Each base and reference case includes at least one transient since transients are needed to initiate a LOCA event. At this time the transients are poorly defined.

Fred Simonen has seen reports that show seismic stresses, but he has no idea of the magnitude of the non­seismic transients (e. g., water hammer, safety relief valve transients). He and Gery Wilkowski would like help in establishing a rough order of magnitude for these types of transients. This information could be best expressed as a percentage of the Service Level stresses. Pete Riccardella indicated that SRV transients could be on the order of a small earthquake, just with a higher frequency. Bruce Bishop agreed to provide some water-hammer transient stresses for the pressurizer. Gery Wilkowski volunteered to provide some summary information from past probabilistic LBB analyses

Bengt Lydell indicated that the water hammer frequency is about 5E-3. There are more water hammer events on the secondary side, but there are design basis events that can cause water hammer on the primary side. It as noted again that without a transient a large LOCA is highly unlikely. The cracks will just leak until they are detected, and then will be repaired. Long surface cracks that don’t leak are drivers for large LOCAs.

Guy Deboo (Consolidated Edison) volunteered to provide stresses and frequencies for transients (e. g., feedwater line water hammer, SRV, and seismic from the LaSalle plant). Gery Wilkowski to provide some tables from NUREG/CR-6004 showing the N+SSE stresses for about 30 piping systems. This data was originally developed for the ASME Section XI Working Group on Pipe Flaw Evaluation. Additionally, Gery Wilkowski and Guy Deboo will provide stresses (not frequency) for some large faulted loads that are not really expected to occur over the life of plant. Gery is to examine the N+SSE stresses from the USNRC LBB submittal database. Guy noted that a 1SSE amplitude earthquake, based on seismic hazard curves, is expected to occur once over 40 years (design basis). The frequency (not amplitudes) of the seismic hazard curves are generally considered to be conservative. Gery noted that the design basis apparently is conservative since he is sure that an SSE event has ever occurred at any US (or other) plant. Hence, the seismic event frequency could perhaps be down graded to 0.5/2,600 events/year rather than 1/40 events per year.

For the smaller transients, Dave Harris will extract stresses from a NUREG report by Fred Simonen. Guy Deboo, Pete Riccardella, and Sam Ranganath will provide some data on normal operating transients. Pete Riccardella will get some data showing comparison of design versus actual transients based on some thermal fatigue analysis from a Sandia report.

Bruce Bishop has some plant specific ISI data that he could provide which provides transient information, but he won’t be able to provide it expeditiously due to other commitments. In fact, it is unlikely he will be able to provide this during the timeframe of this effort. Pete Riccardella concluded that the actual transients were not as severe as the design basis transients, but there are typically more of them.

It was also agreed that the group should have isometric drawings for the LOCA-sensitive piping. Bengt Lydell will inventory his electronic drawing database. The purpose is to look at generic systems to get an idea of how many welds are involved, pipe sizes, etc. Guy DeBoo will also evaluate the ISO drawings in his archive. Bengt will coordinate with Guy Deboo on this effort. For the time being it was decided that we would not seek additional isometric drawings until we determine if we are missing any of the major piping systems. Guy Deboo will then help obtain drawings (e. g. ISI drawings) for missing systems that at least indicate the number of welds. In general, multiple isometrics of similar systems are useful since each plant design is somewhat unique.

Dave Harris thought we only needed census data, i. e., number of welds as a function of pipe sizes, etc. Dave will provide a census that he has developed of number of welds for the base case piping systems. Dave and Bengt Lydell will coordinate on this action. Gery Wilkowski indicated that there is a MRP report with locations and numbers of bimetal welds that may be useful to review. Gery will provide a table of Inconel weld locations in different piping systems from the MRP-44 report.

Rob Tregoning asked if the panel should consider redefining the base and reference cases. The biggest concern is that the loadings and the mitigation/maintenance may need to more accurately reflect the operating experience. The panel could redefine these variables in a way that is more consistent with the quantitative analyses that was presented for the base cases earlier. The approach is to define these base and reference cases load and mitigation variables to reflect historical plant operating experience for the first 25 years of plant life (e. g. the current LOCA estimate). The consensus opinion agreed with this proposal. Guy Deboo added that he would like to see the base cases run out to 40 and 60 years, with a seismic event included. It was again noted that the objective of the “current estimate” analyses (i. e., out to 25 years of plant life) was to provide a benchmark against historical data.

Rob also asked if the panel wanted to consider more than one degradation mechanism for each reference case since the operating experience contains contributions from all applicable degradation mechanisms. Rob Tregoning wants to make sure each panel member is making the same relative comparisons with the reference cases and that these relative comparisons are natural. However, the group consensus was that it is easier to use the reference cases for anchoring when only considering one degradation mechanism and that no other changes in the reference cases should be adopted.

Vic Chapman argued that if we run the probabilistic fracture models for the first 25 years for benchmarking purposes, and we see a failure, then we should exclude that result since in reality we have not seen any failures to date. Lee Abramson agreed with this thought. The operating experience-based estimates developed by Bill Galyean and Bengt Lydell are inherently benchmarked in this manner. The probabilistic fracture models (David Harris and Vic Chapman) still need to benchmark their data.

Bruce Bishop would like Karen Gott, Bill Galyean, and Bengt Lydell to extract the failure mechanisms as a function of piping system from their databases. Bill will have one of his colleagues do this. Bengt and Karen will query their databases to get a list of degradation mechanisms as a function of piping system. Bruce Bishop would like someone to publish a list of plants by design type. Rob Tregoning indicated that he would provide this information.

Several people wanted Dave and Vic to run their models using refined stress histories. Pete Riccardella agreed to redefine the loads for the HPI/MU nozzle. Gery Wilkowski volunteered to get some more realistic surge line stresses for the surge line elbow case. It should actually be for a crack in the girth weld at the elbow, not a crack in the body of the elbow.

Bill Galyean and Bengt Lydell will determine the frequency of IGSCC leaks and surface cracks as a function of time and pipe size using their respective databases. The surface crack data should be characterized by the length and depth of the flaws if possible. The Swedish data should be separately characterized from the US plant data, since the US plants may not have characterized the flaw shapes by ISI if the piping was being removed from service. In Sweden, flaws from removed pipe systems have been characterized. A further requirement of this query to examine the recirculation system piping studied in the base case would also be helpful. Bengt will provide all this information in an Excel spreadsheet.