ML25210A321
| ML25210A321 | |
| Person / Time | |
|---|---|
| Site: | 99902100 |
| Issue date: | 08/26/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| EPID L-2024-TOP-0010 | |
| Download: ML25210A321 (1) | |
Text
OFFICIAL USE ONLY - PROPRIETARY INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION
SUMMARY
REPORT ON THE REGULATORY AUDIT OF TERRAPOWER, LLC TOPICAL REPORT TP-LIC-RPT-0008, "PARTIAL FLOW BLOCKAGE METHODOLOGY," REVISION 1 Applicant:
TerraPower, LLC Applicant Address:
15800 Northup Way, Bellevue, WA 98008 Plant Name:
Natrium Project No.:
99902100
1.0 BACKGROUND
By letter dated March 26, 2024, TerraPower, LLC (TerraPower) submitted topical report (TR)
TP-LIC-RPT-0008, Partial Flow Blockage Methodology," Revision 1 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24085A822) 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 partial flow blockage events in the Natrium reactor. 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 (ML24107B049).
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) 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.
Paragraph 50.43(e) of 10 CFR states that applicable applications that propose reactor designs that differ significantly from light-water reactor designs licensed before 1997, or use simplified, inherent, passive, or other innovative means to accomplish their safety functions will be approved only if the performance of each safety feature of the design has been demonstrated by and the interdependent effects among the safety features of
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION the design are acceptable as demonstrated by analysis, appropriate test programs, experience, or a combination thereof. In addition, sufficient data should exist 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 will require docketing to support the NRC staffs safety evaluation.
4.0 SCOPE OF THE AUDIT AND AUDIT ACTIVITIES On July 15, 2024, the NRC staff transmitted an audit plan to TerraPower (ML24197A184). The audit was conducted from July 25, 2024, to April 10, 2025, in a virtual format and followed guidance in the Office of Nuclear Reactor Regulations Office Instruction LIC-111, Regulatory Audits, Revision 1 (ML19226A274). TerraPower responded to the audit plan questions over the course of multiple meetings. To support addressing the NRC staff questions, TerraPower made the following documents available in its electronic reading room (ERR):
Hanus, et al., 1977, Local Sodium Boiling in a Partially Blocked Simulated [Liquid Metal Fast Breeder Reactor] Subassembly (Thors Bundle 3B), ORNL/TM-5862, Oak Ridge National Laboratory, June 1977.
NAT-6681, TerraPower, LLC, Natrium Partial Flow Blockage Analysis, Revision 2.
NAT-7767, TerraPower, LLC, Mongoose++ Theory Manual, Revision 1.
NAT-14450, TerraPower, LLC, Supporting Neutronics Calculations for Partial Flow Blockage Method, Revision 0.
NAT-10620, TerraPower, LLC, Benchmark Analysis for the Validation of Partial Flow Blockage Methodology, Revision 0.
NATD-ENG-RPT-0006, TerraPower, LLC, Phenomena Identification and Ranking Table (PIRT) Report for Natrium Partial Flow Blockage within a Subassembly Evaluation Model, Revision 0.
Tang, et al., Thermal Analysis of Liquid Metal Fast Breeder Reactors, American Nuclear Society, 1978.
Van Erp, et. al., An Evaluation of Pin-to-Pin Failure Propagation Due to Fission Gas Release in Fuel Subassemblies of Liquid-Metal-Cooled Fast Breeder Reactors, Nuclear Engineering and Design, Volume 31, Issue 1, Pages 128-149, November 1974.
In addition, TerraPower provided written summaries of its responses to some of the NRC staff questions in the ERR. By letter dated May 9, 2025, TerraPower submitted a revision to the subject TR, NAT-9395, Partial Flow Blockage Methodology, Revision 0 (ML25129A064), 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, Natrium Technical Lead Stephanie Devlin-Gill, Senior Project Manager, Audit Project Manager Walter Williams Reactor Systems Engineer, Audit Technical Lead
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION The participants from TerraPower for this audit were Cole Blakely, Patrick Donnelly, Nick Garstka, Tim Guidotti, Sterling Harper, Moonkyu Hwang, Mike Jarrett, Brian Johnson, Jaehyok Lim, Hugh Luo, Nick Kellenberger, Sam Miller, Matthew Presson, Mine Yilmaz, SuJong Yoon, and Hongbin Zhang.
On April 10, 2025, 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.
During the audit exit meeting, the NRC staff informed TerraPower that the staff intends to impose limitations and conditions in the SE. The limitations and conditions must be addressed by any licensee or applicant referencing the TR. The staff summarized that the limitations and conditions will relate to the design of the reactor core and associated operating conditions; and that assumptions made in the TR remain valid, particularly ((
)).
OFFICIAL USE ONLY - PROPRIETARY INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question 1
Pertaining to the entire TR:
a) Provide additional information, and justification, on
((
)).
b) Provide justification for ((
)).
c) Provide clarification regarding the use of Computational Fluid Dynamics (CFD).
d) Provide clarification, at a high level, how the semi-empirical model is implemented in Mongoose++.
1a) TerraPower stated it would revise the TR to
((
)).
2 TR section 1,
Introduction:
Provide additional detail regarding licensing basis event frequencies cited to support partial flow blockage analyses (e.g., is the accumulation of debris used to determine licensing basis event frequency specific to wire wrap degradation or does it include other blockage initiating events).
TerraPower stated it would revise the TR to remove the dependency on event frequency for this EM.
TerraPower clarified that partial flow blockage events are ((
)).
3 TR section 3.1, Assumptions, assumption 3.1: The partial flow blockage occurrence is assumed to be
((
)).
TerraPower stated it would revise TR assumption 3.1 to clarify that ((
)).
4 TR section 3.1, assumption 3.3:
a) Provide additional detail and justification for assuming
((
4a) The TR revision discussed above under audit question 1a, ((
)),
addresses this question. The revised text sufficiently describes the ((
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question
)).
b) Provide additional explanation to justify the use of
((
)).
)).
4b) TerraPower stated it would revise assumption 3.3 to address this item.
5 TR section 3.1, assumption 3.6: The ((
)). Provide additional justification for assumption 3.6.
TerraPower stated it would revise TR section 5.3.2.2, "Supporting Reactivity Feedback Calculations, to discuss the ((
)).
6 TR section 3.1, assumption 3.9: Provide additional information on ((
)).
TerraPower stated it would revise the TR to clarify that
((
)).
7 TR section 3.1, assumption 3.12:
a) ((
)).
7a) The TR revision discussed above under audit question 1a addresses this item, as ((
)).
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question b) ((
)).
7b) TerraPower stated it would revise TR assumption 3.12 to provide the requested additional justification.
8 TR section 3.1, assumption 3.13: The ((
)).
TerraPower stated it would revise the TR to address this question (also see related audit item 2 above).
9 TR section 3.1, assumption 3.14: The ((
)).
TerraPower stated it would revise the TR to clarify that
((
)) is outside the scope of this EM.
TerraPower also stated it would revise the TR to remove the reliance on ((
)).
10 TR section 5.1.1, EMDAP Step 1: Specify Analysis Purpose, Transient Class, and Power Plant Class: The TerraPower stated it would revise TR section 5.1.1.2 Transient Class, to clarify that ((
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question NRC staff seeks clarification as to whether the ((
)).
)) are considered in the scope of the TR as
((
)).
11 TR section 5.1.1: Table 5-1, Local Fault Events due to Partial Flow Blockage, contains multiple entries that state: ((
)). This does not appear to be a complete sentence; please clarify.
TerraPower stated it would revise the TR to resolve this typographical error.
12 TR section 5.1.2, EMDAP Step 2: Specify Figures of Merit: The TR puts forward fuel temperature as a figure of merit (FOM) for the partial flow blockage analysis.
a) Fuel-cladding-chemical-interaction (FCCI) and constituent redistribution will result in local solidus temperatures in the fuel system that are significantly less than equilibrium U-Zr solidus lines. The NRC staff requests TerraPower clarify the lower solidus temperatures considered, particularly in the TATNF analysis, in addition to cladding wastage and creep. In table 5-3, "Figures of Merit, the ((
)).
Additionally, the NRC staff notes that during the audit on TP-LIC-RPT- 0004, "Design Basis Accident Methodology for In-Vessel Events without Radiological Release," Revision 0 (ML24064A195) TerraPower proposed changes clarifying that the ((
)). Please discuss the discrepancy and provide an appropriate justification.
b) Clarify if the fuel temperature is still considered as a FOM in the methodology since it is only briefly cited in the modeling or sample analyses provided in the TR.
12a) TerraPower clarified that the TATNF criteria is used to screen for ((
)).
12b) TerraPower stated it would revise the TR to clarify that fuel centerline temperature is a FOM.
13 TR section 5.1.2, table 5-3: Fuel, cladding, and coolant temperature are listed as FOMs without discussion on how these will be quantified (e.g., peak temperatures, TerraPower stated it would revise the TR to include concise definitions for the FOMs in TR table 5-3.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question inner, mid-wall, outer cladding temperature, fuel centerline temperature, peak coolant, average coolant).
While these may be inferred throughout the TR, provide a concise definition of each FOM for clarity to ensure they capture appropriate phenomena.
14 TR section 5.1.3, EMDAP Step 3: Identify Systems, Components, Phases, Geometries, Fields, and Processes that Must be Modeled: The ingredients found within select EM characteristics (i.e., constituents, phases, geometrical configurations, fields, and transport process) become fully coolant focused (i.e., cladding and fuel are removed from the scope). Fuel and cladding are included again in section 5.3.2, EMDAP Step 11:
Establish EM Structure. Clarify that all modeled ingredients are consistent and propagate throughout the TR, or if not, why some are excluded.
TerraPower stated it would revise the TR to clarify that fuel and cladding temperatures are considered and modeled throughout the EM.
15 TR section 5.1.4, EMDAP Step 4: Identify and Rank Phenomena and Processes: Provide the PIRT report for audit review.
TerraPower provided the staff access to the PIRT report during the audit.
16 TR section 5.1.4, table 5-7, PIRT Rankings with Rationales for Partial Flow Blockage within a Fuel Assembly: ((
TerraPower stated it would revise the TR to clarify that
((
)) is not a credible mechanism for a partial flow blockage.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question
)).
17 TR section 5.1.4, table 5-7: Provide additional discussion regarding ((
)).
TerraPower stated it would revise the TR to clarify that
((
)).
18 TR section 5.2, "EMDAP Element 2: Develop Assessment Base, ((
].
TerraPower stated it would revise the TR to clarify why
((
)).
19 TR section 5.2.2, EMDAP Step 6: Perform Scaling Analysis and Identify Similarity Criteria: Discuss why surrogate FOMs were developed for cladding temperature but not fuel temperature.
TerraPower stated it would revise the TR to include a description and reasoning for surrogate FOMs in TR section 5.2.2.2 Scaling Analysis Scope and Overview.
20 TR section 5.2.2: To ensure adequate independence of validation data, please clarify whether or not the ORNL or Power Reactor and Nuclear Fuel Development Corporation data were used in the past to inform or develop Mongoose++.
TerraPower clarified that no validation data was used in the development of Mongoose++.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question 21 TR section 5.2.2.4.1, Consistent Geometry with Viscous and Conduction Terms: Provide confirmation regarding the assumption of variables being held constant in the nondimensional Naiver-Stokes equations, such as
((
)).
TerraPower clarified the validity of holding these variables constant in the nondimensional Naiver-Stokes equations.
22 TR section 5.2.3.2, Phenomena Validation Matrix for Existing Data: ((
))?
TerraPower stated it would revise the TR to clarify that
((
)) are not credible and therefore does not need to perform such an assessment.
23 TR section 5.3.1, EMDAP Step 10: Establish an EM Development Plan: Provide access to the Mongoose++
theory manual and associated sensitivity studies.
TerraPower provided access to the Mongoose++ theory manual and associated sensitivity studies for audit review.
24 TR section 5.3.1.2.2, Evaluation Matrix: Table 5-21, Evaluation of Code Ability to Model Phenomena within PIRT for Partial Flow Blockage, indicates that
((
))?
TerraPower provided information on testing that was completed to validate Mongoose++.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question 25 TR section 5.3.1.2.4, Mongoose++ Selection: Provide reference 22, Tang, Y. S., Coffield, R. D. and Markley, R. A., Thermal Analysis of Liquid Metal Fast Breeder Reactors, 1978. Provide additional justification for why there are minimal impacts from ((
)).
TerraPower provided the staff with the reference during the audit. The reference supported the assertion made in the TR.
26 TR section 5.4.1.1.1.3.2, Axial Turbulent Momentum Mixing: This section ((
)).
TerraPower confirmed the value in question was appropriate and clarified that ((
)).
27 TR section 5.4.4, EMDAP Step 16: Determine Capability of Field Equations to Represent Processes and Phenomena and the Ability of Numeric Solutions to Approximate Equation Set: Regarding the source for values in table 5-34, Fuel Properties for Evaluation of Numerical Techniques of Mongoose++ for Partial Flow Blockage, ((
))?
TerraPower clarified how the fuel properties listed in TR table 5-34 are used in the EM.
28 TR section 5.4.4: ((
TerraPower stated that it would revise the TR to clarify how ((
)) are evaluated in the EM, including that ((
)).
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Number Question Resolution of Question
)).
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY 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.