For each plant type and for piping and non-piping Dr. Harris selected a reference system and attempted to scale other systems relative to that reference system. He tried to use estimates based on operating experience to the maximum extent, and then scaled the relative frequencies for the LOCA categories using results from the PFM analyses. In many instances, operating experience was not applicable, so he then relied more on the PFM results. If he felt that a given system was not a significant contributor to the leak frequency for a given LOCA category, then he was less concerned about the accuracy of the frequency estimate for that system.

PWR Non-seismic LOCA: For the PWR case, he used the hot leg as the reference system for large leak flow rates. Operating experience is not readily applicable. The PFM analyses for the hot leg showed a very wide range of results depending on the assumptions and input to the analyses. Therefore, he scaled the hot leg results by use of the RPV reference case results. Results presented at the wrap-up meeting in February 2004 provided an estimate of the RPV > 500,000 gpm (1,900,000 lpm) as 10-10 (per plant-year) for the first 25 years. He doubled this value to account for 2 hot legs. He assumed that the leak frequency for 100 gpm (380 lpm) LOCA is 3 У orders of magnitude higher, and then interpolated on a log-log scale. This fixes the frequency-leak rate for the hot leg at 25 years. He assumed that the frequency for > 500,000 gpm (1,900,000 lpm) in the time increment 25-40 years is twice that for 0-25 years, and four times as large in the increment 40-60 years. The leak frequency for 100 gpm (380 lpm) is assumed to be independent of time.

The cold leg is then assumed to have frequencies 1/3 those of the hot leg, because the cold leg operates at a somewhat lower temperature. The surge line is assumed to have leak frequencies 100 times as large as the hot leg, because the surge line sees a lot more cycles than the hot leg, and is just as hot. These estimates then define the very large leak frequencies for the entire plant.

At the low end of the leak rate scale, he assumed the plant results to be bounded by the past operating experience for steam generator tubes. An estimate from the wrap-up meeting for the steam generator LOCA frequencies is 3.5×10-3 per plant-year.

He used the HPI make-up nozzle as a surrogate for all 2 to 6 inch diameter lines. He used the reference case results from the PFM results that was presented at the wrap-up meeting, but reduced the leak frequencies by an order of magnitude at 5,000 gpm (19,000 lpm). He assumed that the SIS accumulator and RHR systems have about an order of magnitude less contribution than the surge line, so they have a small contribution to the overall plant.

This procedure provides his best estimate. The 5% and 95% estimates are scaled up and down from the best estimate. He estimated the 5% to be 1 У orders of magnitude below the best estimate (multiply by

0. 03), independent of time and leak rate. He varied the multiplier for the 95% estimate, making it larger for the larger leak categories. The multiplier varied from 30 to 1000. He believes that we have a better handle on the smaller leak rates, because they are bounded by steam generator tubing experience, which is plentiful.

BWR Non-seismic LOCA: He selected the recirculation system for the reference for BWRs. For intermediate leak rates 100 to 25,000 gpm (380 to 95,000 lpm), the 12 inch diameter portion of the recirculation system dominates. He used the base case results from the wrap-up meeting, but reduced them by an order of magnitude, because his PFM analysis underestimated the benefit of the post-remedial action residual stress.

The feedwater system is also important, because it has lots of welds, and is prone to FAC (which is not related to welds). Since this is a dominant system, he assumed it to be comparable to the surge line in a PWR (which is a dominant system for that type of plant). The steam line is about the same size and same material as the feedwater, but is not prone to FAC, therefore he assumed the steam line to have leak frequencies that are two orders of magnitude below the feedwater system. He assumed the RHR line to be the same as the PWR surge line, which is about the same size. NUREG/CR-6674 shows very low probabilities of through-wall cracks in the HPCS/LPCS system, so the contribution of this system was assumed to be negligible. The recirculation, feedwater, steam line and RHR are assumed to be the dominant systems, and no estimates were made for other systems.

The estimated uncertainty bands are generally tighter than the PWR estimates, because they are based more on experience for the dominant system (recirculation). They are independent of time, but do vary somewhat with leak category.

Non-piping: Dr. Harris felt less confident making estimates for non-piping components, because most of his experience is related to piping. He relied heavily on results provided by others in the wrap-up meeting, and used CRDM nozzle PWSCC, rector pressure vessel and steam generator tubing data for reference purposes. He also scaled relative to piping in many instances. He did not estimate time dependencies. For instance, for pumps and valves, he figured that they are less failure prone than the piping system in which they are located (passive failures). He estimated probabilities, and then calculated relative contributions of failure scenarios.