To shorten the outage periods of its reactors in order to increase their availability, Electricité de France reduced the time between the reactor shutdown and the end of the fuel unloading. This results in increasing the residual power of the fuel stored in spent fuel pool during plant unit shutdowns.
Therefore, equipment participating in the fuel storage safety is currently operated beyond the limits defined in the safety reports of the nuclear plants. This abnormal situation was highlighted by the experience feedback and by the safety review performed on 900-MWe reactors. In order to deal with such a situation, operating requirements have been temporarily implemented. In the meantime, definitive actions will be defined to improve the conception and operation as a whole of the spent fuel storage facilities in nuclear plants.
In this context, IRSN analyzed the suggestions and studies carried out by Electricité de France to improve the safety of the spent fuel storage and to remove the current requirements concerning the operation of the spent fuel pools of 900 MWe reactors.
Therefore, IRSN studied the incidental or accidental operating situations that may affect the cooling function of the spent fuel pool. The risk of uncovering assemblies stored in the fuel building was assessed by a specific probabilistic risk assessment. The result of these works leads to propose new modifications to improve the monitoring (cooling monitoring, pool instrumentation…) of storage facilities and the management of incidents or accidents (especially support means and procedures) which may affect them.
IRSN assessment conclusions were submitted to the permanent group in charge of the nuclear reactors in November 2002. It appears from this assessment that the control of the cooling of the spent fuel stored in the pool will be significantly improved compared to the current situation, when the modifications considered by Electricité de France are applied.
Until these conclusions and related modifications are taken into account, it was decided to maintain some compensatory measures defined in 1998. Moreover, these conclusions and modifications shall be adapted in order to apply them to the most recent reactors (1300 and 1450 MWe).
Besides, regardless of the residual power stored in spent fuel pool, it was also highlighted that some accidents such as a rapid drainage of the spent fuel pool, a fire or flooding affecting the premises of the fuel storage building might be uncontrolled in the current facility conditions.
The detailed study of these situations may result in new modifications in order to improve their prevention and management.