ML24305A290
| ML24305A290 | |
| Person / Time | |
|---|---|
| Site: | 99902078 |
| Issue date: | 10/31/2024 |
| From: | NRC |
| To: | NRC/NRR/DNRL/NRLB |
| References | |
| Download: ML24305A290 (11) | |
Text
From:
Getachew Tesfaye Sent:
Thursday, October 31, 2024 5:19 PM To:
Request for Additional Information Cc:
Dennis Galvin; David Drucker; Stacy Joseph; Mahmoud -MJ-Jardaneh; Griffith, Thomas; Sfowler@nuscalepower.com; Bode, Amanda; NuScale-SDA-720RAIsPEm Resource
Subject:
Nonproprietary - NuScale XPC TR-124587, Extended Passive Cooling and Reactivity Control Methodology, Revision 0 - Request for Additional Information No. 033 (RAI-10298-R1)
Attachments:
TR-124587-NP (XPC) - RAI-10298-R1 - FINAL NON-PROPRIETARY.pdf Attached please find NRC staffs nonproprietary request for additional information (RAI) concerning the review of NuScale Topical Report TR-124587, Extended Passive Cooling and Reactivity Control Methodology, Revision 0 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23005A308). The encrypted proprietary version will be submitted in a separate email.
Please submit your technically correct and complete response by the agreed upon date to the NRC Document Control Desk.
If you have any questions, please do not hesitate to contact me.
Thank you, Getachew Tesfaye (He/Him)
Senior Project Manager NRC/NRR/DNRL/NRLB 301-415-8013
Hearing Identifier:
NuScale_SDA720_RAI_Public Email Number:
43 Mail Envelope Properties (BY5PR09MB5682DBE309074291E9B7CC728C552)
Subject:
Nonproprietary - NuScale XPC TR-124587, Extended Passive Cooling and Reactivity Control Methodology, Revision 0 - Request for Additional Information No. 033 (RAI-10298-R1)
Sent Date:
10/31/2024 5:18:45 PM Received Date:
10/31/2024 5:18:49 PM From:
Getachew Tesfaye Created By:
Getachew.Tesfaye@nrc.gov Recipients:
"Dennis Galvin" <Dennis.Galvin@nrc.gov>
Tracking Status: None "David Drucker" <David.Drucker@nrc.gov>
Tracking Status: None "Stacy Joseph" <stacy.joseph@nrc.gov>
Tracking Status: None "Mahmoud -MJ-Jardaneh" <Mahmoud.Jardaneh@nrc.gov>
Tracking Status: None "Griffith, Thomas" <tgriffith@nuscalepower.com>
Tracking Status: None "Sfowler@nuscalepower.com" <sfowler@nuscalepower.com>
Tracking Status: None "Bode, Amanda" <abode@nuscalepower.com>
Tracking Status: None "NuScale-SDA-720RAIsPEm Resource" <NuScale-SDA-720RAIsPEm.Resource@nrc.gov>
Tracking Status: None "Request for Additional Information" <RAI@nuscalepower.com>
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BY5PR09MB5682.namprd09.prod.outlook.com Files Size Date & Time MESSAGE 700 10/31/2024 5:18:49 PM TR-124587-NP (XPC) - RAI-10298-R1 - FINAL NON-PROPRIETARY.pdf 152480 Options Priority:
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1 REQUEST FOR ADDITIONAL INFORMATION No. 033 (RAI-10298-R1)
BY THE OFFICE OF NUCLEAR REACTOR REGULATION NUSCALE STANDARD DESIGN APPROVAL APPLICATION DOCKET NO. 05200050 TR-124587, EXTENDED PASSIVE COOLING AND REACTIVITY CONTROL METHODOLOGY, REVISION 0, ISSUE DATE: 10/31/2024
=
Background===
By letter dated October 31, 2023, NuScale Power, LLC (NuScale or the applicant) submitted Part 2, Final Safety Analysis Report (FSAR) of the NuScale Standard Design Approval Application (SDAA) for its US460 standard plant design. Chapter 15, Transient and Accident Analyses (Agencywide Documents Access and Management System Accession No. ML23304A365) references the NuScale topical report "Extended Passive Cooling and Reactivity Control Methodology," TR-124587, Revision 0 (ML23005A308) (XPC LTR), submitted January 5, 2023. The applicant submitted the US460 plant SDAA in accordance with the requirements of Title 10 Code of Federal Regulations (10 CFR) Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants, Subpart E, Standard Design Approvals. The NRC staff has reviewed the information in TR-124587-P, Revision 0, and determined that additional information is required to complete its review.
Regulatory Basis General Design Criteria (GDC) 1, Quality Standards and Records, and GDC 30, Quality of Reactor Coolant Pressure Boundary, in 10 CFR Part 50, Appendix A require RCS components design, fabrication, erection, and testing to meet the highest quality standards practical.
GDC 10, Reactor Design, requires that the reactor core and associated coolant, control, and protection systems shall be designed with appropriate margin to assure that specified acceptable fuel design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences.
GDC 15, Reactor Coolant System Design, requires sufficient margin is in the RCS design to assure that the design conditions of the RCPB are not exceeded during any condition of normal operation, including anticipated operational occurrences.
GDC 34, Residual Heat Removal, requires that a system to remove residual heat shall be provided with the safety function to transfer fission product decay heat and other residual heat from the reactor core at a rate such that specified acceptable fuel design limits and the design conditions of the reactor coolant pressure boundary are not exceeded.
GDC 35, Emergency Core Cooling, requires that a system to provide abundant emergency core cooling shall be provided. The system safety function shall be to transfer heat from the reactor core following any loss of reactor coolant at a rate such that (1) fuel and clad damage that could interfere with continued effective core cooling is prevented and (2) clad metal-water reaction is limited to negligible amounts. In addition, suitable redundancy in components and features, and suitable interconnections, leak detection, isolation, and
2 containment capabilities shall be provided to assure that for onsite electric power system operation (assuming offsite power is not available) and for offsite electric power system operation (assuming onsite power is not available) the system safety function can be accomplished, assuming a single failure.
10 CFR 50.46(b)(4) requires that calculated changes in core geometry shall be such that the core remains amenable to cooling.
10 CFR 50.46(b)(5) requires that, after any calculated successful initial operation of the ECCS, the calculated core temperature shall be maintained at an acceptably low value and decay heat shall be removed for the extended period required by the long-lived radioactivity remaining in the core.
Question XPC.LTR-1 Issue The TR-124587 methodology is missing key information related to testing and validation for the NRELAP5 code for the staff to make a finding of acceptability of the Evaluation Model.
Specifically, the topical report is missing:
a) A demonstration of how the effects of both the SG and DHRS being in operation, after ECCS is actuated, are adequately addressed for the duration of the long-term events, up to the full 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. There is no testing presented in the topical report to validate the models for events where the SG and DHRS are in operation for the long-term (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) after ECCS actuation. The uncertainty introduced by this unaddressed phenomenon has not been quantified, nor has it been shown how the uncertainty has been adequately addressed/included in the NRELAP5 analyses that use the topical report methodology.
b) Validation of the coarse ECCS long term cooling (LTC) model utilized for LTC calculations against LTC tests. (( The coarse ECCS LTC model needs an adequate validation basis for the parameters important to LTC calculations over a range of LTC conditions. The topical report figure 5-15 shows a comparison of the riser collapsed liquid level between the LOCA topical report model and the LTC coarse model. ((
}} Given that the limiting LTC event results show that the level above the top of active fuel (TAF) is small, the modeling uncertainty introduced by the coarse integral LTC topical report model needs to be addressed as part of the validation of the model. The integrated model is highly sensitive to the coarse nodalization and should be accounted for in the uncertainty within the methodology in order to validate the model used for LTC calculations for figures of merit.
Information Requested a) In the absence of test data, provide an evaluation that contains sensitivity analyses over various conditions of NRELAP5 and independent calculations to demonstrate that the system heat transfer behavior is similar to a first principles simplified conceptual model that evaluates the various heat transfer coefficients (slowly varying steady state heat removal) and that quantifies the sensitivity to the multiple heat transfer coefficients over a broad
3 range. Provide sensitivity analyses over various conditions that quantify the sensitivity to the multiple heat transfer coefficients for the integrated NPM 20 response for long term cooling figures of merit. Alternately, provide additional sensitivity calculations that show progressively higher and lower sensitives to the heat transfer coefficient values for the minimum level case, maximum temperature case, minimum temperature case, and the boron transport thermal hydraulic cases. The results should either show that the responses are relatively insensitive over an extremely large range, or the uncertainty should be accounted for in the methodology results. Revise the topical report to include the results and summary. b) The nodalization has a noticeable impact on the calculation results for the LTC and adds uncertainty/bias that needs to be accounted for in the methodology for the validation. Provide the validation of the integrated coarse ECCS long term cooling model utilized for LTC calculations that accounts for the introduced bias/uncertainty due to the nodalization. The validation basis that accounts for the nodalization impacts for the coarse ECCS LTC model needs to have an adequate validation basis for the parameters important to LTC calculations over a range of LTC conditions including various different event types (LOCA and non-LOCA cases). If a sufficient validation basis that accounts for the introduced bias/uncertainty due to nodalization is not available, then develop and propose an appropriate penalty as part of the LTR. If updates to the methodology are made, revise the topical report and affected SDAA sections. Question XPC.LTR-2 Issue The TR-124587 methodology is missing needed information relative to the methodology assumptions for RCS and ESB mixing in the following areas: a) The methodology does not provide an adequate basis for model validation vs test data ((
}} Riser holes have an impact on thermal hydraulic conditions, for example but not limited to, initial conditions, natural circulation and RCS response; these impacts and any others need to be addressed.
b) Validation basis information/evaluations for the assumed condensate flow rate into the boron dissolver basket and containment mixing tubes is missing. There is no justification provided for the assumed fraction of the available containment wall area above each condensate rail that is used in determining the condensate collector flow rate, considering the non-uniformity of the containment wall shell as well as the other structures in the containment that influence condensation and condensate flow. c) The methodology is missing the basis information/evaluations that validates the ((
4
}} Additionally, the methodology is missing the basis information that validates the transport assumptions of fluid from one mixing volume to the next, (( }} (The gradients and flow patterns/behavior in the volumes have not been validated.)
Information Requested a) Provide justification via sensitivity studies demonstrating that NRELAP5 can adequately calculate the actual expected NPM response with respect to the integrated topical report model. The total integral response of the LTR model (from beginning of the event until the end of the long-term cooling phase) to various design basis events and conditions should be validated considering that the tests did not include flow holes and no other activities have been performed to quantify the impact on the total integral response and conditions of the RCS during design basis events. The potential impact on the RCS response and conditions for the integrated model on the figures of merit calculated by the topical report model and downstream activities should be shown analytically as described below: Provide sensitivity analyses and evaluations of the impact of riser holes on the integral effects tests responses used for Evaluation Model validation for the XPC LTR. The various impacts of riser holes on the integral test validation response should be captured in the impacts on the integrated NRELAP5 evaluation model results for XPC analyses. Provide sensitivities and evaluations for the impact of riser holes on the total integral response of the LTR models (XPC LTR model) to various design basis events and conditions that show that the test validation response to riser holes is captured. The impact on the RCS response and conditions for the integrated model on the figures of merit calculated by the XPC LTR model should be shown analytically. Revise the LTR to include the above information. b) Provide the validation basis information/evaluations for the assumed condensate flow rate into the boron dissolver basket and containment mixing tubes that justifies the assumed fraction of the available containment wall area above each condensate rail that is used in determining the condensate collector flow rate, considering the non-uniformity of the containment wall shell as well as the other structures in the containment that influence condensation and condensate flow. The potential impact on the NPM response due to the uncertainty added by the condensate flow rate assumptions on the figures of merit should be quantified to support justifications and provided in the response.
5 Specifically, (1) Provide the methodology description that specifically states how the credited fraction of the available containment wall area is determined; state specifically how the geometry is used and how influences on condensation and condensate flow rate are considered. (2) Provide the methodology description for how the credited minimum CNV surface area interacts with the channels that are directed toward the boron dissolver baskets. One of the channels enters the basket but is not used for diluting the boron. Describe how the minimum CNV surface area is distributed to the channels. (3) Provide the methodology description for how the dissolver condensate collection capacity is determined and a description of how the dissolver condensate collection capacity is used. (4) Provide the methodology description which states how the ESB mixing tube containment wall area is determined and how the methodology accounts for major obstructions. (5) Provide the methodology description for how the mixing tube condensate collection capacity is determined or a description of how the dissolver condensate collection capacity is used. Provide the requested information through revisions to the XPC LTR or SDAA. c) To justify the transport and mixing assumptions in the XPC LTR, provide an analytical basis and evaluations that validate ((
}} The gradients and flow patterns/behavior in the volumes should be validated analytically (such as through computational fluid dynamics). The analytical basis needs to account for the density difference between borated and pure water, because not accounting for the boron density leads to a result that is less conservative than a best estimate calculation. The density difference impact should be part of the validation methodology and uncertainties due to density and temperature should be quantified and the impacts on the figures of merit identified.
Revise the XPC LTR to include the requested information. Question XPC.LTR-6 Issue Table 4-17 in the XPC LTR states that the fuel decay heat source used in the methodology is biased high or low based on the direction of conservatism for the acceptance criterion. The topical report does not state how the methodology biases the decay heat source low, except to state ((
}}
Lower decay heat would result in a faster cooldown after a non-LOCA event during the eight-hour DHRS cooldown period. When ECCS actuates at eight hours, this would result in less steam condensate flow to the boron baskets and mixing tubes and reduce flow through the RRVs needed to get boron into the core. This situation has not been addressed in the topical report methodology. Additionally, the analysis of core reactivity after a non-LOCA transient assumes ((
6 }} However, operation at reduced power or a short maintenance shutdown could result in higher core xenon with a reduced core boron concentration to offset peak xenon. This situation has not been addressed in the topical report methodology. Information Requested a) Update the topical report to state how the methodology biases the decay heat source low. b) Provide an evaluation of the impacts on reduced decay heat from (1) plant operation at reduced power operation and (2) a plant restart after a maintenance outage. Discuss the impact of a maintenance outage of one to three weeks on decay heat after restart and impact on the methodology analyses. Revise the SDAA and/or topical report Additionally, evaluate the impact on the methodology analyses of a non-LOCA transient with initial conditions of peak xenon and reduced core boron concentration before end-of-cycle. Revise the SDAA and/or topical report accordingly. c) Revise the SDAA and/or topical report to include the methodology for how the decay heat source is determined when biased low, and the methodology, analysis and results for the impact on criticality during extended passive cooling of (1) reduced decay heat conditions and (2) peak xenon and the corresponding reduced initial boron near end-of-cycle conditions. The methodology for how xenon transients are analyzed needs to be provided or a guaranteed minimum boron concentration must be provided regardless of plant operation down powers (cycle capacity factor). The revisions need to justify the assumed operational paradigm that any transients initialized at partial power are from short term deviations from full power operation (short term down powers or short duration shutdowns) and how this will be assured in the design (identification of the operational restrictions and how they are enforced). The revisions need to include the operational restrictions and limits for shutdowns or downpowers near end-of-cycle that are used to ensure decay heat levels are sufficient to maintain subcriticality. Question XPC.LTR-18 Issue During its audit (ML23067A300), the staff observed ((
}}
((
7
}}
Information Requested Provide basis information for whether the NRELAP5 SG-DHRS NPM nodalization in the long-term cooling phase ((
}} The staff also needs to understand the influence of thermal hydraulic boundary conditions on the DHRS performance, e.g., ECCS operation, riser uncovery and cooling pool heat-up. Provide supporting information that shows whether (( }} Provide additional bases for the validation adequacy given that the information provided does not show good agreement for comparisons to test data response and phenomena (( }}
oscillation phenomena that has been seen in the tests which is not in good agreement. Revise the XPC LTR to include the basis and justification information. Question XPC.LTR-21 Issue During its audit (ML23067A300), the staff observed the following: (( }} Information Requested Given the minimal margin to recriticality that appears to exist under certain XPC scenarios, please provide additional explanation and justification for how NuScale develops and applies (( }} ((
}}
8 Question XPC.LTR-23 Issue By letter dated January 6, 2023 (ML23011A012), NuScale submitted NRELAP5 files via DVD to support review of the XPC LTR and other LTRs that support the SDAA. NuScale informed NRC in February 2024, that a code error was found. Subsequently, NuScale informed NRC that a new code version, NRELAP5 v1.7, was being released and would be used for the SDAA and supporting topical reports. By letter dated August 6, 2024, (ML24228A242), NuScale submitted the executable file for NRELAP5 v1.7. Although the executable file was provided, the referenced transmittal from January 6, 2023, included not only the executable, but also (( }} Information Requested In order for the staff to complete its technical review of the analytical approach and base its findings on submitted information relevant to the version of the code utilized in developing the EM, NuScale is requested to re-submit any of the remaining files impacted by this code version change. All files affected by the revision to v1.7, not just the executable, need to be resubmitted to provide a complete submittal package for all four of these LTRs. For example (but not limited to this), (( }} Note that similar requests are being made for all four LTRs, and responses can reference each other. Question XPC.LTR-24 Issue As a result of the NRC staffs Quality Assurance Program (QAP) implementation inspection in February 2024, the NRC staff understands that the XPC LTR is controlled as a licensing product within NuScale, and the document itself is not considered to be the XPC Evaluation Model (EM). It is the NRC staffs understanding that information presented in the XPC LTR are derived from engineering documents. As such, the NRC staff needs to determine if there is information presented or conclusions stated in the XPC LTR, which is drawn from engineering documents that are not subject to design verification consistent with 10 CFR 50 Appendix B in accordance with Part II, Quality Assurance Program Description Details, Section 2.3.1, Design Verification of Topical Report MN-122626, Revision 1-A, NuScale Power, LLC Quality Assurance Program Description (ML24033A318) (hereafter referred to as QAPD TR). This includes information subject to Part III, Nonsafety-Related Structures, Systems, and Components (SSC) Quality Control, Section 3.1.3, Design Control in the QAPD LTR, regardless of whether the verification process has been conducted in accordance with this section of the QAPD TR, since the Section 3.1.3 verification is not required to conform to 10 CFR 50 Appendix B. Information Requested Provide a list of any such documents that have not undergone design verification as described above. For any of these documents not already available in eRR for staff audit, please make
9 those additional documents available for audit. NuScale may choose to provide any relevant information regarding which portions of the LTR or conclusions those documents support. In addition, provide a listing of any section of the XPC LTR that are either not considered part of the EM or not directly supported by engineering documents that are part of the EM. Question XPC.LTR-28 Issue XPC LTR Section 4.2.4 states that NRELAP5 ((
}}
XPC LTR Section 4.2.3.5 states that ((
}}
The staff notes that the methodology (( }} does not support a finding that analysis assumptions compensate for code biases and uncertainties or are conservative. (( }} As such, NRC staff does not have justification or basis to support the conclusion that using the topical report evaluation model adequately compensates for the differences between the data and the NRELAP5 results. It is unclear to the staff how (( }} compensate and are conservative relative to code biases and uncertainties assessed through NIST-2 code-to-data comparisons. Information Requested Explicitly describe the XPC NRELAP5 pool modeling requirements and conservatisms used in the methodology. Provide justification and bases sufficient to demonstrate that conservative pool modeling ensures that the pool boundary conditions compensate and are conservative relative to code biases and uncertainties assessed through NIST-2 code-to-data comparisons and studies quantifying NRELAP5 sensitivity to uncertainties in NIST-2 conditions. Revise the XPC LTR to include the basis and justification.}}