ML24053A348
| ML24053A348 | |
| Person / Time | |
|---|---|
| Issue date: | 12/06/2023 |
| From: | Victor Cusumano Licensing Processes Branch |
| To: | |
| Lenning K, NRR/DORL/LLPB | |
| Shared Package | |
| ML24053A343 | List: |
| References | |
| WCAP-18446-P, WCAP-18446-NP, Rev. 0 | |
| Download: ML24053A348 (1) | |
Text
NRC FORM 757 (06-2019)
Page 1 of 8 Use ADAMS Template NRC-006 (ML063120159)
NRC FORM 757 U.S. NUCLEAR REGULATORY COMMISSION (06-2019)
NRC MD 10.158 NON-CONCURRENCE PROCESS COVER PAGE The U.S. Nuclear Regulatory Commission (NRC) strives to establish and maintain an environment that encourages all employees to promptly raise concerns and differing views without fear of reprisal and to promote methods for raising concerns that will enhance a strong safety culture and support the agencys mission.
Employees are expected to discuss their views and concerns with their immediate supervisors on a regular, ongoing basis. If informal discussions do not resolve concerns, employees have various mechanisms for expressing and having their concerns and differing views heard and considered by management.
Management Directive (MD) 10.158, NRC Non-Concurrence Process, describes the Non-Concurrence Process (NCP).
The NCP allows employees to document their differing views and concerns early in the decision-making process, have them responded to (if requested), and include them with proposed documents moving through the management approval chain to support the decision-making process.
NRC Form 757, Non-Concurrence Process, is used to document the process.
Section A of the form includes the personal opinions, views, and concerns of a non-concurring NRC employee.
Section B of the form includes the personal opinions and views of the non-concurring employees immediate supervisor.
Section C of the form includes the agencys evaluation of the concerns and the agencys final position and outcome.
NOTE: Content in Sections A and B reflects personal opinions and views and does not represent the official agencys position of the issues, nor official rationale for the agency decision. Section C includes the agencys official position on the facts, issues, and rationale for the final decision.
1.
Was this process discontinued? If so, please indicate the reason and skip questions 2 and 3:
Process was not discontinued 2.
At the completion of the process, the non-concurring employee(s):
Continued to non-concur 3.
For record keeping purposes:
This record has been reviewed and approved for public dissemination
NRC FORM 757 (06-2019)
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NRC FORM 757 U.S. NUCLEAR REGULATORY COMMISSION (06-2019)
NRC MD 10.158 NON-CONCURRENCE PROCESS (Continued) 1.
NCP Tracking Number NCP-2023-006 Date 2023-12-06 Section A - To Be Completed by Non-Concurring Employee 2.
Title of Subject Document U. S. NUCLEAR REGULATORY COMMISSION DRAFT SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION FOR WESTINGHOUSE ELECTRIC COMPANY TOPICAL REPORT WCAP-18446-P/WCAP-18446-NP, REVISION 0, INCREMENTAL EXTENSION OF BURNUP LIMIT FOR WESTINGHOUSE AND COMBUSTION ENGINEERING 3.
ADAMS Accession Number ML23278A257 4.
Document Signer Gerond George - BRANCH CHIEF 5.
Document Signers Office NRR 6.
Document Signers Email Gerond.George@nrc.gov 7.
Name of Non-Concurring Employees John Lehning - SENIOR NUCLEAR ENGINEER; Kevin Heller - NUCLEAR ENGINEER
- 8.
Non-Concurring Employee Offices NRR 9.
Employee Emails John.Lehning@nrc.gov; Kevin.Heller@nrc.gov 10.
Non-Concurring Employees Role for the Subject Document Document Author 11.
Name of Non-Concurring Employee Supervisors Scott Krepel - BRANCH CHIEF
- 12.
Non-Concurring Employee Supervisor Offices NRR 13.
Supervisor Emails Scott.Krepel@nrc.gov
- 14.
I would like my non-concurrence considered and would like a written evaluation in Sections B and C.
15.
When the process is complete, I would like management to determine whether public release of the NCP Form (with or without redactions) is appropriate (Select No if you would like the NCP Form to be non-public):
Yes 16.
Reasons for the Non-Concurrence, Potential Impact on Mission, and the Proposed Alternatives Please see attached.
17.
Submitted By / Submitted On John Lehning - SENIOR NUCLEAR ENGINEER; Kevin Heller - NUCLEAR ENGINEER 2023-12-06
NRC FORM 757 (06-2019)
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NRC FORM 757 U.S. NUCLEAR REGULATORY COMMISSION (06-2019)
NRC MD 10.158 NON-CONCURRENCE PROCESS (Continued) 1.
NCP Tracking Number NCP-2023-006 Date 2024-01-02 Section B - To Be Completed by Non-Concurring Employees Supervisor 2.
Title of Subject Document U. S. NUCLEAR REGULATORY COMMISSION DRAFT SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION FOR WESTINGHOUSE ELECTRIC COMPANY TOPICAL REPORT WCAP 18446 P/WCAP-18446-NP, REVISION 0, INCREMENTAL EXTENSION OF BURNUP LIMIT FOR WESTINGHOUSE AND COMBUSTION ENGINEERING FUEL DESIGNS" 3.
ADAMS Accession Number ML23278A257 4.
Name of Non-Concurring Employees Supervisor Scott Krepel - BRANCH CHIEF 5.
Non-Concurring Employees Supervisor Email Scott.Krepel@nrc.gov 6.
Office NRR 7.
Comments for the NCP Reviewer to Consider I have noted my considerations in moving forward with the subject document in the attached file.
8.
Reviewed By / Reviewed On Scott Krepel - BRANCH CHIEF 2024-01-02
NRC FORM 757 (06-2019)
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NRC FORM 757 U.S. NUCLEAR REGULATORY COMMISSION (06-2019)
NRC MD 10.158 NON-CONCURRENCE PROCESS (Continued) 1.
NCP Tracking Number NCP-2023-006 Date 2024-02-08 Section C - To Be Completed by NCP Coordinator 2.
Title of Subject Document U. S. NUCLEAR REGULATORY COMMISSION DRAFT SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION FOR WESTINGHOUSE ELECTRIC COMPANY TOPICAL REPORT WCAP 18446 P/WCAP-18446-NP, REVISION 0, INCREMENTAL EXTENSION OF BURNUP LIMIT FOR WESTINGHOUSE AND COMBUSTION ENGINEERING FUEL DESIGNS" 3.
ADAMS Accession Number ML23278A257 4.
Name of NCP Coordinator Benjamin Parks (He/Him/His) - SR TECH ADV FOR SYSTEMS AND FUELS 5.
NCP Coordinators Email Benjamin.Parks@nrc.gov 6.
Office NRR
- 7.
Agreed Upon Summary of Issues This non-concurrence was submitted by the authors of the draft safety evaluation (SE) on Westinghouse Electric Company (Westinghouse) Topical Report WCAP-18446-P/WCAP-18446-NP, Revision 0, Incremental Extension of Burnup Limit for Westinghouse and Combustion Engineering Fuel Designs (ML23278A057). The submitters non-concurred on NRC managements direction to delete a Limitation/Condition (L/C) from the draft SE, which would have stated as follows: "Licensees implementing WCAP-18446-P/WCAP-18446-NP, Revision 0, shall assess the potential for fuel dispersal from fuel rods with less than 62 gigawatt-days per metric tonne uranium (GWd/MTU) rod-average burnup and justify that the estimated quantity of dispersed fuel does not result in non-compliance with the acceptance criteria in 10 CFR 50.46." Without the L/C in the draft SE, there is not adequate assurance that licensees implementing WCAP-18446-P/NP methodology would comply with regulations relevant to this topic, including the coolable core geometry requirement in Section 50.46, Acceptance Criteria for Emergency Core Cooling Systems for Light Water Nuclear Power Reactors, to Title 10, "Energy of the Code of Federal Regulations. The submitters provided alternatives to including the above Condition/Limitation in the draft SE, which the submitters and NRC management have considered, including an evaluation describing the submitters views of the acceptability of each. In the above L/C, the term dispersed fuel refers to a phenomenon that could occur following a design basis event such as a loss-of-coolant accident or a rod ejection accident. A part of fuel fragmentation, relocation and dispersal (FFRD), fuel dispersal can occur when a fuel rod experiences a heatup transient that causes the fuel cladding to swell and in some cases rupture. If the in-reactor exposure of the uranium dioxide fuel is high enough, the fuel can fragment into small particles that may then be expelled through the ruptured fuel cladding into the coolant. The submitters focus their narrative on dispersal because fuel fragmentation and relocation alone do not lead to the issues raised by the submitters; the fuel must be dispersed to be of further concern. Experimental data and analysis concerning FFRD, including a Research Information Letter and two NRC staff analyses, demonstrates that fuel in appreciable amounts could be dispersed following a hypothetical loss-of-coolant accident. This information demonstrates that fuel dispersal can be a credible outcome, given the occurrence of certain postulated loss-of-coolant accidents. While some of the analyses to which the submitters referred attempted to quantify possible amounts of fuel dispersed following a loss-of-coolant accident, existing methods relied upon in licensees analyses for demonstrating compliance with the acceptance criteria in 10 CFR 50.46(b) were approved before potential impacts of fuel dispersal were adequately recognized. Notwithstanding the above accumulation of experimental and analytic information concerning the effects of fuel dispersal, there are remaining uncertainties regarding potential safety implications in the event significant amounts of fuel are dispersed into a post-loss-of-coolant accident reactor coolant system. The submitters summarize ongoing research and data collection activities, such as the development of a phenomena identification and research table (PIRT), which indicate that there are remaining uncertainties preventing the creation of an analytic means that would be acceptable to the staff to demonstrate acceptable consequences following a fuel dispersal event. Due to the complexity of underlying phenomena and substantial logistical challenges associated with performing representative tests, experimental data remains in short supply. The submitters provide a regulatory history of 10 CFR 50.46 to offer context as to why it is challenging to conclude whether the prediction of dispersed fuel in any amount could be considered to conform to or comply with regulatory requirements such as 10 CFR 50.46(b)(4). This includes a
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Page 5 of 8 Use ADAMS Template NRC-006 (ML063120159) review of the discussion concerning the technical and rational basis for promulgating the acceptance criteria, based on the 1973 Opinion of the Commission on the Emergency Core Cooling System rule making. It also includes references to the treatment of the issue within NRCs Generic Issues Program, recent technical publications providing assessments that significant amounts of predicted fuel dispersal may not be considered to be a part of an acceptable emergency core cooling system performance demonstration, and finally Commission correspondence and other policy/rulemaking documents concerning direction to staff to address fuel dispersal within two rulemaking efforts underway: 10 CFR 50.46c and the increased enrichment rulemaking effort. Based on a review of these references, it is not currently possible to determine whether the prediction of fuel dispersal in any amount could be evaluated and found acceptable under the currently existing regulations, given the NRCs current posture within the two rulemaking activities and the absence of regulatory guidance or a clear staff position establishing a means to do so. The potential for fuel dispersal following a loss-of-coolant accident exists in operating plants today. The submitters emphasize that they are not asserting that such plants are necessarily out of compliance with existing regulations. However, consistent with the rationale summarized above, it is not possible to conclude that licensees affected by fuel dispersal are reasonably assured of maintaining a coolable core geometry, as required by 10 CFR 50.46(b)(4), among other requirements. The potential for fuel dispersal is not the sole instance of a safety issue whose magnitude could increase from burnup extensions. The submitters provided a summary of the effects that neglecting a burnup-dependent phenomenon, nuclear fuel thermal conductivity degradation, could have on safety analysis results if the models used in the analyses did not account for it. The NRC issued three information notices addressing the potential for this phenomenon to lead to the exceedance of 10 CFR 50.46(b) acceptance criteria. The submitters also noted that affected licensees have incorporated effective interim solutions to this issue, but a number of plants have not yet adopted methods that account for this phenomenon. In light of this operating experience, the submitters indicate that allowing further burnup increases without establishing whether fuel dispersal could affect operating reactor compliance with existing regulations does not appear consistent with the agencys public safety mission. Westinghouse proposed justifying the acceptability of WCAP-18446-P/NP by relying on an expectation that
[
] The submitters indicated agreement that Westinghouse provided substantial evidence that allowing incremental burnup increases in accordance with WCAP-18446-P/NP [
] However, the methodology was not considered acceptable for two reasons: (1) if licensees cannot currently be assured of being in compliance with 10 CFR 50.46(b)(4) and other applicable requirements, it would still be impossible to conclude that licensees implementing the methodology would be able to demonstrate compliance with the relevant requirements, and (2) because existing margins to regulatory requirements may be degraded or negative for plants implementing WCAP-18446-P/NP, sufficient margin is not available to offset the existing uncertainties associated with fuel behavior at increased burnup. In justifying the acceptability/appropriateness of imposing a L/C on the staffs approval of WCAP-18446-P/NP, the submitters indicated that they had considered whether introducing a new matter - that licensees would need to address the potential for fuel dispersal and its attendant compliance with NRC requirements - would be subject to any measures set forth in 10 CFR 50.109 or Management Directive 8.4, inasmuch as the regulation and NRC MD require justifications to impose new staff positions on licensees. Based on consultation with the Backfitting Community of Practice, the justifications required by 10 CFR 50.109 and MD 8.4 do not apply because this matter is addressed within a voluntary topical report. As further justification for the appropriateness of imposing the subject L/C on the staffs approval of WCAP-18446-P/NP, the submitters identified several other topical reports that contained similarly rationalized limitations and conditions to provide evidence that the L/C was appropriately justified. These examples illustrate that the agency has considerable latitude to impose reasonable conditions and limitations that are necessary to reach a finding of applicability on a proposed topical report. Based on the issues summarized above, imposition of the subject L/C could enable approval of WCAP-18446-P/NP, in that approving the topical report with the L/C in place would provide a means to determine whether a licensee proposing to implement it would comply with 10 CFR 50.46(b), among other requirements applicable to fuel dispersal. This approach is appropriate because a finding of acceptability in an NRC safety evaluation should be contingent upon a positive determination of compliance with relevant regulatory requirements as the basis for establishing that there is adequate protection of public health and safety. The submitters also indicated that initiating the analyses and evaluations necessary to determine whether a backfit in accordance with 10 CFR 50.109, to address the potential safety implications of fuel dispersal within the entire population of operating domestic power
NRC FORM 757 (06-2019)
Page 6 of 8 Use ADAMS Template NRC-006 (ML063120159) reactors would be an acceptable alternative, would be acceptable provided a definitive and publicly available schedule for doing so was set. The submitters summarized additional alternatives that had been considered, but were not found acceptable. Since these alternatives were not acceptable, they are not repeated here with the exception of Alternative (5), which aligns with the NRC management-preferred approach. This alternative would result in the issuance of the WCAP-18446-P/NP SE, without the above-identified L/C, while continuing to conduct research to better characterize the impacts of fuel dispersal. This ongoing research is discussed in a passage that was added to the draft SE, and the submitters indicated that such discussion about ongoing research would either appear to be a part of the basis for a safety finding or extraneous information, and was hence inappropriate to include.
The submitters indicated support for further testing and analysis, but maintained that issuing the SE for WCAP-18446-P/NP without the subject L/C in place did not appear sufficient to protect public health and safety. The submitters noted that a passage that was added to the modified draft SE stating among other things, fuel dispersal is not a significant safety issue for burnups below 62 GWd/MTU. But in their view, there is insufficient evidence to justify such a definitive conclusion, or one to the contrary. Thus, the impact of fuel dispersal on operating reactors is an open safety question that should be addressed before concluding that licensees implementing WCAP-18446-P/NP would comply with 10 CFR 50.46(b)(4). The submitters assert that addressing the issue is especially necessary given the considerations required to justify backfitting and similar actions in accordance with 10 CFR 50.109 and related agency procedures, which make it less apparent that the NRC would be capable of imposing any corrective actions in the event that future activities identify non-compliance in existing plants. The next planned action to improve the current understanding of fuel dispersal - developing a phenomenon identification and ranking table - is generally an early step toward developing a mature understanding of a technical phenomenon. The submitters indicated that the fact that PIRT development was a planned next step, which would likely necessitate further follow-on research, indicates that the state of knowledge of fuel dispersal suggests a state of indeterminacy in which issuing the modified SE while moving forward with a phenomenon ranking exercise and potentially other undefined follow-on activities cannot assure that operating reactors implementing WCAP-18446-P/NP would be in compliance with regulatory requirements relevant to fuel dispersal. For this reason and those outlined above, the submitters indicated that this alternative was not acceptable. Either of the above options that the submitters deemed acceptable - imposing the subject L/C on the approval of WCAP-18446-P/NP or initiating a backfit evaluation for fuel dispersal implications for operating plants - or a combination thereof, would represent timely resolution of an existing safety question associated with fuel dispersal under postulated loss-of-coolant accident conditions. Absent such evaluations, there is insufficient basis to conclude that implementing WCAP-18446-P/NP would provide for licensees operation within the regulations of the Commission.
8.
Evaluation of Non-Concurrence and Rationale for Decision The submitters provided a commendably detailed and articulate summary of the state of knowledge concerning the possibility for fuel dispersal in a nuclear power reactor following certain large-break loss-of-coolant accidents. As noted in Research Information Letter (RIL) 2021-13, Interpretation of research on Fuel Fragmentation, Relocation, and Dispersal [FFRD] at High Burnup, it is generally understood that fuel exposed beyond a likely threshold of 55 gigawatt days per metric tonne of uranium (GWd/MTU) can be susceptible to a scenario, following such a loss-of-coolant accident, where some of the fuel cladding could balloon and rupture, and if this happens, the more highly exposed fuel is likely to be expelled from the rupture because the fuel pellet has degraded into fine particles as a result of its irradiation. Also commendable, the submitters provide a reasonable and unbiased assessment of the state of knowledge concerning the downstream effects of fuel dispersal. While the susceptibility of fuel to this phenomenon is well-understood, different analyses provide varying estimates of the potential amount of fuel that could be dispersed, which is an indication of significant uncertainties associated with the phenomenon, and it is not generally well understood what could occur to the fuel particles once dispersed. Finally, the submitters provide a well-reasoned policy analysis of the implications of this phenomenon relative to existing NRC regulatory requirements, including especially those contained in 10 CFR § 50.46. In effect, the phenomenon was not understood to occur at the time the requirements in § 50.46 were written, however, as characterized in NUREG-2121, Fuel Fragmentation, Relocation, and Dispersal During the Loss-of-Coolant Accident, examination of the historical record indicates that, at the time the cladding criteria [contained in § 50.46(b)] were developed, there was no expectation of fuel loss
- at least, no expectation of significant fuel loss - during a successfully mitigated [loss-of-coolant accident] (ML12090A018). Because the issue was not explicitly addressed in the regulations, and still is not, there is a reasonable question, which was identified by the submitters, as to whether the regulations as written allow for fuel dispersal in any amount. The submitters clearly take no position on this matter but believe that it is inappropriate to allow licensees to extend allowable burnup by an incremental amount without
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Page 7 of 8 Use ADAMS Template NRC-006 (ML063120159) providing assurance that predicted fuel dispersal for fuel within current burnup limits meets existing regulatory requirements. The practical effect of doing so poses a dilemma: if the staff has no position on whether fuel dispersal is precluded by existing regulations, how would a licensee demonstrate to the staff that a predicted amount of fuel dispersal meets existing requirements?
Given todays state of knowledge, it is likely that a prudent licensee would attempt to meet this staff-proposed stipulation by demonstrating that fuel dispersal is precluded [
] This is because it is certain that the total preclusion of fuel dispersal meets regulatory requirements, but as convincingly established by the submitters, it is presently uncertain whether fuel dispersal in any amount could meet regulatory requirements. Meanwhile, the submitters indicated that Westinghouse provided acceptable assurance that [
] The condition/limitation (C/L) for which the submitters advocated would apply [
] meaning that the C/L would necessitate changes to the way a licensee currently operates in order to provide the sought-after assurance. Because the proposed C/L would practically necessitate changes to the way plants currently operate while any plant that chooses not to implement Westinghouses incremental burnup extension topical report would not have to provide similar assurance, I agree with the supervisors remarks that this C/L would, if included in the topical report SE, be outside the scope of the approval Westinghouse requested from the staff. The NRC staff has never identified, with respect to safety, an urgency to take immediate action to ensure that fuel dispersal is precluded. For example, this matter was permitted to remain outside the scope of the pending 10 CFR 50.46c rulemaking despite that the NRCs state of knowledge was becoming, at the time, sufficiently mature as to understand that FFRD is a real, physical phenomenon to be expected following certain loss-of-coolant accidents. Presently, the staff has been directed by the Commission to address FFRD as a part of the increased enrichment rulemaking that is presently underway. In the past, the staff has attempted to address the issue of fuel dispersal generically on different occasions. Most recently, as noted by the submitters, the issue was referred to the Generic Issues Program, which screened it out because of the then-underway 50.46c rulemaking, which ultimately did not address fuel dispersal. However, while the submitters indicated not identifying a record documenting the basis for not returning the issue to the Generic Issues Program in 2015 when SECY-15-0148 documented the staffs recommendation not to pursue fuel dispersal within 50.46c rulemaking, I found that fuel dispersal as a generic issue was formally closed by memorandum dated September 20, 2016 (ML16204A136). This memorandum indicates that the GI program staff had continued to monitor the rulemaking to ensure the issue was evaluated before closing the issue. Noting that the staff completed an evaluation of FFRD as part of the rulemaking, the memo concluded that no further action was required. The memorandum concludes with "Nonetheless, the NRC staff will continue multilateral research activities and interactions with stakeholders with the goal of developing a regulatory framework to address FFRD, if needed, in the next few years." A number of the documents referenced by the authors, specifically RIL 2021-13, the NURETH paper, and the IE rulemaking regulatory basis document, demonstrate the continuation of these activities. Although these activities may not be occurring with the schedule rigor that the submitters would like, they continue nonetheless, including efforts noted by the submitters to develop a phenomenon identification and ranking table. I believe these ongoing efforts provide an adequate means to address the issues associated with fuel dispersal in a more holistic way, and in a way that applies more generically to all licensees potentially affected by fuel dispersal, not just a subset that may choose to implement Westinghouses incremental burnup topical report. Because these activities have continued to improve the NRC staffs and stakeholders understanding of FFRD, and because the Commission has directed the staff to address FFRD as a part of the ongoing IE rulemaking activity, I also do not believe it is necessary to establish an independent schedule to address FFRD outside the IE rulemaking; the IE rulemaking itself has a publicly available schedule. The submitters provided a remarkable summary and evaluation of the facts underlying their issues and conveyed the necessary information in unbiased terms, with the sole exception of the subjective matters of the appropriateness of issuing the SE with or without the proposed L/C in place, or the alternative of pursuing a more generic resolution on a more rigorous schedule than presently exists. As noted above, the efforts to develop the basis for this nonconcurrence are commendable. I thank the submitters for such a well-reasoned engagement of the nonconcurrence process.
9.
Coordinated By / Coordinated On Benjamin Parks (He/Him/His) 2024-02-08
NRC FORM 757 (06-2019)
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Approved By / Approved On Vic Cusumano - BRANCH CHIEF 2024-02-08
December 4, 2023 FROM:
SUBJECT:
Non-Concurrence on U.S. Nuclear Regulatory Commission Draft Safety Evaluation on Westinghouse Electric Company Topical Report WCAP-18446-P/NP, Revision 0, Incremental Extension of Burnup Limit for Westinghouse and Combustion Engineering Fuel Designs
SUMMARY
The technical staff who performed the safety review for the proposed treatment of fuel dispersal during a loss-of-coolant accident in Westinghouse topical report WCAP-18446-P/NP (ML20351A157) are unable to concur upon the modified version of the draft safety evaluation endorsed by NRC management (ML23278A257). As explained below, the reviewers determined that there is not adequate assurance that licensees implementing the WCAP-18446-P/NP methodology would be in compliance with regulations relevant to this topic, including the coolable core geometry requirement in 10 CFR 50.46(b)(4).
Because adequate assurance does not exist that plants implementing the WCAP-18446-P/NP methodology would operate in compliance with applicable regulatory requirements, the staff who performed the technical review are unable to concur that issuance of the management-endorsed, modified version of the safety evaluation for this topical report would be consistent with the NRCs foundational mission of protecting public health and safety.
BACKGROUND Recent experimental data and analysis concerning fuel fragmentation and dispersal documented in RIL 2021-13 (ML22056A282), [
]
and NRC staff analyses presented at technical conferences including TOPFUEL and NURETH (e.g., ML23086B272, ML23116A214) indicate a potential for substantial quantities of fuel fragments to be dispersed from the cores of operating reactors during a loss-of-coolant accident. While specific conditions associated with each experiment or analysis vary, on the whole, the accumulated evidence constitutes reasonable grounds to call into question the presumption of operating power reactor licensees compliance Section A: Attachment
with certain regulatory requirements, including the requirement in 10 CFR 50.46(b)(4) for maintaining a coolable core geometry. Such questions are particularly salient because the existing methods relied upon in licensees analyses for demonstrating compliance with the acceptance criteria in 10 CFR 50.46(b) were approved before potential impacts of fuel dispersal were adequately recognized. Existing safety analyses based on such methods are incapable of providing confidence that the potential for fuel dispersal has been adequately addressed because these methods completely neglect dispersal of fragmented fuel and any attendant downstream consequences.1 With the benefit of todays knowledge, the non-concurring technical staff (along with other cognizant technical staff within the agency) hold that, absent further efforts to assess fuel dispersal and its attendant impacts (e.g., collecting additional experimental data, performing additional, potentially plant-specific, analysis), it is not generally possible to conclude that licensees affected by the issue are reasonably assured of maintaining a coolable core geometry, as required by 10 CFR 50.46(b)(4), as well as assuring compliance with other regulatory requirements (e.g., those associated with equipment performance, long-term core cooling, etc.).
It should be stressed that the above discussion does not indicate that the non-concurring technical staff are asserting that operating reactors are necessarily out of compliance or that corrective actions per 10 CFR 50.109 are necessarily justified. Rather, the non-concurring technical staff are merely advocating for the assessment and resolution of a well-founded safety question regarding fuel dispersal to assure licensees regulatory compliance prior to approving further increases in fuel burnup that could exacerbate the existing safety question.
As discussed further below, additional ongoing research and analysis is expected to begin to help with addressing safety questions associated with fuel dispersal. However, the NRC does not currently have a defined plan or timeline to collect all requisite information or incorporate this information into a systematic evaluation of whether operating plants comply with regulations, even as defined plans with rough timelines exist for completing regulatory reviews to allow plants to begin further increasing fuel burnup (e.g., ML23135A021).
The sublime difficulty in reaching a compliance determination without substantial additional effort is rooted in both (1) uncertainties and unknowns with respect to physical phenomena governing fuel dispersal and its downstream impacts and (2) the lack of a well-defined agency position concerning how regulations such as 10 CFR 50.46(b)(4) are to be applied with respect to fuel dispersal.
Regarding the former difficulty, the NRC remains actively engaged in efforts to address uncertainties and unknowns associated with fuel dispersal. The NRC has sponsored significant 1 Cf. Section 3.3.2 of the modified version of the safety evaluation endorsed by management that was shared with the non-concurring technical staff on November 17, 2023, which cites such existing safety analysis methods as capable of providing assurance that regulations are satisfied:
The proposed analysis methodology and associated NRC staff evaluation only addresses fuel dispersal in the requested burnup range. Demonstration of compliance with 10 CFR 50.46 requires analysis of all the fuel assemblies in the core, regardless of burnup. Therefore, fuel assemblies with a burnup less than 62 GWd/MTU will continue to require analysis to demonstrate compliance with applicable rules and regulations. Licensees may utilize current approved methodologies to evaluate fuel assemblies for burnups up to 62 GWd/MTU.
research and analysis over approximately 15 years, as documented in RIL 2021-13, technical reports including NUREG-2121 (ML12090A018), and the TOPFUEL and NURETH papers referenced above. The NRC continues to sponsor ongoing research into fuel fragmentation and dispersal, has recently engaged a phenomenon identification and ranking table (PIRT) panel to consider impacts of fuel dispersal, and continues to exercise available analytical tools to better assess expected quantities of dispersed fuel fragments. Despite these continued, significant efforts, due to the complexity of underlying phenomena and substantial logistical challenges associated with performing representative tests, experimental data remains in short supply.
Consequently, at the present time, adequately validated safety analyses (i.e., according to NRC guidance such as Regulatory Guide 1.203, Transient and Accident Analysis Methods), of the pedigree used to assure compliance with the acceptance criteria for peak cladding temperature, maximum local oxidation, and core-wide oxidation in 10 CFR 50.46(b)(1)-(3), do not exist for demonstrating that the dispersal of fragmented fuel does not prevent compliance with the coolable core geometry requirement in 10 CFR 50.46(b)(4).
Understanding the latter difficulty associated with achieving a well-defined agency position concerning existing regulations applicability to fuel dispersal requires a brief historical exposition. While a complete story is well beyond the scope of this document, the non-concurring technical staff will focus upon two key agency decisions that might have offered opportunities to address fuel dispersal on a generic basis, such that it need not have been raised in relation to specific licensing actions such as vendor topical report WCAP-18446-P/NP and any subsequent licensee implementation submittals.
First, although the generic issue program operated upon the fuel dispersal issue from 2011-14, effective issue resolution did not ensue. While fuel dispersal passed the generic issue programs acceptance review, the issue ultimately screened out of the process based upon the expectation that it could be addressed by inclusion in the ongoing 10 CFR 50.46c rulemaking. The letter discussing the screening decision (ML13192A494) stipulates that If it is determined by NRR staff that fuel dispersal is not appropriate to be addressed by the proposed new rule, the NRR staff will document the basis in their rulemaking package responding to the Commissions SRM and return the issue to the Generic Issues Program.
In fact, the agency did subsequently determine in SECY-15-0148 (ML15230A200) that, based upon the perception that immediate regulatory action was not justified, the fuel dispersal issue should not be included in the 10 CFR 50.46c rulemaking that was otherwise considered ready for regulatory implementation. However, the screening letters commitment to restore fuel dispersal to the generic issue program in this eventuality was not fulfilled. The non-concurring technical staff could not locate a record documenting the basis for not returning the issue to the generic issue program in 2015. This unfulfilled commitment to resume addressing fuel dispersal as a generic issue appears to be a significant root cause that has detracted from the timely development of a regulatory conclusion concerning whether and how operating reactor licensees should address potential impacts of fuel dispersal within existing burnup limits.
Second, an agency decision was made in September/October 2022 during the development of the ongoing increased enrichment rulemaking not to pursue a separate SECY paper that the staff had recommended to document explicitly the agency's existing regulatory interpretation regarding fuel dispersal. While the agency-selected rulemaking pathway, which involved
incorporation of some relevant information on fuel dispersal into a regulatory basis document (ML23032A504), has allowed the proposed increased enrichment rulemaking schedule to be maintained, it has not produced a well-defined agency position on fuel dispersal that would promote efficient decision making until such time as the increased enrichment rulemaking effort is completed.
While the specific manner in which the coolable core geometry requirement in 10 CFR 50.46(b)(4) applies to dispersed fuel (e.g., how much dispersal could be acceptable, how much would be excessive) is not the central issue upon which this non-concurrence is founded, further historical discussion in this regard is necessary to support a fulsome contextual understanding. Going back to the origin of the regulatory requirements in 10 CFR 50.46, in its opinion on the matter of the rulemaking hearing on the acceptance criteria for emergency core cooling systems for light-water-cooled nuclear power reactors (ML20236U832), the Commission stated relative to the acceptance criteria for peak cladding temperature and maximum local oxidation that The purpose of these first two criteria is to ensure that the zircaloy cladding would remain sufficiently intact to retain the UO2 fuel pellets in their separate fuel rods and therefore remain in an easily coolable array. Conservative calculations indicate that during the postulated LOCA, the cladding of many of the fuel rods would swell and burst locally with a longitudinal split. The split cladding would remain in one piece if it were not too heavily oxidized, and would still restrain the UO2 pellets.
Concerning the requirement in (b)(4) for a coolable core geometry, the Commission further stated that If there were no emergency core cooling after a LOCA, the core would probably eventually fuse together into a large mass with insufficient external surface area to allow the fission product heat generated within it to be transferred away. Intermediate steps in arriving at such a state might be the oxidation and melting of the zircaloy cladding, allowing the uranium dioxide fuel pellets to fall together into a heap that would be difficult to cool.
Considering all of the required features of the evaluation models, we are inclined to agree that, for any situation that we have been able to anticipate, this criterion should be superfluous. However, in view of the fundamental and historical importance of maintaining core coolability, we retain this criterion as a basic objective.
In its commentary on the Commissions opinion on the emergency core cooling system rulemaking, the generic issue programs 2013 screening analysis of the fuel dispersal issue found that Examination of the historical record indicates that, at the time the cladding criteria were developed, there was no expectation of fuel loss - or at least no expectation of significant fuel loss - during a successfully mitigated LOCA.
As noted above, while relevant, the acceptability of one specific acceptance criterion for allowable fuel dispersal versus another is ultimately not central to the technical staffs decision
to non-concur on the safety evaluation for WCAP-18446-P/NP. Rather, the problem is more fundamental in that the agency does not currently appear to have any clearly articulated, technically defensible regulatory acceptance criteria that would facilitate effective safety decisions concerning fuel dispersal. As such, the NRC staff has no yardstick against which fuel dispersal may be evaluated to assure that compliance with regulatory requirements such as 10 CFR 50.46(b)(4) is currently being achieved.
Were the historical interpretation of 10 CFR 50.46(b)(4) described above still in effect within the NRC, in light of the accumulated evidence of potential impacts of fuel dispersal, the agency would appear to have good cause to evaluate and resolve in a timely manner the potential for regulatory noncompliance at currently operating plants. While continued research into the issue would be appropriate, a decision not to perform a parallel regulatory evaluation in light of the existing body of evidence discussed above would not appear justified. In this regard, waiting for a significant period of time to obtain further information could effectively be construed as regulatory acceptance, absent a validated technical basis, of continued plant operation for that time period with an unassessed potential to be out of compliance with regulations. The agencys course of action, therefore, could be indicative of an unexpressed regulatory interpretation that differs from that expressed in the Commissions Opinion from 1973 (as affirmed in the generic issue programs 2013 screening analysis document).
In particular, the lack of a regulatory evaluation of fuel dispersal up through the present time could be consistent with an agency belief that substantial fuel dispersal during a loss-of-coolant accident is permissible under regulatory requirements such as 10 CFR 50.46(b)(4). However, as discussed above, a well-defined regulatory position to this effect has not been formally established, and it is not obvious whether the staff would be authorized to apply such an interpretation of 10 CFR 50.46(b)(4) at this time (i.e., absent a rulemaking effort). Nor, as discussed above, have validated safety analyses of the type used to demonstrate compliance with the other acceptance criteria in 10 CFR 50.46(b) yet been completed to serve as a technical basis to justify that a regulatory interpretation allowing substantial fuel dispersal would adequately protect public health and safety.
The discussion above has thus elaborated two proximate root causes of this non-concurrence:
The NRC does not currently appear to have a well-defined and technically defensible regulatory position or acceptance criterion concerning fuel dispersal.
Significant challenges remain with respect to calculating the quantity of dispersed fuel and assessing downstream impacts to within a reasonable uncertainty band.
Finally, it is important to observe that an inability to determine that licensees comply with NRC regulations is of potential safety significance because reasonable assurance of adequate protection of public health and safety is, as a general matter, defined by the Commissions health and safety regulations themselves (ML040980367). Logically, while the inability to assure compliance is not a definitive indication of inadequate protection of public health and safety, it does indicate a potential for inadequate protection. The technical staff who reviewed WCAP-18446-P/NP believe that the NRC, consistent with its foundational mission, has the statutory responsibility to evaluate in a timely manner and, as justified, act upon emergent information, such as the potential for substantial quantities of fuel to be dispersed during a postulated loss-
OFFICIAL USE ONLY - PROPRIETARY INFORMATION 6
of-coolant accident, that appears to call into question whether the public health and safety is being adequately protected.
DISCUSSION The objective of the non-concurring technical staff is not to resolve the concerns described above related to fuel dispersal on a generic basis in its specific review of the WCAP-18446-P/NP methodology. Rather, other than establishing background context, these issues are relevant to the non-concurrence primarily in the sense that they appear fully applicable to the subset of operating reactors (i.e., conventional Westinghouse and Combustion Engineering pressurized-water reactors) that may opt to implement the WCAP-18446-P/NP methodology.
In particular, Westinghouses proposed justification for the acceptability of WCAP-18446-P/NP relies significantly upon its expectation that [
] While the technical staff who performed the safety review for WCAP-18446-P/NP agree that Westinghouse has provided substantial evidence that allowing incremental burnup increases in accordance with the proposed methodology [
] the reviewers did not find the as-proposed methodology acceptable for general application for two primary reasons:
If licensees cannot be assured of being in compliance with 10 CFR 50.46(b)(4) and other regulatory requirements currently, for reasons described above, even if implementation of the WCAP-18446-P/NP methodology does not lead to increased fuel dispersal, it would still be impossible to conclude that licensees implementing the methodology will be able to demonstrate compliance with relevant regulatory requirements.
Because existing margins to regulatory requirements may be degraded or negative for plants implementing WCAP-18446-P/NP, sufficient defense-in-depth margin is not available to accommodate significant, irreducible uncertainties associated with fuel behavior at increased burnup.
Concerning the first bullet, licensees have an obligation to satisfy 10 CFR 50.46(b)(4) and other regulatory requirements. In this respect, it is immaterial whether the potential for excessive fuel dispersal that precludes satisfaction of these requirements results from fuel rods in the incremental burnup range proposed in WCAP-18446-P/NP or the currently analyzed burnup range. If excessive fuel dispersal occurs, regardless of the burnup of the source fuel rod(s),
10 CFR 50.46(b)(4) would not be satisfied.
Concerning the second bullet, fragmentation-and dispersal-related phenomena become more prominent with increased burnup (e.g., reference RIL 2021-13). As burnup increases, for instance, fuel fragments tend to become finer and more prevalent, and the margin to cladding rupture may be reduced due to higher fuel rod internal pressures and more significant releases of transient fission gas. Representative data for many relevant phenomena is limited, while associated phenomenological uncertainties tend to increase with burnup as applicable data
thins out and analytical models are pushed to their limits. A concrete example of the potential for burnup-related unknowns and uncertainties to result in potentially significant impacts is the existing safety question associated with fuel fragmentation and dispersal that is a direct consequence of previous burnup increases up to 62 GWd/MTU that were generally implemented in the 1990s. In reaching a conclusion of acceptability for existing fuel burnups, the NRC staff did not anticipate the possibility of outcomes involving the dispersal of hundreds or perhaps thousands of pounds of fuel fragments.
Moreover, nor is fuel fragmentation, relocation, and dispersal the only relevant instance of an emergent safety issue arising following the approval of increased fuel burnup during the 1990s.
The thermal conductivity degradation issue that arose approximately 15 years ago is another emergent issue driven by increased fuel burnup that unexpectedly challenged licensees capabilities to demonstrate compliance with the acceptance criteria specified in 10 CFR 50.46(b). As burnup increases, a fuel pellet incurs cracking that creates gaps that impede heat transport out of the pellet. As a result, the average temperature and stored heat within a cracked fuel pellet increase, which tends to result in more limiting values of peak cladding temperature and maximum local oxidation during a loss-of-coolant accident. The thermal conductivity degradation issue was the subject of three information notices addressed to operating reactor licensees between 2009 and 2012 (ML091550527, ML113430785, and ML121730336) due to concerns that acceptance criteria in 10 CFR 50.46(b) could be exceeded.
While the agency has previously determined that affected licensees have incorporated effective interim solutions to the thermal conductivity degradation issue, as recognized during the WCAP-18446-P/NP review (e.g., RAI 28 and Limitation and Condition 7), up through the present, a number of plants have not yet adopted approved safety analysis methods that explicitly account for thermal conductivity degradation.
In light of these operating experiences associated with past burnup increases, allowing further burnup increases, prior to establishing whether the potential for substantial fuel dispersal has affected the status of operating reactors compliance with existing regulations at current burnup limits, does not appear consistent with the agencys fundamental public safety mission.
The authors of the non-concurrence recognize that it is not the role of technical staff to establish agency policy; conversely, however, established agency policies can strongly impact the acceptable courses of action available to technical reviewers evaluating specific licensing actions. Because established agency policies have not fostered reasonable assurance that licensees adopting WCAP-18446-P/NP would be in compliance with applicable regulations, the non-concurring technical staff believe that issuance of a favorable safety evaluation for the methodology would require additional, concomitant actions, beyond those proposed by Westinghouse in its submittals associated with the topical report. Absent such actions, adequate assurance is lacking that plants implementing the methodology would be in compliance with relevant regulatory requirements that serve, in part, as the basis for concluding that public health and safety is adequately protected.
In general, the NRC can address safety concerns with the potential to affect nuclear power plants existing design-or licensing-bases by two distinct types of regulatory actions:
(1) As a voluntary, forward-looking change.
(2) As a mandatory backfit in accordance with 10 CFR 50.109.
The first approach, which relies upon voluntary, forward-looking licensee implementation, could immediately address the fuel dispersal issue only for operating reactor licensees that choose to adopt WCAP-18446-P/NP. For example, approval of licensee submittals requesting implementation of WCAP-18446-P/NP could be conditioned upon adequate demonstration that any fuel dispersal experienced during a loss-of-coolant accident would satisfy regulatory requirements.2 Applying such a conditioned-approval approach to place the burden of proof of compliance upon implementing licensees appears appropriate because (1) reasonable assurance of operating reactor licensees compliance with regulatory requirements concerning fuel dispersal is lacking at present and (2) a finding of acceptability in an NRC safety evaluation should be contingent upon a positive determination of compliance with relevant regulatory requirements as the basis for reasonable assurance that the public health and safety is adequately protected. Under this approach, an inability to determine that a licensees application of the method would comply with relevant regulatory requirements would constitute grounds for not approving the implementation of the topical report for that plant. The non-concurring staff note that, as discussed with community-of-practice experts in the Office of Nuclear Reactor Regulation during the concurrence process for the draft safety evaluation, because WCAP-18446-P/NP is a vendor topical report that licensees are not compelled to adopt, this voluntary forward-looking approach would not be subject to the restrictions defined in Management Directive 8.4 (ML18093B087).
The second approach, which relies upon the backfit process, would consider the imposition of corrective actions to resolve the fuel dispersal issue on all potentially affected operating reactors. A generic backfitting approach could be appropriate because fuel dispersal is believed to affect a broader population of operating reactors than the subset of plants that could implement the WCAP-18446-P/NP methodology. However, as opposed to the voluntary, forward-looking approach described above, the backfit process would require significant additional efforts to justify the imposition of corrective actions to resolve the issue. Under this approach, the NRC would quantify fuel dispersal for affected plants and evaluate the safety significance of any attendant downstream impacts. Furthermore, if justified relative to the criteria in 10 CFR 50.109, the NRC would impose any necessary corrective actions. The non-concurring technical staff have no presupposition as to the outcome of such a backfit evaluation.
Whether or not corrective actions are indicated, the performance of a backfit evaluation appears justified considering factors such as (1) the fundamental importance of maintaining a coolable core geometry to reactor safety and (2) the agencys principles of good regulation, such as openness and clarity, which are furthered when valid rationale is documented to support agency decisions concerning whether corrective actions are necessary to address a potential safety question.
In the estimation of the non-concurring technical staff, either the forward-looking or backfit approach could be used to address potential concerns associated with fuel dispersal. Other resolution pathways, such as the NRCs generic issue program (as described in Management Directive 6.4 (ML14245A048)) or performing additional research outside of a formal process, may also have been viable when the fuel dispersal issue was first considered in 2011. However, at present, such approaches seem ineffective because their associated issue resolution timeline appears likely to extend well beyond the implementation timelines for WCAP-18446-P/NP and 2 Note that a similar conditioned-approval approach could be applied for other vendor-proposed topical reports that are directly associated with fuel burnup increases.
other planned industry initiatives associated with increased fuel burnup. These and additional alternative resolution pathways considered by the non-concurring technical staff are briefly evaluated in the following section of this non-concurrence document.
Based upon the evidence and reasoning described above, the technical staff who reviewed WCAP-18446-P/NP believe it is appropriate that the agency pursue timely resolution of an existing safety question associated with fuel dispersal under postulated loss-of-coolant accident conditions at operating reactors using either forward-looking or backfit approaches, or a combination thereof. Timely evaluation of fuel dispersal is appropriate to resolve any potential for noncompliance with the Commissions regulations prior to approving additional fuel burnup increases that could exacerbate an unresolved safety question with fuel dispersal at currently approved burnups. Absent timely regulatory evaluations to assess compliance and confirm or refute the necessity of corrective actions to address fuel dispersal, there is insufficient basis to conclude that licensees implementing WCAP-18446-P/NP would operate within the regulations the Commission has established to assure adequate protection of public health and safety.
Because the agency intends to issue a safety evaluation for the WCAP-18446-P/NP methodology that would permit increased fuel burnup without assuring timely resolution of a safety question concerning fuel dispersal within current burnup limits, the technical staff who performed the safety review for this issue are compelled to non-concur.
ALTERNATIVES TO NON-CONCURRENCE CONSIDERED BY TECHNICAL REVIEWERS Prior to non-concurring on the draft safety evaluation for WCAP-18446-P/NP, the technical staff who performed the safety review and responsible NRC management both proposed alternative approaches in attempt to reach agreement on the safety evaluation. Although these efforts ultimately proved unsuccessful, inasmuch as it may be of interest to the disposition of the non-concurrence, a brief discussion of the alternatives considered from the perspective of the non-concurring technical staff is provided below:
(1) Issue the safety evaluation with a limitation and condition requiring that licensees implementing WCAP-18446-P/NP assess fuel dispersal and assure compliance with 10 CFR 50.46(b)(4).
The non-concurring technical staff proposed this voluntary, forward-looking approach because a limitation and condition could serve as an effective vehicle for requiring an assessment of fuel dispersal and evaluation of attendant consequences relative to regulatory requirements for any licensee implementing WCAP-18446-P/NP. The technical staffs original safety evaluation input included a limitation and condition, as follows, which has been eliminated in the management-endorsed final draft of the safety evaluation:
Licensees implementing WCAP-18446-P/WCAP-18446-NP, Revision 0, shall assess the potential for fuel dispersal from fuel rods with less than 62 GWd/MTU rod-average burnup and justify that the estimated quantity of dispersed fuel does not result in non-compliance with the acceptance criteria in 10 CFR 50.46.
The technical staffs original safety evaluation had further included discussion clarifying that this limitation and condition could be addressed, contingent upon the Commissions direction, by information demonstrating acceptable treatment of fuel dispersal in accordance with any viable alternative(s) considered under the ongoing increased enrichment rulemaking effort:
In light of this developing initiative, the NRC staff has phrased Limitation and Condition 11 in a performance-based fashion that would allow licensees implementing WCAP-18446-P/WCAP-18446-NP to consider information and regulatory positions developed under the rulemaking process in the resolution of the limitation and condition. In particular, Limitation and Condition 11 does not imply that licensees must apply the modeling approaches or acceptance criteria described in WCAP-18446-P/WCAP-18446-NP for that demonstration.
Hence, the recommended inclusion of the above-cited limitation and condition in the safety evaluation for WCAP-18446-P/NP was not intended as a mandate that licensees implementing the methodology would need to demonstrate zero fuel dispersal; nor did it attempt to specify any particular limit on fuel dispersal. Rather the proposed limitation and condition could be construed as a flexible, performance-based requirement that, prior to further increasing allowable burnup limits, licensees must assess fuel dispersal, which has, up through the present, not been evaluated with technically defensible methods within the licensing basis for any operating plant, and justify according to relevant, available regulatory acceptance criteria that compliance exists with applicable regulatory requirements.
Finally, in the view of the non-concurring technical staff, recent topical report reviews have incorporated limitations and conditions, albeit on different subjects, that are of a similar logical and conceptual nature to that proposed by the non-concurring technical staff for the safety evaluation on WCAP-18446-P/NP. For example:
Limitation and Condition 2 in the staffs final safety evaluation for ANP-10339P (ML23233A151),
Limitation and Condition 4.7 in the staffs final safety evaluation for EMF-2103P, Revision 3 (ML16172A329),3 and Limitation and Condition 7 in the staffs draft safety evaluation for WCAP-18446-P/NP (ML23278A257).
While none of these examples is identical to the current situation, the instances identified here, many more of which could be brought to bear, illustrate that the agency has considerable latitude to impose reasonable limitations and conditions that are necessary to reach a finding of acceptability on a proposed topical report methodology. As such, there would not appear to be good cause to argue that a conceptually similar limitation and condition, as discussed in this option, would be uniquely non-viable.
3 See also similar limitations and conditions in topical reports for other fuel vendors: Limitation and Condition 14 in WCAP-16996-P (ML16343A238) and Limitation and Condition 10.5 in NEDE-33005P, Revision 1 (ML17055A387).
In conclusion, the non-concurring staff would find the forward-looking, conditioned-approval approach an acceptable alternative to non-concurrence because it would reasonably assure that plants implementing the WCAP-18446-P/NP methodology would be in compliance with relevant regulatory requirements intended to assure public health and safety.
(2) Issue the safety evaluation with a commitment that the NRC would initiate a timely backfit evaluation to assess for all affected licensees whether imposition of corrective actions to address impacts of fuel dispersal within existing burnup limits is justified.
The non-concurring technical staff would find this approach an acceptable alternative to non-concurrence because it would establish a clear, publicly stated commitment to perform a timely, comprehensive assessment of (1) whether existing licensees are in compliance with regulatory requirements and (2) any corrective actions necessary to assure adequate protection of public health and safety and regulatory compliance against relevant criteria in 10 CFR 50.109. Because the scope of this backfit evaluation would include all potentially affected licensees, its completion would encompass all evaluations and any corrective actions necessary to address fuel dispersal for the subset of plants that could potentially implement WCAP-18446-P/NP (i.e., the specific population of direct concern to the non-concurrence). A public commitment in the safety evaluation for WCAP-18446-P/NP to complete a comprehensive backfit evaluation in a timely manner would provide adequate assurance that the safety evaluation could be issued at the present time because a reasonable expectation would exist that backfit evaluation results would be available prior to the agencys passing judgment on licensees requests for implementation of WCAP-18446-P/NP.
(3) Delay issuance of the safety evaluation for WCAP-18446-P/NP until such time as an adequate technical basis for evaluating fuel dispersal can be developed to determine with reasonable assurance that licensees implementing the topical report would be in compliance with 10 CFR 50.46(b)(4) and other relevant regulatory requirements.
The discussion above has elaborated significant challenges the NRC is facing with respect to both defining acceptance criteria and performing calculations for fuel dispersal and its downstream impacts. The non-concurring technical staff would find the delayed-issuance approach an acceptable alternative to non-concurrence because it would defer a regulatory determination concerning the acceptability of the WCAP-18446-P/NP methodology until such time as validated analytical capabilities can be established to quantify how much fuel may be dispersed under loss-of-coolant accident conditions, assess attendant downstream impacts, and make a regulatory determination concerning whether 10 CFR 50.46(b)(4) and other existing regulatory requirements would be satisfied.
(4) Issue the safety evaluation and re-enter fuel dispersal into the NRCs generic issue program for further evaluation.
The non-concurring technical staff would not find this approach an acceptable alternative to non-concurrence because it would not appear capable of determining in a timely manner (1) how much fuel may be dispersed under loss-of-coolant accident conditions and (2) the significance of attendant downstream impacts relative to existing regulatory requirements. The NRC staff notes in particular that licensees are seeking to implement fuel burnup increases within the next few years; whereas,Section IV of Management Directive 6.4 indicates that the three-stage process for resolving generic issues may typically take between 6.75 and 14.5 years. Hence, while potentially viable in 2011, this alternative no longer appears capable of providing adequate basis for issuance of the safety evaluation in its present form and does not constitute an acceptable alternative to non-concurrence.
(5) Issue the safety evaluation while continuing to conduct research, such as an ongoing phenomenon identification and ranking exercise and simulations using a coupled analysis approach applying neutronic, thermal-hydraulic, and fuel performance codes, to better characterize the impacts of fuel dispersal.
The management-endorsed, modified version of the safety evaluation shared with the non-concurring technical staff on November 17, 2023, follows the basic approach described in this alternative. While the non-concurring technical staff support further testing and analysis to resolve the fuel dispersal issue, as explained below, the management-endorsed approach for issuing the safety evaluation for WCAP-18446-P/NP does not appear sufficient to protect the public health and safety.
To begin, the non-concurring technical staff believe that ongoing plans to complete research, and any speculation concerning possible future insights to be derived therefrom, should not influence the present decision concerning whether to issue a favorable safety evaluation for WCAP-18446-P/NP. Rather, this safety evaluation, which should include a regulatory conclusion about whether plants implementing the WCAP-18446-P/NP methodology would be assured of being in compliance with existing regulations, should only be based upon consideration of the currently available knowledge of fuel dispersal. Logically, if regulatory findings in the modified safety evaluation depend upon the outcome of future research, the safety evaluation should not be issued until the future research is completed. If, on the other hand, regulatory findings in the modified safety evaluation do not depend upon future research, it is not clear why such information should be included, apparently as part of the basis for a regulatory conclusion. For instance, as discussed in the safety evaluation content guide for power reactor license amendment reviews (ML20136A161) incorporated by reference into NRR office instruction LIC-101 (ML19248C539),4 The SE should be written succinctly. It should only include the information necessary to explain or support the regulatory findings.
4 While LIC-101 and its associated content guide technically do not apply to a safety evaluation for a topical report, the non-concurring technical staff nevertheless consider its intent relevant.
Concerning ongoing and future research, the modified draft safety evaluation states the following:
If the NRC identifies information as a result of this research [i.e., a phenomenon identification and ranking table (PIRT) panel to investigate impacts of fuel dispersal] that challenges the current NRC position that fuel dispersal is not a significant safety issue for burnups below 62 GWd/MTU, then the NRC will take appropriate regulatory action regarding methodologies currently approved for these burnups.
Consistent with foregoing discussion, the non-concurring technical staff would agree that appropriate regulatory action should be taken when a safety issue is identified after the NRC has made a soundly based regulatory decision that a requested licensing action would be in compliance with relevant requirements. However, the possible capability to take retroactive corrective action, subject to economic cost-benefit considerations under certain conditions, does not constitute an equivalent substitute for making a sound, safety-focused, up-front decision. As discussed above relative to the WCAP-18446-P/NP methodology, the agency lacks positive reasonable assurance that operating plants are currently in compliance with 10 CFR 50.46(b)(4) and other relevant regulations. While the modified safety evaluation summarily asserts that fuel dispersal is not a significant safety issue for burnups below 62 GWd/MTU, as discussed above, there is in fact no validated safety analysis demonstrating the impacts of fuel dispersal on operating reactor safety. Therefore, from the view of the non-concurring technical staff, the available evidence remains insufficient to justify either the definitive conclusion in the modified safety evaluation (i.e., not a significant safety issue) or its converse. In other words, the impact of fuel dispersal on operating reactors is an open safety question that the non-concurring technical staff believe should be addressed before concluding that licensees implementing WCAP-18446-P/NP would comply with 10 CFR 50.46(b)(4) and other regulatory requirements. Moreover, in light of the cost-benefit considerations mandated for backfitting by 10 CFR 50.109 and associated agency procedures, it is not apparent that the NRC would be capable of imposing any corrective actions (e.g., the appropriate regulatory action alluded to in the modified safety evaluation) in the event that noncompliance with regulations is discovered following NRC acceptance and licensee implementation of WCAP-18446-P/NP. Therefore, it is crucial that the NRC make evidence-based, up-front regulatory decisions, independent of speculative outcomes concerning future research or backfit evaluations, that operating reactor licensees would be in compliance with relevant regulatory requirements including 10 CFR 50.46(b)(4) prior to allowing the implementation of fuel burnup increases through methods such as WCAP-18446-P/NP.
The non-concurring technical staff further note, regarding the phenomenon ranking exercise noted in the above quotation, that the agencys decision to perform such an exercise appears to reflect limited knowledge and significant open questions with respect to fuel dispersal and its impacts. In particular, a phenomenon identification and ranking exercise is typically performed early in the process of analyzing a potential safety concern. At an initial stage, phenomena are ranked and tabulated by experts in
terms of their perceived importance to the calculation of relevant acceptance criteria.
Considering its abstract and generalized nature, a phenomenon ranking table, on its own, appears very unlikely to be capable of either confirming or refuting the significance of fuel dispersal as a safety issue for a specific subset of operating reactors (e.g., the population of concern to this non-concurrence). Rather, definitive confirmation or refutation of a safety issue would typically be accomplished through further downstream research or analysis, for which the phenomenon ranking exercise would merely serve as a foundation. At this time, however, potential future experiments and analytical model development efforts sufficient to address the impacts of fuel dispersal in a definitive manner for operating reactors remain undefined and unscheduled. This state of indeterminacy is again in contrast with the agencys roadmap for implementing increased fuel burnup and other design advancements, which incorporates a number of planned steps and a rough timeline to achieve implementation by 2027 for pressurized-water reactors.
Regarding the outcome of the phenomenon ranking exercise for fuel dispersal impacts that is discussed above, the modified safety evaluation further states that it is not expected for the conclusions of the PIRT (and any related follow-up research efforts) to conflict with the staffs findings related to WCAP-18446-P/WCAP-18446-NP, Revision 0.
However, no objective basis for the stated expectation is apparent. Further, logically, if the staffs knowledge in a certain area were already reasonably assured, it would follow that significant additional research or analysis efforts would be superfluous because they would be unnecessary to reach a regulatory conclusion. However, the facts that (1) the agency has indeed deemed additional research necessary, and (2) the additional, necessary research is currently at an early stage, suggest there may not be an objective basis for reasonable assurance that fuel dispersal would not impact regulatory compliance for plants implementing WCAP-18446-P/NP. In fact, the modified safety evaluation ultimately makes no conclusion regarding whether operating reactors are currently in compliance with applicable regulations:
the NRC staffs approval of WCAP-18446-P/WCAP-18446-NP, Revision 0 does not imply any consideration about the acceptability of fuel dispersal for fuel cladding below 62 GWd/MTU peak rod average burnup As such, this alternative (i.e., issuing the safety evaluation while moving forward with a phenomenon ranking exercise and potentially other undefined follow-on activities) cannot assure that operating reactors implementing WCAP-18446-P/NP would be in compliance with regulatory requirements relevant to fuel dispersal. This alternative, therefore, does not provide adequate basis for issuance of the safety evaluation in its present form and does not constitute an acceptable alternative to non-concurrence.
(6) Issue the safety evaluation under the presumption that dispersal of fuel from rods that are within existing burnup limits will receive appropriate disposition via incorporation into the ongoing increased enrichment rulemaking.
The non-concurring technical staff support including explicit consideration of dispersed fuel from rods within existing burnup limits in the increased enrichment rulemaking.
However, the Commissions ultimate decision in that regard or with respect to the larger rulemaking effort is not knowable in the present. Confidence in the viability of this potential approach is therefore insufficient at this time, as the agencys staff is currently making a regulatory decision concerning fuel dispersal in connection WCAP-18446-P/NP. Uncertainty in the viability of this approach is likely to persist, as additional proposals to allow increased fuel burnup are expected come before the staff prior to the Commissions decision on the proposed increased enrichment rulemaking. The non-concurring technical staff therefore do not consider this alternative as providing adequate basis for issuing the safety evaluation at this time. The approach is not capable of assuring that plants implementing the WCAP-18446-P/NP methodology would be in compliance with existing regulations considering currently available knowledge of fuel dispersal and does not constitute an acceptable alternative to non-concurrence.
large break loss of coolant accident (LOCA) frequencies; the fact that any significant FFRD would occur near the end of a cycle, further reducing its frequency; the conservatism of the assumptions used by the NRC in determining the amount of fuel dispersal; the relatively modest upper bound on the amount of fuel dispersal estimated to occur based on studies conducted by RES; and the fact that the fuel dispersal would generally be expected to occur from fuel rods distributed throughout the core (i.e., no significant local buildup in dispersed fuel fragments that would lead to significant flow blockages or other concerns with core coolability).
The NRC is currently engaged in multiple activities to investigate the FFRD issues for both current licensed exposures and proposed higher burnups. This includes a Phenomena Identification and Ranking Table (PIRT) exercise, an Informal Assistance Request to assess current capabilities to evaluate the safety concerns with the information from the PIRT, and planned efforts to further assess the consequences of fuel dispersal. If necessary, the NRC Section B: Attachment The non-concurring NRC staff make several good arguments in support of their position on this non-concurrence, and I do not intend to evaluate each individual point made by the non-concurrers. Rather, I will focus on two specific areas which I believe to be the most relevant to the disposition of this non-concurrence.
The first consideration is related to the regulatory finding under consideration. The WCAP-18446-NP/P methodology only concerns itself with evaluation of fuel rods that enter the rod average exposure range [
] and the intent of the methodology is to demonstrate that the fuel rods which enter this range do not burst, thus precluding fuel dispersal. The non-concurrers do not appear to dispute a finding that the methodology is acceptable for this purpose, nor do the non-concurrers state that this approach is not consistent with regulatory requirements. The non-concurrers state that the applicant provided reasonable evidence that implementation of this methodology by licensees would [
] Therefore, the acceptability of using the WCAP-18446-NP/P methodology to demonstrate compliance for rod average exposures
[ ] is unrelated to the analytical treatment for rod average exposures below 62 GWd/MTU. Licensees would presumably use methodologies described in other NRC approved topical reports to address fuel at rod average exposures below 62 GWd/MTU. For this reason, the concerns that the non-concurrers are trying to address are not related to the scope of the WCAP-18446-NP/P submitted for review by the NRC.
The second consideration is related to the safety concern. The non-concurrers make a valid point that the NRC is aware of information (i.e., RIL 21-13) which suggests that fuel fragmentation, relocation, and dispersal (FFRD) occurs at lower fuel exposures than previously assumed. In SECY-15-0148, the NRC staff states that the amount of potential fuel dispersal is not sufficient to present an immediate safety concern. Further review in light of the information in RIL 21-13 did not change this conclusion. In summary, the finding of no immediate safety concern was based on the following information:
would initiate a backfit evaluation to address any identified safety issues for fuel rod average burnups below 62 GWd/MTU. In addition, the NRC expects to address the regulatory concernsrelated to FFRD on a more generic basis as part of the increased enrichment rulemaking process currently underway.