E. 1 Summary

In this base case 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 LOCA size categories, RCS subsystems and components, and degradation mechanisms. This allocation is performed using data on primary system leaks and cracks from both U. S. and foreign PWR and BWR reactors.

E. 2 Assumptions and Observations

As with all analyses, there are a number of implicit assumptions associated with this approach. First is that past performance is representative of future performance. The common scenario for the occurrence of a LOCA starts with postulating the existence of a flaw or defect in the primary reactor coolant boundary. This flaw is then subjected to a stress that results in the catastrophic failure of the primary pressure boundary, producing a LOCA. The U. S. LWR operating experience to date consists of approximately 100 reactors with an average age of about 23 years. During this time the RCS of these plants have experience numerous transients and loads, which have produce a wide range of stresses. Whether these plants operate for 40 years (or 60 years with license extensions) this available operating experience represents a significant portion of the average plants lifetime. It is therefore reasonable to assume that the stresses that have already occurred are representative of those that will occur in the future. Similarly, various degradation mechanisms have affected RCS pipe, welds and components. However, when these degradation mechanisms have been detected, mitigation programs have subsequently been implemented (e. g., IGSCC in BWRs). Therefore, the number of flaws and defects in the RCS is likely to be cyclic over time. As the degradation mechanism manifests itself, the number of defects grows, as the degradation mechanism is addressed and mitigated, the number of defects is reduced. Again, the assumption here is that current 23 years of operating (on average, per reactor) are representative of the remaining operating life.

Another observation is the occurrence of zero LOCAs for both PWRs and BWRs. Although this does not prove that the LOCA frequencies are the same for both designs, it likewise does not support different LOCA frequencies. Therefore, for this analysis, the operating experience data (i. e., zero failures) will be pooled to generate a single LOCA frequency.

Furthermore, this analysis, just as every LOCA frequency estimate performed to date, assumes that the frequency of a LOCA decreases as pipe size increases. This might be attributable to a couple of issues. First, for small diameter pipe, some failure mechanisms exist that don’t apply to larger diameter pipe (e. g., compression fitting failures and socket welds). Second, the same flaw in both a small diameter pipe and a large diameter pipe represents a large percentage of the pipe diameter in the small diameter pipe. Third, inspection is probably more thorough in larger diameter pipe so that the chance of a defect going undetected is less in the larger diameter pipe. For all of these reasons (and probably others), the total LOCA frequency is reduced as LOCA size category increases. The scaling factor of Уг order of magnitude (assuming a lognormal probability distribution on LOCA frequency) appears to be reasonably consistent with historical LOCA frequency estimates.

This assumption of a half-order of magnitude (i. e., approximately a factor of 3) decrease in frequency for each increase in LOCA size is an assumption based on the general practice employed in estimating LOCA frequencies over the past 30 years starting with the Reactor Safety Study (Ref. E.1). This assumption is further supported by work done by Beliczey and Schulz (Ref. E.2). In this study, a combination of operating experience and fracture mechanics is used to demonstrate that the conditional probability of a rupture, given a leak, decreases as pipe diameter increases. This conclusion is reached because the size of detectable cracks and leaks remains relatively constant as a function of pipe size. Therefore, the relative crack or leak size as a function of the pipe circumference decreases, and the safety margin increases, as the pipe diameter increases.

Additionally, Beliczey and Schulz developed a quantitative conditional failure probability — which decreases by approximately У2 order of magnitude for each successively larger LOCA size — that was based on the propensity of through-wall fatigue flaws to lead to successively larger LOCA sizes.

Although the quantitative conditional failure probability is not applicable to all failure mechanisms and systems, this simple relationship has been extensively employed. This assumption was also employed in these analyses.

The final premise of this base case analysis is that the relative frequency of precursor data (i. e., leaks and cracks) is an indicator of the relative frequency of LOCA events. In the calculations that follow, the total LOCA frequency is allocated to the different RCS subsystems and components, and the different degradation mechanisms according to the relative frequency of observed leaks and cracks attributable to these subsystems and mechanism. Note that in order to determine the relative frequencies, complete crack and leak data are not needed, only consistent data that has not been biased by the over reporting of one attribute relative to another. Completeness in the data is neither required nor important, only consistency.