Probabilistic safety assessment of French PWRs: Importance of non-power states

The probabilistic safety assessment of French 900 MW(e) PWRs was carried out by the Safety Analysis Department of the Commissariat a l'Energie Atomique, and financed by the Central Service for Nuclear Installation Safety. It was a Level 1 probabilistic study of a generic unit from the 900 MWE(e) series. One feature of the study was that it took into account all reactor states. The results showed that non-power states make an important contribution to the total calculated core meltdown probability. The sequences which make the greatest contribution to that probability are those related to losses of primary coolant, inadvertent dilutions, and coolant losses during shutdown. In all of these cases, core meltdown can only be avoided by operator intervention since, in these states, there is no automatic system for dealing with the accident situation. Although probabilistic assessments for non-power states are beset with major uncertainties (physics of the phenomena, frequency of initiating events, human reliability), the results have caused particular attention to be brought to bear on reactor shutdown situations. For certain sequences, where the contribution seems to be of major importance, immediate measures have been taken by the operator. This applies in particular to sudden dilution sequences during hot shutdown, and loss of coolant during shutdown when the level in the primary circuit is at a minimum. (author)