In order to decompose the elicitation topics further, the group determined that it would be useful to further decompose non base case piping systems and variables. This was accomplished by defining a set of reference conditions for each LOCA-sensitive piping system identified in Tables B.1.7 and B.1.8. The reference conditions are similar to the base cases in that they define a unique set of conditions (materials, geometric variables, mitigation and maintenance procedures, and degradation mechanisms) that can be analyzed. They are different from the base cases in that absolute LOCA frequencies will not be developed for the reference cases by the base case team. The reference cases for various systems will be compared to determine the relative LOCA-severity among piping systems. LOCA-severity variability within any specific system can then be gauged with respect to the reference case for that system. It will be up to each panel member to determine the method for developing these relative comparisons.

The BWR reference cases (Table B.1.11) represent specific combinations of the possible BWR piping as previously listed in Table B.1.7. In general, one material and degradation mechanism has been chosen is ideally representative of each piping system. The effect of fabrication defects and repair (discussed earlier) should be considered for its effect on the other degradation mechanism in all cases. Nominal pressure, thermal, RS, and DW loading should be considered for all cases. One loading transient was identified for each system. All the transients should be fairly well identified based on past discussion, but the overload transient for the control rod drive (CRD) piping needs to be better defined. In all cases, the snubber is considered to be functional.

Table B.1.11 BWR Reference Case Conditions

Specific piping, safe end, and weld materials were not determined by the group. Table B.1.11 represents an initial attempt to select these materials based on the general discussion. However, the group did decide to consider uncrevised Type 304 where proper in the reference cases. The mitigation and maintenance for all systems should assume NWC. Standard ISI with technical specification leakage detection should be considered along with augmented inspection as defined in generic letter 88-01. The mitigation and maintenance also has listed alloy 182 and stress improved, but these concepts need to be better defined and summarized.

The PWR reference case conditions are provided in Table B.1.12. The reference cases were again distilled from the LOCA-sensitive PWR piping systems (Table B.1.8). The philosophy behind this table was consistent with the BWR reference case development (Table B.1.11) with a few notable exceptions.

The effects of fabrication defects and repair on the other listed degradation mechanisms should again be considered in all piping systems. However, several PWR reference cases list multiple other degradation mechanisms which is a departure from the BWR approach (Table B.1.11). The PWR piping reference cases also account for nominal loading supplied by pressure, thermal, residual stress, and dead weight loading to each system. However, very few PWR systems have associated loading transients while all BWR systems do. It may be necessary to add associated PWR transients for consistency. The PWR piping, safe end and weld materials were also not specified. Some initial choices have been made in Table B.1.12, but feedback from the group is required in order to finalize selection. The mitigation and maintenance to be considered for each reference case consists of ISI with TSL. No other special mitigation procedures were identified.

Table B.1.12 PWR Reference Case Conditions

It is important that the baseline and reference case conditions be clearly defined prior to the start of the elicitation so that each panel member understands the general attributes of each of these cases. As mentioned previously, the elicitation questions will be structured to query the variability and uncertainty associated with each piping system with respect to the reference cases. Each panel member will compare the reference and base cases to assess the relative importance of each piping system to the total LOCA frequencies. Every effort will be made to accommodate all requests and information will be shared among the group. Additionally, any areas or issues which are not clear to a panel member should be raised to Rob Tregoning as soon as it arises.