On March 6, 2007, Leibstadt NPP (GE BWR/6, 1165 MWel, net) was operating at the full rated power. Plant staff was preparing planned instrumentation functional tests at the SEHR division 51, which required the simulation of SEHR actuation signals. The SEHR-system (Special Emergency Heat Removal System) is a bunkered, protected system designed to ensure the safe shutdown and residual heat removal during special emergency conditions, e.g. airplane crash, explosion, fire, third party action, etc. During the test preparation work, the plant staff did not follow exactly the procedures and inadvertently activated the so called “SEHR ADS”, which resulted in the opening of 8 out of 16 SRV’s during full power operation.
At 9:44 am, 22 seconds after the inadvertent opening of the SRVs, the reactor automatically scrammed due to a RPV low water level signal (level 3). The shift personnel realized that 8 out of 16 SRV’s of the Automatic Depressurization System (ADS) were open. This led to a fast depressurization of the RPV and to a blow-down of coolant through the open SRV’s. The reactor coolant inventory was maintained by the HPCS and RCIC systems, which were actuated manually by the shift personnel. Besides the reactor scram, the following automatic actions were initiated during the event: MSIV isolation, turbine trip, feedwater pump trip, containment isolation, SGTS start, and RWCU isolation. After closing the SRV’s, which required two attempts, the plant was shut down to the subcritical, cold condition.
During the incident, the RPV and other mechanical components experienced significant temperature transients, caused by the fast cooling down of the RCS, and by the supply of cold RCIC coolant. The incident is provisionally rated as INES 1.
Before start-up of the plant, the licensee had to demonstrate that no stresses on any components were above the design limits during the transient. Based on the analysis submitted to the Swiss Federal Nuclear Safety Inspectorate (HSK), HSK agreed to the start-up of the plant on the evening of Thursday, March 8, 2007.