Approach for the Analysis of a Shutdown Scenario in the Framework of the Validation of Severe Accident Management Guidelines in Belgium

During last four years, in the framework of the periodic safety review of the Belgian Nuclear Power Plants, the Severe Accident Management Guidelines implemented in Belgium have been involved in a series of detailed validation exercises as suggested by the Westinghouse Owner Group SAMG Scenario Templates. The purpose of this task is essentially to evaluate the severe accident management capabilities of the units and to ensure that personnel in the utility’s emergency response organization (crisis team and eventually the control room staff for certain type of accidents) are trained with the use of the above mentioned guidelines. The supporting calculations to the validation exercises have been performed by Tractebel Engineering by means of the MELCOR 1.8.5 code, which is developed under the sponsorship of the United States Nuclear Regulatory Commission (USNRC). Most of the implemented scenarios and related validation exercises account for full power operating states and are based on previous PSA studies. These included Station Black-Out accidents (SBO), Small Break Loss of Coolant Accidents (SBLOCA), Large Break Loss of Coolant Accidents (LBLOCA), and Interface System Loss of Coolant Accidents (ISLOCA), possibly including additional losses of available emergency safeguards features (ECCS, containment sprays, fan coolers, chemical and volumetric control system). In order to cover the entire spectrum of possible scenarios, it has been judged necessary to consider also a type of accident not originated at nominal power but initiated while the plant is in shutdown conditions. The specific Plant Operating State characterizing this scenario has been defined by a mid-loop operation with the reactor pressure vessel head still in place, and including the opening of the pressurizer manhole, the installation of the nozzle dams in all steam generators, the isolation of the reactor building, and the operation of the Residual Heat Removal system. The initiating event of this accident is the loss of the Residual Heat Removal system one day after the normal reactor shutdown. A point demanding a special attention is the fact the entry criterion to redirect towards the opening of the SAMG (based on core exit temperature measurement in full power states) does not straightforward apply in this case and an alternative criterion is necessary. In particular this paper presents the approach and results obtained accounting for the proposed criterion based on the launch of the internal emergency plan and on the timing for the crisis team to be operational and take the decision.