141 LOCA Frequencies Estimated from Operating Experience (PSAM-0282)

Loss of coolant accidents (LOCA) have been an issue of concern since the very beginning of the commercial nuclear power industry. Designing against the possibility of such an accident has been codified into the federal regulations governing the operation light water reactors. However, concern has been raised about the justification for requiring licensees to spend significant resources on an issue that (at least in the case of large LOCAs) appears to be of such a small likelihood as to be relatively risk insignificant. Before any rational decisions can be made about the value of regulations aimed at the risk posed by LOCAs a reasonable estimate of the frequency of these events needs to be made.

That new estimates for LOCA frequencies are needed can be seen by simply calculating (assuming LOCAs follow a Poisson process) the expected number of small LOCA that should have been observed using the NUREG-1150 small LOCA frequency (1E-3/yr was used for both PWRs and BWRs) and the U.S. operating experience for LWRs (2924 LWR calendar years). This calculation predicts an expected number of small LOCAs during this time interval of 2.9, and a probability of seeing at least one small LOCA of 95%. Considering that there have been zero small LOCA (>100gpm) events in the U.S. during this time, these estimates are clearly unnecessarily conservative.

In this study, LOCA frequencies are calculated using a “top-down” approach. Specifically, a total LOCA frequency is calculated using U.S. commercial nuclear power plant (NPP) operating experience. This total frequency is then allocated to the three typical LOCA size categories. The resulting LOCA frequencies are shown on the table 1 below.