ML24255A017
| ML24255A017 | |
| Person / Time | |
|---|---|
| Site: | 99902100 |
| Issue date: | 11/27/2024 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| EPID L-2023-TOP-0050 | |
| Download: ML24255A017 (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
SUMMARY
REPORT, TOPICAL REPORT TP-LIC-RPT-0004 DESIGN BASIS ACCIDENT METHODOLOGY FOR IN-VESSEL EVENTS WITHOUT RADIOLOGICAL RELEASE, REVISION 0 Applicant:
TerraPower, LLC Applicant Address:
15800 Northup Way, Bellevue, WA 98008 Plant Name:
Natrium Project No.:
99902100
1.0 BACKGROUND
By letter dated September 29, 2023, TerraPower, LLC (TerraPower) submitted topical report (TR) TP-LIC-RPT-0004 Design Basis Accident Methodology for In-Vessel Events without Radiological Release, Revision 0 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23272A260) to the U.S. Nuclear Regulatory Commission (NRC) staff. The TR summarizes the approach taken to satisfy the guidance outlined in Regulatory Guide (RG) 1.203, Transient and Accident Analysis Methods, Revision 0 (ML053500170),
regarding the Evaluation Model Development and Assessment Process (EMDAP) for in-vessel design basis accident (DBA) events without radiological release in the Natrium reactor. On October 31, 2023, 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 (ML23303A165).
TerraPower requested the NRC staffs review and approval of the evaluation model (EM) 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 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).
2.0 AUDIT REGULATORY BASES 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 provided for the prevention of accidents and the mitigation of the consequences of accidents.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 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. 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. 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.
3.0 AUDIT PURPOSE AND OBJECTIVES The purpose of the audit was for the NRC staff to gain a more detailed understanding of TerraPowers EM presented in the TR. A secondary purpose of the audit was to identify any information that would require docketing to support the NRC staffs safety evaluation.
4.0 SCOPE OF THE AUDIT AND AUDIT ACTIVITIES On March 5, 2024, the NRC staff transmitted an audit plan to TerraPower (ML24064A195). The audit was conducted from March 25, 2024, to June 27, 2024, in a virtual format and followed the guidance in the Office of Nuclear Reactor Regulation Office Instruction LIC-111, Regulatory Audits, Revision 1 (ML19226A274). TerraPower responded to the audit plan questions over the course of multiple audit meetings. To support addressing the NRC staff questions, TerraPower made the following documents available in its electronic reading room (ERR):
NAT-2786, Revision 0, Phenomena Identification and Ranking Table (PIRT) for Natrium Licensing Basis Events without Fuel Failure NAT-2807, Revision 0, Fuel Performance Hot Channel Factors for Type 1 Fuel Assemblies NAT-3228, Revision 1, Natrium Fuel Material Properties Handbook NAT-3275, Revision 0, Core Assembly Pressure Drop and Heat Transfer Correlations NAT-4653-P Natrium Core Thermal Hydraulic Methods NAT-5883, Revision 0, SAS Plant Model Description for the Natrium Reactor NAT-6217, Revision 0, SAS Nuclear Island Control System Model Documentation for Natrium PSAR/CPA NAT-6642, Revision 1, SAS4A/SASSYS-1 5.7.1 Software Dedication Technical Evaluation Report NAT-6644, Revision 1 SAS4A/SASSYS-1 5.7.1 Software Dedication Report NAT-6899, Revision 0, In-Vessel DBAs and Non-DBA LBEs [Licensing Basis Events]
without Radiological Release Application Methods NAT-7753, Revision A, Event RPD-SGB-CN: Small Gas Bubble in High Worth Assembly, Power Runback Failure: Response WHC-EP-0117, FFTF Inherent Safety Tests: Results of Cycle 8A Steady-State Reactivity Feedback Measurements
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION In addition, TerraPower provided written summaries of its responses to some of the NRC staff questions in the ERR. By letter dated October 11, 2024, TerraPower submitted a revision to the subject TR, NAT-9390, Design Basis Accident Methodology for In-Vessel Events without Radiological Release, Revision 2 (ML24295A202), which incorporated changes to the TR to address questions discussed during the audit.
Members of the audit team included the NRC staff listed below.
Reed Anzalone, Senior Nuclear Engineer, Audit Lead Roel Brusselmans, Project Manager, Audit Manager Stephanie Devlin-Gill, Senior Project Manager Zach Gran, Reactor Scientist (Severe Accident)
Michelle Hart, Senior Reactor Engineer Alec Neller, Reactor Systems Engineer The participants from TerraPower for this audit were Joo Seok Baek, Patrick Donnelly, Nick Kellenberger, Selim Kuran, Antoine Margeride, Sam Miller, Michal Pawluczyk, Matthew Presson, Richard Schultz, and Hongbin Zhang.
On June 27, 2024, the NRC staff held an audit exit meeting with TerraPower and summarized the audit purpose, activities, and high-level results. The NRC staff did not acquire any documents during the audit.
5.0
SUMMARY
OF OBSERVATIONS As indicated in the NRC staffs audit plan, the audit was focused on specific inquiries pertaining to the content of the TR. The NRC staff audited information through the TerraPower ERR and held discussions with TerraPower staff to understand and resolve questions. The table below replicates the transmitted audit questions and summarizes the resolution of the questions.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question 1
In TR section 2.2, Figures-of-Merit: EMDAP Step 2, fuel centerline temperature is selected as a figure-of-merit. Is there potential for another portion of the fuel to be hotter than the centerline due to fuel composition changes from uranium and zirconium migration? If so, how will this temperature deviation be accounted for?
TerraPower stated that, ((
)). TerraPower stated it would revise the TR to reflect conservatisms used for fuel centerline temperature as a figure-of-merit.
2 TR section 2.3, Systems, Components, Phases, Geometries, Fields, and Processes Modeled: EMDAP Step 3, provides a hierarchical system decomposition of items that need to be modeled in the EM. Are there any transient scenarios which could require the flow of both sodium and argon in a pipe to be modeled?
TerraPower stated it would revise the TR to clarify that no transient scenario required modeling the flow of sodium and argon in a pipe.
3 In section 2.3 of the TR, the sodium-salt heat exchangers are included in the hierarchical decomposition. Given the SAS4A-SASSYS-1 (SAS) code appears unable to model salt in a loop attached to an intermediate loop, provide a description of how the salt side of the heat exchanger will be modeled.
TerraPower stated it would revise the TR to clarify how the sodium-salt heat exchanger is modeled in SAS.
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TR section 2.4, Identification and Ranking of Phenomena and Processes: EMDAP Step 4, discusses the performance of five Phenomena and Identification and ranking Tables (PIRTs), three by external experts and two by internal TerraPower experts. Provide more details regarding the performance of the PIRTs. The two internal PIRT scenarios were not discussed in the TR; therefore, include information regarding what scenarios were considered by the internal panel.
TerraPower stated it would revise the TR to clarify what PIRTs were performed. TerraPower additionally provided NAT-2786, Phenomena Identification and Ranking Table (PIRT) for Natrium Licensing Basis Events without Fuel Failure, on the ERR for the NRC staff to audit.
5 In TR section 2.5, Preliminary Evaluation of Highly-ranked Phenomena, eight highly-ranked phenomena will be quantified using sensitivity studies that envelope the range of interest via input to the EM. Provide additional information on how this will be performed in SAS.
TerraPower stated it would revise the TR to clarify how TerraPower models the eight highly-ranked phenomena discussed in TR section 2.5. TerraPower also stated it would revise the TR to discuss how TerraPower plans on using sensitivity studies for these phenomena.
6 Provide more information on the role computational fluid dynamics (CFD) calculations plays for analysis of in-vessel DBAs without radiological release. Section 3.4.2, Top-down Description of PHT [Primary Heat Transfer System] Loop Flow Dynamics, states that the one-dimensional characterization of the large pool sections needs to be reevaluated via CFD calculations to represent multi-dimensional effects in the flow/temperature distribution. ((
)).
TerraPower stated it would revise the TR to clarify how CFD will be used for this EM. TerraPower also stated that it plans to use CFD codes for scoping calculations to look for distortions in highly-ranked phenomena.
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Section 3.5.1 of the TR discusses the scaled IET, identifying seven highly-ranked phenomena that will be assessed based on data from the IET. The Preliminary Natrium Code Assessment Matrix (table 3-5) ((
)). Clarify which phenomena will be assessed with data from the IET.
TerraPower stated it would revise the TR to clarify what phenomena will be assessed with data from the IET.
8 Based on TR section 3.5.1.1, ((
))?
TerraPower stated it would revise the TR to reflect that
((
)).
9 TR section 3.5.1.2, ((
)).
TerraPower stated it would revise the TR, to remove
((
)).
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((
)). TR section 3.5.1.3, ((
)).
TerraPower stated it would revise the TR to clarify
((
)).
11
((
)).
TerraPower stated that data from ((
)) is no longer under consideration for inclusion in the EM assessment matrix. ((
)). Accordingly, TerraPower stated it would revise the TR.
12 In TR section 3.3, Assessment Base Objectives:
EMDAP Step 5, a complete assessment base is described as having [e]xperimental data from at least one Category 1 IET and the supporting Category 1 SETs deemed necessary are available to support assessment of all the highly-ranked phenomena identified in Element 1. Are any IETs being considered for the
((
)) phenomena related to the RAC system to meet this objective?
TerraPower stated that no IETs are planned for the phenomena related to the RAC system. TerraPower stated it would revise the TR to clarify that all RAC-related phenomena will be validated using data from RAC validation tests. TerraPower also stated it would revise the TR to discuss the specific RAC tests planned.
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((
)). Based on the NRC staff review, it appears that the tests discussed in TR sections 3.5.2, EBR-II Tests: SHRT-17, SHRT-45R, and balance of plant (BOP), 3.5.3, FFTF Tests:
LOFWOS Test #10-12, and 3.5.4, Phenix Tests:
Natural Circulation Tests, ((
)).
TerraPower stated it would revise the TR to clarify how TerraPower intends to use data from ((
)) in the EM assessment matrix, including discussion on scaling considerations.
14 TR section 3.5.4 states that tabular data for the Phenix experiment is available in Reference 18. Reference 18 of the TR is a technical report documenting Toshiba electromagnetic pump development (as referenced in TR section 3.5.7, Toshiba 4S Test Facility Tests). It does not seem to contain any relevant data on the Phenix natural circulation test. Is there a different document this section should be referencing?
TerraPower stated that section 3.5.4, Phenix Tests:
Natural Circulation Tests, contained a typographical error where an incorrect reference was cited.
TerraPower stated it would revise the TR to cite the correct reference.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 15 TR table 3-5 lists the ((
)). According to section 3.3 of the TR, to meet Category 1 requirements, the physical geometry of the experimental facility used to generate data relevant to the Natrium design, including unique features, is assessed considering both the design of the system components and the comparison to experimental facility similarity criteria. How was ((
))?
TerraPower stated it would revise the TR to list ((
)).
16
((
))?
TerraPower stated that data from ((
)) is no longer under consideration for inclusion in the EM assessment matrix. ((
)). Accordingly, TerraPower has stated it would revise the TR.
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 10 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 17 TR section 3.5.10, PCN 37-Pin Bundle Experiments, does not seem to fully capture the two different experimental rigs used (CCTL-CFR and PLANDTL-DHX) which are discussed in TR Reference 37, instead listing the objectives of the CCTL-CFR experiment as objectives of the PLANDTL-DHX experiment.
Additionally, TR table 3-5 states that ((
)). These phenomena do not appear to be related to the objectives of either the CCTL-CFR or PLANDTL-DHX experiments.
- a. How will PLANDTL-DHX data be used to assess the reactivity feedback phenomena listed in table 3-5?
- b. For the reactivity feedback phenomena, how was it determined that the PLANDTL-DHX facility met Category 1 scaling requirements for the Natrium design?
- c. Currently, PLANDTL-DHX is the only listed experiment for the three reactivity feedback phenomena. Are any other experiments or legacy datasets being considered for these phenomena?
- a. TerraPower stated that data from PLANDTL-DHX was incorrectly included in the EM assessment matrix for assessing the highly-ranked phenomena related to ((
)). TerraPower stated it would revise the TR to reflect this change.
- b. This question is no longer relevant as TerraPower stated it will not be using data from PLANDTL-DHX to assess phenomena related to ((
)).
- c. TerraPower stated it would revise the TR to discuss a series of tests performed at the FFTF.
TerraPower also stated it would revise the TR to include this experiment for assessing the phenomena related to ((
)).
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 11 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 18 TR section 3.5.12, UIUC [University of Illinois Urbana-Champaign] Natural Circulation Tests, discusses
((
))?
TerraPower stated it would revise the TR to reflect that data from the UIUC experiment will be ((
)).
19 Provide more information on the functional, performance, and validation requirements of the EM development plan as outlined in TR section 4.1, EM Development Plan: EMDAP Step 10. ((
)).
TerraPower provided NAT-6642 on the ERR for the NRC staff to audit. This document allowed the NRC staff to gain a better understanding of the design requirements TerraPower has chosen for this EM.
NAT-6642 is a revision to NAT-5497 referenced in the original submittal. TerraPower stated it would revise the TR to update this reference.
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((
)).
((
)).
21 Table 4-2, Applicable SAS4A/SASSYS-1 Modules and Phenomenological Models, lists applicable SAS modules and models that will be used for analyzing in-vessel DBAs without radiological release:
- a. Beyond this list, what other modules will be used for analyzing in-vessel DBAs without radiological release (e.g., SSTHRM, TSTHRM, MFUEL)? Based on the NRC staff evaluation of the SAS user manual, table 4-2 appears to be missing key modules required for transient analysis.
- b. How will the ((
)) module be employed?
The NRC staff reviewed ((
)) of the SAS user manual and it appears that the ((
)) module is focused on modeling ((
)).
- a. TerraPower clarified which modules are used in SAS as part of the EM. Table 4-2 only specifies which modules from SAS User Manual table 2.2.1, SAS4A/SASSYS-1 Modules, TerraPower is employing.
- b. TerraPower stated that the ((
)) module is not presently used as part of the EM. TerraPower stated that potential uses of the module are being considered.
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 13 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 22 Provide additional information on how TerraPower intends to model the sodium-salt heat exchanger and the sodium-air heat exchanger. The SAS user manual indicates only four heat exchangers can be modeled in a design and assumes a sodium-water heat exchanger in the intermediate loop.
TerraPower stated it would revise the TR to clarify how the sodium-salt heat exchanger is modeled in SAS.
23 Section 5.4.7, RVACS/RACS Models, of the SAS user manual outlines three different models available for reactor vessel auxiliary cooling and RAC systems.
Which model is TerraPower planning on implementing for analyzing in-vessel DBAs without radiological release?
TerraPower stated it would revise the TR to clarify what RAC model TerraPower is using within SAS.
24 TR sections 4.2.1.2.d, Closure Relations, and 4.3, Closure Models and Conservativisms - EMDAP Step 12, state closure relationships are discussed in the SAS user manual, providing context only on three new correlations being added to SAS. The NRC staff reviewed the user manual and did not find concise discussions on what correlations or closure relationships are being used. Provide additional information on what correlations are being used in SAS for your analysis.
TerraPower stated it would revise the TR to provide an overview of correlations used in SAS. TerraPower also provided NAT-6899 for NRC staff audit in the ERR.
NAT-6899 provided staff with a better understanding of how correlations discussed in section 4.3 are used within the EM.
25 Section 4.3 of the TR discusses three correlations which are being added to SAS for the EM. How will this be implemented in SAS (i.e., what correlations will they be replacing)? Provide context for these changes.
TerraPower stated it would revise the TR to provide a background on why the three new correlations were added to SAS.
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 14 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 26 In TR section 4.3.1.3, Wire-wrapped Pin-Bundle Pressure Drop, ECI((
))?
- a. ECI((
)).
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 15 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 27 Has commercial grade dedication (CGD) been completed for SAS? Section 5.5, Integrated EM - Top-down: Assess Applicability of EM to Simulate System and Global Capability: EMDAP Steps 17 and 18, states both the final summary and conclusion of CGD is documented in the SAS4A/SASSYS-1 Software Dedication Report [52] and [t]he CGD will be performed if a version of the code is adopted for the application.
TerraPower stated that CGD has been completed for SAS code version 5.7.1. TerraPower additionally provided NAT-6644 on the ERR for the NRC staff to audit. The NRC staff confirmed SAS code version 5.7.1 has undergone CGD. Accordingly, TerraPower stated it would revise the TR to provide this clarification.
28 TR section 4.3.2, Conservativisms, Biases, and Hot Channel Factors, discusses applying safety hot channel factors to the temperature rise across the coolant, film, clad, gap, and fuel layers. Will a corrosion layer form on the clad? If so, has TerraPower considered the temperature rise across this layer?
TerraPower stated that ((
)). TerraPower additionally provided NAT-3228 in the ERR for the NRC staff to audit.
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 16 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 29 Provide clarification for the following:
- a. Table 3-5 appears to ((
)).
- b. In TR section 5.2, Closure Relations (Bottom-up: Model Fidelity and Accuracy): EMDAP Step 14, two references to past chapters appear to be misnumbered (i.e., experiments are described in chapter 4 (vice actually being in chapter 3) as well as describing Steps 10 and 12 in chapter 5 (vice these steps actually being detailed in chapter 4).
Top-down: Field Equations/Numeric Solutions Capabilities - EMDAP Step 16, Category 2 is described as [t]he physical properties (e.g., the thermodynamic state and a similar working fluid),
and. Is this the complete definition? The NRC staff notes that descriptions for Categories 1 and 3 are complete sentences.
- a. TerraPower confirmed a typographical error existed in table 3-5. TerraPower stated it would revise the TR to correct this error.
- b. TerraPower confirmed that the two citations discussed in Question 29(b) referenced incorrect chapters. TerraPower stated it would revise the TR to correct these errors.
- c. TerraPower stated it would revise the TR to clarify the definition for Category 2 scaling.
OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 17 OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 6.0 REQUESTS FOR ADDITIONAL INFORMATION RESULTING FROM AUDIT As a result of the audit, the NRC staff did not identify any requests for additional information related to this TR. However, TerraPower revised the subject TR to address questions discussed during the audit.
7.0 OPEN ITEMS AND PROPOSED CLOSURE PATHS There are no open items resulting from this audit.