ML24197A156
| ML24197A156 | |
| Person / Time | |
|---|---|
| Site: | Kemmerer, 99902100 File:TerraPower icon.png |
| Issue date: | 07/15/2024 |
| From: | Stephanie Devlin-Gill NRC/NRR/DANU/UAL1 |
| To: | |
| References | |
| EPID L-2024-TOP-0009 | |
| Download: ML24197A156 (1) | |
Text
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION TERRAPOWER, LLC - AUDIT PLAN FOR TOPICAL REPORT TP-LIC-RPT-0007, "DESIGN BASIS ACCIDENT METHODOLOGY FOR EVENTS WITH RADIOLOGICAL RELEASE," REVISION 0 (EPID NO. L-2024-TOP-0009)
Applicant:
TerraPower, LLC Applicant Address:
15800 Northup Way, Bellevue, WA 98008 Plant Name:
Natrium Project No.:
99902100
Background:
By letter dated March 22, 2024, TerraPower, LLC (TerraPower) submitted topical report (TR)
TP-LIC-RPT-0007, Design Basis Accident Methodology for Events with Radiological Release,"
Revision 0 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24082A262) to the U.S. Nuclear Regulatory Commission (NRC) staff. The TR provides evaluation models (EMs) developed for the Natrium reactor to evaluate design basis accidents (DBAs) with the potential for radiological release. On April 22, 2024, the NRC staff found that the material presented in the TR provides technical information in sufficient detail to enable the NRC staff to conduct a detailed technical review (ML24107B043).
TerraPower requested the NRCs review and approval of the EMs presented in the TR for use by future applications utilizing the Natrium design. The applicants overall licensing methodology follows the technology-inclusive, risk-informed, and performance-based approach outlined in Revision 0 of Regulatory Guide (RG) 1.233 Guidance for a Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light Water Reactors, Revision 0 (ML20091L698).
Purpose:
The purpose of the audit is for the NRC staff to gain a more detailed understanding of TerraPowers EMs developed for the Natrium reactor to evaluate DBAs with the potential for radiological release. A secondary purpose of the audit is to identify any information that will require docketing to support the NRC staffs safety evaluation.
Regulatory Audit Basis:
The basis for the audit includes:
Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(4) and 10 CFR 50.34(b)(4), which apply to applicants for construction permits and operating licenses, respectively. These sections require analysis and evaluation of the design and performance of structures, systems, and components (SSCs) of the facility with the objective of assessing the risk to public health and safety resulting from operation of the facility and including determination of the margins of safety during normal operations and transient conditions anticipated during the life of the facility, and the adequacy of SSCs
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION provided for the prevention of accidents and the mitigation of the consequences of accidents.
Paragraph 50.43(e) of 10 CFR requires that reactor designs that differ significantly from light-water reactor designs licensed before 1997 may only be approved if there has been appropriate demonstration of their safety features. The requirements in 10 CFR 50.43(e)(1)(i) and (ii) require demonstration of safety feature performance and interdependent effects through analysis, appropriate test programs, experience, or a combination thereof. The requirements in 10 CFR 50.43(e)(1)(iii) requires that sufficient data exists on the safety features to assess the analytical tools for safety analysis over a sufficient range of plant conditions.
Regulatory Audit Methodology:
The regulatory audit will follow the guidance in Office of Nuclear Reactor Regulation Office (NRR) Instruction LIC-111 Regulatory Audits, Revision 1 (ML19226A274), and focus on information provided by TerraPower in the electronic reading room.
Information and Other Material Necessary for the Regulatory Audit:
The NRC staff requests TerraPower to make available the information or subject matter experts necessary to respond to the enclosed audit inquiries. These inquiries primarily fall into the following general categories:
Additional detail on the assumptions used in the TR; Additional detail on the scope and requirements for the methodologies discussed in the TR; Additional detail on the various codes used by the methodologies discussed in the TR; and General clarification of various subjects included in the TR.
Team Assignments:
Reed Anzalone Senior Nuclear Engineer, Natrium Technical Lead Stephanie Devlin-Gill Senior Project Manager, Audit Manager Zachary Gran Reactor Scientist Michelle Hart Senior Reactor Engineer Alec Neller Reactor Systems Engineer, Audit Lead Logistics:
Entrance Meeting 4:00 PM ET, July 23, 2024 Exit Meeting September 30, 2024 Audit meetings will take place in a virtual format, using Microsoft Teams or another similar platform. Audit meetings will be scheduled on an as-needed basis after the entrance meeting and once the NRC staff has had the opportunity to review any documents placed in the online reference portal. The audit will begin on July 23, 2024, and continue as necessary, with activities occurring intermittently during the audit period. The audit period may be reduced or
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION extended, depending on the progress made by the NRC staff and TerraPower in addressing audit questions.
Deliverables:
At the completion of the audit, the audit team will issue an audit summary within 90 days after the exit meeting but will strive for a shorter duration. The audit summary will be declared and entered as an official agency record in ADAMS and be made available for public viewing through the publicly available records component of ADAMS.
If you have questions about this audit, please contact me at 301-415-5301 or via email at Stephanie.Devlin-Gill@nrc.gov.
Sincerely,
/RA/
Stephanie Devlin-Gill, Senior Project Manager Advanced Reactors Licensing Branch 1 Division of Advanced Reactors and Non-Power Production and Utilization Facilities Office of Nuclear Reactor Regulation Project No.: 99902100
Enclosure:
As stated cc: TerraPower Natrium via GovDelivery
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Audit Questions Related to TP-LIC-RPT-0007 Revision 0, Design Basis Accident Methodology for Events with Radiological Release
- 1. Topical Report (TR) section 2.4, "[Design Basis Accident (DBA)] Event Selection, indicates that DBAs are considered for analysis using the DBA with radiological release methodology based on whether they have descriptions indicating they involve a potential release of radioactive material. The NRC staff requests that TerraPower, LLC (TerraPower) clarify the process used for determining whether DBAs have the potential for radionuclide releases.
- 2. TR table 2-1, Natrium DBAs with Radioactive Material Release, includes three DBAs that do not appear to be addressed by any of the evaluation models (EMs) discussed in the TR:
((
)), Excessive Sodium-Water Reaction in the [Pool Immersion Cell (PIC)] (DBA);
((
)), Loss of [Ex-Vessel Storage Tank (EVST)] Cooing While Storing Fuel Assembly (DBA); and
((
)), [Gaseous Radiological Waste Processing System (RWG)]
leak from the holdup tank and is released in the [Fuel Handling Building (FHB)]
(DBA).
The NRC staff requests additional information from TerraPower on how these DBAs with release were modeled.
- 3. The NRC staff requests that TerraPower confirm the list of DBAs provided in TR table 2-1 is comprehensive and completely captures all DBAs considered for the Natrium design. For example, the US SFR Owner, LLC construction permit application and corresponding preliminary safety assessment report ML24088A065) includes RRS-SPLA-CN, [Primary Sodium Processing System (SPS-P)] Leak in the [Reactor Auxiliary Building (RAB)], in table 3.5-4, Summary of DBAs, which is not included in this TR.
- 4. Assumption 3.2, discussed in TR chapter 3, Assumptions Requiring Verification, states that the range of phenomena involved in the Natrium DBAs with release is assumed to be restricted to the initiating and early transition phases of accident progression. This is also referenced in TR section 5.1.2, Assumptions, as the reason why ((
)) is not considered. The NRC staff requests that TerraPower clarify Assumption 3.2 and its basis. Specifically, the NRC staff is requesting to understand how the early transition phase is defined and how the Natrium design features, and the selected events provide a basis for restricting the range of phenomena to only the initiating and early transition phases of accident progression.
- 5. Assumption 3.4 refers to system leakage scenarios. The NRC staff requests that TerraPower clarify what it means by this phrase and discuss why it is appropriate to assume leakage is not a part of, or consequence of, a different event.
- 6. Assumption 3.5 states that ((
)) The NRC staff
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION requests that TerraPower clarify how ((
)).
- 7. Assumption 3.6 states that ((
The NRC staff requests that TerraPower clarify whether this assumption also applies ((
))
- 8. Assumption 3.9 states that the partial flow blockage analysis assumes maximum assembly power, which minimizes local peaking. Considering that flow blockage is a highly localized phenomenon, The NRC staff requests that TerraPower justify why it is appropriate to maximize assembly power instead of local peaking.
- 9. Assumption 3.10 states that misloads can be analyzed ((
)) The NRC staff requests that TerraPower provide additional clarification on the meaning of this assumption.
- 10. Assumption 3.14 states that [t]he In-Vessel Transients with Radiological Release methodology assumes that only Type 1 fuel is used. The NRC staff requests that TerraPower clarify if this methodology will be updated prior to the use of Type 1B in lead test assemblies as discussed in the Natrium Topical Report: Fuel and Control Assembly Qualification (ML23025A409)?
- 11. TR table 4-1, Figures of Merit for In-Vessel DBAs, lists figures of merit for in-vessel DBAs without release, referenced from TerraPowers TR titled Design Basis Accident Methodology for In-Vessel Events without Radiological Release (ML23272A260). The NRC staff requests that TerraPower clarify if the figures of merit in the cited TR are revised, will this TR be revised.
- 12. TR table 4-1 provides a brief overview of the time-at-temperature no failure (TATNF) criterion, which is an integral part of TerraPowers safety analysis. The NRC staff requests that TerraPower provide documentation that details how the TATNF criterion is determined.
- 13. The NRC staff requests that TerraPower provide clarification on Figure 4.1-1, EM Calculational Devices and Analysis Workflow. Specifically, the NRC staff requests that TerraPower clarify how the various EMs and codes are ((
)).
- 14. Multiple sections of the TR discuss the use TerraPowers Acquired Software Quality Assurance (QA) Plan under Safety Analysis and Risk. TR section 4.2, Evaluation Model Development, states that this plan provides a process framework supporting the QA requirements for software that are used for analyses in the Natrium plan. The NRC staff requests that TerraPower provide this document for the NRC staff audit review.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION
- 15. TR section 4.4, Evaluation Model Assessment, states that an assessment matrix is created for each methodology based on the [phenomena identification and ranking table (PIRT)] results The NRC staff requests that TerraPower clarify what it means by each methodology and if assessment matrices were created for fuel misloads, fuel handling accidents (FHAs), and sodium leaks.
- 16. The NRC staff does not fully understand how ((
)) will be employed for the in-vessel transient with radiological release EM as described in TR section 5.1 In-vessel Transients with Radiological Release Methodology. TR section 5.1.1, Purpose and Scope, describes how ((
)) As shown in Figure 5.1-1, DSAW Data Flow,
((
)) This section also notes that, if necessary ((
)) that integrates severe accident modules is planned to be employed. This seems to contrast with TR section 5.1.4, EM Description, which states that ((
)) The NRC staff requests that TerraPower clarify:
how ((
)) will be used in transient analyses.
the DSAW process for transient analyses and how it interfaces with ((
))
The TR also discusses ((
)) Has the determination been made to use ((
))
how ((
)) Additionally, the NRC staff notes that ((
))
TR section 5.1.4 states that ((
))
TR section 5.1.4 discusses the use of ((
))
- 17. The NRC staff requests that TerraPower provide documentation, such as user and technical manuals, on Mongoose++, DSAW, ((
)).
))
analyze several unprotected events (ULOF, ULOHS [Unprotected Loss of Heat Sink],
UTOP [Unprotected Transient Over Power], etc.) that potentially involve fuel failure. The NRC staff requests that TerraPower provide these analyses for staff audit.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION
- 19. Section 5.1.6 of the TR notes that some DBAs that lead to potential fuel failures are associated with asymmetric boundary conditions at the inlet of the core but then states that these multidimensional effects cannot be assessed with ((
)) TerraPower proposes to identify and address non-conservatisms associated with this issue using a ((
)) The NRC staff requests that TerraPower clarify why that does not appear to be developed and why TerraPower has not included it in the TR.
The NRC staff requests that TerraPower provide additional information regarding the development of the ((
))
If asymmetric transients have the potential of being analyzed non-conservatively using ((
)) the NRC staff requests that TerraPower clarify how TerraPower will maintain reasonable assurance that analyses performed in support of the licensing applications refencing this TR methodology are adequate.
)) Given that
((
)), the NRC staff requests that TerraPower clarify the nature of ((
))
- 21. ((
)) The NRC staff requests that TerraPower clarify ((
))
- 22. TR section 5.3.1, Purpose and Scope, states that the purpose of the fuel misload methodology includes analyzing the consequences of fuel loaded in the right location but with the wrong orientation; however, this was not discussed further in section 5.3, Fuel Misload Methodology. The NRC staff requests that TerraPower clarify what analyses have been done for fuel misloads where fuel assemblies were in the right location but had the wrong orientation.
- 23. Core misload analyses are typically performed to identify the impacts of the worst fuel misload that is not detectable by instrumentation or startup testing. The NRC staff requests that TerraPower clarify if the misloads discussed in this TR are considered to be detectable. If so, the NRC staff requests that TerraPower provide a brief overview of the methods that would be used to identify misloads during fuel loading, startup, or operation. If not, the NRC staff requests that TerraPower provide further discussion on the conditions the misloaded fuel is assumed to experience in the analysis.
- 24. The NRC staff requests that TerraPower clarify if the fuel misload analysis methodology considers the potential for fuel assemblies to not be fully seated.
- 25. TR section 5.3.4, EM Description, describes the fuel misload methodology used for the Natrium reactor. It states that ten fuel misload cases were analyzed, ((
)) The NRC staff requests that TerraPower provide documentation on the referenced fuel misload analyses to assist the NRC staff in better understanding the application of the fuel misload EM. Ensure that the
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION referenced documentation is clear on the acceptance criteria that were used for the analyses.
- 26. TR section 5.4.3, Acceptance criteria, lists two acceptance criteria for FHAs: dropped fuel assemblies must not result in fuel cladding mechanical failure and must not create unacceptable core conditions that impact safe reactor operations, such as local criticality or reduced flow. TR section 5.4, Fuel Handling Accident Methodology, focuses on analyzing mechanical failure for fuel assemblies without any further discussion on the potential for unacceptable core conditions. The NRC staff requests that TerraPower clarify what analyses have been performed for FHAs related to the creation of unacceptable core conditions.
))
The NRC staff requests that TerraPower clarify if TerraPower has verified that
((
)) is able to model these.
- 29. Section 5.4.5, EM Description, states that some experimentally determined factors, such as the impact damping coefficient are inputs to the finite element model of Natriums fuel assembly. The NRC staff requests that TerraPower clarify what experimental factors are inputs to this model and what experiments have been done to determine these factors.
- 30. Section 5.4.5 discusses a finite element model of the Natrium fuel assembly which has been used for preliminary analysis of FHAs. The NRC staff requests that TerraPower provide any existing documentation available on this model for NRC staff audit review.
- 31. The NRC staff requests that TerraPower clarify what role, if any, ((
)) will play in the FHA EM. The NRC staff notes that it is included as a suitable software in table 4-2, Representative Events with Potential Fuel Failure and Radiological Release, and in TR section 6.1, Summary of Codes Selected. This appears to contrast with footnote 5, which states that ((
))
- 32. TR section 5.5, Sodium Liquid and Gas Leak Methodology, states that the analysis of sodium and gas leaks includes the extent of leaks and releases based on the event initiation - the location, timing, system conditions, and propagation. The NRC staff notes that this characterization of a potential leak is not discussed in the TR. The NRC staff requests that TerraPower clarify what work has been done on this topic.
- 33. TR section 5.5.3, EM Scope and Requirements, states that the EM established for analyzing sodium and gas leaks should have the capability of modeling important processes and phenomena identified during the PIRT for SPS leaks, shown in table 2-5 in TerraPowers TR, Radiological Source Term Methodology Report, Revision 1 (ML24017A116). The NRC staff requests that TerraPower clarify other processes or phenomena that were considered in the modeling of leaks from the SCG or IHT. The
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION NRC staff notes that the ((
))
TR section 6.2, EM Conservatism Study, documents several conservatisms associated with the in-vessel transients. It then states: An effort is underway to demonstrate that the conservative approach described above is sufficiently conservative for the Natrium design. Ongoing work is planned to be complete prior to TerraPower's submittal of an operating license application, and that information will be included in a future licensing submittal. The NRC staff requests that TerraPower make the results of this effort to date available for the NRC staff audit and discuss in further detail why the conservatisms listed are sufficient for the analyses performed using this TR.
- 34. Appendix A, TATNF and Related Analyses, brings together several different concepts, and it is not clear how these concepts are related to the TATNF criterion, each other, or the main body of the TR. The NRC staff requests that TerraPower provide clarification on the purpose of appendix A and its relationship to the DBA with radiological release EM.
- 35. The NRC staff requests that TerraPower provide additional information on how the legacy data in appendix B, Initial Experimental Database for Fuel Performance and Radiological Release/Transport Methodology, is planned for use in assessing DBAs with release and to which EMs this assessment matrix relates.