The first step in the expert panel elicitation was to develop an amalgamated set of base case LOCA frequencies upon which the elicitation responses are anchored. The generic base case LOCA frequencies developed for the panel represented the work of four teams: two teams used an empirical approach based on operating plant experience with leakage and other precursor events, while two other teams used theoretical, PFM analyses. Each of these approaches has different strengths and weaknesses, such that a better estimate of base case LOCA probabilities can be achieved by selectively combining the results in a manner that optimizes the strengths of both. The method and rationale for combining the base case results of the individual teams were documented, ultimately producing a revised set of LOCA frequencies for the five piping base cases.

LOCA frequencies for each of the LOCA sensitive piping systems identified for PWRs and BWRs were then estimated. This was done by picking the base case which is most representative of the specific LOCA sensitive system, considering plant type, material of construction, operating conditions and relevant degradation mechanisms, and then scaling the base case frequencies for each LOCA category based on judgment of any substantive differences between the base case and the system under evaluation. One of the main factors accounted for in this scaling process was differences in the size of the systems, in terms of number of welds of various pipe sizes (based on a system-by-system weld census that was provided to the panel). Scaling was also used to account for system specific factors, such as whether repairs or mitigation have been applied to address degradation mechanisms considered in the base cases, and the timing of such actions. An estimate of the probability of breaks in small diameter socket welded piping (instrument, vent and drain lines) due to vibration fatigue was made, which was not included in any of the base cases. This estimate was based on prior experience with this relatively common failure mechanism.

It was felt that there is a relatively large uncertainty band in all of the above probability estimates; plus or minus an order of magnitude. Included in this uncertainty band is the potential development of new, as yet unseen degradation mechanisms in the future, which obviously weren’t considered in the base cases.

A set of base cases for non-piping LOCAs was developed, the methodology for which is documented in Appendix I to this NUREG report. These base cases included potential breaks due to small vessel penetrations such as CRD nozzles, medium size breaks due to larger diameter nozzles (excluding safe-end ruptures which are included in the piping base cases), and very large breaks due to pressure vessel ruptures (specifically addressing irradiation embrittlement of the RPV). The resulting base cases were then used to estimate contributions to LOCA frequency from non-piping LOCAs.

The detailed rationale used in developing the elicitation response for each system was documented in a report, to permit the reconstruction of the logic in the future if it becomes necessary.