LOCA Issue development required the group to brainstorm important LOCA issues. The group first defined a structure for categorizing issues in the form of a flowchart (Figure B.1.1). It was stressed that the LOCA frequencies in this exercise will consider only passive system failure. Active system failure will not be considered for the following reasons:

1. The panel has no specific expertise in these types of failures.

2. Failure of these components is not as rare and there is adequate data to assess their contribution to the LOCA frequencies.

3. Active components are subject to ongoing maintenance which should diminish the likelihood of future failure rate increases.

However, LOCA frequency contributions from active components will be combined with the passive component contributions to develop final LOCA estimates which can be supplied as PRA input. The estimation of these contributions will occur separately, but will be summarized for the panel at the wrap — up meeting.

The group divided the passive system LOCA sources (Figure B.1.1) into two classes: piping and non­piping. Non-piping contributions include RPVs, steam generators, bolting flange failures, valves, pumps, etc. The distinction between piping and non-piping categorization is useful because piping has unique issues.

Two of the six respondents who responded to this question only provided frequencies out to Category 5 LOCAs even though BWR non-piping could cause a Category 6 (Vessels) LOCA; thus Rob only provides results for Category 5 LOCAs here.

Concern with thermal aging of cast stainless steel is when it is present in concert with some other degradation mechanism.

CRDM refers to stud tube housing on bottom head of BWRs.

Panelist F didn’t consider RPV for Category 1 LOCAs; question was whether they didn’t think important or did they not have a means of making an evaluation.

There is much less spread in results for RPV than valves and pumps; panelists spent more time and more work in past on RPV than pumps and valves.

Panelist C sees a decrease in freq with time but may be an artifact of the fact that he anchored against BWR recirculation line that shows a decrease in LOCA frequency with time; he provides no rationale for why he would expect non-piping LOCA frequency to decrease with time.

This work was supported by the United States Nuclear Regulatory Commission (NRC) through the Component Integrity Branch of the Division of Engineering of the Office of Nuclear Regulatory Research under several different contracts. Battelle Memorial Institute support was provided under NRC Contract NRC-04-02-074 (NRC Job Code Y6538). Mr. William Galyean of Idaho National Engineering Environmental Laboratory was supported under NRC Job Code Y6332. Dr. Cory Atwood was supported under NRC Job Code Y6492. Dr. Alan Brothers was supported under NRC Job Code Y6604.

The authors would first like to thank the members of the expert elicitation panel whose insights formed the basis for the results and conclusions reached in this report. The elicitation panel members also provided valuable editorial and technical comments that have been incorporated into this summary document. The panel members included:

Westinghouse Electric Co. LLC OJV Consultancy Limited Exelon Nuclear

Idaho National Engineering Environmental Laboratory Swedish Nuclear Power Inspectorate Engineering Mechanics Technology, Inc.

ERIN® Engineering and Research, Inc.

Structural Integrity Associates, Inc.

Gesellschaft fur Reaktorsicherheit (GRS) mbh Formerly GE Nuclear Energy/Now XGEN Engineering Pacific Northwest National Laboratory Engineering Mechanics Corporation of Columbus

A special debt of gratitude is expressed to the base case team members (Dr. David Harris, Mr. Bengt Lydell, Mr. William Galyean, and Dr. Peter Riccardella) for the extra effort they provided to conduct the base case and sensitivity analyses discussed in Sections 3 and 4 of this report. In addition, Dr. Riccardella provided base case frequencies for a series of non-piping components and Mssrs. Galyean and Lydell developed the non-piping precursor database. Summaries of the base case team members’ analyses are contained in Appendices D — I. Each appendix has been written by the responsible base case team member and these contributions are much appreciated and are vital to the technical basis of this NUREG. Mr. Lydell and Dr. Harris also tirelessly discussed and corrected the authors’ characterization of their efforts for accuracy and clarity.

The authors would also like to recognize the facilitation team members who assisted with the development of the technical issues and associated elicitation questions and who also participated in the individual elicitations sessions. Along with the authors, Dr. Ken Jacquay of Casco Services, Inc. was a principal member of the facilitation team

The authors are also appreciative of Dr. Corwin Atwood of Statwood Consulting and Dr. Alan Brothers of Pacific Northwest National Laboratory, who provided an external peer review of the elicitation
approach and analysis of results. They suggested a number of sensitivity studies to further validate and clarify the conclusions reached in this report. They also provided many helpful comments that have been used to clarify this report. Dr. Atwood, in particular, provided many valuable insights which were used to develop and refine the techniques used to analyze the elicitation results.

Many NRC staff also provided valuable comments and contributions that were used to revise the draft report, including Dr. Arthur Buslik, Mr. Stephen Dinsmore, Mr. Gary Hammer, Mr. Glenn Kelly, and Mr. Arthur Salomon,. In particular, the authors would like to thank Dr. Arthur Buslik for his contribution to Section 5.6.4.4 and Mr. Arthur Salomon for his reviewing and editing assistance. The authors are also grateful to Dr. David Rudland of Engineering Mechanics Corporation of Columbus for assisting with selected Monte Carlo calculations to evaluate the analysis of individual panelist responses.

Finally, the authors would like to thank Dr. Al Csontos of the NRC, who served as the program manager over the final months of this program, and to Ms. Charlotte Matthews and Ms. Patricia Zaluski, both of Battelle-Columbus, for their invaluable assistance in the preparation of this report.

Dave prefaced his comments with the thought that he thinks that the base case results are surprisingly close considering the differences in the approaches. Dave indicated that an important input to any PFM analysis is the stress history. This requirement is contrary to the operating-experience approaches where the stress history is indirectly reflected in the incidence of cracking events. Dave’s analysis is performed on individual pipe locations which are then integrated to determine the overall system frequency.

Some key points from the crack initiation and crack growth portion of Dave’s presentation are that piping failures occur due to the initiation and growth of cracks. Cracks initiate due to stress corrosion or fatigue. Growth is controlled by fracture mechanics (other than early SCC). The question was asked as to what assumption Dave used as to the size of the crack once it initiates. Dave’s PRAISE code assumes that fatigue cracks are 0.3 inch (7.6 mm) deep (per a criteria proposed by Argonne National Laboratories [ANL]). This is based on an assumed 25 percent load drop definition of crack initiation from an S-N specimen test. In addition, in PRAISE the SCC rules differentiate between early SCC growth and fracture mechanics growth since the early growth is faster than calculated by fracture mechanics analysis. The SCC rules in PRAISE assume a 0.001 inch (0.025 mm) deep surface crack with some distribution function on length. The latest version of PRAISE (2002) includes updates to the S-N curves that incorporate environmental effects. Pete Riccardella noted that Art Deardorff of SAI was doing an update of some of the environmental S-N results from EPRI. For crack growth, the focus of PRAISE is semi­elliptical part-through surface cracks. PRAISE considers crack growth in both the depth and length directions (K at both the maximum depth and at the ends of crack.)

Dave pointed out that fatigue failure of welds is dominated by growth from pre-existing crack-like fabrication defects. Thus, the flaw distribution of initial fabrication defects is an important parameter to define. PRAISE stipulates that the final failure is controlled by tearing instability. PRAISE treats the stresses at maximum load as load-controlled stresses from stability analysis perspective.

The leak rate in PRAISE is computed based on the length of the leaking through-wall crack on the inside pipe surface using the SQUIRT leak-rate code. The SQUIRT code has recently been updated with numerous technical enhancements as part of the USNRC Large Break LOCA program. PRAISE includes some of the mechanistic dependent crack morphology parameters from some of the earlier versions of SQUIRT, but not the new COD-dependent roughness, number of turns, and flow path length to thickness ratio parameters. In addition SQUIRT has been made more user friendly by incorporating a graphical user interface. Note, WinPRAISE is PC-PRAISE with a Windows pre-processor for entering input parameters.

Dave used a stratified sampling technique that allows for the evaluation of extremely small probabilities events such as the case of fatigue crack growth from pre-existing defects (10E-17 frequencies). The approach also assumes that all cracks with leak rates greater than 5 gpm (19 lpm) are discovered and subsequently removed from service which implies that getting a higher Category LOCA (e. g., a 1,500 gpm [5,700 lpm] LOCA) would most likely result from the growth of a long surface crack that pops through the wall thickness and immediately becomes a TWC with a length equal to length of the surface crack on inside pipe surface. (The only other means of achieving a higher Category LOCA would be through some sort of transient event.)

Dave postulates that inaccuracies in leak-rate calculations will not significantly impact final LOCA frequencies. Even assuming the leakage detection capability equals zero should not have a large effect. Sam Ranganath felt that a surface crack grows 3 or 4 times faster in the length direction than it does in depth. He wasn’t sure if that was due to multiple initiation sites, or the surface growth rate being higher. The analysis in PRAISE considers crack growth in both the depth and length directions (K at both depth and ends of the crack) with an RMS value of K in each direction.

The detection probabilities shown in slide 14 are based on depth only, not length. It is likely that current technology has better performance. It was also noted the fatigue crack growth parameter used could be improved based on newer results. Dave Harris indicated that sensitivity studies show that using improved crack growth parameters in PRAISE (e. g., including environmental effects) will result in changes in LOCA frequencies on the order of a factor of 2. Slide 16 presents an example S-N initiation curve for a low alloy steel. Vic Chapman raised the concern that there may not be a plateau or fatigue limit with the higher number of cycles. Gery Wilkowski commented that the default flow stress values (slides 17 and 18) are very tight from a standard deviation perspective, especially in light of what was seen in NUREG/CR-6004 from an analysis of PIFRAC data.

The NUREG-6674 stresses have been downgraded from the design basis stresses by Jack Ware of INEEL to make them more realistic with fewer transients. The stresses may have been elastically calculated values, which can go well above yield and still be allowed for secondary stresses by the Code. PRAISE uses the most realistic values available. The final results (frequencies) from Dave’s analyses are very dependent on stress input values. Dave commented that one only needs stresses at the high stress locations. These high stress locations dominate the final frequency answers.

Slide 21 shows the surge line stresses. These stresses are probably for the flank of the elbow, not the weld. These values can’t be used for the girth weld location (i. e., they are too high). The stresses shown are stress amplitudes (the stress ranges will be twice these values). In addition to stresses and number of occurrences, one also needs some input as to the spatial distribution for these stresses. One of the short comings of PRAISE is that PRAISE doesn’t have a model for FAC for the feedwater lines. The fatigue initiation models are only in latest versions of PC-PRAISE, i. e., from NUREG/CR-6674 published in June 2000.

An ad hoc procedure was used with pc-PRAISE in order to obtain results for larger leak rates (stratified sampling for fatigue crack growth is not available for fatigue crack initiation). One of the handouts provided shows this ad hoc procedure.

A question arose with slide 26 concerns the fact that Dave’s analysis shows that the cumulative failure probabilities continue to increase after 20 years after a weld overlay repair is applied at 20 years. Past experience at Battelle as part of the Degraded Piping Program showed that weld overlays are very effective. They have much higher strength than the base metal. Another aspect of their application is that they apply a very high compressive stress at the crack plane. These high compressive stresses should restrict any further crack growth of the surface crack. In addition, it was noted that these high compressive stresses may preclude the environment from getting to the crack tip. Dave noted that he put in a linear approximation of the stresses through the thickness, the increased thickness of the overlay, and the crack growth equations for Type 316 NG into his analysis. WIN-PRAISE uses an adjusted weld residual stress pattern (linear gradient) for the case of post-weld overlay residual stresses (see slide 38). Dave will also present the failure probabilities without the weld overlay so that an assessment of its effect can made.

Bruce Bishop asked how much was Dave’s results affected by inspection. Dave didn’t think that the final frequencies would be affected that much. Dave’s results showed that there was a minimal change in LOCA frequencies for the hot leg as a result of the application of a 5SSE earthquake. This was not surprising to Dave since he has found similar behavior in a previous study. Lee Abramson pointed out that one means of seeing the effect of the earthquake is to compare conditions for equal probabilities. For example, for the 40 year time period analysis, no earthquake results in 1.3E-18 LOCA frequency for the no leak case while for the same 40 year time period analysis, a 5SSE earthquake results in 1.3E-18 frequency for a DEGB.

Dave, generally found that the LOCA frequencies were not highly dependent on JIc and dJ/da. Gery Wilkowski thought that if the toughness was low enough that one was operating in the EPFM regime then the LOCA frequencies may be more dependent on toughness. Gery indicated that he thought that the toughness values used in Dave’s base cases were too high for weld crack locations, or aged cast stainless steel pipe and fittings.

PRAISE can’t account for time dependent material properties. Thus, to account for aging, one would need to input aged properties in at time equal to zero.

The very low LOCA frequencies for the hot leg in slide 31 may be an artifact of the failure mechanism (fatigue) chosen for analysis. Higher frequencies may be seen for some other mechanism, such as PWSCC. This is a case that we may want to analyze in future analyses.

As outlined above, this LOCA frequency assessment is limited to specific portions of BWR and PWR Code Class 1 piping. The BWR base cases include contributions from potential pipe breaks in Loop B of the respective RR and FW System. Pipe break frequency contributions from normally pressurized sections of HPCI, RCIS, RHR or RWCU piping are not considered in this study. Piping system design information beyond that itemized above is not accounted for in this study. The PWR base cases include contributions from 3-of-3 RC hot legs and 2-of-2 HPI/NMU lines, respectively.

Excluded from the analysis are LOCA frequency contributions due to degradation and failure of cast stainless steel components such as valve bodies. While there is some documented evidence of degradation of such components, (e. g., [D.10]) the frequency of a through-wall defect in valve bodies and pump casings is viewed as being considerably lower than for welds in Class 1 systems.

Since joining the Pacific Northwest National Laboratory (PNNL) in 1976, and before that at the Battelle Columbus Division beginning in 1966, Dr. Simonen has worked in the areas of fracture mechanics and structural integrity. His research has addressed the safety and reliability of nuclear pressure vessels and piping as well as other industrial and aerospace structures and components.

During the 1990’s Dr. Simonen was a leader on the behalf of NRC and the ASME in the implementation of risk-informed methods for the inspection of nuclear piping. Dr. Simonen supported NRC staff by writing all chapters on piping reliability that are now part of DG-1063 Regulatory Guide on RI-ISI of Nuclear Power Plant Piping. These chapters provided the first formal set of guidelines to industry for probabilistic structural mechanics calculations for estimating piping failure probabilities. His recommendations impacted the selection of critical piping components that are given high priority for nondestructive examinations. On behalf of NRC and ASME Research, Dr. Simonen led a national effort during 1996-97 to benchmark PFM computer codes. The exercise concluded with PNNL performing the first ever statistically based calculations to quantify the uncertainties in calculated piping failure probabilities.

Since the early 1980’s he has led several studies for the USNRC on the effects of PTS on the failure probability of RPVs. This work has advanced the technology of PFM and methods for estimating the number and sizes of flaws in vessel welds. Dr. Simonen’s research has corrected longstanding deficiencies in traditional methods used to estimate the number and sizes of the welding flaws that govern the structural reliability of high-energy reactor piping and vessels.

Dr. Simonen was invited during 1995 and 1998 by the IAEA to meetings in Russia and Sweden to participate with a group of experts who evaluated the application of the LBB concept to RBMK reactors. The Central Research Institute of the Electric Power Industry invited Dr. Simonen to Japan during 1998 to present lectures on the reliability of reactor piping and methods to quantify the benefits of ISI programs.

Dr. Simonen has published over 200 papers, articles and reports in the open literature. He is a member/fellow with the ASME and serves on numerous ASME committees and codes and standards bodies, and has been awarded a number of prestigious awards from ASME.

Dr. Simonen holds a PhD. and Masters Degree in Engineering Mechanics from Stanford University and a B. S. in Mechanical Engineering from Michigan Technological University.

GERY WILKOWSKI
PRESIDENT

ENGINEERING MECHANICS CORPORATION OF COLUMBUS

COLUMBUS, OHIO

Dr. Wilkowski is an internationally recognized expert on the fracture behavior of piping in the nuclear as well as oil and gas industries. His areas of expertise include: full-scale pipe and pressure vessel fracture testing, nondestructive examination, JR-curve testing, high-rate toughness testing, experimental design and instrumentation, elastic-plastic estimation scheme analysis, impact testing, ASME Section XI flaw analyses, LBB analyses, and pipe system fracture behavior under seismic loading.

He was heavily involved in the development and verification of the fracture mechanics analyses for circumferential cracks in nuclear pipe for ASME Section XI. He was also a member of the following review committees:

(1) NRC Pipe Crack Task Group member that developed the NRC LBB procedure,

(2) NRC Peer Review Committee for proposed new seismic design rules for nuclear piping,

(4) NRC CRDM cracking review team member,

(5) NRC Davis-Besse clad integrity review team member,

(6) Consultant to AECB on CANDU pressure tube guillotine break phenomena, and

(7) Member of DOE’s Peer Review Groups for: Savannah River plant, New Production Reactor plant, Advanced Neutron Reactor, and uranium hexafluoride storage cylinders.

Dr. Wilkowski is a fellow of ASME. Currently he is a member of the following ASME Boiler and Pressure Vessel Code Section XI groups: Plant Operating Criteria Special Working Group, Flaw Evaluation Working Group, and Secretary of the Pipe Flaw Evaluation Working Group. He is the past chairman of the ASME Materials Fabrication Committee, and past chairman of the Pipe and Support Subcommittee of the ASME Operations, Applications, and Components Committee, all of which are part of the ASME Pressure Vessel and Piping Division. He was a coordinator for the 14th, 16th, and 17th Structural Mechanics in Reactor Technology (SMiRT) Conferences. He is a registered professional engineer in the State of Ohio since 1979.

Dr. Wilkowski has more than 200 technical publications, most on piping fracture. He is currently on the Editorial Board of the International Journal of Pressure Vessels and Piping. He is a past Associate Technical Editor of the ASME Journal of Pressure Vessel Technology, and guest editor of the Nuclear Engineering and Design journal. He was editor or co-editor of eleven ASME special technical publications. He was co-editor of four NRC Conference Proceeding Reports on LBB.

Dr. Wilkowski has both a B. S. and M. S. degree in Mechanical Engineering from the University of Michigan and a PhD in Nuclear Engineering from the University of Tokyo.

Vugraph #3 is a summary of revised results since July 2003.

Could use VC Summer Hot Leg/RPV nozzle weld crack as benchmark, but need to be careful to consider all aspects such as differences in weld residual stresses due to repairs.

VG12 shows an order magnitude difference in LOCA frequency for a difference of 2 ksi (14 MPa) in normal operating stress; this was thought to be pretty sensitive result.

Dave assumed a linear residual stress field through the thickness due to the weld overlay repair.

The solid line in VG13 is PRAISE result while prior and post symbols come from Bengt’s results.

Included for comparison, Figure D.18 shows the calculated flaw rates (non-through wall). Listed in Table D.10 are the RR and FW weld failure rates that represent the state-of-knowledge at T = 15 years of operation. Listed in Table D.11 are the RC and HPI/NMU prior weld failure rates. The failure rates represent the frequency per weld-year of a through-wall flaw resulting in a leakage of less than or equal to the Technical Specification limit for unidentified leakage. In summary, the derivation of prior weld failure rates includes the following steps:

• Determine the number of through-wall leaks from PIPExp database. Includes performing a trend analysis.

• From the PIPExp database, determine the appropriate exposure terms.

• Establish the susceptibility of different weld locations to degradation.

• Combine the output from the previous steps to determine prior failure rates applicable to welds in the RR and FW systems.

1.00E-02

1.00E-06

Figure D.18 Prior Frequency of Non-Through Wall IGSCC in RR Piping

The group first defined piping to include vessel penetrations (e. g CRDM housings, instrumentation lines), piping, and safe ends. The boundary between piping and non-piping components (e. g. vessels) was defined as the nozzle (or component) side of the safe-end/piping to nozzle weld. The group then decided that piping should be categorized by the specific plant system. The piping system is important because it defines the functionality and operating history, or influence factors. The plant system is also often associated with specific piping designs and materials, or attribute functions. The relationship between the piping attribute and influence characteristics will determine its failure propensity.

For a given plant system, the variables which affect the LOCA probability fall into one of the following five categories: geometry, materials, loading history, degradation mechanisms, and mitigation or maintenance procedures. The group decided to list all the possible contributors for each variable category and then link the dependencies with a given plant system. Obviously there is a synergistic effect among these variables. The piping system requirements result in geometrical and material selection constraints.

The geometrical and material choices mesh with the system functionality and operating history to determine component loading history. This specific combination dictates the degradation mechanisms that emerge. Mitigation and maintenance procedures are developed to counteract these mechanisms. The effectiveness of these strategies, however, is a function of all the other variables discussed.

Figure B.1.1 Passive LOCA Contributions

Material Variables: It was quickly determined that the materials category should also include fabrication procedures as well. Material variables are important because they give rise to specific degradation mechanisms. Fabrication variables can lead to variations in defect formation and residual stress which can result in certain locations having a greater propensity for degradation. The important variables which affect the LOCA propensity are summarized in Table B.1.2. Table B.1.2 indicates whether the material is found in the base metal portion of the piping system or the welds. Issues that are strictly related to fabrication are separated in the table.

The circumferential versus axial welded pipe issue was raised to differentiate between seamless and non­seamless pipe. While the bulk of current piping is seamless, there is some remaining seam welded piping in service. The concern is that seam-welded piping is more susceptible to degradation and leaking than is seamless. The shop versus field welded issue was raised due to possible weld quality differences between welds made in controlled conditions during piping fabrication and those made on site during piping system assembly. Finally, it was noted that defects, residual stress irregularities, and poor material properties can be associated with repair welding. Experience has shown that field cracking and leakage is often associated with repair welds.

Geometric Variables: Table B.1.3 lists the geometric variables which influence the LOCA frequency distributions. These variables affect piping stress, system compliance, the propensity for a given degradation mechanism, and the likelihood of leaking versus catastrophic rupture. Many of the variables are obvious and include general system information such as piping diameter and thickness (NPS and schedule), the number of welds and their location, the types and numbers of specific piping components, and the layout of supports and snubbers. The system configuration is related to the layout, but also specifically considers where active components such as pumps, valves, and flow offices are located. The variable “connections” in this table are meant to distinguish between welded connections which are more typical and flanged connections which can segregate piping from active components.

Table B.1.3 Geometric Piping Variables

The final two variables in this table are a combination of geometric and material/fabrication issues: the existence of crevice welds (e. g. at thermal sleeves), and the difference between drawn piping and piping which is field-fabricated. Drawn piping is cold worked to size and is usually limited to smaller diameter piping (< xx” NPS). Field-fabricated piping may be hot forged or cold work to some extent, but the final size is often achieved by machining. This is more typical for larger diameter pipes (> yy”). Crevice or filet welds are for sleeves and other piping system attachments. Partial penetration welding can result in a greater propensity for flaws and higher residual stresses and constraint than through-wall welding.

Material Degradation Mechanisms: The next variable group describes material degradation mechanisms. As mentioned earlier, the degradation mechanisms are often associated with the piping system material. Each material is susceptible to each of the mechanisms listed, although the degree of susceptibility will greatly vary. The system loading history can also favor certain mechanisms. Table B.1.4 summarizes the mechanisms developed during the group brainstorming session. The mechanisms

are segregated by the primary mechanism type. Additional sub-categories are used to identify either specific degradation mechanisms under the appropriate main category or features associated with the main category.

Fatigue degradation was separated into low cycle fatigue which is primarily driven by thermal loading fluctuations due to plant heat-up and cool-down cycles and high cycle mechanical fatigue which could result from general loading fluctuation on the piping. This loading fluctuation could be induced by vibration, pressure or temperature fluctuation (e. g. striping). High and low cyclic loading are often differentiated with respect to the loading magnitude relative to yield strength or number of cycles. For this elicitation, a rule of thumb differentiation of 1,000 cycles is sufficient to differentiate between low — cycle and high-cycle events. Both crack initiation and crack growth portions of life are important contributors to fatigue life, although crack initiation occupies a greater percentage of life in high cycle fatigue.

Stress corrosion cracking (SCC) is listed as a main category and it includes IGSCC which was prevalent in BWRs in the late 70’s, TGSCC which affects casting components, PWSCC which has more recently surfaced in PWRs, and ECSCC. The localized corrosion category includes both general pitting and crevice corrosion which is likely in tight, stagnate areas. While general corrosion is listed as its own category, boric acid corrosion is the principal contributor. Both internal (ID) and external (OD) boric acid corrosion are included in this sub-category. External corrosion can result from leaking fluid from another

component in the plant which then impacts the pipe. Fretting wear describes material erosion usually resulting from external contact with other components. This wear usually occurs with vibration loading but should be distinguished from high cycle fatigue which results in crack formation.

Material aging relates to changes in the intrinsic quasi-static material properties at the time when the component is placed into service. These properties include constitutive properties (stress/strain behavior), cyclic, crack growth resistance (da/dN versus ДК), and resistance to crack initiation and tearing under monotonic loading (J-R behavior). Aging can occur due to thermal or radiation embrittlement; long term, or low temperature creep. Dynamic strain aging refers to the stress/strain and J-R curve resistance changes which result from dynamically applied loading. It is most evident in the ductile to brittle transition temperature shift of ferritic steels. Hydrogen embrittlement is somewhat related to the other material aging mechanisms in that strength and toughness can be affected over time. The distinction is that hydrogen embrittlement only becomes prevalent after a crack has formed. The other aging degradations do not require a preexisting defect although the system impact is certainly enhanced in their presence.

The flow sensitive category is used to capture those mechanisms which are sensitive to the flow characteristics of the piping medium. The erosion/cavitation sub-category refers to material erosion due to cavitation which occurs when vapor bubbles collapse. This is distinct from FAC where downstream turbulence results in erosion/corrosion that is not accompanied by low pressure boiling. The category of unanticipated or new mechanisms covers those mechanisms which could surface in the future which are either unknown at the present time or not deemed to be important at this time. This category is purposely vague to capture the panel’s general uncertainty of the completeness of our understanding of future piping degradation mechanisms. For instance, just a few years ago, PWSCC would not likely have been considered to be an important degradation mechanism. Now however, it is of primary concern.

Fabrication defects and the repair of those defects (or lack thereof) are distinct from repair welding mentioned earlier in the material/fabrication issue table. This issue covers the likelihood of repair of fabrication defects and the possibility that these fabrication defects could lead to failure due to one of the other mechanisms listed in Table B.1.4. The repair component of this issue only considers the possibility that these defects are repaired (or not), not any new defects generated by the repair process. Defects generated by repair welds have been captured in the geometric variability section (Table B.1.2).

Loading History Variables: The next variable which contributes to the piping LOCA frequency estimates is the system loading history. The term loading history considers both the magnitude and frequency of the loading applied to the piping system over its service history. The different types of applied loading are summarized in Table B.1.4. Again, as in Table B.1.4, the loading variables are divided into main and sub categories.

The thermal loading category considers loading from differential expansion between dissimilar piping materials. This loading is potentially exacerbated at the ends of the piping if connected to a rigid component (e. g. steam generator or RPV). This loading is categorized as restraint-free expansion in Table B.1.4. Radial thermal gradients are induced in piping due to the temperature difference between the pipe ID and OD. Insulation can diminish this gradient. Thermal stratification can occur under low flow rate conditions when hotter liquid flows on top of cooler water. Boundary layer fluctuations in this interface can induce thermal cyclic loading which is referred to as thermal striping.

Water hammer, dead weight, and SRV loading are considered as separate main categories with no corresponding sub-categories. Water hammer is distinguished from other pressure transients because of its potential severity. Safety relief valve loading describes the pressure transient which occurs when SRVs are opened or closed. Dead weight loading is contributed by the weight of the pipe and any unsupported attachments. The pressure loading category includes normal operating pressure and any pressure transients other than those specifically listed in Table B.1.5 (e. g. water hammer, SRV loading, internal overloads, and cavitation).

Residual stress is a prominent loading category and includes contributions from locally-induced welding process stresses related to the as-designed weld (Design sub-category in Table B.1.5) and additional contributions due to weld repair. The Design sub-category also encompasses contributions from the pipe system compliance on the weld restraint during component assembly. System compliance will obviously influence the residual stress distribution which if formed at a particular weld joint. Fabrication residual stresses include cold-springing needed to align piping during plant construction. This residual stress contribution may not be apparent from the system design which is why it is distinguished from the Design sub-category. Finally, a unique sub-category is entitled “Mitigation-Induced” to account for residual stresses which may be applied during plant operation to mitigate certain failure mechanisms. These stresses are induced by processes like weld overlay repairs (used during IGSCC mitigation) and mechanical stress improvement (applied for VC Summer). These stresses are certainly associated with the weld joint being treated, but also affect the residual stress distribution in surrounding welds and piping.

Overloads can result from external failures in other plant systems and internal accidents. These accidents can result in loading which is potentially in excess of the structural design limits of the piping. External overloads include those induced by pipe whip and jet impingement. Both of these categories require failure outside of the reactor pressure boundary as a precursor event. The loading on the reactor pressure boundary piping occurs either by pipe whip of the failed components or through water or steam jet impingement caused by the breech in the other system. A special type of internal overload sub-category is Deflagration. This describes the loading due to hydrogen combustion which occurred at the Hamaoka

and Brunsbuettal plants. This category would cover not only direct failure of pressure boundary piping, but also precursor failure of secondary piping that leads to conditional failure (from shrapnel or jet impingement) of pressure boundary piping.

Another loading history category is support structure loading. This includes loading due to a malfunctioning snubber or misadjusted hanger that leads to piping system loading that is beyond the intended design limits. Vibration loading includes classical displacement loading due to nearby active component vibration (Mechanical sub-category) and loading due to cavitation which may exist within the piping system.

The final loading category is seismic which will be treated uniquely from the other loading variables listed. The seismic loading category includes both inertial and displacement loading components.

Seismic loading is treated separately within PRA models and the LOCA contribution from seismic loading is also calculated separately. However, many analysts frequently calculate conditional failure probabilities due to seismic loading and this is often the principal transient of interest in most piping systems. The panel therefore seemed generally comfortable with considering the possible effects of seismic loading if the loading magnitude was specified. For these reasons, it was decided to query the panel about seismic effects separate from any other loading history contributions to LOCA. The seismic contributions can then be segregated from the final results and used to examine the effect of conditional seismic events on the LOCA frequency distributions.

Mitigation and Maintenance Issues: The final piping variable which was discussed separately is in the area of mitigation and maintenance. This is an important topic area because these procedures have been developed to ensure piping system integrity and prevent piping rupture. The effectiveness of any particular procedure is often a function of the degradation mechanism, although some issues developed are not specific to any mechanism. These procedures can sometimes result in unintended consequences which actually exacerbate the piping failure likelihood. It should be stressed that the procedures and issues in this table are not just concerned with current practice, but also future application and possible improvements. Each panel member must express his or her expectations about future LOCA performance up to the end of the plant license-renewal period.

The first four mitigation procedures (Table B.1.6) are related to piping system inspection and maintenance. In-service inspection and RI-ISI considers both current application of these programs and future industry trends. Currently, more US plants are adopting RI-ISI. The effectiveness of these techniques to find and determine the extent of degradation is considered in this category. Often a technique’s effectiveness is quantified by the POD for a certain degradation mechanism. Leak detection considers the broad array of leak detection methods (including plant walkdowns) and their effectiveness in uncovering piping degradation. Online monitoring considers the effect that current system performance indicators (pressure, temperature, etc.) may have on preventing failures as well as future systems that could be utilized to monitor degradation in real-time.

Planned maintenance accounts for programs which monitor degradation and then replace piping segments once the degradation exceeds allowable limits. This is a popular approach for dealing with FAC in carbon steel piping. Planned maintenance also considers any component cleaning or preparation for inspections and the effect on the failure likelihood. Planned maintenance can be either beneficial or detrimental. For instance, maintenance requires closed systems to be opened which could introduce air if gas blanketing is not sufficient. Maintenance can then lead to more future problems than if the maintenance had not occurred.

The next group of procedures is concerned with changing either the piping medium environment or the metal/medium interface in order to impede degradation. Water chemistry is concerned with additions or changes in the basic water chemistry in order to reduce the degradation rate of a certain mechanism. For instance, hydrogenated and noble metal additions to BWR water have proven effective in impeding the rate of IGSCC. This category also considers fluctuations in water chemistry over the plant’s operating cycle and the affect that this may have on failure rates. Decontamination is related to water chemistry and considers the removal of impurities in the water supply and the possible impact that this could have on the degradation rate of certain mechanisms.

The application of internal linings and coatings is used to segregate a susceptible piping material from the environment using a coating or overlay of a more resistant material. The coating or lining performs the same role as stainless steel cladding does in protecting carbon steel piping. Weld overlays, induction heating stress improvement (IHSI), and mechanical stress improvement (MSI) all attempt to change or relieve weld joint residual stress in order to impede crack growth. Normally, they attempt to create compressive residual stresses at the inside surface, or throughout the entire, piping segment. However, as mentioned earlier, they can also affect the residual stress in other sections of the piping.

The next group of mitigation and maintenance procedures is concerned with anticipated future improvements in materials, repair techniques, and inspection methods that could reduce the likelihood of future LOCAs. These improvements are captured by the categories for Improved Weld Techniques/Materials and Improved Inspection Techniques. The Pipe Replacement category considers not just the removal of possibly degraded piping, but also the replacement with new materials that are less susceptible to known, important degradation mechanisms for a certain system. This could also be coupled with new design and layout configurations to reduce residual stresses and improve accessibility for inspections. Socket weld replacement is a specific piping replacement program that is currently being considered. The effectiveness and scope of its implementation still is uncertain which is the reason that this has been included as a separate category.

The next two mitigation procedures are related to current and future plant operating performance.

Thermal stratification mitigation is a specific technique employed by some plant operators in order to improve thermal mixing and reduce stratification and striping stresses which can occur in the surge line and other piping systems. General plant operating performance is a category that captures all other similarly related issues. This would include issues such as the effect of possible power upgrades, possible future changes in the heat-up and cool-down cycle, the possibility of increased time periods between outages, etc.

The final two issues (Table B.1.6) are related to utility and regulatory safety culture in general. There are many specific issues that the group lumped into these broad categories. One such issue within the utility safety culture is human error. Human error was defined by the expert panel as the likelihood that incorrect action is taken during mitigation and maintenance. This includes improper application of techniques and procedures, misinterpretation of obvious indications (beyond the POD included in ISI), and omission of a prescribed procedure. Other issues within the broad category of utility safety culture include the adoption and implementation of risk-informed management strategy which requires a detailed understanding of real-time plant risk and the objective to embrace changes that reduce overall plant risk. The impact of economic considerations is important in terms of choosing which mitigation strategies to pursue. All decisions weigh plant risk with economic considerations to hopefully arrive at the optimal mitigation strategy. However, this mitigation strategy might not lead to the lowest possible plant risk. Flow accelerated corrosion monitoring programs illustrate this concept. While the absolute lowest plant risk could be achieved for a system by replacing the pipe with FAC-resistant materials, many plants have chosen to monitor the degradation and replace only when the failure risk becomes unacceptable.

Also part of the general safety culture are the lessons-learned from past problems. This experience may decrease the response time for mitigating future problems. For instance, the industry experience with mitigating IGSCC in the early 1980’s may provide some useful strategies for PWSCC issues that are currently surfacing. Response time is generally an important mitigation concept. When degradation mechanisms are identified, the failure likelihood due to these mechanisms may continue to increase with time until effective mitigation strategies are employed which reduce their propensity. Obviously, shorter response times are preferred. The industry required roughly three to four years to fully implement IGSCC cracking mitigation strategies after the issue was fully identified. A final related issue is technology transfer which is the training of and knowledge transfer to the next generation of plant operators and engineers. As the workforce continues to age and is replaced by less experienced workers, it is possible that plant risk may be affected.

The regulatory safety culture also encompasses many of the same issues discussed under utility safety culture. Certainly lessons-learned, regulatory response time, and technology transfer equally apply to the regulatory culture. The regulatory environment is also affected by the agency’s interaction with the public and the changing public perception of risk. Management philosophy and the adoption of risk — informed regulations may also influence the regulatory safety culture.

The group next determined if the effectiveness of specific mitigation or maintenance procedures varied as a function of degradation mechanisms and materials being evaluated. This dependency is reflected in the second column of Table B.1.6. Plant and regulatory safety culture were considered to be general issues which do not vary significantly with the degradation mechanism. However, the utility safety culture was considered by the panel to vary from plant to plant, as indicated in Table B.1.6. Regulatory safety culture was not determined to be a plant-specific function.

BWR Piping Systems: The variables just discussed (geometry, materials, loading, degradation mechanism, and mitigation) are important for determining the overall LOCA frequencies. However, these variables are linked to each specific LOCA-sensitive piping system (Figure B.1.1). The next group task was to identify LOCA-sensitive piping and assign only pertinent variables to each system. This task spanned both the Wednesday (2/5) and Thursday (2/6) meeting days, but it is summarized here for continuity. It should also be noted that Sam Ranganath provided most of the initial input for BWRs.

The data presented in Tables B.1.7 (for BWRs) and B.1.8 (for PWRs) summarize information developed by the panelists at several brainstorming sessions. Therefore, they represent panelist expertise rather than a systematic analysis of a database of piping system geometries. In some cases, there are minor discrepancies between the pipe size ranges provided and actual plant survey data. For example, a review of the LBB database developed by Emc2 shows that there are 16-inch diameter surge lines, such as those at South Texas Units 1 and 2, while Table B.1.8 indicates that the range in surge line diameters is 10 to 14 inches. However, these relatively small discrepancies in the outer ranges of these bound have no significant effect on the results and conclusions which focused on developing generic LOCA frequency estimates. The diameters provided in this table do represent nominal pipe dimensions (e. g., 12-, 16-, 28- inch diameter) for the sake of simplicity, rather than inside pipe dimensions which would have some fractional component. This convention was utilized by consensus among the panelists.

The BWR piping systems that can result in a LOCA were first identified (Table B.1.7). These include the recirculation (RECIRC), feed water, steam line, high pressure (HPCS), and low pressure core spray (LPCS), RHR, RWCU, CRD, standby liquid control (SLC), instrument lines in both the reactor and in other piping systems (INST), drain lines, head spray lines, steam relief valve lines, and the reactor core isolation cooling (RCIC) system. It should be noted that while drain lines are associated with each system, they were segregated into a separate category due to their common functionality. The materials commonly used for the piping within each system are identified (column 2 of Table B.1.7). Similarly, the safe end material (column 4) and the weld material (column 5) are also indicated. The intent of this identification was to be comprehensive and also indicate the most prevalent materials wherever possible. Table abbreviations are provided at the end of the table.

Some additional clarification is required for certain entries in this table. In the recirculation piping system, the safe end materials (stainless steel or Alloy 600) are furnace-sensitized during manufacture. The feedwater safe end is manufactured either by interference fit, butt welded, or by a triple sleeve weld overlay. The HPCS and LPCS contain both creviced and non-creviced welds between the piping and safe end. Also, the bulk of this system’s piping material is carbon steel. The CRD system consists of a crevice Alloy 82/182 weld to the RPV head while the stub tube (“safe end” in this system) is stainless steel and alloy 600 which is welded and furnace sensitized. It should be noted that no stainless steel clad carbon steel, cast stainless steel, or bimetallic stainless steel welds were indicated in any of these systems, although they are listed in Table B.1.2.

The next variable listed in Table B.1.7 is the nominal piping size present in the system (column 3 in Table B.1.7). This is the only geometric variable (Figure B.1.1 and Table B.1.3) indicated in Table B.1.7. It was not possible to do an exhaustive listing of the possible geometric variables (as with materials) due to the complexity and plant-specific nature of variables related to layout, configuration, weld location, and component type. Each expert panel member must individually determine variability and influence of these parameters. The rationale for listing the piping size is only to provide the panel with an indication of the piping size for maximum leak rate assessment. Common piping sizes for a system are separated by commas in Table B.1.7. Size ranges are separated by a dash and the maximum piping size is given as < 4 inch for many of the smaller systems. It should be noted that the feedwater system typically consists of 10 or 12 inch diameter pipes. However, a range from 12 to 24 inch is also possible.

Significant degradation mechanisms that could be associated with piping materials in each system are included in the sixth column of Table B.1.7. Unanticipated mechanisms (UA) and fabrication defect and repair (FDR) issues are present in every system. Stress corrosion cracking is listed for all stainless and carbon steel materials, while global corrosion (GC) is associated solely with carbon steel piping. Localized corrosion (LC) is included for carbon steel and stainless steel piping for all systems, except SLC, CRD, and instrumentation lines. Material aging (MA) was considered for the higher temperature lines that see constant use for both stainless and carbon steels. Flow sensitive (FS) degradation is present in all carbon steel piping systems with constant use. Mechanical fatigue was judged to be significant in all of the smaller piping systems (< 4 inch diameter). However, it was also considered important in the SRV and feedwater systems. Thermal fatigue (TF) was judged to be important in the feedwater, RHR, RWCU, HPCS/LPCS, and head spray piping.

P = pressure

S = Seismic

T = Thermal

DW = dead weight

SUP = support loading

SRV = SRV loading

WH = water (and steam) hammer

O = overload

V = vibration

TFL = thermal fatigue loading from striping TS = thermal stratification REM = all remaining mitigation strategies possible (eg. not unique to piping system)

Significant loading sources for BWR piping systems are also listed (column 7). All systems undergo residual stress (RS), pressure, thermal, seismic, SRV and dead weight (DW) loading. Water (or steam) hammer (WH) was considered to be important in the feedwater, steam line, and HPCS/LPCS systems. Support loading (SUP) was mainly considered to occur through the snubber support. This is important for the recirculation (RECIRC), RHR, RWCU, and HPCS/LPCS systems. Vibrational loading is listed for the smaller diameter piping systems. This loading is always coupled with mechanical fatigue degradation (column 6). However, vibrational loading is conspicuously absent from the feedwater and SRV lines which both have MF in the list of significant degradation mechanisms.

Overloads (O) are possible for all systems, but they are likely to be external due to pipe whip, jet impingement, or secondary system failure. However, the drain line, CRD, instrument lines, and SLC were deemed to be more likely to be susceptible to internal overloads. The thermal loading was broken down into thermal fatigue loading due to striping (TFL) in the feedwater system, and thermal fatigue loading due to stratification (TS) in the RHR, RWCU, and HPCS/LPCS systems. The head spray line, which is also judged to be TF susceptible, does not have a corresponding thermal fatigue loading source considered.

There were no mitigation and maintenance procedures that were identified by the panel as being unique for any particular BWR piping system. Standard mitigation and maintenance for all systems is ISI with credit given for technical specification leakage (TSL) detection. The technical specification leakage threshold is 1 gallon per minute. The effect of all remaining (REM) mitigation and maintenance procedures and issues (Table B.1.6) on the LOCA likelihood should be considered by the panel.

PWR Piping Systems: LOCA-sensitive PWR piping systems were also determined (Table B.1.8) along with associated piping safe end, and weld materials; pipe size; significant degradation mechanisms; significant loading sources; and system-dependent mitigation and maintenance procedures. The format of this table is identical to the BWR summary table (Table B.1.7) and the abbreviations have been retained.

Unique abbreviations are defined at the bottom of Table B.1.8. It should be noted that the group did not discuss broad differences between Westinghouse (W), B&W, and Combustion Engineering (CE) designs. In fact the only place it is noted (Table B.1.8) is in the use of A600 safe ends in the CVCS system in CE and B&W plants. However, any plant design distinctions may be important for certain LOCA classes and should be considered by each expert during analysis.

The LOCA-sensitive PWR systems listed include reactor coolant piping (RCP) hot leg, RCP cold leg, and RCP crossover legs, the surge line, SIS accumulator line (ACCUM) and SIS DVI line, drain lines, CVCS, RHR, SRV lines, pressurizer spray lines (PSL), CRDM lines, reactor head (RH), in-core instrumentation (ICI), and instrumentation (INST) lines. The hot leg was segregated from the other RCP components by the group due to its higher operating temperature. The SIS system was divided into the ACCUM and DVI components to account for the piping size, material, and functionality differences. The RH group was intended to capture all the non-CRDM lines that penetrate the upper reactor vessel head. This grouping is distinct from the ICI system. The INST line grouping here considers mainly lines within piping systems and not the reactor. It is worth noting that this grouping is different from the grouping for BWRs where INST lines capture both piping and reactor lines.

The materials utilized in PWR piping systems are similar to those in counterpart BWR systems. One difference is the inclusion of cast stainless steel (C-SS) in RCP, surge line, and ACCUM PWR piping. Also, stainless steel clad carbon steel (SSC-CS) is prominent in certain plant designs within the RCP. There is also less use in general of carbon steel in PWRs. It should be noted that the only material listed in Table B.1.2 which is not explicitly listed in either BWR (Table B.1.7) or PWR (Table B.1.8) piping systems is bimetallic stainless steel welds.

The degradation mechanisms are again tied to the material and functional considerations of the piping system. The FDR and UA categories are included for all systems, as is thermal fatigue. Stress corrosion cracking was affiliated primarily with stainless steel piping, but also for carbon steel. Material aging was listed for several higher-temperature, constant-service piping systems (PSL, RHR, RCP, SIS) and mechanical fatigue was deemed important for smaller diameter piping, including the CRDM. Flow sensitive degradation, specifically FAC, was determined to be important in only the SIS system piping, while fretting wear (FW) is listed only for the ICI system.

Significant loading sources are consistent with the BWR piping sources. Pressure, seismic, thermal, RS, DW, and overload loading histories are sources for all systems. Smaller lines again are again considered to be susceptible to vibration loading and this loading is linked to the MF degradation mechanism. The RCP system is considered to have additional support loading contributions, mainly due to snubber malfunction. Both thermal stratification and thermal fatigue loading due to striping and heat-up/cool- down were listed as significant for the surge line.

Thermal fatigue loading is also important for the RHR and drain lines according to the group, while the reactor head and pressurizer surge lines are influenced by thermal stratification. The PSL also must consider water hammer. Only the SRV lines need to consider SRV transients which is quite different that the BWR classification. All of the major loading variables (Table B.1.5) were considered in either BWR or PWR systems. However, hanger misadjustment and cavitation loading were not specifically mentioned. They would certainly fall under the broader loading categories listed in Table B.1.5, but may need to be considered individually by each expert during the elicitation.

As with BWR piping, ISI with credit for leak detection is existent for all piping systems. All remaining mitigation and maintenance issues should also be considered for their effect on the LOCA frequencies. However, some specific mitigation procedures have been highlighted. This includes thermal stratification mitigation which some operators practice to limit surge line loads. Also, reactor vessel head replacement (HREPL) is a solution being considered to alleviate CRDM cracking concerns. The group will need to consider the extent and effectiveness (now and in the future) of each of these specific procedures.

Ratios of non-piping to piping for various category LOCAs are for 25 years only.

Pete made the point that most of the plots that Rob has shown are for 25 years, while he thought 40 and 60 years more important since 25 years is in past and the associated problems have been addressed while 40 and 60 years are for future; Rob responded that not that much difference between 25 and 40 years with some effect for certain category LOCAs at 60 years.

Sam commented that he was somewhat surprised that non-piping contribution less than piping contribution for BWRs in that piping has some active mechanisms that have been successfully mitigated in past; Karen responded that non-piping components more robust.

Bill Galyean warned about combining group distributions and panel distributions that may introduce a bias in that various members of group defined boundaries of system differently.

Much discussion on whether to chose group median or individual medians; Rob and Lee haven’t done panel distributions yet.

Comparison of BWR and PWR — effect of mitigation encompassed in results for BWRs, but not PWRs — BWRs have been doing mitigation for 15 to 20 years whereas PWRs are just starting with mitigation for PWSCC.

Inclusion of S/G tube rupture for Category 1 LOCAs will be problematic for some people in that PRA people aren’t used to accounting S/G tube failures in with rest of data; typically S/G tube rupture data is presented separately; in future Rob will present data both with S/G tube rupture data and without.

Categories 1, 2, and 3 are historically same as small, medium, and large break LOCAs respectively.

For PWR MB LOCAs, major contributor is CRDM, not S/G tube ruptures.

Discussion of whether MB LOCA was Category 2 or Category 3; some thought that MB LOCA was more in line with Category 3 LOCA.

Bengt felt we are comparing apples and oranges as we try to compare our results with historical results; NUREG/CR-5750 didn’t look at non-piping per se whereas we did, although Bill Galyean indicated that if there had been indications of TWC in non-piping components then he would have included that data in his analysis in 5750; some thought that due to apples and oranges nature of our approach with 5750 that we shouldn’t present these comparisons but Rob argued that if we don’t present these comparisons then others will; Some argued that we should present frequencies for multiple LOCA categories when we compare with small, medium, and large break LOCAs for 5750

Lee reviewed the feedback questionnaire.

Day 3: February 12, 2004

Presentation 17: Emergency and Faulted Loading: Elicitation Approach and Responses

Water hammer type loadings should be in normal operating loading history.

What we are asking panel members to estimate is only the conditional failure probability given a stress with magnitude i (PL/Si)

Ken commented that the seismic anchor motion (SAM) stress which is a secondary stress may be a bigger contributor than some primary stresses such as inertial stresses.

On VG entitled Elicitation Requirements, we are asking panel to do first bullet, we will do 2nd bullet, and plants would do 3rd and 4th bullets.

Asking them for a given system and degradation mechanism for their estimate of L50, P50, Ppl, Lpb Ltsl, and Ptsl and then we will interpolate to get entire curve.

Some people argued during their elicitations that non-piping and large piping are non contributors to LOCA frequency due to seismic; we will look at results from piping and then decide what and if we will do anything for non-piping considerations.

Bruce argued that can get some very high loads due to malfunctions of snubbers.

Pbc and Lbc are probabilities for base case and likelihood of base case