I tend to take the reciprocal of these risk values. The overall risk is typically expressed something like 3.8×10 -5 per reactor-year (3.8E-05 per reactor-year in scientific notation). The overall risk is determined by the sum of the odds of pathways leading to core damage. Path AB leads to another event tree – the Anticipated Transient Without Scram (ATWS) event tree because the reactor protection system failed to cause the immediate shut down of the reactor and additional mitigating systems are involved. The letters added to the initiating event letter define what additional failure(s) led to reactor core damage. Paths AE, ACE, and ACD result in reactor core damage. It is assumed that the initiating event happens, so each path starts with A. The column on the right describes the various paths that could be taken for the initiating event. Again, the event tree branches upward for the chances of the pumps successfully fulfilling this function and downward for failure.įinally, post-accident heat removal examines the chances that reactor core cooling can be sustained following the initial response. Two emergency coolant pumps (C and D) can each provide makeup cooling water to the reactor vessel to replenish the lost inventory. The event tree branches upward based on the odds of the reactor protection system successfully performing this action and downward for its failure to do so.

Nuclear reactor meltdown per success