The earlier sections of this document contain many sets of results for each base case component. The multiple cases were generated primarily as a series of sensitivity studies. For these results are to be useful in the LOCA elicitation, a reference case must be selected for each component as being representative for that component. This section briefly discusses which case for each component is suggested as the reference case, and system leak frequencies are presented for each reference case.

The joint frequency is calculated from the cumulative results reported above by use of Equation F.2. The system frequencies are then obtained by multiplying by the number of highly stressed joints in the system (this approximation works because the failure probabilities are generally small).

Each component is discussed, with a summary table provided after all components are discussed.

F.4.1 Hot Leg Pressure Vessel

As shown in Tables F.5 — F.7, the large leak (> 380 lpm [100 gpm] and larger) probabilities for this component varied considerably, depending on the crack growth mechanism (cycle-dependent fatigue or time-dependent stress corrosion cracking [PWSCC]), and whether crack initiation or growth from pre­existing defects was considered. The fatigue crack growth results (Table F.5) were very low (~10-18), and the PWSCC crack initiation results (Table F.6) were quite large (~10-5). Since it is expected that this component will totally dominate the very large (> 380,000 lpm [100,000 gpm]) leak category, the selection of the reference case is critical for very large leak estimates. The PWSCC with fabrication defects has intermediate failure probability results (~10-10), and is recommended as the reference case.

The case without residual stresses is selected. Table F.6 shows that residual stresses do not have a large influence. The time dependency of the large leak cumulative probability is very small, which suggests that the leak frequency is very small. For estimation purposes, the leak frequencies are estimated by taking the value of the cumulative at 60 years, dividing it by 60, and assuming the value to be applicable independently of time. This will overestimate the leak frequency at long time and underestimate it at short time.

For extension to system failure frequency, it is assumed that there are three comparably stressed joints in the large main coolant piping.