Tritten, said that it was first informed about the event on 18 February. However, on 17 December, the Ministry of Finance and Energy (MFE), the state regulators of Schleswig-Holstein, were informed of a leak in the Brunsbüttel head spray system. MFE was not satisfied by Hamburgische Electricitäts-Werke's (HEW's) appraisal of the event and maintains that, for a month, HEW refused to reduce power or shut down the reactor in order to perform a walkdown of the containment area.
Trittin has ordered the MFE to explain why it did not force HEW to shut the plant last December.
The BMU has issued a report stating that, after considerable pressure by the MFE, on 18 February, HEW agreed to reduce the power level to 10% to allow an inspection to be carried out. The inspection established that a length of between two and three metres of 10cm diameter piping was shattered into about 25 pieces. While the cause of the incident is still unclear, HEW believes that a hydrogen explosion inside the piping caused the rupture. The affected pipe was confirmed to be part of the head spray cooling system.
The BMU report states: "The event has a safety significance because, not only was an explosive mixture of hydrogen and oxygen generated, but the explosion took place in piping that is directly behind the pressure boundary. Had the explosion taken place between three to four metres closer to the pressure vessel.....there would have been a LOCA (loss of coolant accident) event requiring operation of ECCS (emergency core cooling system)." Following the discovery of the pipe rupture, the reactor was immediately shut down. The BMU will only allow the reactor to be restarted if the following conditions are met:
• The damage mechanism must be understood, and appropriate measures taken to prevent it happening again.
• All damage must be identified and repaired.
• An investigation into why, after control room operators detected that a radioactive release under containment had taken place, the plant was not immediately shut down.
Reports in the German media have quoted sources claiming that technicians at Kernkraftwerk Brunsbüttel (KKB), the Brunsbüttel plant operating company, wanted to lower power to hold an inspection, but had been overruled by HEW management.
HEW has categorically rejected the allegations. It said that, in full accordance with standard safety procedures, the shift leader immediately reduced output to 50% following the discovery of a "steam leak" in a head spray cooling system pipe. Full power was resumed only after a detailed analysis of the available information.
HEW has pointed out that the initial incident and all relevant information until the inspection on 18 February had been reported to the MFE. The utility also said that the decision to continue operation until that date was supported by the technical inspection agency TÜV (Technischer Überwachungsverein).
HEW has denied that there was any safety significance, as the system is only used to help cool down the pressure vessel closure head during shutdown and has no function during normal operation.
The plant's head spray cooling system uses an early KWU design. The system is connected to the pressure vessel by an isolation valve, and is only used during shutdowns to reduce the time needed for cooling by about two hours.
Following the findings at Brunsbüttel, in-containment piping at all German BWRs has been examined by the utilities' safety experts and the nuclear safety authorities. Preliminary conclusions are that the Brunsbüttel design was unique, and that the incident is not likely to have safety implications for other BWRs.
The incident has been given a provisional rating of INES level 1. A report submitted to the International Atomic Energy Agency justifies the rating on the grounds of degradation of defence in depth.
Brunsbüttel operating company KKB is owned by HEW (67%) and E.ON (33%). HEW is majority-owned by Vattenfall.