Probabilistic safety assessment of risks associated with reactivity accidents

Several types of reactivity accident are taken into account at the design stage for pressurized water reactors. As part of the post-Chernobyl activities, a new series of physical studies of reactivity accidents has been carried out and, using the results which have been obtained, a probabilistic safety assessment has been undertaken for all standard states of the pressure vessel. The main results obtained from these studies for the 900 MW(e) series are as follows. Sequences resulting from control cluster movements, cluster ejections and steam line ruptures, and sequences which might result in a core meltdown owing to excessive insertion of reactivity have low probabilities. The probabilistic study of uncontrolled dilutions revealed two significant sequences: one caused by progressive dilution during cold shutdown for refuelling, and another caused by the sudden injection of an unborated water plug into the core. Although the probabilistic assessment of the latter sequence rests on highly pessimistic assumptions, the major risk involved has prompted Electricite de France to implement a modification of all units as a matter of urgency. Moreover, additional studies are in progress on the physics of these phenomena and the exhaustiveness of the dilution scenarios. (author). 2 tabs