ML25183A352
| ML25183A352 | |
| Person / Time | |
|---|---|
| Issue date: | 06/30/2025 |
| From: | Jeffrey Poehler NRC/RES/DE |
| To: | |
| JEFF POEHLER 3014158353 | |
| Shared Package | |
| ML25183A340 | List: |
| References | |
| Download: ML25183A352 (6) | |
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Z AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP Panelist 1 Panelist 2 Panelist 3 Panelist 4 Panelist 5 Panelist 1 Panelist 2 Panelist 3 Panelist 4 Panelist 5 Average Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK RCPB-1 Research should be continued on improving our understanding of PWSCC initiation mechanisms.
Notes: These items apply generally to both Alloy 600 and Alloy 690 base and weld metals. Items of particular interest include:
- The early stages of intergranular oxide growth
- The eects of stress on iniaon mechanisms (this is separate from initiation time empirical modeling parameter effects)
- Creep cavity formaon in lower-CW materials
- Creep cavity formaon at lower temperatures
- The eect of grain boundary carbides M
M PK While mechanisms are good determine for more accurate models, I think we can engineer around the problem such that our lack of knowledge would be ameliorated.
L M
PK In regards to SG tubing, this issue is mostly non-relevant since the only welding is at the tube end and all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L M
PK While initiation mechanisms are debated for SCC in Ni-alloys, the current regulatory requirements typically assume initiation. Use for xLPR could be considered, but the mechnism is not going to be modeled, just the data. Peening is an exception that might be concerned with surface effects, but that is focused on the macroscopic issue of surface and near surface residual stress + operating stress. The RCPB-1 research topics should be goals of industry to improve the materials available. Once they have a new material and basis for its further reduced susceptibility, then we have more of a basis for confirmatory research in this area.
L M
PK H
M PK The better understanding of the factors that can affect PWSCC initiation (e.g., stress, strain, temperature and water chemistry) is important to prevent, mitigate and manage PWSCC.
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2 1.3 2.0 2.0 RCPB-2 Conduct research on thermal fatigue and corrosion fatigue as these mechanisms will likely continue to operate as plants age and possibly operate in previously unexpected areas or under unexpected conditions.
see red text L
L K
The Industry has an inspection program in place to identify thermal fatigue that appears to work overall and it gets revised on an as-needed basis.
H L
PK In regards to SG tubing, this issue is mostly non-relevant since the primary chemistry is closely controlled and eddy current inspections are performed on a regular basis. On the secondary side of the SG tubing, chemical hideout is a concern and typically adressed via sludge lancing and chemical cleanings.
However, chemical transients and lack of maintenance (cleanings) can have a significant effect, especially on Alloy 600.
M M
PK Industrys guidance document MRP-146 has required multiple updates due to operating experience. Continued vigilance to ensure thermal fatigue programs are adequate should consider NRC staff capabilities to verify the scope of a program to address any thermal stress thresholds. This could include ensuring computational fluid dynamics skills to an experimental validation program to ensure that MRP-146 scope boundaries are bounding for extended plant lifetimes.
H L
K M
M PK Generally speaking, thermal fatigue and environmentally assisted fatigue are not regarded to cause a rupture in large bore piping (e.g., main loop piping).
However, the operating experience indicates that fatigue cracks have been observed in the branch lines connected to the main loop piping.
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2 1.8 1.4 1.6 RCPB-3 Perform testing/research on actual plant materials to help validate aging management programs, inspections intervals and predictive models. This would require committing to ex-plant materials harvesting efforts.
H L
PK While testing lab specimens has helped characterize PWSCC over a variety of heats and welds, testing ex-plant materials would help verify the results obtain from lab specimens. We know what we would want to test, but new issues might arise so our knowledge is PK.
M M
UK In regards to SG tubing, pulled tube specimens are the gold standard for analyizing cracking but based on OpE with 690TT tubing, the need is low, even though the importance of controlling cracking (were it to occur) in 690 tubing is high. Hence, a medium ranking.
M L
PK Field materials that actually have cracked are of max value. Any work on actual plant materials to validate aging management programs should be on reasonably known aging mechanisms.
The value of the work is to ensure relevance of laboratory testing or engineering judgement to validate the current regulatory requirements. This can lead to the reduction of those requirements by removing unnecessary conservatisms thereby increasing H
M PK M
M PK Plant materials and related characterization efforts will be useful to confirm the applicability of lab test results to actual plant components and their aging management (e.g., confirmation of the absence of (1) crack initiation, (2) excessive residual strain and (3) material property changes).
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2 2.0 1.6 2.2 RCPB-4 Research should focus on dissimilar metal weld interfaces and heat-affected zones, as these may exhibit hardening and carbide coarsening during thermal aging.
M M
PK Thermal aging is an important mechanism to evaluate, but research results obtained so far w.r.t.
PWSCC CGR are showing that the high chrome alloys appear to be fairly resistant to increased PWSCC susceptibility L
L K
In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
M H
PK Thermal aging of Alloy 690 and associated weld materials has been found to increase hardness in some laboratory aging experiments in Oregon State University and VTT.
Increased hardness levels are consistent with 20% or higher cold work conditions in timelines that are within plant operating lifetimes. Higher cold work conditions lead to increased susceptibility of Alloy 690 materials, additionally current EPRI factors of improvement for Alloy 690 materials consider cold work up to 12% only.
There is great uncertainty about the possible impact of the current laboratory results with limited harvested materials to validate the initial findings. Testing of aged materials, preferably some harvested, for crack initiation and growth of aged materials with increased hardness should be considered for long term operation, 80+ years.
H M
PK M
H PK Material hardening and related microstructure changes can promote environmentally assisted degradation such as PWSCC. This effort would enhance the understanding of the degree of such effects and the associated mechanisms.
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2 1.7 2.2 1.8 RCPB-5 Investigate the impact of preexisting welding defects on SCC initiation in high-chrome weld metals. These defects may act as high-stress regions that induce creep cavities and degrade materials over long-term operations.
M L
PK We have performed some work in this area using blunt notch specimens and found that pre-existing defects can lead to crack initiation, but the initiated cracks tend not to grow. Some limited work is needed on the variety of welding defects that would help provide the assurance needed to make determinations for instances when these types of defects are found in service.
L L
K In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
H M
PK Weld defects in high-chrome weld metals are known to exist in many replacement components, repairs and mitigations. The effectiveness of these materials with these known conditions is being addressed with new weld alloys to address weldability and fabrication cracking conditions.
However, the existing fleet is known to have several components with these fabrication cracks. Replacement heads have identified over 800 fabrication cracks in pre-service volumetric examinations. Embedded flaw repairs have had subsurface defects become exposed to primary coolant after cycling due to startups/shutdowns.
Evaluating the effect of these cracks on these repairs/mitigations should consider the very thin layers of these materials installed for consideration of operation in 80+ years.
L M
PK M
M PK Welding defects in Alloy 690 has not been a source of environmentally assisted cracking in actual plant components in the operating experience. However, the current understanding needs to be enhanced in terms of what causes the welding defects (e.g.,
alloy chemistry and welding process parameters) and how the defects can be prevented.
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2 1.5 1.6 1.8 RCPB-6 Research on degradation mechanisms associated with low alloy steel should be considered such as temper bead welding which can cause an elevated yield strength and environmental cracking (EC) susceptibility.
- The eect of service environment on fracture properes Context: Cracks grew slightly into the LAS from A52 weld during CGR testing M
L PK Temper bead welds should possess sufficient toughness and resistance to PWSCC, but some research results a level of susceptibility that should be explored. I feel we have partial knowledge of this research topic, but we know enough to narrow the research to look at specific aspects to rule out concern. Uncertainty is low because we are well aware of the safety significance.
L L
K In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since the tubing is a Ni-based alloy and the channel head is clad with either stainless steel or a Ni-based alloy. There have been a few instances of channel head cladding being breached during manufacture which has lead to minor corrosion of the LAS under the cladding but engineering analysis has shown stresses remain low and corrosion is minimal because of low oxygen levels in the primary coolant.
L M
PK Dilution testing and the effects of increased hardness for temper bead welding are and should continue to be evaluated for initiation and crack growth rates. Impurities need to be better defined for additional work, with reasonable concerns for concentrations given reactor coolant chemistry controls. Should also define service environment of concern.
L M
PK L
M PK Low alloy steels show relatively high resistance to environmental cracking (also called environmentally assisted cracking) in the reactor coolant water environments especially those with a yield strength level of less than 150 ksi. However, the effects of welding processes and water chemistry excursions on environmental cracking in low alloy steels may need to be further studied for better understanding.
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2 1.0 1.6 1.8 RCPB-7 Research on stainless steels to provide a better understanding of SCC initiation in stainless steel piping, focusing on factors such as thermal stra"caon,temperature, dissolved oxygen, surface condion, long-term aging, and welding residual strain.
M L
PK The issues related to SS SCC in France do not appear to be applicable to US plants. However, OpE in the US on sensitized SS BMI raises this to a medium for me.
Also, perhaps long-term thermal aging is a potential issue.
L L
PK In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since in the few channel heads clad with stainless steel in the US, there has been no OpE of SCC.
H L
K Many of the mechanisms listed are known, but research is needed to identify what components can be affected by these conditions within extended plant lifetimes. A leak from a SS safe end to a BMI identified a new issue of sensitization of SS due to original plant heat treatment.
Dissolved oxygen levels in trapped or crevice conditions in PWRs are a concern of some experts. Similarly long term aging of stainless steels for 80+
years of PWR operational environments has been raised as an area of limited data.
M M
PK L
M PK Stainless steels show relatively high resistance to SCC in the PWR reactor coolant water environment. One factor of interest may be the heat input of the welding process because it can affect the hardness of the adjacent metal and susceptibility to SCC.
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2 2.0 1.0 1.7 RCPB-8 Conduct modeling and simulation to validate assumed mechanisms and make accurate predictions about PWSCC crack initiation and growth. This includes building comprehensive databases and benchmarking different models. This also includes adjusting NDE strategies and creating NDE tools to accurately assess cracking kinetics.
M L
PK While modeling is helpful, the uncertainty in PWSCC models is very large. I think making accurate predictions is out of the realm of possibility. Models may help identify trends and bound the problem, but are far from being useful for "accurate" predictions. I gave it a M for safety significance because PWSCC is a safety concern of which there little uncertainty. Our knowledge is growing and for high chrome alloys, it's showing they are very resistance to PWSCC.
M H
K In regards to SG tubing, this issue is mostly non-relevant since all PWRs in the US use a plug-on-detection strategy regarding cracking in SG tubing. In the event that the US ever pursued a crack sizing strategy in SG tubing, this issue would be of the highest importance, due to the low wall thickness of SG tubing.
Additionally, the uncertainty associated with eddy current inspection of SCC is pretty high, especially with shallow cracking.
H L
K I think we always want to have the tools to predict and assess crack initiation and growth for all degradation mechanisms. Additionally, NDE tools are necessary to evaluate each material for degradation. I just think this is too general of a category and should be focused on a particular degradation or component material.
M M
PK H
M PK Modeling and simulation capabilities can provide a more relevant risk insight on the material degradation and component failure at a piping system or plant level (e.g.,
probabilistic fracture mechanics analysis that can estimated loss-of-coolant accident frequencies).
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2 2.3 1.7 1.3 RCPB-9 Conduct research to evaluate how cold work and applied stress affect SCC initiation time for different alloy groups (e.g., nickel-base alloys, stainless steels). Understanding these differences is important for developing unified models and determining the improvement factors for second-generation nickel-based alloys. This includes how cold work is applied (e.g., forging vs tensile strain). The effect of dynamic strain should also be evaluated.
M M
PK Cold work has shown to be a significant PWSCC accelerating factor. We are fairly certain that cold worked materials can be highly susceptible to SCC.
CW seems to lead to different cracking mechanisms among the nickel alloys (IG vs GB creep).
L L
K In regards to SG tubing, this issue is mostly non-relevant since all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L L
K I think we know cold work is bad for these materials. A more focused question would be to evaluate any new Second-Generation materials that claim to address the cold work issue.
Further, assessment of any new materials for new plants to evaluate cold work may be useful. Current requirements note good practices to minimize cold work as much as possible.
L M
K M
H PK The level of cold work (and strain) can affect the susceptibility of materials to SCC initiation and growth. The levels of cold work and strain (e.g.,
residual strain due to welding) in the actual components may involve relatively large uncertainties and plant/component-specific variations.
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1.3 score
H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L
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AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK score: H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L 13 14 15 16 17 RCPB-10 Evaluate how plant modifications and upgrades, including power uprates and low-leakage core (which may lead to SCC acceleration by thermal mixing) affect degradation.
L H
PK This does not seem like it would be a big safety concern given that the plant upgrades and modifications that I could think of would likely improve the situation w.r.t. materials such as steam gen replacements, vessel head replacements, etc. A power uprate might increase stress and temperature but I have no knowledge by how much.
L L
K In regards to SG tubing, this issue is mostly non-relevant since tubes in replacement and new SGs are Alloy 690, power uprates typically result in minimal Thot increases, and neutron leakage doesn't affect the SGs.
M L
K Plant uprates are regularly reviewed for effects of temperature change and fatigue/flow issues. The steam dryer issues with BWRs is an example of how those effects were identified after implementation, so it would be good to be proactive to identify concerns.
However, the scope of the item should be on a particular upgrade and potential concern.
M L
PK L
M PK Plant modifications and upgrades such as power uprates may promote material degradation (e.g., accelerated crack growth due to an increase in temperature, vibration or irradiation). However, generally speaking, plant modifications and upgrades appear to have a relatively small effect on material degradation unless there are plant-specific issues (e.g., inadequate design of replacement components).
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2 1.3 1.7 1.3 RCPB-11 Perform research on how temperature affects SCC initiation mechanisms and kinetics to help develop models. This research would help develop a more fundamental and broader understanding of temperature effects focusing on validating kinetics and surface microstructure evolution (e.g., grain boundary oxidation, diffusion, etc.).
H L
PK Temperature effects are a controlling service parameter for PWSCC in general. Current research results are not elucidating the effect of temperature on PWSCC initiation sufficiently to provide a high level of confidence in Arrhenius relationships assumed to characterize temp effects.
H L
PK What Eric said.
M L
PK Needs better focus on a particular material of concern. We know temperature has an effect, but we have models to address this concern.
Scatter of data usually in these models is sometimes large or small depending on the material.
L M
PK H
M PK The effect of temperature on SCC is one of the important factors to be considered in the development of inspection frequencies as well as the conduct of analytical evaluation of detected flaws. A better understanding of the effects of relatively high and low temperatures on SCC can be also used to optimize inspection strategies.
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2 2.7 1.0 2.0 RCPB-12 Perform research on how temperature affects SCC CGR mechanisms and kinetics to help develop models with emphasis on potential microstructural changes that might occur during long-term thermal aging.
M L
PK Long term thermal aging is a concern but not an immediate safety concern, but we really don't know a lot about the mechanisms operating at very long service lives. The temperature effects and the mechanisms of aging appear to be chemical composition dependent, but we do not have a lot of data on the mechanisms we think are operating. We have some data on LRO, and it appears to not be an issue for iron contents above 9%, but there are potentially low-iron weld metals of interest being developed. ANL is measuring increased hardness in aged materials but not LRO. We don't really know why, yet.
M M
UK What Eric said, but for SGs, the importance might be a little higher since the wall thickness is less.
M M
PK Needs better focus on a particular material of concern. Depending on the material, we may know the thermal aging effect, and we have models to address this concern.
L M
K H
M PK Please see the rationale for Item 11 above. The microstructural changes can affect the material susceptibility to SCC, too.
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2 2.0 1.7 2.3 RCPB-13 Perform research to improve accelerated testing methods to avoid altering microstructures too much while still obtaining useful data. This involves finding a balance between accelerating tests and maintaining the integrity of the materials being tested.
H M
PK We know that accelerated testing is not ideal, but we are forced to do it to obtain data in a reasonable time frame. I think this is an important issue to address because we are trying to make decisions based on the data so we need to better understand how to best correlate/translate accelerated testing to actual service conditions.
M M
UK If we can ever get the material harvested from a plant with significant operational history, it would be valuable to confirm whether our accelerated testing has unintended consequences.
I'm not sure how applicable this is to SGs.
H M
PK It would be very useful to evaluate accelerating testing methods to ensure our testing is of representative microstructures of plant aged materials. As similarly discussed in the harvesting item, this would improve regulatory efficiency and stakeholder confidence.
H M
PK M
M PK Accelerated testing approaches have been used to simulate and characterize the material properties and resistances to degradation (e.g., neutron and thermal aging embrittlement) in an accelerated manner. The accelerated test results may need to be compared with the test and characterization results of harvested plant materials to confirm the applicability of the accelerated testing.
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2 2.7 2.0 2.3 RCPB-14 Research may be needed to evaluate the EC resistance of components after the application of mitigation surface treatments such as peening.
M M
UK I didn't review the TR on peening so I'm not familiar with how successful surface treatments are for improving the resistance to PWSCC. I ranked it as a high safety concern because we are relying on surface treatments to mitigate cracking as opposed to material replacement....is that correct? I am very interested in the long-term stability of the treated surface layers and how well compressive stresses are maintained.
H M
PK With regards to SG tubing, shot peening with metal media was very successful for a unit with 600MA tubing that was shot peened prior to being placed in service. Other units were the shot peening was performed after the SG was placed in service did not result in the same level of benefit.
I found no OpE on laser peening of SG tubing.
L L
K I think we would need to see an instance of where peening has failed under plant operating conditions to help focus this item.
M M
PK M
M PK Surface treatments can be an effective means to mitigate environmental cracking.
Additional research activities may be needed to better understand the effectiveness of surface treatment approaches, including the review of operating experience of mitigated components.
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Z AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR AS AT AU AV AW AX AY AZ BA BB BC BD BE BF BG BH BI BJ BK BL BM BN Eric Focht Andy Johnson Jay Pat Seung Panelist 1 Panelist 2 Panelist 3 Panelist 4 Panelist 5 Average Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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Knowledg e (avg) error bars Import Uncert Know error More research could be used to gain knowledge on a topic expected to be important to safety More research could reduce uncertainty and clarify topic importance More research could improve knowledge but topic not expected to be safety significant Topic expected to have safety significance but likely could be addressed without additional research Topic may have safety significance but sufficient knowledge exists to manage Topic not expected to have safety significance and sufficient knowledge exists K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK RCPB-1 Research should be continued on improving our understanding of PWSCC initiation mechanisms.
Notes: These items apply generally to both Alloy 600 and Alloy 690 base and weld metals. Items of particular interest include:
- The early stages of intergranular oxide growth
- The eects of stress on iniaon mechanisms (this is separate from initiation time empirical modeling parameter effects)
- Creep cavity formaon in lower-CW materials
- Creep cavity formaon at lower temperatures
- The eect of grain boundary carbides M
M PK While mechanisms are good determine for more accurate models, I think we can engineer around the problem such that our lack of knowledge would be ameliorated.
L M
PK In regards to SG tubing, this issue is mostly non-relevant since the only welding is at the tube end and all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L M
PK While initiation mechanisms are debated for SCC in Ni-alloys, the current regulatory requirements typically assume initiation. Use for xLPR could be considered, but the mechnism is not going to be modeled, just the data. Peening is an exception that might be concerned with surface effects, but that is focused on the macroscopic issue of surface and near surface residual stress + operating stress. The RCPB-1 research topics should be goals of industry to improve the materials L
M PK H
M PK The better understanding of the factors that can affect PWSCC initiation (e.g., stress, strain, temperature and water chemistry) is important to prevent, mitigate and manage PWSCC.
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2 RCPB-2 Conduct research on [thermal] fatigue and corrosion fatigue as these mechanisms will likely continue to operate as plants age and possibly operate in previously unexpected areas or under unexpected conditions.
see red text L
L K
The Industry has an inspection program in place to identify thermal fatigue that appears to work overall and it gets revised on an as-needed basis.
H L
PK In regards to SG tubing, this issue is mostly non-relevant since the primary chemistry is closely controlled and eddy current inspections are performed on a regular basis. On the secondary side of the SG tubing, chemical hideout is a concern and typically adressed via sludge lancing and chemical cleanings. However, chemical transients and lack of maintenance (cleanings) can have a significant effect, especially on Alloy 600.
M M
PK Industrys guidance document MRP-146 has required multiple updates due to operating experience.
Continued vigilance to ensure thermal fatigue programs are adequate should consider NRC staff capabilities to verify the scope of a program to address any thermal stress thresholds. This could include ensuring computational fluid dynamics skills to an experimental validation program to ensure that MRP-146 scope boundaries are bounding for extended plant lifetimes.
H L
K M
M PK Generally speaking, thermal fatigue and environmentally assisted fatigue are not regarded to cause a rupture in large bore piping (e.g., main loop piping). However, the operating experience indicates that fatigue cracks have been observed in the branch lines connected to the main loop piping.
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2 1.8 1.4 1.6 0.2 1.8 1.4 1.6 0.2 2
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2 RCPB-3 Perform testing/research on actual plant materials to help validate aging management programs, inspections intervals and predictive models. This would require committing to ex-plant materials harvesting efforts.
H L
PK While testing lab specimens has helped characterize PWSCC over a variety of heats and welds, testing ex-plant materials would help verify the results obtain from lab specimens. We know what we would want to test, but new issues might arise so our knowledge is PK.
M M
UK In regards to SG tubing, pulled tube specimens are the gold standard for analyizing cracking but based on OpE with 690TT tubing, the need is low, even though the importance of controlling cracking (were it to occur) in 690 tubing is high. Hence, a medium ranking.
M L
PK Field materials that actually have cracked are of max value. Any work on actual plant materials to validate aging management programs should be on reasonably known aging mechanisms. The value of the work is to ensure relevance of laboratory testing or engineering judgement to validate the current regulatory requirements. This can lead to the reduction of those requirements by removing unnecessary conservatisms H
M PK M
M PK Plant materials and related characterization efforts will be useful to confirm the applicability of lab test results to actual plant components and their aging management (e.g.,
confirmation of the absence of (1) crack initiation, (2) excessive residual strain and (3) material property changes).
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2 2.0 1.6 2.2 0.3 2.0 1.6 2.2 0.3 RCPB-4 Research should focus on dissimilar metal weld interfaces and heat-affected zones, as these may exhibit hardening and carbide coarsening during thermal aging.
M M
PK Thermal aging is an important mechanism to evaluate, but research results obtained so far w.r.t.
PWSCC CGR are showing that the high chrome alloys appear to be fairly resistant to increased PWSCC susceptibility L
L K
In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
M H
PK Thermal aging of Alloy 690 and associated weld materials has been found to increase hardness in some laboratory aging experiments in Oregon State University and VTT.
Increased hardness levels are consistent with 20% or higher cold work conditions in timelines that are within plant operating lifetimes.
Higher cold work conditions lead to increased susceptibility of Alloy 690 materials additionally current EPRI H
M PK M
H PK Material hardening and related microstructure changes can promote environmentally assisted degradation such as PWSCC. This effort would enhance the understanding of the degree of such effects and the associated mechanisms.
2 2
2 1
1 1
2 3
2 3
2 2
2 3
2 1.7 2.2 1.8 0.4 1.7 2.2 1.8 0.4 RCPB-5 Investigate the impact of preexisting welding defects on SCC initiation in high-chrome weld metals. These defects may act as high-stress regions that induce creep cavities and degrade materials over long-term operations.
M L
PK We have performed some work in this area using blunt notch specimens and found that pre-existing defects can lead to crack initiation, but the initiated cracks tend not to grow. Some limited work is needed on the variety of welding defects that would help provide the assurance needed to make determinations for instances when these types of defects are found in service.
L L
K In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
H M
PK Weld defects in high-chrome weld metals are known to exist in many replacement components, repairs and mitigations. The effectiveness of these materials with these known conditions is being addressed with new weld alloys to address weldability and fabrication cracking conditions. However, the existing fleet is known to have several components with these fabrication cracks. Replacement heads have identified over 800 fabrication cracks in pre-service volumetric examinations. Embedded flaw repairs have had subsurface defects become exposed to primary coolant after cycling due to startups/shutdowns. Evaluating the effect of these cracks on these repairs/mitigations should consider the very thin layers of these materials installed for consideration of operation in 80+ years.
L M
PK M
M PK Welding defects in Alloy 690 has not been a source of environmentally assisted cracking in actual plant components in the operating experience. However, the current understanding needs to be enhanced in terms of what causes the welding defects (e.g., alloy chemistry and welding process parameters) and how the defects can be prevented.
2 1
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3 2
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2 1.5 1.6 1.8 0.3 1.5 1.6 1.8 0.3 RCPB-6 Research on degradation mechanisms associated with low alloy steel should be considered such as temper bead welding and irradiation which can cause an elevated yield strength and environmental cracking (EC) susceptibility.
- The eect of service environment on fracture properes Context: Cracks grew slightly into the LAS from A52 weld during CGR testing M
L PK Temper bead welds should possess sufficient toughness and resistance to PWSCC, but some research results a level of susceptibility that should be explored. I feel we have partial knowledge of this research topic, but we know enough to narrow the research to look at specific aspects to rule out concern. Uncertainty is low because we are well aware of the safety significance.
L L
K In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since the tubing is a Ni-based alloy and the channel head is clad with either stainless steel or a Ni-based alloy.
There have been a few instances of channel head cladding being breached during manufacture which has lead to minor corrosion of the LAS under the cladding but engineering analysis has shown stresses remain low and corrosion is minimal because of low oxygen levels in the primary coolant.
L M
PK Dilution testing and the effects of increased hardness for temper bead welding are and should continue to be evaluated for initiation and crack growth rates. Impurities need to be better defined for additional work, with reasonable concerns for concentrations given reactor coolant chemistry controls. Should also define service environment of concern.
L M
PK L
M PK Low alloy steels show relatively high resistance to environmental cracking (also called environmentally assisted cracking) in the reactor coolant water environments especially those with a yield strength level of less than 150 ksi.
However, the effects of welding processes and water chemistry excursions on environmental cracking in low alloy steels may need to be further studied for better understanding.
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2 1.0 1.6 1.8 0.3 1.0 1.6 1.8 0.3 RCPB-7 Research on stainless steels to provide a better understanding of SCC initiation in stainless steel piping, focusing on factors such as thermal stra"caon,temperature, dissolved oxygen, surface condion, long-term aging, and welding residual strain.
M L
PK The issues related to SS SCC in France do not appear to be applicable to US plants. However, OpE in the US on sensitized SS BMI raises this to a medium for me. Also, perhaps long-term thermal aging is a potential issue.
L L
PK In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since in the few channel heads clad with stainless steel in the US, there has been no OpE of SCC.
H L
K Many of the mechanisms listed are known, but research is needed to identify what components can be affected by these conditions within extended plant lifetimes. A leak from a SS safe end to a BMI identified a new issue of sensitization of SS due to original plant heat treatment. Dissolved oxygen levels in trapped or crevice conditions in PWRs are a concern of some experts. Similarly long term aging of stainless steels for 80+ years of PWR operational environments has been raised as an area of limited data.
M M
PK L
M PK Stainless steels show relatively high resistance to SCC in the PWR reactor coolant water environment. One factor of interest may be the heat input of the welding process because it can affect the hardness of the adjacent metal and susceptibility to SCC.
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2 2.0 1.0 1.7 0.2 2.0 1.0 1.7 0.2 RCPB-8 Conduct modeling and simulation to validate assumed mechanisms and make accurate predictions about [PWSCC] crack initiation and growth.
This includes building comprehensive databases and benchmarking different models. This also includes adjusting NDE strategies and creating NDE tools to accurately assess cracking kinetics.
M L
PK While modeling is helpful, the uncertainty in PWSCC models is very large. I think making accurate predictions is out of the realm of possibility. Models may help identify trends and bound the problem, but are far from being useful for "accurate" predictions. I gave it a M for safety significance because PWSCC is a safety concern of which there little uncertainty. Our knowledge is growing and for high chrome alloys, it's showing they are very resistance to PWSCC.
M H
K In regards to SG tubing, this issue is mostly non-relevant since all PWRs in the US use a plug-on-detection strategy regarding cracking in SG tubing. In the event that the US ever pursued a crack sizing strategy in SG tubing, this issue would be of the highest importance, due to the low wall thickness of SG tubing.
Additionally, the uncertainty associated with eddy current inspection of SCC is pretty high H
L K
I think we always want to have the tools to predict and assess crack initiation and growth for all degradation mechanisms.
Additionally, NDE tools are necessary to evaluate each material for degradation. I just think this is too general of a category and should be focused on a particular degradation or component material.
M M
PK H
M PK Modeling and simulation capabilities can provide a more relevant risk insight on the material degradation and component failure at a piping system or plant level (e.g.,
probabilistic fracture mechanics analysis that can estimated loss-of-coolant accident frequencies).
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2 2.3 1.7 1.3 0.3 2.3 1.7 1.3 0.3 RCPB-9 Conduct research to evaluate how cold work and applied stress affect SCC initiation time for different alloy groups (e.g., nickel-base alloys, stainless steels). Understanding these differences is important for developing unified models and determining the improvement factors for second-generation nickel-based alloys. This includes how cold work is applied (e.g., forging vs tensile strain). The effect of dynamic strain should also be evaluated.
M M
PK Cold work has shown to be a significant PWSCC accelerating factor. We are fairly certain that cold worked materials can be highly susceptible to SCC.
CW seems to lead to different cracking mechanisms among the nickel alloys (IG vs GB creep).
L L
K In regards to SG tubing, this issue is mostly non-relevant since all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L L
K I think we know cold work is bad for these materials. A more focused question would be to evaluate any new Second-Generation materials that claim to address the cold work issue. Further, assessment of any new materials for new plants to evaluate cold work may be useful.
Current requirements note good practices to minimize cold work as much as possible.
L M
K M
H PK The level of cold work (and strain) can affect the susceptibility of materials to SCC initiation and growth. The levels of cold work and strain (e.g., residual strain due to welding) in the actual components may involve relatively large uncertainties and plant/component-specific variations.
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2 1.3 1.3 1.3 0.2 1.3 1.3 1.3 0.2 RCPB-10 Evaluate how plant modifications and upgrades, including power uprates and low-leakage core (which may lead to SCC acceleration by thermal mixing) affect degradation.
L H
PK This does not seem like it would be a big safety concern given that the plant upgrades and modifications that I could think of would likely improve the situation w.r.t. materials such as steam gen replacements, vessel head replacements, etc. A power uprate might increase stress and temperature but I have no knowledge by how much.
L L
K In regards to SG tubing, this issue is mostly non-relevant since tubes in replacement and new SGs are Alloy 690, power uprates typically result in minimal Thot increases, and neutron leakage doesn't affect the SGs.
M L
K Plant uprates are regularly reviewed for effects of temperature change and fatigue/flow issues. The steam dryer issues with BWRs is an example of how those effects were identified after implementation, so it would be good to be proactive to identify concerns. However, the scope of the item should be on a particular upgrade and potential concern.
M L
PK L
M PK Plant modifications and upgrades such as power uprates may promote material degradation (e.g., accelerated crack growth due to an increase in temperature, vibration or irradiation). However, generally speaking, plant modifications and upgrades appear to have a relatively small effect on material degradation unless there are plant-specific issues (e.g.,
inadequate design of replacement components).
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1 1
2 1
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2 1.3 1.7 1.3 0.3 1.3 1.7 1.3 0.3 RCPB-11 Perform research on how temperature affects SCC initiation mechanisms and kinetics to help develop models. This research would help develop a more fundamental and broader understanding of temperature effects focusing on validating kinetics and surface microstructure evolution (e.g., grain boundary oxidation, diffusion, etc.).
H L
PK Temperature effects are a controlling service parameter for PWSCC in general. Current research results are not elucidating the effect of temperature on PWSCC initiation sufficiently to provide a high level of confidence in Arrhenius relationships assumed to characterize temp effects.
H L
PK What Eric said.
M L
PK Needs better focus on a particular material of concern. We know temperature has an effect, but we have models to address this concern.
Scatter of data usually in these models is sometimes large or small depending on the material.
L M
PK H
M PK The effect of temperature on SCC is one of the important factors to be considered in the development of inspection frequencies as well as the conduct of analytical evaluation of detected flaws. A better understanding of the effects of relatively high and low temperatures on SCC can be also used to optimize inspection strategies.
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2 2.7 1.0 2.0 0.2 2.7 1.0 2.0 0.2 RCPB-12 Perform research on how temperature affects SCC CGR mechanisms and kinetics to help develop models with emphasis on potential microstructural changes that might occur during long-term thermal aging.
M L
PK Long term thermal aging is a concern but not an immediate safety concern, but we really don't know a lot about the mechanisms operating at very long service lives. The temperature effects and the mechanisms of aging appear to be chemical composition dependent, but we do not have a lot of data on the mechanisms we think are operating.
We have some data on LRO, and it appears to not be an issue for iron contents above 9%, but there are potentially low-iron weld metals of interest being developed. ANL is measuring increased hardness in aged materials but not LRO. We don't really know why, yet.
M M
UK What Eric said, but for SGs, the importance might be a little higher since the wall thickness is less.
M M
PK Needs better focus on a particular material of concern. Depending on the material, we may know the thermal aging effect, and we have models to address this concern.
L M
K H
M PK Please see the rationale for Item 11 above. The microstructural changes can affect the material susceptibility to SCC, too.
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2 2.0 1.7 2.3 0.3 2.0 1.7 2.3
0.3 score
H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-1 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-2 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-3 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-4 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-5 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-6 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-7 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-8 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-9 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-10 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-11 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-12 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 PIRT Chart
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AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR AS AT AU AV AW AX AY AZ BA BB BC BD BE BF BG BH BI BJ BK BL BM BN Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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Knowledg e (avg) error bars Import Uncert Know error More research could be used to gain knowledge on a topic expected to be important to safety More research could reduce uncertainty and clarify topic importance More research could improve knowledge but topic not expected to be safety significant Topic expected to have safety significance but likely could be addressed without additional research Topic may have safety significance but sufficient knowledge exists to manage Topic not expected to have safety significance and sufficient knowledge exists K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK score: H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L 16 17 RCPB-13 Perform research to improve accelerated testing methods to avoid altering microstructures too much while still obtaining useful data. This involves finding a balance between accelerating tests and maintaining the integrity of the materials being tested.
H M
PK We know that accelerated testing is not ideal, but we are forced to do it to obtain data in a reasonable time frame. I think this is an important issue to address because we are trying to make decisions based on the data so we need to better understand how to best correlate/translate accelerated testing to actual service conditions.
M M
UK If we can ever get the material harvested from a plant with significant operational history, it would be valuable to confirm whether our accelerated testing has unintended consequences. I'm not sure how applicable this is to SGs.
H M
PK It would be very useful to evaluate accelerating testing methods to ensure our testing is of representative microstructures of plant aged materials. As similarly discussed in the harvesting item, this would improve regulatory efficiency and stakeholder confidence.
H M
PK M
M PK Accelerated testing approaches have been used to simulate and characterize the material properties and resistances to degradation (e.g., neutron and thermal aging embrittlement) in an accelerated manner. The accelerated test results may need to be compared with the test and characterization results of harvested plant materials to confirm the applicability of the accelerated testing.
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2 2.7 2.0 2.3 0.3 2.7 2.0 2.3 0.3 RCPB-14 Research may be needed to evaluate the EC resistance of components after the application of mitigation surface treatments such as peening.
H M
UK I didn't review the TR on peening so I'm not familiar with how successful surface treatments are for improving the resistance to PWSCC. I ranked it as a high safety concern because we are relying on surface treatments to mitigate cracking as opposed to material replacement....is that correct? I am very interested in the long-term stability of the treated surface layers and how well compressive stresses are maintained.
H M
PK With regards to SG tubing, shot peening with metal media was very successful for a unit with 600MA tubing that was shot peened prior to being placed in service. Other units were the shot peening was performed after the SG was placed in service did not result in the same level of benefit.
I found no OpE on laser peening of SG L
L K
I think we would need to see an instance of where peening has failed under plant operating conditions to help focus this item.
M M
PK M
M PK Surface treatments can be an effective means to mitigate environmental cracking.
Additional research activities may be needed to better understand the effectiveness of surface treatment approaches, including the review of operating experience of mitigated components.
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2 2.3 1.7 2.0 0.3 2.3 1.7 2.0 0.3 Knowledge 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-13 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-14
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Z AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR AS AT AU AV AW AX AY AZ BA BB BC BD BE BF BG BH BI BJ BK BL BM BN Eric Focht Andy Johnson Jay Pat Seung Panelist 1 Panelist 2 Panelist 3 Panelist 4 Panelist 5 Average Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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Knowledg e (avg) error bars Import Uncert Know error More research could be used to gain knowledge on a topic expected to be important to safety More research could reduce uncertainty and clarify topic importance More research could improve knowledge but topic not expected to be safety significant Topic expected to have safety significance but likely could be addressed without additional research Topic may have safety significance but sufficient knowledge exists to manage Topic not expected to have safety significance and sufficient knowledge exists K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK RCPB-1 Research should be continued on improving our understanding of PWSCC initiation mechanisms.
Notes: These items apply generally to both Alloy 600 and Alloy 690 base and weld metals. Items of particular interest include:
- The early stages of intergranular oxide growth
- The eects of stress on iniaon mechanisms (this is separate from initiation time empirical modeling parameter effects)
- Creep cavity formaon in lower-CW materials
- Creep cavity formaon at lower temperatures
- The eect of grain boundary carbides M
M PK While mechanisms are good determine for more accurate models, I think we can engineer around the problem such that our lack of knowledge would be ameliorated.
L M
PK In regards to SG tubing, this issue is mostly non-relevant since the only welding is at the tube end and all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L M
PK While initiation mechanisms are debated for SCC in Ni-alloys, the current regulatory requirements typically assume initiation. Use for xLPR could be considered, but the mechnism is not going to be modeled, just the data. Peening is an exception that might be concerned with surface effects, but that is focused on the macroscopic issue of surface and near surface residual stress + operating stress. The RCPB-1 research topics should be goals of industry to improve the materials L
M PK H
M PK The better understanding of the factors that can affect PWSCC initiation (e.g., stress, strain, temperature and water chemistry) is important to prevent, mitigate and manage PWSCC.
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2 RCPB-2 Conduct research on [thermal] fatigue and corrosion fatigue as these mechanisms will likely continue to operate as plants age and possibly operate in previously unexpected areas or under unexpected conditions.
see red text L
L K
The Industry has an inspection program in place to identify thermal fatigue that appears to work overall and it gets revised on an as-needed basis.
H L
PK In regards to SG tubing, this issue is mostly non-relevant since the primary chemistry is closely controlled and eddy current inspections are performed on a regular basis. On the secondary side of the SG tubing, chemical hideout is a concern and typically adressed via sludge lancing and chemical cleanings. However, chemical transients and lack of maintenance (cleanings) can have a significant effect, especially on Alloy 600.
M M
PK Industrys guidance document MRP-146 has required multiple updates due to operating experience.
Continued vigilance to ensure thermal fatigue programs are adequate should consider NRC staff capabilities to verify the scope of a program to address any thermal stress thresholds. This could include ensuring computational fluid dynamics skills to an experimental validation program to ensure that MRP-146 scope boundaries are bounding for extended plant lifetimes.
H L
K M
M PK Generally speaking, thermal fatigue and environmentally assisted fatigue are not regarded to cause a rupture in large bore piping (e.g., main loop piping). However, the operating experience indicates that fatigue cracks have been observed in the branch lines connected to the main loop piping.
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2 RCPB-3 Perform testing/research on actual plant materials to help validate aging management programs, inspections intervals and predictive models. This would require committing to ex-plant materials harvesting efforts.
H L
PK While testing lab specimens has helped characterize PWSCC over a variety of heats and welds, testing ex-plant materials would help verify the results obtain from lab specimens. We know what we would want to test, but new issues might arise so our knowledge is PK.
M M
UK In regards to SG tubing, pulled tube specimens are the gold standard for analyizing cracking but based on OpE with 690TT tubing, the need is low, even though the importance of controlling cracking (were it to occur) in 690 tubing is high. Hence, a medium ranking.
M L
PK Field materials that actually have cracked are of max value. Any work on actual plant materials to validate aging management programs should be on reasonably known aging mechanisms. The value of the work is to ensure relevance of laboratory testing or engineering judgement to validate the current regulatory requirements. This can lead to the reduction of those requirements by removing unnecessary conservatisms H
M PK M
M PK Plant materials and related characterization efforts will be useful to confirm the applicability of lab test results to actual plant components and their aging management (e.g.,
confirmation of the absence of (1) crack initiation, (2) excessive residual strain and (3) material property changes).
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2 2.0 1.6 2.2 0.3 2.0 1.6 2.2 0.3 RCPB-4 Research should focus on dissimilar metal weld interfaces and heat-affected zones, as these may exhibit hardening and carbide coarsening during thermal aging.
M M
PK Thermal aging is an important mechanism to evaluate, but research results obtained so far w.r.t.
PWSCC CGR are showing that the high chrome alloys appear to be fairly resistant to increased PWSCC susceptibility L
L K
In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
M H
PK Thermal aging of Alloy 690 and associated weld materials has been found to increase hardness in some laboratory aging experiments in Oregon State University and VTT.
Increased hardness levels are consistent with 20% or higher cold work conditions in timelines that are within plant operating lifetimes.
Higher cold work conditions lead to increased susceptibility of Alloy 690 materials additionally current EPRI H
M PK M
H PK Material hardening and related microstructure changes can promote environmentally assisted degradation such as PWSCC. This effort would enhance the understanding of the degree of such effects and the associated mechanisms.
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2 1
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2 1.7 2.2 1.8 0.4 1.7 2.2 1.8 0.4 RCPB-5 Investigate the impact of preexisting welding defects on SCC initiation in high-chrome weld metals. These defects may act as high-stress regions that induce creep cavities and degrade materials over long-term operations.
M L
PK We have performed some work in this area using blunt notch specimens and found that pre-existing defects can lead to crack initiation, but the initiated cracks tend not to grow. Some limited work is needed on the variety of welding defects that would help provide the assurance needed to make determinations for instances when these types of defects are found in service.
L L
K In regards to SG tubing, this issue is mostly non-relevant since the tubing is only welded at the tube end and the frictional force generated by hydraulic epansion has been shown to compensate in case the welds fail.
H M
PK Weld defects in high-chrome weld metals are known to exist in many replacement components, repairs and mitigations. The effectiveness of these materials with these known conditions is being addressed with new weld alloys to address weldability and fabrication cracking conditions. However, the existing fleet is known to have several components with these fabrication cracks. Replacement heads have identified over 800 fabrication cracks in pre-service volumetric examinations. Embedded flaw repairs have had subsurface defects become exposed to primary coolant after cycling due to startups/shutdowns. Evaluating the effect of these cracks on these repairs/mitigations should consider the very thin layers of these materials installed for consideration of operation in 80+ years.
L M
PK M
M PK Welding defects in Alloy 690 has not been a source of environmentally assisted cracking in actual plant components in the operating experience. However, the current understanding needs to be enhanced in terms of what causes the welding defects (e.g., alloy chemistry and welding process parameters) and how the defects can be prevented.
2 1
2 1
1 1
3 2
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2 1.5 1.6 1.8 0.3 1.5 1.6 1.8 0.3 RCPB-6 Research on degradation mechanisms associated with low alloy steel should be considered such as temper bead welding and irradiation which can cause an elevated yield strength and environmental cracking (EC) susceptibility.
- The eect of service environment on fracture properes Context: Cracks grew slightly into the LAS from A52 weld during CGR testing M
L PK Temper bead welds should possess sufficient toughness and resistance to PWSCC, but some research results a level of susceptibility that should be explored. I feel we have partial knowledge of this research topic, but we know enough to narrow the research to look at specific aspects to rule out concern. Uncertainty is low because we are well aware of the safety significance.
L L
K In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since the tubing is a Ni-based alloy and the channel head is clad with either stainless steel or a Ni-based alloy.
There have been a few instances of channel head cladding being breached during manufacture which has lead to minor corrosion of the LAS under the cladding but engineering analysis has shown stresses remain low and corrosion is minimal because of low oxygen levels in the primary coolant.
L M
PK Dilution testing and the effects of increased hardness for temper bead welding are and should continue to be evaluated for initiation and crack growth rates. Impurities need to be better defined for additional work, with reasonable concerns for concentrations given reactor coolant chemistry controls. Should also define service environment of concern.
L M
PK L
M PK Low alloy steels show relatively high resistance to environmental cracking (also called environmentally assisted cracking) in the reactor coolant water environments especially those with a yield strength level of less than 150 ksi.
However, the effects of welding processes and water chemistry excursions on environmental cracking in low alloy steels may need to be further studied for better understanding.
2 1
2 1
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1 2
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2 1.0 1.6 1.8 0.3 1.0 1.6 1.8 0.3 RCPB-7 Research on stainless steels to provide a better understanding of SCC initiation in stainless steel piping, focusing on factors such as thermal stra"caon,temperature, dissolved oxygen, surface condion, long-term aging, and welding residual strain.
M L
PK The issues related to SS SCC in France do not appear to be applicable to US plants. However, OpE in the US on sensitized SS BMI raises this to a medium for me. Also, perhaps long-term thermal aging is a potential issue.
L L
PK In regards to SG tubing and the primary bowl of the SG channel head, this issue is mostly non-relevant since in the few channel heads clad with stainless steel in the US, there has been no OpE of SCC.
H L
K Many of the mechanisms listed are known, but research is needed to identify what components can be affected by these conditions within extended plant lifetimes. A leak from a SS safe end to a BMI identified a new issue of sensitization of SS due to original plant heat treatment. Dissolved oxygen levels in trapped or crevice conditions in PWRs are a concern of some experts. Similarly long term aging of stainless steels for 80+ years of PWR operational environments has been raised as an area of limited data.
M M
PK L
M PK Stainless steels show relatively high resistance to SCC in the PWR reactor coolant water environment. One factor of interest may be the heat input of the welding process because it can affect the hardness of the adjacent metal and susceptibility to SCC.
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2 2.0 1.0 1.7 0.2 2.0 1.0 1.7 0.2 RCPB-8 Conduct modeling and simulation to validate assumed mechanisms and make accurate predictions about [PWSCC] crack initiation and growth.
This includes building comprehensive databases and benchmarking different models. This also includes adjusting NDE strategies and creating NDE tools to accurately assess cracking kinetics.
M L
PK While modeling is helpful, the uncertainty in PWSCC models is very large. I think making accurate predictions is out of the realm of possibility. Models may help identify trends and bound the problem, but are far from being useful for "accurate" predictions. I gave it a M for safety significance because PWSCC is a safety concern of which there little uncertainty. Our knowledge is growing and for high chrome alloys, it's showing they are very resistance to PWSCC.
M H
K In regards to SG tubing, this issue is mostly non-relevant since all PWRs in the US use a plug-on-detection strategy regarding cracking in SG tubing. In the event that the US ever pursued a crack sizing strategy in SG tubing, this issue would be of the highest importance, due to the low wall thickness of SG tubing.
Additionally, the uncertainty associated with eddy current inspection of SCC is pretty high H
L K
I think we always want to have the tools to predict and assess crack initiation and growth for all degradation mechanisms.
Additionally, NDE tools are necessary to evaluate each material for degradation. I just think this is too general of a category and should be focused on a particular degradation or component material.
M M
PK H
M PK Modeling and simulation capabilities can provide a more relevant risk insight on the material degradation and component failure at a piping system or plant level (e.g.,
probabilistic fracture mechanics analysis that can estimated loss-of-coolant accident frequencies).
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2 2.3 1.7 1.3 0.3 2.3 1.7 1.3 0.3 RCPB-9 Conduct research to evaluate how cold work and applied stress affect SCC initiation time for different alloy groups (e.g., nickel-base alloys, stainless steels). Understanding these differences is important for developing unified models and determining the improvement factors for second-generation nickel-based alloys. This includes how cold work is applied (e.g., forging vs tensile strain). The effect of dynamic strain should also be evaluated.
M M
PK Cold work has shown to be a significant PWSCC accelerating factor. We are fairly certain that cold worked materials can be highly susceptible to SCC.
CW seems to lead to different cracking mechanisms among the nickel alloys (IG vs GB creep).
L L
K In regards to SG tubing, this issue is mostly non-relevant since all tubes are stress relieved after formation (prior to bending). Low row tubes are also stress relieved again after bending in the U-bend region.
L L
K I think we know cold work is bad for these materials. A more focused question would be to evaluate any new Second-Generation materials that claim to address the cold work issue. Further, assessment of any new materials for new plants to evaluate cold work may be useful.
Current requirements note good practices to minimize cold work as much as possible.
L M
K M
H PK The level of cold work (and strain) can affect the susceptibility of materials to SCC initiation and growth. The levels of cold work and strain (e.g., residual strain due to welding) in the actual components may involve relatively large uncertainties and plant/component-specific variations.
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2 1.3 1.3 1.3 0.2 1.3 1.3 1.3 0.2 RCPB-10 Evaluate how plant modifications and upgrades, including power uprates and low-leakage core (which may lead to SCC acceleration by thermal mixing) affect degradation.
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PK This does not seem like it would be a big safety concern given that the plant upgrades and modifications that I could think of would likely improve the situation w.r.t. materials such as steam gen replacements, vessel head replacements, etc. A power uprate might increase stress and temperature but I have no knowledge by how much.
L L
K In regards to SG tubing, this issue is mostly non-relevant since tubes in replacement and new SGs are Alloy 690, power uprates typically result in minimal Thot increases, and neutron leakage doesn't affect the SGs.
M L
K Plant uprates are regularly reviewed for effects of temperature change and fatigue/flow issues. The steam dryer issues with BWRs is an example of how those effects were identified after implementation, so it would be good to be proactive to identify concerns. However, the scope of the item should be on a particular upgrade and potential concern.
M L
PK L
M PK Plant modifications and upgrades such as power uprates may promote material degradation (e.g., accelerated crack growth due to an increase in temperature, vibration or irradiation). However, generally speaking, plant modifications and upgrades appear to have a relatively small effect on material degradation unless there are plant-specific issues (e.g.,
inadequate design of replacement components).
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2 1.3 1.7 1.3 0.3 1.3 1.7 1.3 0.3 RCPB-11 Perform research on how temperature affects SCC initiation mechanisms and kinetics to help develop models. This research would help develop a more fundamental and broader understanding of temperature effects focusing on validating kinetics and surface microstructure evolution (e.g., grain boundary oxidation, diffusion, etc.).
H L
PK Temperature effects are a controlling service parameter for PWSCC in general. Current research results are not elucidating the effect of temperature on PWSCC initiation sufficiently to provide a high level of confidence in Arrhenius relationships assumed to characterize temp effects.
H L
PK What Eric said.
M L
PK Needs better focus on a particular material of concern. We know temperature has an effect, but we have models to address this concern.
Scatter of data usually in these models is sometimes large or small depending on the material.
L M
PK H
M PK The effect of temperature on SCC is one of the important factors to be considered in the development of inspection frequencies as well as the conduct of analytical evaluation of detected flaws. A better understanding of the effects of relatively high and low temperatures on SCC can be also used to optimize inspection strategies.
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2 2.7 1.0 2.0 0.2 2.7 1.0 2.0 0.2 RCPB-12 Perform research on how temperature affects SCC CGR mechanisms and kinetics to help develop models with emphasis on potential microstructural changes that might occur during long-term thermal aging.
M L
PK Long term thermal aging is a concern but not an immediate safety concern, but we really don't know a lot about the mechanisms operating at very long service lives. The temperature effects and the mechanisms of aging appear to be chemical composition dependent, but we do not have a lot of data on the mechanisms we think are operating.
We have some data on LRO, and it appears to not be an issue for iron contents above 9%, but there are potentially low-iron weld metals of interest being developed. ANL is measuring increased hardness in aged materials but not LRO. We don't really know why, yet.
M M
UK What Eric said, but for SGs, the importance might be a little higher since the wall thickness is less.
M M
PK Needs better focus on a particular material of concern. Depending on the material, we may know the thermal aging effect, and we have models to address this concern.
L M
K H
M PK Please see the rationale for Item 11 above. The microstructural changes can affect the material susceptibility to SCC, too.
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0.3 score
H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-1 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-2 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-3 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-4 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-5 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-6 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-7 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-8 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-9 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-10 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-11 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-12
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AA AB AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR AS AT AU AV AW AX AY AZ BA BB BC BD BE BF BG BH BI BJ BK BL BM BN Recommendation ID title of recommendation
- Scope, objectives, and/or topic modifications Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance Uncertainty Knowledge Rationale for scoring Importance (score)
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Knowledg e (avg) error bars Import Uncert Know error More research could be used to gain knowledge on a topic expected to be important to safety More research could reduce uncertainty and clarify topic importance More research could improve knowledge but topic not expected to be safety significant Topic expected to have safety significance but likely could be addressed without additional research Topic may have safety significance but sufficient knowledge exists to manage Topic not expected to have safety significance and sufficient knowledge exists K/PK/UK K/PK/UK K/PK/UK K/PK/UK K/PK/UK score: H/M/L score: H/M/L score: H/M/L score: H/M/L score: H/M/L 16 17 18 RCPB-13 Perform research to improve accelerated testing methods to avoid altering microstructures too much while still obtaining useful data. This involves finding a balance between accelerating tests and maintaining the integrity of the materials being tested.
H M
PK We know that accelerated testing is not ideal, but we are forced to do it to obtain data in a reasonable time frame. I think this is an important issue to address because we are trying to make decisions based on the data so we need to better understand how to best correlate/translate accelerated testing to actual service conditions.
M M
UK If we can ever get the material harvested from a plant with significant operational history, it would be valuable to confirm whether our accelerated testing has unintended consequences. I'm not sure how applicable this is to SGs.
H M
PK It would be very useful to evaluate accelerating testing methods to ensure our testing is of representative microstructures of plant aged materials. As similarly discussed in the harvesting item, this would improve regulatory efficiency and stakeholder confidence.
H M
PK M
M PK Accelerated testing approaches have been used to simulate and characterize the material properties and resistances to degradation (e.g., neutron and thermal aging embrittlement) in an accelerated manner. The accelerated test results may need to be compared with the test and characterization results of harvested plant materials to confirm the applicability of the accelerated testing.
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2 2.7 2.0 2.3 0.3 2.7 2.0 2.3 0.3 RCPB-14 Perform research on the effects of PWR water chemistry such as oxygen in PWR make-up water and Pt distribution with On-Line NobleChem (OLNC) with a goal towards optimization.
[EMF] After reviewing this further, I think this can be combined with RCPB-7 since it appears that this area w.r.t. oxygen in PWRs was aimed at SS.
Context: Some experts believe the oxygen content in PWR might be too high and exacerbating PWSCC issues.
L L
PK DOUBLE CHECK PWR VS BWR W.R.T. PETER'S CONCERN L
L PK M
M K
I think we know oxygen in PWR water could increase the susceptibility of some materials to degradation. Optimization of chemistry control and recommendations for those licensee processes seems to be more a responsibility of industry to ensure the reliability of their plant. One point in favor of review is the new Ni-alloy CGRs include a hydrogen term to address rates. The general hydrogen concentrations generally lower CGRs.
M M
PK M
M UK N/A per the plan to consolidate Items 7 and 14.
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3 1.3 1.3 1.7 0.2 1.3 1.3 1.7 0.2 RCPB-14 Research may be needed to evaluate the EC resistance of components after the application of mitigation surface treatments such as peening.
H M
UK I didn't review the TR on peening so I'm not familiar with how successful surface treatments are for improving the resistance to PWSCC. I ranked it as a high safety concern because we are relying on surface treatments to mitigate cracking as opposed to material replacement....is that correct? I am very interested in the long-term stability of the treated surface layers and how well compressive stresses are maintained.
H M
PK With regards to SG tubing, shot peening with metal media was very successful for a unit with 600MA tubing that was shot peened prior to being placed in service. Other units were the shot peening was performed after the SG was placed in service did not result in the same level of benefit.
I found no OpE on laser peening of SG L
L K
I think we would need to see an instance of where peening has failed under plant operating conditions to help focus this item.
M M
PK M
M PK Surface treatments can be an effective means to mitigate environmental cracking.
Additional research activities may be needed to better understand the effectiveness of surface treatment approaches, including the review of operating experience of mitigated components.
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2 2.3 1.7 2.0 0.3 2.3 1.7 2.0 0.3 Knowledge 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-13 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Importance Knowledge RCPB-14