ML25097A243
| ML25097A243 | |
| Person / Time | |
|---|---|
| Issue date: | 07/17/2025 |
| From: | Vance Petrella NRC/RES/DE |
| To: | |
| References | |
| PRE-2024-001 | |
| Download: ML25097A243 (8) | |
Text
Initial Review of PRE-2024-001, Nuclear Criticality in Fuel Pools Due to Drain Down Events and Boil Off Section 1: Background Information Assigned Responsible Project Manager: Vance Petrella Pre-Generic Issue Number: PRE-2024-001 Pre-Generic Issue
Title:
Nuclear Criticality in Fuel Pools Due to Drain Down Events and Boil Off Section 2: Proposed Generic Issue See ML25024A015 Section 3: Allegation or Physical Security issue:
- 1. Is the proposed generic issue an allegation?
No
- 2. Is the proposed generic issue a physical security issue?
No
- 3. Is the pre-generic issue obviously a generic issue? (Examples of obviously not being a generic issue: request for information, worker harassment claims, or personnel safety issues)
Yes Section 4: Challenges to the timeline for the Screening and Assessment Stages Challenges:
- 1. No challenges can be determined at this time.
Section 5: NUREG-0933 Search Search and results o Search criteria:
Boral o Results:
Found Section 3, Issue 196 Conclusion This is not applicable.
This issue talks about the Boral inside a spent fuel canister and not the spent fuel pool racks.
Search and results o Search criteria:
absorber plate o Results:
Found Section 3, Issue 196 Conclusion This is not applicable.
This issue talks about the Boral inside a spent fuel canister and not the spent fuel pool racks.
Search and results o Search criteria:
In the Related Technical Area I selected the following Spent Fuel Storage and Handling o Results:
Found Section 2, Item A-28 and A-36 Item A-28 o Conclusion This is not applicable.
It talks about the conversion of the spent fuel pools to allow more storage.
Item A-36 o Conclusion This is not applicable.
This talks about heavy loads above the spent fuel pool.
Found Section 3, Issue, 82, 173, 186, 196, 202 and 203 Issue 82 o Conclusion This is applicable Issue 173 o Conclusion This is not applicable.
I could not determine how this issue was similar to the present safety concern.
Issue 186 o Conclusion This is not applicable.
This talks about heavy loads.
Issue 196
o Conclusion This is not applicable.
This issue talks about the Boral inside a spent fuel canister and not the spent fuel pool racks.
Issue 202 o Conclusion This is not applicable.
This talks about leakage through the spent fuel pool liner.
Issue 203 o Conclusion This is not applicable.
This talks about heavy loads.
Section 6: Other Research Proposed Sub issue 1:
Summary of the proposed Sub issue: The NRC has not taken the following into account for the probability of the initiating event for a spent fuel storage pool draining or boil off event:
Insider sabotage Missile attacks by domestic terrorists Missile attacks by foreign terrorists Extended regional grid blackouts resulting from cyber attacks Extended regional grid blackouts resulting from electromagnetic pulses by a Carrigton-scale solar storm Extended regional grid blackouts resulting from electromagnetic pulses by a high-altitude thermonuclear blast by an adversary NRC Findings: This issue has been looked at multiple times in the past for example, NRCs response to PRM-50-108 (ML16061A114/ML16022A185) and the closing out of the 2.206 petition from Mark Leyse (ML23202A067). The NRC indicated in the closing out of PRM-50-108 that the risk of a spent fuel pool (SFP) severe accident is low. The NRCs conclusion in the closing out of the 2.206 petition from Mark Leyse was that it is the NRCs position that U.S.
Nuclear Power Plants (NPPs) can safely shut down following an electromagnetic pulse (EMP) or extreme solar storm event.
NRC Basis:
Insider sabotage, missile attacks by domestic terrorists, missile attacks by foreign terrorists, and extended regional grid blackouts resulting from cyber attacks
In the NRC response to PRM-50-108, the NRC addresses the probability of strong geomagnetic disturbance, a nuclear device detonated in the earths atmosphere, a pandemic, or a cyber or physical attack. The following is a summary of the petition:
The petitioner stated that the requested regulations pertaining to SFP accident evaluation models are needed because the probability of the type of events that could lead to SFP accidents is relatively high.The petitioner also provided several examples of events that could lead to a long-term station blackout and, ultimately, an SFP accident, such as a strong geomagnetic disturbance, a nuclear device detonated in the earths atmosphere, a pandemic, or a cyber or physical attack.
The following is a summary of the NRCs response:
As supported by numerous evaluations referenced in this document, the NRC has determined that the risk of an SFP severe accident is low. While the risk of a severe accident in an SFP is not negligible, the NRC believes that the risk is low because of the conservative design of SFPs; operational criteria to control spent fuel movement, monitor pertinent parameters, and maintain cooling capability; mitigation measures in place if there is loss of cooling capability or water; and emergency preparedness measures to protect the public.
Extended regional grid blackouts resulting from electromagnetic pulses by a carrigton-scale solar storm and extended regional grid blackouts resulting from electromagnetic pulses by a high-altitude thermonuclear blast by an adversary In the NRC response to 2.206 petition from Mark Leyse, NRC states:
Grid resilience (including under extreme solar storm conditions) is outside the NRCs authority and is regulated by the Federal Energy Regulatory Commission (FERC) with participation from the North American Electric Reliability Corporation (NERC).
All U.S. NPPs are equipped to respond to an extended loss of AC power (ELAP), in accordance with 10 CFR 50.155 (b)(1) and (c). Further, it is the NRCs position that U.S.
NPPs can safely shut down following an extreme solar storm event and maintain spent fuel pool cooling. In the Federal Register for the Mitigation of Beyond-Design-Basis Events (MBDBE) rule (84 FR 39684; August 9, 2019), the commission stated that the requirements in §50.155(b)(1) and (c) and the associated regulatory guidance, address the issues raised by the (PRM-50-96) petitioner because these regulations require licensees to establish offsite assistance to support maintenance of the key functions (including both reactor and SFP cooling) following an extended loss of AC power, which has been postulated as a consequence of geomagnetic disturbances. These requirements include diesel fuel resupply during and following beyond design basis events. This does not include any diesel fuel resupply efforts that may come as part of the federal governments response efforts.
In the NRC response to 2.206 petition from Mark Leyse, NRC states:
it is the NRCs position that U.S. NPPs can safely shut down following an EMP or extreme solar storm event.
The NRC has been examining impacts of EMPs for more than 40 years, starting in the late 1970s when the agency studied how EMPs could affect nuclear power plant safe-shutdown systems. In February 1983 the NRC issued the studys conclusion: nuclear power plants safety systems can fulfill their intended safety functions after an EMP event (NUREG/CR-3069, SAND82-2738/1 Vol. 2, Interaction of Electromagnetic Pulse with Commercial Nuclear Power Plant Systems). The agency revisited the issue in 2007 to account for the increasing use of digital computer systems in nuclear plants, which potentially could be more susceptible to an EMP (Assessing Vulnerabilities for Present Day Digital Systems to Electromagnetic (EM) Threats at Nuclear Power Plants, F.
Wyant, M. Walker, L. Bacon, M. Dinallo December 2009 (non-public - Security-Related content)). Most recently, the agency worked closely again with Sandia National Laboratories on further EMP related testing and the Sandia report concluded that the test results supported previous determinations that US NPPs will safely shutdown following an EMP event (SAND2022-10244, Ensuring a Nuclear Power Plant Safe State Following an EMP Event, December 2022 (non-public - Official Use Only)).
Proposed Sub issue 2:
Summary of the proposed Sub issue: The NRC staff failed to adequately model the progression and consequences of fuel fires caused by pool draining or boil-off in MELCOR.
NRC Findings: This issue has been looked at in the past when closing out PRM-50-108 and the 2.206 petition from Mark Leyse. The NRC stated in the closing out of PRM-50-108 that the MELCOR computer code is the NRCs best estimate tool for severe accident analysis. It has been validated against experimental data, and it represents the current state of the art in severe accident analysis. In response to the 2.206 petition from Mark Leyes, the NRC stated that the NRC disagrees with the petitioners statements that MELCOR is flawed. MELCOR is the NRCs best estimate tool for severe accidents analysis and has been validated against experiential data.
NRC Basis:
The federal register for the closing out of PRM-50-108 states the following:
The MELCOR computer code is the NRCs best estimate tool for severe accident analysis. It has been validated against experimental data, and it represents the current state of the art in severe accident analysis. In NUREG-2161, the NRC stated that MELCOR has been developed through the NRC and international research performed since the accident at Three Mile Island in 1979. MELCOR is a fully integrated, engineering-level computer code and includes a broad spectrum of severe accident phenomena with capabilities to model core heatup and degradation, fission product release and transport within the primary system and containment, core relocation to the vessel lower head, and ex-vessel core concrete interaction. Furthermore, MELCOR has
been benchmarked against many experiments, including separate and integral effects tests for a wide range of phenomena. Therefore, the NRC has determined that MELCOR is acceptable for its intended use.
Additional information about the capabilities of the MELCOR code to model SFP accidents can be found in the NRC response to stakeholder comments in Appendix E to NUREG-2161. The NRC also addressed questions regarding MELCOR in Appendix D to NUREG-2157, Volume 2, Generic Environmental Impact Statement for Continued Storage of Spent Nuclear Fuel (ADAMS Accession No. ML14196A107).
In the NRC response to 2.206 Petition from Mark Leyse, NRC states:
The petitioner lists many concerns about MELCOR modeling. Among the arguments being made are that MELCOR cannot not capture the ignition timing, total chemical reaction energy, and the consequences of spent fuel pool accidents because it does not explicitly model the nitriding of zirconium fuel-cladding, that it does not explicitly model the effects of an oxide layer, and that MELCOR does not model criticality.
The NRC has addressed these specific concerns regarding MELCOR in Appendix D to NUREG-2157, Volume 2, Generic Environmental Impact Statement for Continued Storage of Spent Nuclear Fuel (ML14196A107). To briefly summarize the NUREG-2157, Volume 2 response to related comments D.2.39.18 and D.2.39.24, the NRC considers that the physical models in MELCOR capture the most important phenomena necessary to realistically evaluate the initiation and progression of SFP fires. Although zirconium nitriding was not explicitly modeled in the NRCs spent fuel pool fire analyses, the NRC believes such a model is not necessary since initiation and progression has been validated against air-oxidation experiments which do involve the effects of nitriding, that oxygen-starved scenarios where the inclusion of a nitriding model would change results compared to current modeling necessarily involve less energy release and also necessarily involve less release of radioactive material to the environment than scenarios involving a failed reactor building which readily allow both air (oxygen) ingress to the fuel and radioactive materials release to environment, and for which limiting cases or sensitivity analyses can be used to bound results. The NRC concluded that it is reasonable to rely on the quantitative results from NUREG-1738, Technical Study of Spent Nuclear Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants (ML010430066) for NUREG-2157, Volume 1 because NUREG-1738 makes appropriate, bounding conservative assumptions regarding SFP fire progression.
Additional information about the capabilities of the MELCOR code to model SFP accidents can be found in the NRC response to stakeholder comments in Appendix E to NUREG-2161 Consequence Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I Boiling Water Reactor, (ML14255A365).
The NRC also addressed MELCOR modeling concerns in the August 24, 2016 SECY-16-0100 (ML16188A300), Staff Review and Response to National Academy of Sciences Study of the Lessons Learned from the Fukushima Nuclear Accident for Improving Safety and Security of U.S. Nuclear Power Plants. In this report, the staff concluded that spent fuel continues to be stored safely and securely at nuclear power plants in both spent fuel pools and dry casks. The petitioner referenced the National Academy of Sciences report but did not reference SECY-16-0100 responding to this report.
The PRB is aligned with previous NRC responses regarding MELCOR which convey the following: The NRC disagrees with the petitioners statements that MELCOR is flawed.
MELCOR is the NRC's best estimate tool for severe accident analysis and has been validated against experimental data. The findings from the experimental programs and the MELCOR assessments have been documented in NUREG/CR-7143 for BWR assemblies and NUREG/CR-7215 & NUREG/CR-7216 for PWR assemblies. Sensitivity analyses are used to account for uncertainties inherent to the progression of severe accidents. Alternative conservative assumptions are used in modeling when needed.
Proposed Sub issue 3:
Summary of the proposed Sub issue: The NRC staff failed to recognize that there is a possibility of criticality when refilling the spent fuel pool.
NRC Findings: This issue has been looked at in the past in section 4.3.10.7, Inadvertent Criticality, in Enclosure 1 for COMSECY-13-0030, Regulatory Analysis for Japan Lessons-Learned Tier 3 Issue on Expedited Transfer of Spent Fuel (ML13273A628). The NRCs conclusion was that the consequences of an inadvertent criticality event would be insignificant relative to consequences of a zirconium fire.
NRC Basis:
The COMSECY-13-0030, Section 4.3.10.7 states the following:
Potential criticality is limited by moderator availability and pool configuration. Many U.S.
SFPs rely on the presence of neutron absorbing materials that are part of the storage rack structure to meet sub-criticality requirements under normal and credible abnormal events. The performance of these materials following a large beyond design basis seismic event has not been fully analyzed. It is possible that the environmental conditions after the beyond design basis seismic event could cause degradation of these materials. However, the presence of a moderator is necessary for an inadvertent criticality event to occur, and an adequate moderator would only be present during the drain down/boil off phase or during recovery actions. While neither of these scenarios has been analyzed, the sustainable power of the inadvertent criticality event would be limited to a level significantly below the operating reactor, since the SFP is an open system and significant heat generation would create steam voids that provide
inadequate moderation. Therefore, the additional fission product inventory in the fuel would not be significant. In addition, the required moderator for criticality limits the effect of any inadvertent criticality event because the water would provide shielding and reduce the fraction of radioactive material that would be released.
Consequences of an inadvertent criticality event would be insignificant relative to consequences of a zirconium fire: Fuel assemblies that experienced zirconium cladding ignition could have sufficient cladding damage where further agitation, such as seismic aftershocks, would relocate fuel fragments in a non-uniform configuration. In this scenario, a large majority of the radioactive source term material would have already been released during the zirconium fire. The release from a subsequent inadvertent criticality event would be primarily a hazard to onsite workers with little offsite impact.
The staff expects that any sustained inadvertent criticality event would be orders of magnitude lower than the power generated in the reactor with a corresponding lower production of short half-lived releasable material, making the inadvertent criticality event an insignificant contributor to the consequences of the zirconium fire. Therefore, the NRC staff judges that the consequences of a potential inadvertent criticality event following a zirconium fire fuel need not be considered. Furthermore, if a SFP criticality event did occur and generated short-lived radionuclides that are associated with offsite early fatalities, the emergency response as modeled effectively prevents any early fatality risk. This occurs in part because the modeled accident progression results in releases that are long compared with the time needed for relocation.
References:
- 1. PRM-50-108 Fuel-Cladding Issues in Postulated Spent Fuel Pool Accidents (NRC-2014-0171. (ML16061A114/ML16022A185)
- 2. Nuclear Regulatory Commission, Email on July 21, 2023 from Perry Buckberg to Mark Leyse, 2.206 Petition Initial Assessment - Transfer Spent Fuel to Dry Storage. (ML23202A067)
- 3. Nuclear Regulatory Commission, Encloser 1 for COMSECY-13-0030, Regulatory Analysis for Japan Lessons-Learned Tier 3 Issue on Expedited Transfer of Spent Fuel. (ML13273A628)