The next presentation was made by Bill Galyean of INEEL in which he reviewed Appendix J of NUREG/CR-5750. Some of the key points from his presentation and subsequent discussion include:

• There are a number of varieties of LOCA initiating events, including:

• Traditional pipe break LOCAs

• Stuck open PORVs and SRVs

• Steam generator tube ruptures

• Reactor coolant pump seal failures

• Interfacing system LOCAs (ISLOCAs) — where primary system coolant is inadvertently introduced into the secondary side piping and a secondary pipe fails creating a leak path of primary coolant outside containment

• Reactor vessel rupture

• While failure data exists for some of those categories, data for pipe break LOCAs and other similar events simply does not exist because it has never occurred.

• There is methodology for estimating the frequency of an event that has never occurred. A Bayesian update of a non-informative prior can be employed. This assumes that the mean value for the distribution is Уг of a failure over the service life. This can be result in a very conservative estimate because the assumed failure frequency in the prior is so high (pf = 0.5). If the failure rate is not constant over time, one also needs to account for time dependency and this methodology is not equipped for this.

• A primary Appendix J assumption is that you needed a leak before you can get a break. A conditional pipe break probability given a leak was based on the Beliczey-Schulz correlation.

• There was also a presentation of passive LOCA failures that can occur in non-piping systems as well as a list of possible data sources for this information.

Discussion: The elicitation panel discussed the validity of this assumption for degradation mechanisms that result in long surface flaws which are not as likely to leak prior to failure. Also, the expectation is that leaking flaws will be fixed after they are discovered during a plant walkdown or through other leak detection methods.

Discussion: Bruce Bishop indicated the need for very clear definitions of what constitutes a large, medium, small, and very small break LOCA. The concern is that the system response to a DEGB where the flow rates can reach 860,000 gpm (3,250,000 lpm) (according to Westinghouse calculations) is very different from a 5,000 gpm (19,000 lpm) leak which is also often characterized as a large break LOCA. Rob Tregoning indicated that clear definitions will be developed as part of this exercise.

Discussion: Gery Wilkowski relayed information provided by Helmut Schulz that the Beliczey and Schulz correlation of conditional probability of a rupture given a leak was developed for cyclic fatigue crack growth.

Discussion: Rob Tregoning emphasized that Bill Galyean’s presentation was provided to recap the last NRC-sponsored work in this area. This NUREG/CR-5750, Appendix J approach is not endorsed for the expert elicitation process; however, it represents one manner in which LOCA frequencies have been developed. Tregoning also emphasized that because substantial LOCAs have not occurred, past operating experience data needs to be augmented by information from other areas. If information was available simply from operating experience, there would be no need for the elicitation.

Discussion: The point was also raised that the panel needs to consider LOCA sources other than traditional pipe LOCAs.