ML25210A206

P Enclosuterrapower, Llc. - Final Safety Evaluation of Topical Report Nat-9395, Partial Flow Blockage Methodology, Revision 0

ML25210A206
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Issue date:	08/07/2025
From:	Office of Nuclear Material Safety and Safeguards
To:	TerraPower
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EPID L-2024-TOP-0010
Download: ML25210A206 (1)

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OFFICIAL USE ONLY - PROPRIETARY INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION TERRAPOWER, LLC. - FINAL SAFETY EVALUATION OF TOPICAL REPORT NAT-9395, "PARTIAL FLOW BLOCKAGE METHODOLOGY," REVISION 0 (EPID NO. L-2024-TOP-0010)

SPONSOR AND SUBMITTAL INFORMATION Sponsor:

TerraPower, LLC (TerraPower)

Sponsor Address:

15800 Northup Way, Bellevue, WA 98008 Project No.:

99902100 Submittal Date:

March 26, 2024 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.:

ML24085A822, ML25129A064 Brief Description of the Topical Report: On March 26, 2024, TerraPower submitted the topical report (TR) TP-LIC-RPT-0008, Partial Flow Blockage Methodology," Revision 1 to the U.S. Nuclear Regulatory Commission (NRC) staff (the staff). The TR provides an overview and description of evaluation models (EMs) developed to evaluate partial flow blockage events within the Natrium sodium fast reactor (SFR). On April 22, 2024, the staff found that the material presented in the TR provides technical information in sufficient detail to enable the staff to conduct a detailed technical review of (ML24107B049). The staff conducted a regulatory audit (ML24197A184, ML25210A318) on the TR from July 25, 2024, to April 10, 2025. TerraPower submitted a revision of the TR, which was renumbered from TP-LIC-RPT-0008 to NAT-9395, Revision 0, to clarify portions of the TR as discussed during the audit. NAT-9395, Revision 0, summarizes the partial flow EM methodology in the context of the Regulatory Guide (RG) 1.203, Transient and Accident Analysis Methods, (ML053500170) Evaluation Model Development and Assessment Process (EMDAP) process.

REGULATORY EVALUATION Regulatory Basis The regulations that are applicable to the review of this TR are:

Title 10 of the Code of Federal Regulations (10 CFR) section 50.34(a)(4) and 10 CFR 50.34(b)(4), which requires certain information to be submitted by applicants for construction permits and operating licenses, respectively. These sections require, in part, 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 the operation of the facility and including the determination of the margins of safety during normal operations and transient conditions anticipated during the life of the facility, and the adequacy of the SSCs provided for the prevention of accidents and the mitigation of the consequences of accidents.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Regulation 10 CFR 50.43(e), which requires that reactor designs that differ significantly from light-water reactor designs licensed before 1997, or that use simplified, inherent, passive or other innovative means to accomplish their safety functions have an appropriate demonstration of their safety features. Sections 50.43(e)(1)(i) and (ii) require a demonstration of safety feature performance and interdependent effects through analysis, appropriate test programs, experience, or a combination thereof. Section 50.43(e)(1)(iii) requires that sufficient data exist regarding the safety features of the design to assess the analytical tools for safety analyses over a sufficient range of plant conditions, including certain accident sequences.

Guidance Documents RG 1.203, Transient and Accident Analysis Methods, provides the EMDAP as an acceptable framework for developing and assessing EMs for reactor transient and accident analyses. RG 1.203 outlines the four elements of an EMDAP, which is broken into 20 component steps. In the subject TR, TerraPower describes the EM for partial flow blockages in the Natrium reactor and the assessments that have been or will be performed in the context of the EMDAP steps.

For background, the Kemmerer Power Station Unit 1 (KU1) construction permit application (CPA) (ML24088A059)1 was submitted by TerraPower on behalf of US SFR Owner, LLC, for a Natrium reactor following the process outlined in Nuclear Energy Institute (NEI) 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development (ML19241A472), as endorsed by the NRC 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 (ML20091L698). This guidance defines risk-informed, performance-based, and technology-inclusive processes for the selection of licensing basis events (LBEs); safety classification of SSCs; and the determination of defense-in-depth adequacy for non-light-water reactors. NEI 18-04 provides a frequency-consequence target curve that is used to assess events, SSCs, and programmatic controls. LBEs are categorized by the frequency of occurrence, separated into anticipated operational occurrences, design-basis events (DBEs), and beyond-design-basis events. DBAs are derived from DBEs by prescriptively assuming that only safety-related (SR) SSCs are available to mitigate postulated event sequence consequences to within the 10 CFR 50.34, Contents of applications; technical information dose limits, using conservative assumptions. The purpose of the subject TR is to develop an EM that supports a conservative analysis for the evaluation of DBA events, as defined in NEI 18-04, which includes a partial flow blockage.

1 TerraPower, on behalf of US SFR Owner, LLC, a wholly owned subsidiary of TerraPower, submitted the CPA for KU1 on March 28, 2024 (ML24088A059). The NRC staffs review of that CPA is ongoing. The staff is not making any determinations on the acceptability of the Natrium reactor design in this safety evaluation (SE). The description of the Natrium reactor in this SE is based on the description in the TR (NAT-9395, Revision 0).

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION TECHNICAL EVALUATION INTRODUCTION TerraPower requested that the NRC staff review the proposed methodology as an appropriate and adequate means for future applicants using the Natrium design (as described in the TR) to evaluate DBA events involving partial flow blockages. As described in the TR chapter 1, Introduction and section 5.1.3, EMDAP Step 3: Identify Systems, Components, Phases, Geometries, Fields, and Processes that Must be Modeled, the EM evaluates partial flow blockages within the Natrium core. The core contains metallic uranium-zirconium (U-Zr) fuel slugs clad in HT9, a martensitic stainless-steel alloy, with an internal sodium bond. The fuel pins are wrapped in HT9 wire to provide stable lateral pin-to-pin and pin-to-duct spacing, forming a hexagonal assembly of fuel rods. Each of the hexagonal fuel assemblies is surrounded by a hexagonal duct tube, with an inlet nozzle at the bottom and handling socket at the top. These fuel assemblies are inserted into a liquid sodium-cooled reactor core.

As discussed in TR chapter 1, Introduction, and 2, Purpose and Scope, the purpose of the TR is to outline the plan for EM development such that the EM can verify that fuel integrity would be maintained in partial flow blockages and is capable of informing temperature limits for monitoring fuel elements sufficiently for analysis and mitigation of such a DBA. The TR identifies the partial flow blockage as a critical local fault that has been previously recognized in reactor designs such as the Clinch River Breeder Reactor Project (CRBRP), Power Reactor Innovative Small Module (PRISM), and Sodium Advanced Fast Reactor (SAFR). TR chapter 2 defines credible and bounding blockages. The TR documents the development of the partial flow blockage EM through a detailed, multi-step process in alignment with the EMDAP described in RG 1.203. The TR covers all four elements of the EMDAP, which include: establishing EM capability requirements (Element 1), developing the assessment base (Element 2), creating the desired EM (Element 3), and assessing the EMs adequacy (Element 4).

The TR is comprised of eight chapters of which 4-6 cover EMDAP, and three subpart appendices:

Chapter 1 provides an overview of the Natrium reactor design, safety systems, safety issues related to partial flow blockages, and precedence for partial flow blockage analysis.

Chapter 2 identifies that the purpose of the analysis is to ensure fuel integrity is maintained during a partial flow blockage and that the purpose of the document is to outline an EM for partial flow blockage that complies with RG 1.203.

Chapter 3, Assumptions, defines the assumptions used to define the EMs scope and applicability.

Chapter 4, Partial Flow Blockage Events, discusses how partial flow blockage events are characterized within the EM, providing insights into the parameters and phenomena being modeled. This chapter is critical for understanding how specific blockage scenarios are simulated to assess their potential impacts on reactor safety.

Chapter 5, Evaluation Model Adequacy, discusses the planned and completed activities along with activity alignment with the EMDAP. It includes sections summarizing

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION the development of the EM capability, assessment base, model structure, and adequacy evaluation. These discussions provide a clear view of the EMs progress, including the computer codes selected for the model, development activities, and the ongoing validation efforts.

Chapter 6, Summary, discusses the EM adequacy decision.

Chapter 7, Conclusions and Limitations, identifies limitations and applicability of the EM.

Chapter 8, References, lists the TR references.

Chapter 9, Appendices, provides sample derivations, correction factors for the semi-empirical model, and a sample partial flow blockage analysis.

This SE reviews the TR against the EMDAP in RG 1.203. The section Assumptions, in this SE covers elements of the TR that apply across EMDAP steps. The remaining sections of this SE are delineated into consecutive EMDAP steps for clarity. The staff notes various EMDAP steps are still in development and require additional justification to be applied to future licensing applications. An applicant or licensee referencing the methodology developed in the TR must submit documentation and justify that these steps of the EMDAP have been completed to a state that is appropriate for the intended licensing application. This condition and the relevant EMDAP sections are outlined as Limitation and Condition 1, at the end of this SE.

Relationship to Other TerraPower TRs The Partial Flow Blockage Methodology TR is related to several other TerraPower methodology TRs that collectively provide a strategy for evaluating the consequences of potential accidental radiological releases for the proposed Natrium reactor design. NAT-9394, Design Basis Accident Methodology for Events with Radiological Release, Revision 0, (ML25063A329) provides a discussion of these relationships and includes figure 4.1-1, EM Calculational Devices and Analysis Workflow, which illustrates the connections between EMs.

The partial flow blockage methodology does not identify the LBEs, DBAs, or other quantified event scenarios that result in radiological release for a given reactor licensing application.

Rather, the LBEs, DBAs, and other quantified events appropriate for the licensing application are identified using the licensing modernization project methodology described in NEI 18-04.

The DBAs are then analyzed using one of several methodologies. These methodologies include the partial flow blockage methodology, described in this TR, the DBA with radiological release methodology, described in NAT-9394, which is undergoing review by the NRC staff, and the DBA without radiological release methodology, described in NAT-9390, Design Basis Accident Methodology for In-Vessel Events without Radiological Release, Revision 2, (ML24295A202) and evaluated by the NRC staff in in ML25106A038.

TR section 5.1.2, EMDAP Step 2: Specify Figures of Merit, discusses the figures of merit (FOMs) for the partial flow blockage methodology (e.g., peak cladding temperature, see SE section 2.2). If an analysis for a given partial flow blockage LBE demonstrates these FOMs are not violated, no fuel failure occurs, and analysis is accomplished by this EM. However, if the FOMs are violated, further analysis is needed to determine the extent of fuel failure and potential radiological release. This is performed by DBA with radiological release methodology,

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION which is used to determine the extent of cladding or fuel failure, which are inputs into the source term methodology described in NAT-9392, Radiological Source Term Methodology Report, Revision 0 (ML24261B944), and evaluated by the NRC staff in ML25063A323. The output of the source term methodology is radiological releases to the atmosphere (source terms), which are input to the radiological consequence EMs described in TerraPower report NAT-9391, Radiological Release Consequences Methodology Topical Report, Revision 0, (ML24208A181) and evaluated by the NRC staff in ML25106A262, which is used to determine dose consequences associated with releases.

This TR also references NAT-2806, Natrium Topical Report: Fuel and Control Assembly Qualification, Revision 0, (ML24354A192) and TP-LIC-RPT-0011, Core Nuclear and Thermal Hydraulic Design Report, Revision 0, (ML24088A085) regarding fuel failure phenomena and steady-state core analysis, respectively. TP-LIC-RPT-0011 is under review by the NRC staff as part of the KU1 CPA.

STAFF EVALUATION Assumptions TR section 3.1, Assumptions, discusses the assumptions TerraPower used to define the scope of the TR EMs, determine conservative boundaries, or identify areas where future work is planned. The staff reviewed the assumptions and scope in the TR and determined they are reasonable because they ensure conservatism with respect to blockage type, location, and conditions. The assumptions consider the worst-case credible scenarios, such as ((

)). Additionally, the methodology includes evaluations for ((

)), ensuring a comprehensive analysis of potential partial flow blockage events. It is stated in the TR that ((

)) of the EM. The staff determined that this restriction is acceptable, because the EM is

((

)).

Regarding the types of blockages considered in the TR, design specific information is required to determine applicability of the EM and credibility of blockage types, e.g., ((

)). The staff reviewed TerraPowers determination that a ((

)) bounds all credible partial flow blockage types that may occur in the Natrium reactor under the assumptions listed in TR chapter 3. The staff concluded that for this TR the assumption of a ((

)) bounding all credible partial flow blockage types that may occur in the Natrium reactor is reasonable. As the blockage type and outcomes are design specific and reliant on assumptions, the staff is imposing Limitation and Condition 2, at the end of this SE. Limitation and Condition 2 states that any licensee or applicant citing this TR must adopt a core and fuel design closely resembling that of the Natrium reactor as detailed in this TR. This includes matching core geometry, fuel type, coolant type, flow rates, power ranges, and temperature ranges, and adhering to the specified assumptions for the resulting EM to apply. Any departure from these aspects of the Natrium reactor and fuel design must be justified by illustrating that the departures do not affect the analysis found in this TR.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION EMDAP Element 1: Establish Requirements for EM Capability The first element of the EMDAP is to establish requirements for EM capability including identification of mathematical modeling methods, components, phenomena, physical processes, and parameters needed to evaluate event behavior relative to chosen figures of merit (FOMs).

Element 1 ensures that the EM can appropriately analyze selected events and that the validation process addresses the key phenomena for those events. This element is broken into four steps. TR section 5.1, EMDAP Element 1: Establish Requirements for Model Capability, addresses Element 1 with its steps described in the sections below.

1.1.1 Step 1: Specify Analysis Purpose, Transient Class, and Power Plant Class The first step of EMDAP Element 1 outlines the process of specifying the analysis purpose, transient class, and power plant class for establishing EM requirements and capabilities and maintaining focus throughout the EM. TR section 5.1.1.1, Analysis Purpose, identifies that the purpose of the partial flow blockage analysis is to satisfy pertinent regulatory requirements by confirming that system responses to LBEs with partial flow blockage within a fuel assembly meet all relevant acceptance criteria during normal operating conditions. The types of blockages deemed credible are outlined in TR section 2.1, Types of Blockages, which defines credible blockages as ((

)) and are bounded by the posed blockage type,

((

)).

TR section 5.1.1.2, Transient Class, specifies the transient class as local fuel fault, and lists events, identified to date, that are considered in this EM. These events are summarized below:

((

)).

((

)).

((

)).

TR section 5.1.3, Power Plant Class, identifies the power plant class as a Natrium pool-type SFR. Specifics defining the power plant class are provided in TR sections 5.1.3.1, System, and 5.1.3.2, Subsystems, and are discussed in section 2.3 of this SE. Literature, such as J. T.

Han, Blockages in LMFBR [Liquid Metal Fast Breeder Reactor] Fuel Assemblies-A Review of Experimental and Theoretical Studies, Oak Ridge National Laboratory (ORNL)/TM-5839 (ML063060252), supports that ((

)) subchannel blockages are appropriate, bounding local faults, and that recirculation occurs in the wake region without resulting in sodium boiling for similar reactor designs.

The staff reviewed information in the TR and determined that the analysis purpose, transient class, and power plant class described in the TR meet the guidance provided in Step 1 of RG 1.203 and is therefore acceptable. Specifically, the staff determined that the methodology discussed in Step 1 of the TR will result in analysis of appropriate bounding partial flow blockage events for the Natrium reactor design as described in the TR and that the information in Step 1 of the TR is consistent with information gathered based on other pool-type SFR efforts (e.g., CRBRP, PRISM, SAFR). As discussed above, Limitation and Condition 2 limits TR

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION applicability to the Natrium design as described in the TR, including the operating conditions, unless otherwise justified. The staff also imposed Limitation and Condition 3, which limits TR applicability to flow blockage events bounded by those explicitly identified and analyzed in TR section 2.1 unless otherwise justified.

1.1.2 Step 2: Specify Figures of Merit The second step of the EMDAP involves selecting Figures of Merit (FOMs), which are defined in RG 1.203 as quantitative standards of acceptance that are used to define acceptable answers for a safety analysis. Because the EM covered in this TR seeks to ensure that partial flow blockage events do not result in radiological release, TerraPower focused on FOMs that can be used to ensure that fuel cladding remains intact, and that there are no significant disruptions to the core or primary coolant pressure boundary.

The TR defines FOMs for safety analysis in section 5.1.2, EMDAP Step 2: Specify Figures of Merit, table 5-3, Figures of Merit. The FOMs are identified as fuel temperature, peak cladding temperature (PCT), and coolant temperature. TerraPower selected these FOMs because they are measures of fuel melting and pin failure (fuel temperature and PCT) or could impact the integrity of the fuel pins (coolant temperature). To ensure that cladding does not fail, TerraPower developed acceptance criteria for PCT based on a time-at-temperature approach.

The acceptance criteria for time-at-temperature no-failure (TATNF) for PCT accounts for strain, cladding wastage, and thermal creep. TATNF acts as a screening criterion, determining whether a more detailed analysis is required. TerraPower states that a TATNF screening is triggered when certain conditions are met, which allows for ((

)). This allows for the prediction of ((

)) triggered when the TATNF is exceeded is outside the scope of the TR. TATNF and the ((

)) are discussed in more detail in NAT-9390 and NAT-9394.

The staff reviewed the FOMs and determined that they are appropriate for partial flow blockages occurring without radioactive release as they can be used to assess proximity to fuel failure with an included margin of uncertainty. Because fuel temperature and cladding temperature are thermally coupled, reliance on cladding temperature in various sections of the TR accounts for fuel temperature. Additionally, the most prevalent potential fuel failure phenomenon, ((

)), is best screened for with cladding temperature, rather than fuel centerline temperature. This is captured by TerraPower in the TATNF criteria. As such, the staff determined that TerraPowers approach to EMDAP Step 2 is appropriate and adequately considers relevant fuel performance phenomena and temperature limits.

1.1.3 Step 3: Identify Systems, Components, Phases, Geometries, Fields, and Processes The third step of the EMDAP process is to identify EM characteristics. This is done via hierarchical system decomposition, in which a system is broken down into subsystems, subsystems into modules, etc. Ingredients at each hierarchical level are decomposed into the ingredients of the next level down. By defining the number and type of ingredient at each level, the basic characteristics of the EM can be established.

TR section 5.1.3, EMDAP Step 3: Identify Systems, Components, Phases, Geometries, Fields, and Processes that Must be Modeled, identifies the hierarchical ingredients and defines them

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION for each level discussed in RG 1.203. ((

)).

The NRC staff reviewed Step 3 and determined that the hierarchal levels and ingredients defined in the TR are acceptable because they were derived in a manner consistent with the process described in RG 1.203. The applicability of this step is subject to Limitation and Condition 2.

1.1.4 Step 4: Identify and Rank Key Phenomena and Processes In the fourth step of the EMDAP, key phenomena and processes are identified and ranked with respect to their influence on FOMs. This is accomplished by developing a phenomena identification and ranking table (PIRT). A given scenario is divided-up into characteristic time periods where dominant phenomena and processes remain relatively constant. For each time period, phenomena and processes are identified for each component. The phenomena and processes that the EM should simulate are determined by examining experimental data, expert opinion, and code simulations related to the specific scenario. After identification, the phenomena and processes are ranked by importance determined with respect to their effect on the relevant FOMs.

TR section 5.1.4, EMDAP Step 4: Identify and Rank Phenomena and Processes, discusses how TerraPower proposes to accomplish Step 4 for this EM. TerraPower followed the PIRT nine step procedure as described in NUREG/CR-6944, Next Generation Nuclear Plant Phenomena Identification and Ranking Tables (PIRTs) (ML081140459) and reviewed relevant historic PIRTs. These include PIRTs performed for the TerraPower Traveling Wave Reactor - Prototype (TWR-P) and the Toshiba Super-Safe, Small and Simple (4S) Reactor, as well as the PIRT included in the Initial Important Phenomenon Study on Liquid Metal Reactors. As part of the nine step process, TerraPower completed the following:

1. Defined the issue driving the PIRT: analyzing a partial flow blockage within a Natrium fuel assembly.

2. Defined the PIRT objective: identifying and ranking safety-relevant phenomena and processes to build a technical base for developing the EM.

3. Defined hardware and scenarios: Natrium systems and components, and ((

)), respectively. TR table 5-1, Local Fault Events due to Partial Flow Blockage, describes these scenarios.

4. Defined evaluation criteria: the FOMs described in TR table 5-3.

5. Assessed the current knowledge base by compiling expert input from PIRT panel members.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

6. Identified plausible phenomena: TR table 5-6, Phenomenon Identification and Description for Partial Flow Blockage within a Fuel Assembly, describes the identified phenomena.

7. Developed importance rankings for phenomena by polling panel members.

8. Assessed knowledge level of each phenomenon by polling panel members. TR table 5-7 PIRT Rankings with Rationales for Partial Flow Blockage within a Fuel Assembly, presents both the importance rankings and knowledge levels.

9. Documented results of the PIRT with rankings and rationales as found in TR table 5-7.

TerraPower ranked the identified phenomena against established evaluation criteria, equivalent to their FOMs. TR table 5-7 provides the rationales for importance rankings and knowledge levels for each identified phenomenon.

The NRC staff reviewed TerraPowers PIRT development process and determined that it is acceptable because it follows the guidance in Step 4 of RG 1.203. The staff also determined that the PIRT phenomena are appropriate for a partial flow blockage because they are consistent with the Natrium design and past SFR operating experience. The NRC staff notes that the identified importance rankings and states of knowledge for the phenomena were appropriately developed through expert solicitation and are consistent with staffs technical understanding.

EMDAP Element 2: Develop Assessment Base The second element of EMDAP as discussed in RG 1.203 is to develop an assessment base consistent with requirements determined from Element 1. This assessment base is used to validate calculational devices or codes used by the EM and may consist of a combination of legacy experiments and new experiments. The validation is done under EMDAP Element 4. The database, particularly separate effect tests (SETs), may also be used to develop closure relations to be included in the EM during Step 12 (Element 3).

1.1.5 Step 5: Specify Objectives for Assessment Base In RG 1.203, Step 5 of the EMDAP involves identifying the objectives for the database that will be used to assess the EM and if necessary, develop correlations. This database should include results from integral effect tests (IETs) and SETs. It can optionally include benchmarks with other codes or plant transient data, if available. Additionally, it should include simple test problems to illustrate the fundamental calculational device capacity.

TR section 5.2.1 states that an assessment database is needed to develop correlations for numeric, flow anomalies, and field equations, and to provide an overall assessment of the EM.

The TR identifies that the database should include SETs, IETs, benchmarks with other codes, plant transient data, and simple test problems. The staff determined that TerraPowers objectives for the assessment base are acceptable because these objectives are consistent with Step 5 of RG 1.203 that states SETs and IETs are required for EM assessment and may not be substituted with benchmarks or test problems.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION 1.1.6 Step 6: Perform Scaling Analysis and Identify Similarity Criteria In RG 1.203, Step 6 of the EMDAP ensures that the experimental data and models based on that data will be applicable to the full-scale analysis of plant transients. This requires scaling analyses to demonstrate the relevancy and sufficiency of the collective experimental database for representing behavior expected during postulated transients, and to investigate the scalability of the EM and its component codes for representing important phenomena. This process involves both top-down and bottom-up approaches. A top-down scaling methodology derives non-dimensional groups that govern similitude between facilities, shows that these groups scale the results among experimental facilities, and determines whether the ranges of group values provided by the experiment set encompass the corresponding plant and transient-specific values. The bottom-up scaling analyses address issues related to localized behavior and are used to explain differences among tests in different experimental facilities. These bottom-up approaches help infer expected plant behavior and determine whether experiments provide adequate plant-specific representation.

TR section 5.2.2, EMDAP Step 6: Perform Scaling Analysis and Identify Similarity Criteria, discusses the EMs scaling analysis, which includes both top-down and bottom-up approaches to show scalability and the development of a similarity criterion for the posed blockages and blockage validation data. TR section 5.2.2.1, Scaling Analysis Purpose, states that data from ORNL/TM-5839 and a Power Reactor and Nuclear Fuel Development Corporation (PNC) report (Miyaguchi and Takahashi, 1978)2 are used for validation of the EM. TR section 5.2.2.2, Scaling Analysis Scope and Overview, outlines the scope of scaling analysis and discusses how PCT, the central FOM for scaling analyses, will be used to assess the data and how geometry will affect the analysis as guided by the PIRT. TR section 5.2.2.3, Scaling Analysis Background Information, summarizes supporting information for the scaling analysis, including fundamental units, reference values, nomenclature, geometric differences between assessment data and Natrium, and the effects of varying wire wrap pitch.

TR section 5.2.2.4, Non-dimensional Independent Parameters, provides non-dimensional equations and parameters used in the scaling analysis. This includes scenarios with

((

)). TR section 5.2.2.5, Scaling Distortions, investigates the ((

)). TR tables 5-11, Summary of Independent Variable, and Non-dimensional Parameters for Sets of Non-dimensional Equations, and 5-12, Scaling Distortions between the Natrium Design and ORNL/PNC Data, present the scaling distortions between the Natrium design and the ORNL and PNC data.

The NRC staff determined that TerraPowers approach to EMDAP Step 6 is acceptable because the TR adequately describes a scaling analysis that ensures data and models are applicable to Natrium, demonstrates the relevancy of historical experiments to be used, uses a top-down and bottom-up approach to evaluate global system behavior, derives non-dimensional groups of 2 Miyaguchi, K. and Takahashi, J., Thermal-Hydraulic Experiments with Simulated LMFBR [liquid metal fast breeder reactor] Sub-Assemblies Under Nominal and Non-Nominal Operating Conditions, International Working Group on Fast Reactors Specialists Meeting, PNC, February 1978.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION equations that that govern similitude between test data and Natrium, and identifies and addresses key differences between test data and Natrium.

1.1.7 Step 7: Identify Existing Data and/or Perform IETs and SETs to Complete the Database In RG 1.203, Step 7 of the EMDAP is focused on finalizing the database necessary for assessing the EM. Experiments and data are selected to best address important phenomena identified in Step 4. The process of completing the database includes identifying existing data that fulfills the stated objective in Step 5. If available data is insufficient, additional IETs and SETs should be performed to complete the database. In selecting experiments, a range of tests should be employed to demonstrate that the code is not tuned to a single test. For integral behavior assessment, counterpart tests (similar scenarios and transient conditions) in different experimental facilities at different scales should be selected.

TR section 5.2.3, EMDAP Step 7: Identifying Existing Data and/or Perform IETs and SETs to Complete the Database, identifies available experimental data for benchmarking analysis. This includes a set of 19-pin bundle sodium and water experiments, discussed in the references listed in TR table 5-13, THORS [Thermal-Hydraulic Out-of-Reactor Safety] Facility 19-Pin Flow Blockage Configuration, as well as 37-pin bundle sodium experiments discussed in the report by Miyaguchi and Takahashi (1978). The TR provides an assessment of each experiment and maps them to the moderately or highly ranked phenomena identified in the PIRT from EMDAP Step 4. TR table 5-14, Phenomena Validation for Partial Flow Blockage within a Fuel Assembly, provides the draft database, indicating which experiments are mapped to specific phenomena.

Later steps of the EMDAP focus on the ORNL 3A and 5B bundle tests, as well as the PNC 37-pin bundle tests. The 3A bundle was configured for the Fast Flux Test Facility (FFTF), an SFR operated by the U.S. Department of Energy in the 1980s and 1990s. The 3A bundle consisted of 19 pins, with 6 internal channels blocked. The 19-pin 5B bundle was configured for the CRBRP, with an edge blockage of 14 channels. The PNC 37-pin bundle experiments consisted of a 37-pin assembly with a 24-channel central blockage and 50% edge blockage. All three experiments were sodium cooled and contained electrically heated pins. The staff reviewed publicly available documents on these experiments and determined that TerraPowers inclusion of these experiments in their draft database is acceptable as their design is similar to that of the Natrium fuel assemblies described in the TR.

The TR states that there are sufficient relevant historic tests to validate Mongoose++3 for this EM. However, the TR indicated that some of the identified tests do not have readily available data or may require further clarification. TerraPower notes that it is ensuring the data is available and appropriate prior to an application where this methodology is applied. TR section 5.2.3.1.4 identifies data used for code development and prior benchmarking.

The NRC staff determined that TerraPowers approach to EMDAP Step 7 is acceptable because the experiments discussed in the TR are expected to provide adequate assessment data for the moderately and highly ranked phenomena identified in Step 4. However, as noted by the TR, some of the identified tests currently lack available information. Licensing submittals referencing 3 Mongoose++ is a core thermal hydraulics subchannel analysis code developed by TerraPower for performing core thermal hydraulic analyses for SFRs, including partial flow blockages. The structure of Mongoose++ is discussed further in section 2.4.2 of this SE.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION this TR and using these experiments in the EM database will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.8 Step 8: Evaluate Effects of IET Distortions and SET Scaleup Capability In Step 8 of the EMDAP, the effects of IET distortions and SET scaleup capability are evaluated.

The purpose of this step is to assess distortions in the IET database arising from scaling or atypical initial and boundary conditions and to evaluate effects of distortions in the context of experimental objectives determined in Step 5. The SET scaleup capability is evaluated in correlation with phenomena identified in the PIRT and in conjunction with findings from Step 6 of EMDAP.

TR section 5.2.4, EMDAP Step 8: Evaluate Effects of IET Distortions and SET Capability, states that portions of Step 8 are discussed in TR section 5.2.2, covering EMDAP Step 6. The section further indicates that additional work will be completed in future licensing submittals to assess model fidelity, accuracy, and scalability prior to future licensing submittals.

The staff determined that TerraPowers approach to EMDAP Step 8 is adequate because it aligns with RG 1.203 guidance on evaluating the effects of IET distortions and SET scaleup capability. The staff has not made a determination with respect to TerraPowers execution of EMDAP Step 8 because it has not been performed. As discussed in Limitation and Condition 1, future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed.

1.1.9 Step 9: Determine Experimental Uncertainties Step 9 of the EMDAP involves determining experimental uncertainties for the database. If quantified experimental uncertainties are too large compared to requirements for EM assessment, this particular data set or correlation should be rejected. TR section 5.2.5, EMDAP Step 9: Determine Experimental Uncertainties as Appropriate, establishes the magnitude of the experimental uncertainties for certain experiments identified in Step 7, evaluates their impact on key FOMs, and determines whether the data is suitable for model validation.

For Step 9, TerraPower evaluated experimental data from the ORNL 19-pin sodium test series for bundles 3A (6-channel, central blockage) and 5B (14-channel, edge blockage). The TR notes that these tests predate American Society of Mechanical Engineers Nuclear Quality Assurance (NQA-1) standard. TerraPower generally limited the reported uncertainties to instrument and acquisition system accuracy, without explicitly quantifying experimental distortions. Engineering judgment was used to assess the impact of potential distortions on total uncertainty and to determine whether the reported data was sufficiently reliable for model assessment.

For the ORNL 19-pin sodium tests, TerraPowers uncertainty quantification considered measurement errors associated with temperature, flow distribution, and blockage-induced local heating effects. For both bundles 3A and 5B, TerraPower examined a subset of experimental runs with varying power and flow conditions to establish the maximum experimental uncertainties for ((

)).

TR tables 5-17, Experimental Uncertainty Impact for ORNL FFM [fuel failure mockup]

Bundle 3A with a Six-Subchannel Center Blockage, and 5-19, Experimental Uncertainty Impact for ORNL FFM Bundle 5B with a 14-Subchannel Edge Blockage, provide the resulting

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION impact of the maximum uncertainties on the FOMs. In addition to measurement uncertainties, TerraPower identified potential experimental distortions and considered them as part of the total uncertainty assessment. The TR concluded that the ORNL 3A and 5B test series are acceptable for validating the partial flow blockage EM because the measurement uncertainties for the FOMs are ((

)).

The staff determined that TerraPowers approach to EMDAP Step 9 is acceptable because it presents a plan for quantifying uncertainties, consistent with RG 1.203. The staff reviewed TerraPowers uncertainty assessment for the ORNL test data on bundles 3A and 5B and determined it is acceptable because the experimental uncertainties were assessed and used to estimate effects on the FOMs for the calculation, which were found to be small, and the evaluation of experimental distortions appropriately addressed the most significant differences between the experiment and the prototypical environment. However, the staff notes that only a portion of the identified ORNL data was evaluated in this methodology. As such, future licensing submittals using this TR and relying on the other ORNL or any PNC test data will need to justify that this step of the EMDAP has been appropriately addressed. As discussed in Limitation and Condition 1, future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed for all experiments used.

EMDAP Element 3: Develop EM The third element of the EMDAP involves selecting or developing the calculational devices needed to analyze designated transients or events in accordance with the requirements determined in Element 1. The EM is the calculational framework for evaluating the behavior of a reactor system during a postulated transient or DBA. The EM may include one or more computer programs, special models, and all other information needed to apply the calculational framework to a specific event. This includes:

1. Procedures for treating the input and output information (particularly the code input arising from the plant geometry and the assumed plant state at transient initiation).

2. Specification of those portions of the analysis not included in the computer programs for which alternative approaches are used.

3. All other information needed to specify the calculational procedure.

TR section 5.3, EMDAP Element 3: Develop Evaluation Model, discusses TerraPowers approach to Element 3.

1.1.10 Step 10: Establish EM Development Plan Step 10 of the EMDAP involves creating an EM development plan based on the requirements established in Element 1. Because several potentially applicable computer codes exist, TerraPower focused their efforts on the code selection, which includes identifying requirements for code capabilities, software quality assurance pedigree, code experience, and potential additional usage. TR table 5-20, Summary and Description of Codes for Modeling Subchannel Analysis in SFR, identifies various codes that have subchannel analysis capabilities and a description of each. TR table 5-21, Evaluation of Code Ability to Model Phenomena within PIRT for Partial Flow Blockage, summarizes the modeling capabilities of the various codes against

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION the relevant phenomena in this analysis. From the codes listed, TerraPower chose the Mongoose++ computer code for subchannel analysis. TR sections 5.3.1.2.4, Mongoose++

Selection, and 5.3.1.2.5, Mongoose++ Code Development in Support of Partial Flow Blockage, provide further details on the rationale for selecting Mongoose++.

The staff reviewed TerraPowers EM development plan and determined that TerraPower acceptably completed EMDAP Step 10 by addressing and justifying the code selection and identifying an existing quality assurance program and controls for computer codes that perform safety-related or non-safety-related applications.

1.1.11 Step 11: Establish EM Structure In Step 11 of the EMDAP, the EM structure is established. This structure should be based on the principles and requirements established in Element 1, including the following six ingredients:

1. Systems and components: The EM structure should be able to analyze the behavior of all systems and components that play a role in the targeted application.

2. Constituents and phases: The code structure should be able to analyze the behavior of all constituents and phases relevant to the targeted application.

3. Field equations: Field equations are solved to determine the transport of the quantities of interest (usually mass, energy, and momentum).

4. Closure relations: Closure relations are correlations and equations that help to model the terms in the field equations by providing code capability to model and scale particular processes.

5. Numerics: Numerics provide code capability to perform efficient and reliable calculations.

6. Additional features: These address code capability to model boundary conditions and control systems.

TerraPower developed Mongoose++ as the thermal hydraulic system subchannel code that will be applied to partial flow blockage analysis. The TR establishes that the partial flow blockage EM is limited to a single fuel assembly, selecting a bounding case based on PCT. TR section 5.3.2.1.1, System Components, identifies the system components necessary for the EM, consisting of the fuel slug, sodium bond, cladding, rod plenum, wire wrap, inner subchannel, edge subchannel, corner subchannel, and assembly duct.

TR section 5.3.2.1.2, Constituents and Phases, identifies the scope of the analysis being limited to sodium, fuel, and cladding ((

)). TR table 5-21, Evaluation of Code Ability to Model Phenomena within PIRT for Partial Flow Blockage, shows that Mongoose++ is ((

)). The staff audited NAT-14450, Supporting Neutronics Calculations for Partial Flow Blockage Method, to verify TR statements that ((

)).

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION TR section 5.2.1.3, Field Equations, identifies that Mongoose++ solves the fundamental conservation equations for mass, momentum, and energy, incorporating temporal terms to allow for transient simulation. The TR describes how axial and lateral mass transfer, pressure gradients, wall friction, turbulence, and heat conduction mechanisms are included in the model.

The NRC staff audited NAT-7767, Mongoose++ Theory Manual, Revision 1, which provided supplemental details regarding TR section 5.2.1.3.

TR section 5.3.2.1.4, Closure Relations, expresses the high reliance on closure relations and presents them in TR table 5-25, Closure Relations within Mongoose++, ensuring that

((

)) are adequately represented. TR section 5.3.2.1.5, Numerics, covers how the field equations are implemented into the finite volume scheme with staggered grid arrangements. TR sections 5.3.2.1.6, Additional features, and 5.3.2.2, Supporting Reactivity Feedback Calculations, discuss the

((

)).

TR section 5.3.2.3, Mongoose++ Partial Flow Blockage EM within a Natrium Assembly, discusses the limiting case for different LBEs, specifying the number of blocked subchannels for each. The TR states that various ((

)).

The staff reviewed the EM structure of Mongoose++ as presented in the TR. The staff determined that TerraPowers approach to Step 11 is acceptable because it demonstrates that the scope of the EM is clear and adequately supported by the items evaluated under each of the six ingredients, as discussed in this section.

1.1.12 Step 12: Develop or Incorporate Closure Models Step 12 of the EMDAP involves developing and incorporating closure models into the EM.

Closure models or relationships are usually developed using SET data. Correlations may also be selected from existing database literature. TR section 5.3.3, EMDAP Step 12: Develop or Incorporate Closure Models, addresses the closure models and conservatisms used in this EM.

As discussed in the prior SE section on Step 11, closure models within Mongoose++ are found in TR table 5-25.

TR section 5.3.3.1, Conservative Modeling of Heat Transfer in the Wake Region behind the Blockage, outlines four components that must be incorporated and validated in Mongoose++.

These components are ((

)). With these components incorporated, Mongoose++ will be able to ((

))

which are discussed throughout section 5.3.3 of the TR and the subsequent subsections of this SE.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

((

))

TR section 5.3.3.1.2, ((

)) defines this component as the

((

)). It also discusses the history of wake region analysis and implementation of the semi-empirical model, which is ((

)). TR section 5.3.3.1.2.1, Historical Analysis of Wake Region, identified historical data from ORNL, PNC, and Westinghouse, supporting that the length of the wake region is a function of the Reynolds number and blockage diameter. The section further states that the length of the wake region is generally understood to be independent of blockage thickness for relatively thin blockages. This supports TR assumption 3.3, aligns with PIRT rankings, and is relevant to the blockages and bounding cases considered in this methodology. TerraPower used historical data to formulate a correlation for the wave length to blockage diameter ratio.

TR section 5.3.3.1.2.2, Semi-Empirical Model Implementation on Wake Region, discusses

((

)).

Energy Transport via Mixing TR section 5.3.3.1.3, Energy Transport via Mixing, identifies that heat transfer out of the wake region is primarily due to mixing, as supported by historical experiments presented in TR section 5.3.3.1.3.1, Historical Analysis to the Energy Transport. TerraPower stated that these

((

)).

TerraPower used data from an ORNL experiment to determine ((

)). TerraPower used the data from the PNC to confirm that ((

)).

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Semi-Empirical Model Implementation on the Energy Transport TR section 5.3.3.1.3.2, Semi-Empirical Model Implementation on the Energy Transport,

((

)). These results are included in TR table 5-30, Mixing Coefficients and Accompanying Relevant Information from Salt Concentration Measurements from ORNL THORS Water Mockup, which illustrate ((

)).

Convective Heat Transfer in Wake TR sections 5.3.3.1.4, Convective Heat Transfer in the Wake, and 5.3.3.1.5, Local Wake Spatial Factor, discuss how ((

)).

Results TR sections 5.3.3.1.6, Domain of Applicability of Semi-Empirical Models, and 5.3.3.1.7, Results, constrain the semi-empirical model to ((

)). TR section 5.3.3.1.7 provides a comparison of analyses with and without zero bulk flow enforcement, an analysis of a central 6-subchannel blockage (ORNL bundle 3A), and analysis of a 14 subchannel blockage (ORNL bundle 5B). Figure 5-22, Comparison of Mongoose++ Results ((

)), and figure 5-23, Comparison of Measured Cladding and Coolant Temperatures to Calculated Values from Mongoose++ for

((

)), show the ((

)).

TR section 5.3.3.1.7.3, 14-Subchannel Edge Blockage from ORNL 5B, subsequently compares model results to test data for the 14-subchannel edge blockage. TerraPower stated this section shows the model is conservative, ((

)) all measured coolant and cladding temperatures.

The TR also states that the model can overcome the ((

)) as derived in TR section 5.3.3.1.3.2, Semi-Empirical Model Implementation on the Energy Transport. Additional comparisons show that, for ((

)).

Staff Evaluation The staff reviewed the TRs Step 12 and the identified closure models and determined that the EM properly identifies, accounts for, and implements closure relations in a conservative manner justified by validation against relevant experimental data. As such, the staff determined that Mongoose++ is appropriate for the partial flow blockage scenarios outlined in the TR and ((

)) in a bounding and conservative manner.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION EMDAP Element 4: Assess EM Adequacy Element 4 of the EMDAP revolves around evaluating the adequacy of the EM. It consists of two parts: a bottom-up evaluation of the closure relationships used and then a top-down evaluation of the governing equations, numerics, and integrated performance of the EM. After these two parts are completed, the biases and uncertainties of the EM can be determined. A key feature of this adequacy assessment is the ability of the EM to predict appropriate experimental behavior.

In Element 4, Steps 13 through 15 covers the bottom-up evaluation, while Steps 14 through 19 cover the top-down evaluation. Element 4 is addressed in TR section 5.4, EMDAP Element 4:

Assess Evaluation Model Adequacy.

1.1.13 Step 13: Determine Model Pedigree and Applicability to Simulate Physical Processes In Step 13, the closure relationships used in the EM are evaluated based on their pedigree and applicability. The pedigree evaluation relates to the physical basis, assumptions and limitations, and adequacy characterization of the closure model. The applicability evaluation relates to whether the closure model is consistent with its pedigree or whether use over a broader range of conditions is justified. TerraPower implements Step 13 by comparing the EM to historical methodologies and uncertainties while ensuring the implemented model is consistent with the posed range of conditions.

TR section 5.4.1.1, Partial Flow Blockage Evaluation Model and Constitutive Models, discusses fluid flow models, energy exchange models, wire wrap models, and wake region models in Mongoose++. The section identifies constitutive models and closure relationships for general fluid flow in pin assemblies; flow and heat transfer within the wake region; energy exchange outside of the wake region; and crossflow, friction, and mixing caused by wire wraps.

The pedigree and applicability of these models and relationships are evaluated relative to partial flow blockage modeling by addressing the model derivations and associated assumptions. TR section 5.4.1.2, Summary, summarizes model applicability to the Natrium design.

The TR provides a summary of the fluid flow, energy exchange, wire wrap, and wake region models used in the EM. The NRCs review of each of these models is detailed below, along with an overall evaluation of the TRs implementation of Step 13.

Fluid Flow Models For this EM, TerraPower applied the ((

)). The model relies on assumptions about flow regimes and their relative locations, geometric effects, flow phenomena resulting from the wire wraps, and coolant density. The staff reviewed TerraPowers use of the ((

))

and concluded that the model is reasonable for use in a subchannel model of a wire-wrapped pin bundle. The staff reviewed TR section 5.4.1.1.1.1.6, Prediction Accuracy, and determined that the model demonstrates good agreement between predicted friction factors and historical data. This section also provides the application domain of the model, where the staff noted that the model was developed using assemblies with fewer rods than the Natrium Type 1 fuel assembly. TerraPower noted in TR table 5-31, Mongoose++ Constitutive Models, that Natrium-specific axial friction factor relations will be developed to address this issue. However, this issue was previously discussed by the staff in the review of TerraPowers TR NAT-9390, which employed a similar correlation in the SAS4A/SASSYS-1 code. In that report, TerraPower

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION committed to performing confirmatory testing to reduce uncertainty, and the staff found the correlation to be reasonable to apply to Natrium. The staff determined that the use of the

((

)) is acceptable for preliminary analyses because of TerraPowers commitment to develop a Natrium-specific correlation along with the discussion in the staffs SE on NAT-9390 that found application of a similar correlation reasonable for a ((

)). Future licensing submittals referencing this TR will need to provide either ((

)).

TR section 5.4.1.1.1.2, Cross Flow Resistance: Gap Resistance Factor, Kg, identifies assumptions for modeling cross flow resistance in the gaps between fuel pins. TerraPower assumes ((

)). The staff reviewed these assumptions and determined that they are consistent with those made in other subchannel codes approved by the NRC staff, such as ((

)), and therefore are acceptable.

TR section 5.4.1.1.1.3, ((

))

describes the assumptions for ((

)). TerraPower stated that the ((

)) model assumes ((

)). The staff reviewed the assumptions and determined they are acceptable because they are consistent with assumptions commonly made in other subchannel codes approved by the NRC staff. The ((

)) model assumes

((

)). The staff thus considers this approach to be acceptable.

((

))

TR section 5.4.1.1.2, ((

)) provides ((

)). These models are addressed in section 2.4.3 of this SE.

Energy Exchange Models The EM uses the ((

)).

This model is applicable to ((

)). For the Natrium design, TR table 5-31 lists the ((

)). The ((

)) provides a reasonable prediction of ((

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

)). As such, the staff considers the ((

))

to be appropriate for use in the partial flow blockage EM.

Due to the low Prandtl number of liquid sodium, it is important to appropriately model heat conduction in the fluid. This is particularly true for ((

)), which can have a significant impact on subchannel temperature distribution. TerraPower models ((

)) using a ((

)), which ((

)). The staff reviewed the cited references and determined that the ((

)) used provides results that compare favorably to experimental data. The ((

)) falls in the middle of the range of applicability of the

((

)). As such, the staff considers the use of the referenced ((

)) acceptable.

((

))

((

)) affects ((

)) by inducing ((

)). TerraPower categorized these effects as ((

)) and accounted for them using the ((

)). TerraPower developed these correlations using experiments with ((

)). The Natrium fuel assembly is within the range of all these parameters, except for number of pins. TerraPower stated that the ((

)).

Similarly, TerraPower stated that the ((

)). The staff reviewed the ((

)) described in the TR and determined they are acceptable because the partial flow blockage model assumes ((

)), which would ((

)). As such, the staff determined that the ((

)) are acceptable for this methodology.

Staff Evaluation As discussed above, the staff reviewed the EM's evaluation of closure model pedigree and applicability, summarized in TR table 5-31, and determined that the TR's approach to Step 13 is acceptable because it adequately evaluates both the pedigree and the applicability of each closure model used in the EM. However, future licensing submittals referencing this TR will need to provide either ((

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

)), as discussed in Limitation and Condition 1.

1.1.14 Step 14: Prepare Input and Perform Calculations to Assess Model Fidelity and/or Accuracy In Step 14 of the EMDAP, a fidelity evaluation is performed by preparing the necessary input data for the EM and then performing calculations required to assess the fidelity or accuracy of the model. This can be done through validation efforts (i.e., comparing results to experimental data), benchmarking efforts (i.e., comparison to other standards or results obtained from other codes), or some combination thereof. SET input for component devices used in the model should be prepared to represent the phenomena and test facility being modeled. Nodalization convergence studies should be performed when practicable in both the test facility and plant models. Differences between the calculated results and experimental data for important phenomena should be quantified for bias and deviation.

TR section 5.4.2, EMDAP Step 14: Prepare Input and Perform Calculations to Assess Model Fidelity for Accuracy, discusses planned and completed activities related to Step 14. The TR states that modeling strategies for structures, systems, and components during partial flow blockage events must be established. The TR also states that numerical techniques and user options in the EM must be evaluated, including nodalization and time-step convergent studies.

However, such studies are inherently limited as lumped parameter models cannot undergo a nodalization convergence study. In these cases, TerraPower stated it will ensure model applicability. Effects of user inputs on model accuracy and stability are minimized through automation where practical. These activities are discussed further in Step 16, which is evaluated in section 2.5.4 of this SE. The TR additionally states that Step 14 includes benchmarking and validation analysis for the assessment base in parallel with EM development. Regarding these efforts, TerraPower stated that it is necessary to ensure that the phenomena, components, and characteristics of modeled tests and test facilities are applicable to the Natrium design to ensure consistency of inputs between the benchmarking data and the Natrium design.

The staff reviewed TerraPowers preparation of input and performance of calculations to assess model fidelity and accuracy for the Natrium design. The staff determined that TerraPowers approach to Step 14 is acceptable because it is consistent with RG 1.203 and outlines an acceptable approach to develop input models and conduct benchmarking and validation studies. However, as noted by the TR, additional work is needed to complete Step 14 for this EM. As such, the staff has not made a determination with respect to TerraPower's execution of Step 14. Licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.15 Step 15: Assess Scalability of Models In Step 15 of the EMDAP, a scalability evaluation is performed, limited to determining whether the specific model or correlation is appropriate for application to the configuration and conditions of the plant and transient under evaluation. TR section 5.4.3, EMDAP Step 15: Assess Scalability of Models, states that TerraPower plans to assess fidelity and scaling of closure relations and provide a rationale and justification for their applicability to full-scale reactor applications.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION The NRC staff determined that TerraPowers approach to Step 15 is acceptable because planned activities capture the key considerations involved in determining whether models are scalable and will provide rationale and justification for each. However, as noted by the TR, additional work is needed to complete Step 15 for this EM. As such, the staff has not made a determination with respect to TerraPower's execution of Step 15. Future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1. The staff notes that the scalability discussion provided under Step 15 will be informed by Steps 12 and 13, which identifies the EMs closure relations and models and provides some rationale for the pedigree and applicability of each.

1.1.16 Step 16: Determine Capability of Field Equations to Represent Processes and Phenomena and the Ability of Numeric Solutions to Approximate Equation Set Step 16 of the EMDAP determines the capability of the field equations to represent processes and phenomena as well as the ability of numeric solutions to approximate the equation set. For the field equation evaluation, the acceptability of the governing equations in each code is examined to characterize the relevance of the equations for the chosen application. This evaluation should consider the pedigree, key concepts, and processes culminating in the equation set solved by each component code.

TR section 5.4.4, Step 16: Determine Capability of Field Equations to Represent Processes and Phenomena and the Ability of Numeric Solutions to Approximate Equation Set, considers the acceptability of governing equations in each node, convergence, stability, and conservation, and effects of user inputs. The TR states that this step must take place following full development; however, sufficiency requirements are defined and applied to benchmark data.

TR section 5.4.4.1, Sample Fuel Assembly Parameters, provides geometric parameters for Natrium fuel assemblies, blockage parameters, and fuel properties. The staff notes that these may be used as a metric for substantially similar in design to assess applicability of the limitations and conditions listed at the end of this evaluation.

TR section 5.4.4.2, Mongoose++ Residuals, discusses an example simulation that ((

)). TerraPower also discusses limitations in radial and axial refinements, such as the ((

)). The staff reviewed these limitations and determined that they are common to other subchannel analysis codes. As such, the staff considers them reasonable for this EM.

TR section 5.4.4.4, Schemes, discusses the transverse flux discretization schemes available in Mongoose++, which include ((

)). The staff reviewed the transverse flux discretization schemes and determined that the ((

))

is acceptable as it is both conservative and expected to be a ((

)).

The NRC staff determined that TerraPowers approach to Step 16 is acceptable because it demonstrated that the chosen field equations are adequate for the processes identified in the partial flow blockage event and that the numeric solutions are adequate to represent those equations. However, as noted by the TR, additional work is needed to complete Step 16 for this

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION EM. As such, the staff has not made a determination with respect to TerraPower's execution of Step 16. Licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.17 Step 17: Determine Applicability of EM to Simulate System Components In Step 17, an applicability evaluation is performed to consider whether the integrated code can model plant systems and components. The various EM options, special models, and inputs should have the inherent capability to model major systems and subsystems required for the application.

TR section 5.4.5, EMDAP Step 17: Determine Applicability of EM to Simulate System Components, considers whether the integrated EM can model Natrium systems and components. TR section 5.1.3 and SE section 2.3 discuss the relevant systems and components. The process for evaluating the EM options, models, and inputs are discussed in TR sections 5.4.1, EMDAP Step 13: Determine Model Pedigree and Applicability to Simulate Physical Processes, through 5.4.4, and SE section 2.5.

The staff determined that TerraPowers approach to Step 17 is acceptable because it identifies the important considerations for determining whether the EM can model applicable systems and components. However, as noted in the TR, additional work remains to complete the efforts described in Step 17 in support of the overall adequacy determination. As such, the staff has not made a determination with respect to TerraPower's execution of Step 17. Future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.18 Step 18: Prepare Input and Perform Calculations to Assess System Interactions and Global Capability Step 18 of the EMDAP consists of a fidelity evaluation, where EM-calculated data is compared to measured test data from component and integral tests (and to plant transient data if available). For this, data from the EM is compared against the integral database selected in Element 2. Once IET simulations are completed, the differences between calculated data and experimental data should be determined for important processes and phenomena and be quantified for bias and deviation. The ability of the EM to model system interactions are evaluated in this step, and input decks are prepared for the EMs target applications.

TR section 5.4.6, EMDAP Step 18: Prepare Input and Perform Calculations to Assess System Interactions and Global Capability, identifies planned activities to support this step. TerraPower will represent SSCs in the code through nodalization, time-step, and user options such that the base model will be adapted to each application (e.g., experiment-based validation and Natrium).

TerraPower stated that a conservative approach was taken, such that a ((

)). TerraPower stated that nodalization and time-step convergence studies will be performed to ensure model applicability, and impactful user options are automated where possible to minimize error. TerraPower illustrated in TR section 5.4.4 that it is accounting for appropriate parameters. TerraPower will quantify differences between calculated results and experimental data, identified in EMDAP Step 7, TR section 5.2.3. The activities outlined in this section include establishing plant characteristics and modeling assumptions, evaluate numerical techniques and user options, and to perform benchmarking and validation. TerraPowers approach for code development is to model test data in the same

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION manner as Natrium, with comparable inputs, nodalization, time-steps, and user options to maintain consistency.

The staff determined that TerraPowers approach to Step 18 is acceptable because these tasks align with RG 1.203 guidance and will adequately demonstrate the ability to model a partial flow blockage in Natrium. However, as noted in the TR, additional work remains to complete the efforts described in Step 18 in support of the overall adequacy determination. As such, the staff has not made a determination with respect to TerraPower's execution of Step 18. Future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.19 Step 19: Assess Scalability of Integrated Calculations and Data for Distortions Step 19 of the EMDAP involves performing a scalability evaluation limited to whether EM calculations and experimental data exhibit otherwise unexplainable differences among facilities or between calculated and measured data for the same facility. These differences may indicate experimental or code scaling distortions. TR section 5.4.7, EMDAP Step 19: Assess Scalability of Integrated Calculations and Data for Distortions, identifies that benchmarking and validation analysis will be performed and compared against acceptance criteria to identify if distortions are present.

The staff determined that TerraPowers approach to Step 19 is acceptable because these tasks align with RG 1.203 guidance. However, as noted in the TR, additional work remains to complete the efforts described in Step 19 in support of the overall adequacy determination. As such, the staff has not made a determination with respect to TerraPower's execution of Step 19.

Future licensing submittals referencing this TR will need to justify that this step of the EMDAP has been appropriately addressed, as discussed in Limitation and Condition 1.

1.1.20 Step 20: Determine EM Biases and Uncertainties Step 20 of the EMDAP involves determining EM biases and uncertainties. This includes determining whether the degree of overall conservativism or analytical uncertainty is appropriate for the entire EM. TR section 5.4.8, EMDAP Step 20: Determine Evaluation Model Biases and Uncertainties, addresses whether the EM is suitably conservative.

TR section 5.4.8.1, Sensitivities for DBA Six-Subchannel Blockage, quantifies uncertainties for key parameters at a ((

)). TerraPower stated that insignificant parameters, such as ((

)), are adequately identified and assessed, where either the uncertainties themselves are low, or impact of undertenancies are low. Conversely, impactful parameters, including ((

)),

which have a marked impact on PCT are identified and quantified.

TerraPower either bounds these impactful parameters or biases them conservatively with justification. TR table 5-40, Disposition of High and Medium Ranked Phenomena, identifies sensitive parameters for specific phenomena and discusses how conservatism is addressed for each. For example, in the case of ((

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

)).

TR section 5.4.8.2, Major Conservative Biases, discusses conservative biases of model results. When comparing ((

)). While bias from individual parameters compounds, TerraPower stated it has implemented conservative values in appropriate places to limit this effect. For example, ((

)) was shown to significantly influence cladding temperature. The staff reviewed TerraPowers discussion on conservative biases in model results and concluded that TerraPower chose a bounding value to bias the outputs conservatively, reinforcing the overall conservatism rather than challenging it. The staff determined that conservatism is demonstrated throughout the TR, including ((

)).

The staff determined that TerraPower acceptably executed EMDAP Step 20 using a conservative approach because of the conservatisms associated with the assumptions made in the underlying closure models discussed previously in this SE, the conservative assumptions used for the input parameters, and the demonstrated conservatism of the overall model with respect to the available test data.

Adequacy Decision As discussed in RG 1.203 section 1.5, Adequacy Decision, questions regarding EM adequacy should be asked and assessed during and after development to ensure a satisfactory outcome and that activities have not invalidated previous acceptable areas. This should be done in an iterative manner, where answering questions may inform the approach until satisfactory responses are met.

TR section 5.5, Adequacy Decision, presents preliminary questions regarding the adequacy decision. The staff reviewed these questions and determined that they are appropriate and consistent with the questions regarding EM adequacy identified in RG 1.203. TerraPower stated that the question list is preliminary and will be updated as the EM is carried out and further developed before future licensing submittals. The staff determined that this approach is consistent with the guidance in RG 1.203. The staff is not making any determinations on TerraPowers adequacy decision because this step is incomplete. Future licensing submittals referencing this TR will need to provide the status of the adequacy decision and justify that it has been appropriately addressed. See Limitation and Condition 1 of this SE.

LIMITATIONS AND CONDITIONS The staff imposes the following limitations and conditions on the use of this TR:

1. The staff noted that execution of Steps 7, 8, 9, 13, 14, 15, 16, 17, 18, and 19, and adequacy decision of the EMDAP have not been completed. An applicant or licensee referencing the methodology developed in this TR must justify that these steps of the EMDAP have been completed to a state that is appropriate for the intended licensing application.

OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION

2. The staffs determinations in this SE are limited to the Natrium design described in the TR, including the operating conditions. An applicant or licensee referencing the methodology developed in this TR must justify that any departures from design features or operational conditions, such as core geometry, power, temperature, or flow rate, do not affect the conclusions of the TR and this SE.

3. Applicability of this TR is limited to the flow blockage events bounded by those explicitly identified and analyzed in section 2.1 of the TR. If an applicant implementing this methodology identifies a credible flow blockage event(s) not bounded by those defined in this TR, the applicant must justify the applicability of the TR methodology.

CONCLUSION The staff determined that TerraPowers TR provides an acceptable approach to develop a methodology for use by future applicants utilizing the Natrium design as described in the TR and this SE to conservatively assess partial flow blockages because its approach is consistent with RG 1.203. This approval is subject to the limitations and conditions discussed in the previous section of this SE.

Principal Contributors:

R. Anzalone W. Williams A. Neller