ML25071A024

Staff Review of NAT-9390, Design Basis Accident Methodology for In-Vessel Events Without Radiological Release

ML25071A024
Person / Time
Site:	99902100
Issue date:	03/12/2025
From:	Reed Anzalone, Brusselmans R, Neller A NRC/NRR/DANU/UAL1
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Download: ML25071A024 (1)

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Staff Review of NAT-9390, Design Basis Accident Methodology for In-Vessel Events without Radiological Release Roel Brusselmans, Project Manager Reed Anzalone, Senior Nuclear Engineer Alec Neller, Nuclear Engineer Office of Nuclear Reactor Regulation Division of Advanced Reactors and Non-Power Production and Utilization Facilities

Outline

• Review chronology

• Topical report (TR) purpose and review strategy

• Regulatory requirements and guidance

• Overview of Regulatory Guide (RG) 1.203

• Safety evaluation (SE) overview

• Limitations and Conditions (L&Cs)

Review Chronology

• June 30, 2023: Pre-application public meeting (ML23181A189)

• September 29, 2023: Submittal of TR Design Basis Accident Methodology for In-Vessel Events without Radiological Release, (ML23272A260)

• October 31, 2023: TR accepted for review by the NRC staff (ML23303A168)

• March - June 2024: Audit conducted (ML24255A017)

• October 11, 2024: Submittal of revised TR (ML24295A202)

• December 23, 2024: Draft SE issued (ML24358A247)

TR Purpose and Review Strategy

• Purpose of TR:

Requests NRC review and approval of a proposed methodology to evaluate in-vessel DBA events that do not lead to radiological release for future applicants using the Natrium design.

• Review Strategy:

NRC staff reviewed each EMDAP step in the TR against the guidance provided in RG 1.203.

Regulatory Requirements

• 10 CFR 50.34(a)(4)

Requires preliminary analysis and evaluation of the design and performance of structures, systems, and components (SSCs) of the facility to:

assess the risk to public health and safety, determine the margins of safety during normal operations and transients, and assess the adequacy of the SSCs provided for preventing accidents and mitigating their consequences.

• 10 CFR 50.43(e)

Requires a demonstration of safety feature performance through analysis, test programs, experience, or a combination thereof.

Requires sufficient data exists regarding safety features of the design to assess the analytical tools for safety analyses over a sufficient range of plant conditions.

RG 1.203, Transient and Accident Analysis Methods

• Provides guidance for use in developing and assessing evaluation models for accident and transient analyses

• Evaluation Model Concept:

calculational framework for evaluating the behavior of the reactor system during a postulated transient or accident Includes computer programs, special models, and all other information needed to apply the calculational framework to a specific event

• EMDAP includes four elements:

1.

Determine requirements for the EM.

2.

Develop an assessment base consistent with determined requirements.

3.

Develop the EM.

4.

Assess the adequacy of the EM.

RG 1.203: Element 1 Determine requirements for the EM.

• Identify the application envelope for the EM.

• Determine the figures of merit (FOMs).

• Identify the important phenomena and processes needed to evaluate event behavior relative to the FOMs.

Step 1: Analysis Purpose

• RG 1.203: Specify analysis purpose, transient class, and power plant class.

Purpose:

Demonstrate the reactor operates such that all acceptance criteria are satisfied under normal operational conditions, and continued to be satisfied during in-vessel DBAs without radiological release Transient Class: Loss of Offsite Power (LOOP), Rod Withdrawal at Power (RWAP), and Loss of Heat Sink (LOHS)

Power Plant Class: Natrium reactor

• NRC staff compared selected transients against previous sodium fast reactor (SFR) licensing efforts such as the PRISM reactor.

• NRC staff determined that the analysis the TR meets the guidance provided in Step 1 of RG 1.203 and therefore is acceptable.

• Applicability of EM limited to Natrium design as described by TR (L&C 1).

Step 2: Figures of Merit

• RG 1.203: Figures of Merit (FOMs) are quantitative standards of acceptance used to define acceptable answers for a safety analysis.

Fuel Centerline Temperature Coolant Temperature Time at Temperature for Peak Cladding Temperature Time at temperature no failure (TATNF) criteria developed - accounts for strain, cladding wastage, and thermal creep

• TATNF criteria is used to screen whether a DBA needs further analysis, discussed in TerraPowers TR on DBA with radiological release (ML24082A262).

• NRC staff audited internal TerraPower documents detailing TATNF development.

• NRC staff determined that the EMs FOMs are adequate because they can be used to ascertain whether fuel has failed and whether phenomena would challenge the primary coolant boundary.

• Therefore, NRC staff determined TerraPowers approach to Step 2 is acceptable.

Step 3: Identify EM Characteristics

• RG 1.203: EM characteristics are identified through hierarchical system decomposition, with ingredients at each level decomposed into ingredients of the next level down (e.g., systems into subsystems).

• TerraPower provided a hierarchical decomposition for the Natrium plant, scoped to cover the primary and intermediate systems, as well as the reactor air cooling system (RAC), intermediate air cooling system (IAC), and sodium-salt heat exchanger (SHX).

• NRC staff compared the decomposition with the description of the Natrium plant provided in the TR, verifying that all necessary ingredients were included.

• NRC staff determined TerraPowers approach to Step 3 was acceptable as the list of ingredients is consistent with the Natrium plant description and RG 1.203.

Step 4: Key Phenomena

• RG 1.203: Key phenomena and processes are identified and ranked with respect to their influence on the FOMs. This is done by developing a phenomena identification and ranking table (PIRT), in which:

An event is divided into operationally characteristic time periods in which dominant phenomena and processes remain constant.

For each time period, phenomena and processes are identified.

These phenomena are ranked based on their impact on the FOMs.

• TerraPower developed a composite PIRT which encompassed a series of five PIRTs covering the three transients chosen in Step 1 (LOOP, RWAP, and LOHS).

TerraPower identified three characteristic time periods (initiation, transition, and post-scram cooling), which were consistent for all three transients.

Phenomena and processes were identified for each time period, and then ranked based on their importance and state of knowledge (high, medium, low).

Step 4: Key Phenomena

• NRC staff audited the TerraPowers PIRT development process, including the results of all five PIRTs and determined the process is acceptable because it followed the guidance of Step 4 of RG 1.203.

• NRC staff determined the PIRT phenomena are appropriate for the transients considered for the EM because they are consistent with the Natrium design and past SFR operating experience.

• Because TerraPower used an acceptable process to develop the PIRT and arrived at a reasonable set of PIRT phenomena and rankings, the NRC staff determined that TerraPowers approach to Step 4 and the PIRT is acceptable for the methodology scope defined by EMDAP Steps 1 through 3.

RG 1.203: Element 2 Develop Assessment Base, which:

• may consist of a combination of new and legacy experiments,

• should be consistent with requirements determined from Element 1, and

• is used to validate codes used by the EM as part Element 4

Step 5: Objectives for Assessment Base

• RG 1.203: Determine the objectives for database that will be used to assess the EM, which should include separates effects tests (SETs) and integral effects tests (IETs).

• TerraPowers objective is to identify sufficient experimental data to form a complete assessment base for assessing the adequacy of the EM.

TR presents an approach that categorizes the scalability of data into three distinct areas: geometry and phenomena (Category 1), physical properties (Category 2), and phenomena character, event timing, and order (Category 3).

For this assessment matrix, TerraPower plans to include experimental data from at least one Category 1 IET, and all supporting SETs necessary for all highly-ranked phenomena identified in Step 4.

Additional Category 2 and 3 data included to provide credibility for the EM at a variety of scaling factors.

• NRC staff determined that TerraPowers objectives for the assessment base are acceptable because they are consistent with RG 1.203 which states SETs and IETs are required for EM assessment.

Step 6: Scaling Analyses

• RG 1.203: Scaling analyses are performed to ensure experimental data and models based on that data will be applicable to full-scale analysis of plant transients.

A top-down scaling analysis derives no-dimensional groups that govern similitude between facilities.

Bottom-up scaling analyses address localized behavior and are used to explain differences among tests from different experimental facilities to help infer expected plant behavior.

• TerraPower is developing a hierarchical two-tiered scaling (H2TS) approach to perform both top-down and bottom-up scaling analyses.

• NRC staff determined TerraPowers approach to Step 6 is acceptable because the H2TS appropriately approaches scaling from both top-down and bottom-up perspectives to establish similarity criteria.

• NRC staff has not made a determination with respect to TerraPower's execution of Step 6 as it has not been completed. (Subject to L&C 2.)

Step 7: Experimental Data

• RG 1.203: Identify existing data and/or perform IETs and SETs to complete the assessment base.

These experiments should address the important phenomena identified in the PIRT.

A range of tests should be used to demonstrate the EM has not been tuned to a single test.

• TerraPowers EM assessment matrix is planned to include data from:

an IET scaled to the Natrium reactor, four scaled SETs, experiments from previous operating SFRs (EBR-II, FFTF, and Phenix), and historical IETs and SETs.

• NRC staff determined that the identified experiments are expected to provide adequate assessment data for the highly-ranked phenomena identified in Step 4 and that the initial pedigree evaluation and preliminary code assessment matrix are consistent with the guidance provided in RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of Step 7 because the final scaling assessment has not been completed, the scaled IET and SETs still need to be performed, and the pedigree evaluation and the code assessment matrix have not been finalized (Subject to L&C 2).

Steps 8 and 9

• Step 8: Evaluate IET distortions and SET scaleup capability.

TerraPower plans to perform this step following the completion of Step 6 and the completion of the scaled IET and SETs.

• Step 9: Determine experimental uncertainties.

TerraPower plans to follow the American Society of Mechanical Engineers (ASME) Nuclear Quality Assurance (NQA-1) standard for the scaled IET and SET experiments.

For experimental uncertainties associated with legacy experiments, TerraPower plans to use engineering judgement to determine the degree of compliance with NQA-1.

• NRC staff determined TerraPowers approach to these steps are adequate as they align with guidance provided in RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of these steps because these steps have not been completed (L&C 2).

RG 1.203: Element 3 Develop the Evaluation Model.

• The calculational devices needed to analyze the transients identified in Element 1 are selected.

• Includes choosing applicable computer codes, boundary conditions, and procedures for treating input and output information.

Step 10: EM Development Plan

• RG 1.203: An EM development plan is created based on the requirements established in Element 1, including developing the standards and procedures that cover:

Design specifications for the calculational device Documentation requirements Programming standards and procedures Transportability requirements QA procedures Configuration control procedures

• NRC staff audited documents detailing the EMs design specifications and applicable quality assurance (QA) requirements.

• The NRC staff determined that TerraPowers approach to Step 10 is acceptable because TerraPowers software design specifications and QA requirements appropriately address the six key focus areas discussed in RG 1.203.

Step 11: EM Structure

• RG 1.203: EM structure should include:

The ability to model relevant systems and components The ability to model relevant constituents and phases Field equations (mass, energy, and momentum)

Closure relations Numerics (code capability to perform efficient and reliable calculations)

Capability to model boundary conditions and control systems.

• TerraPower identified SAS4A/SASSYS-1 (SAS), Version 5.7.1 as the main system analysis computer code to be used for the EM.

• NRC staff reviewed the SAS Code Manual and the TR to ensure all six ingredients discussed in Step 11 were appropriately addressed.

• NRC staff determined TerraPowers approach to Step 11 is acceptable.

Step 11: Systems and Components

• Basic geometric modeling element used in SAS core modeling is a channel consisting of a fuel pin, its cladding, and the associated coolant and structure around the channel.

• Options for a single-pin or multiple-pin model Single-pin: a single average channel is used to represent the average of many pins, with multiple channels used to model all the pins in the reactor Multiple-pin: each channel represents one or more pins in a subassembly. Multiple-pin subassembly models are joined with single-pin subassembly models to model all pins in the reactor.

Step 3 Hierarchical Breakdown Subsystems:

Reactor core and core components

Reactor enclosure system

Primary heat transport system

Intermediate heat transport system

IAC

Control rod drive system

RAC Components:

Reactor vessel

Intermediate heat exchanger

Other heat exchangers (e.g., IAC, SHX)

Step 11: Systems and Components

• SAS models primary and intermediate heat transport systems through compressible volumes (CVs) connected via liquid or gas segments.

• Segments contain multiple elements.

• CVs: hot and cold pools

• Elements: core subassemblies, pipes, pumps, heat exchanger shell-and-tube sides

• SAS additionally has modules available for modeling the RAC.

Generalized Geometry (SAS Code Manual)

Step 3 Hierarchical Breakdown Subsystems:

Reactor core and core components

Reactor enclosure system

Primary heat transport system

Intermediate heat transport system

IAC

Control rod drive system

RAC Components:

Reactor vessel

Intermediate heat exchanger

Other heat exchangers (e.g., IAC, SHX)

Step 11: Constituents, Phases, Field Equations

• SAS developed specifically for analyzing power and flow transients in liquid metal reactors.

Capable of modeling liquid sodium in both primary and intermediate loops

• SAS allows for selecting parameters for the cover gas, including argon.

• SAS allows for air and its interactions with the RAC to be modeled.

• SAS uses mass, momentum, and energy conservation equations to predict transport of mass, momentum, and thermal energy of liquid sodium, argon, and air.

Step 3 Hierarchical Breakdown Constituents:

Liquid sodium

Air

Argon gas Phases:

Liquid sodium

Gases Field Equations:

Mass

Momentum

Energy

Step 12: Closure Models

• RG 1.203: Closure models are developed and incorporated into the EM. These are developed using SET data or can be selected from existing database literature.

• TerraPowers EM includes closure models that currently exist in the version of SAS available from Argonne National Laboratory as well as new models added to SAS developed from literature.

• The NRC staff:

reviewed the closure models detailed in the SAS Code Manual as well as the available literature on the newly added closure models.

audited internal TerraPower reports to ensure fuel assembly design parameters fell within the ranges of applicability for each correlation.

determined the newly added closure models are acceptable for use in the EM because they generally provide adequate predictions of key parameters.

• Subject to L&C 2, pending results of further testing related to correlation development.

RG 1.203: Element 4 Assess EM Adequacy

• Bottom-up evaluation of closure relationships used in the EM.

• Top-down evaluation of the governing equations, numerics, and integrated performance of the EM.

• Assess the ability of the EM to predict key phenomena identified in Element 1.

Step 13: Model Pedigree and Applicability

• RG 1.203: 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 provided an approach to Step 13, outlining the considerations for evaluating the pedigree and applicability for an example closure relationship.

• NRC staff determined that this approach is acceptable as it is consistent with the considerations discussed in RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of EMDAP Step 13 because it has not been performed. (Subject to L&C 2.)

Steps 14 and 15: Model Fidelity and Scalability

• Step 14: A fidelity evaluation is performed by preparing necessary input data for the EM and then performing calculations to access the accuracy of the model. This can be done through validation with experimental data, benchmarking with other codes, or some combination thereof.

TerraPower states that SAS calculations will be performed and compared against the experiments applicable to Natriums design.

NRC staff determined that this approach was acceptable as it appropriately focuses on validation of the EM relative to experimental data.

• Step 15: A scalability evaluation is performed to determine whether a given model or correlation is appropriate for the application based on plant conditions and the transient under evaluation.

TerraPower states that confirmatory calculations or justifications for the scalability of each closure relationship will be performed once experimental data from Step 7 is available.

NRC staff determined that this approach was acceptable as it is consistent with RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of Steps 14 and 15 because they have not been performed. (Subject to L&C 2.)

Step 16: Field Equation Capability

• RG 1.203: The capability of the field equations to represent important phenomena and the ability of the numeric solutions to approximate the equation set are evaluated.

For the field equation evaluation, the acceptability of the governing equations in each code are examined for the target application.

For the numeric solution evaluation, the convergence, property conservation, and stability of code calculations should be considered.

• TerraPower plans to:

validate the EMs field equations by performing calculations using data from experiments scaled to Natrium, consider the pedigree, key concepts, and processes culminating in the field equations used in SAS, and consider the consistency, property conservation, and stability of the SAS code for the numeric solution evaluation.

• NRC staff determined that TerraPowers approach to Step 16 is acceptable because it is consistent with the considerations discussed in RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of Step 16 because it has not been performed. (Subject to L&C 2.)

Step 17 and 18

• Step 17: An applicability evaluation is performed to consider whether the integrated code is capable of modeling plant systems and components.

• Step 18: A fidelity evaluation is performed, where EM-calculated data is compared to measured test data from available IETs. The differences between calculated data and experimental data should be determined for important processes and phenomena and be quantified for bias and deviation.

• TerraPower plans to first evaluate the capability of the EM to simulate the systems and subsystems of the Natrium plant, and then assess the system interactions and global capabilities of the EM.

• NRC staff determined the approach to Steps 17 and 18 is acceptable because the tasks planned are consistent with the considerations discussed in RG 1.203 and will sufficiently demonstrate the EMs ability to model Natrium and demonstrate the EMs fidelity.

• NRC staff has not made a determination with respect to TerraPower's execution of Steps 17 and 18 because they have not been performed. (Subject to L&C 2.)

Step 19: Scalability Assessment for Integrated EM

• RG 1.203: A scalability evaluation is performed to determine whether there are distortions between EM calculations and experimental data among facilities or between calculated and measured data for the same facility.

• TerraPower plans to use the scalability assessment to ensure that experimental data and EM calculations of highly-ranked phenomena agree show reasonable agreement and that the EM is sufficiently conservative.

• NRC staff determined that TerraPowers approach to Step 19 is acceptable because it is consistent with the considerations discussed in RG 1.203.

• NRC staff has not made a determination with respect to TerraPower's execution of EMDAP Step 19 because it has not been performed. (Subject to L&C 2.)

Step 20: Determine EM Biases and Uncertainties

• RG 1.203: EM biases and uncertainties are determined, including whether the degree of overall conservatism or analytical uncertainty is appropriate for the entire EM.

• TerraPower plans to take a conservative approach for the EM rather than performing uncertainty analyses.

• TerraPower plans to ensure the approach is conservative by:

Inserting conservative biases on the nominal inputs related to highly-ranked phenomena.

Applying hot channel factors to the output to obtain a conservative cladding temperature.

• NRC staff determined that TerraPowers approach to Step 20 was appropriate to ensure that inputs will be biased conservatively and provide an overall conservative result, and is consistent with RG 1.203, which states that suitably conservative transient analyses do not require a complete uncertainty analysis.

• NRC staff has not made a determination with respect to TerraPower's execution of EMDAP Step 20 because the application of this approach and its comparison to experimental results have not been performed. (Subject to L&C 2.)

Conclusion NRC staff determined that the TR provides an acceptable approach to develop a methodology for applicants utilizing the Natrium design to evaluate in-vessel DBA events without radiological release.

Limitations & Conditions:

1. The NRC staffs determinations in this SE are limited to the Natrium design described in Section 1.2 of the TR and this SE, including the use of Natrium Type 1 fuel. An applicant or licensee referencing the methodology developed in this TR must justify that any departures from these design features do not affect the conclusions of the TR and this SE. Additionally, this methodology was developed to analyze certain design basis accidents as discussed in TR section 2.1 and this SE (and as defined in NEI 18-04);

use of this methodology for other kinds of analyses must be justified.

2. The NRC staff noted that execution of the steps 6, 7, 8, 9, 12, 13, 14, 15, 16, 17, 18, 19, and 20 of the EMDAP, as well as sensitivity studies discussed in section 2.5 of the TR and section 3.1.4 of this SE, have not been completed. An applicant or licensee referencing the methodology developed in this 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.

Questions?