Probabilistic evaluation of risk during shutdown

French PWR's have been designed on a deterministic basis. This deterministic approach is now backed up by a probabilistic safety study designed to check a posteriori that the overall risk of a severe outcome is sufficiently low for the unit as a whole, or, where appropriate, to provide for the installation of extra defenses. It was in 1986 that Electricity de France decided to launch the Probabilistic Safety Study at the Paluel power station, first-off of the PWR 1300 MWe series. The objective of this study is to identify all the scenarios which may lead to core damage (level 1 study) and to assess the probability of their occurring. All units states are studied, including cold shutdowns for servicing or reloading. Internal hazards such as fires and flooding are not taken into account. The annual frequency of core damage is assessed at 1.5 10-5/unit year. This value is for all reactor states. With the unit under power, the calculated risk is only 4.6 10-6/unit year. Risk during shutdown does, however, represent 70% of the overall risk. This high risk during shutdown can be put down to: the high level of initiating events; the inhibition of automatic protective devices; diagnostic difficulties; high rates of maintenance outage times. The following describes the three predominant accident sequences brought to light during shutdown, and the measures adopted to reduce the risk of their occurring. (author)