ML25150A335
| ML25150A335 | |
| Person / Time | |
|---|---|
| Issue date: | 06/03/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML25150A335 (18) | |
Text
NRC Staff Review of Topical Report (TR) 007834 Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology (TSAM)
ACRS Meeting of the X-Energy Design Center Subcommittee June 3, 2025 https://www.nrc.gov/reactors/new-reactors/advanced.html
Outline
- Background on TSAM and related topical reports
- Regulatory Basis
- Review Approach
- Summary of Technical Review
- Other Limitations (Referenced Topical Report and Radiological Consequences)
- Conclusions 2
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Background===
Xe-100 Topical Report: Transient and Safety Analysis Methodologies Framework, Revision 1 was submitted October 2021 (ML21288A173) and SE issued March 2023 (ML23086C082).
Limitations identified to address needs for (1) identification of figures of merit, (2) a finalized Phenomena Identification and Ranking Table (PIRT), (3) verification and validation, and (4) justification of the models used.
TSAM originally submitted on April 30, 2024 (ML24121A285) with updated submittal on March 14, 2025 (ML25077A288)
Includes disposition of the limitations from "Transient and Safety Analysis Methodologies Framework Updated submittal includes markups identified during regulatory audit Review leveraged the audit process to enable efficient identification of markups such that requests for additional information were not needed TSAM section 1.5 states, X-energy requested review and approval of the evaluation model to analyze design basis accidents (DBAs) as defined in NEI 18-04 to support preliminary analysis and evaluation of the Xe-100 TSAM is the top-level topical evaluation model report that used in combination with three other topical reports (TSAM section 13.2):
Xe-100 Licensing Topical Report GOTHIC and Flownex Analysis Codes Qualification Xe-100 Licensing Topical Report Mechanistic Source Term Approach Xe-100 Licensing Topical Report Reactor Core Design Methods and Analysis 5
Regulatory Basis Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(1)(ii)(D) requires, in part, that an applicant for a construction permit (CP) perform an evaluation and analysis of a postulated fission product release to evaluate the offsite radiological consequences.
Under 10 CFR 50.34(a)(4) an applicant for a CP must perform a preliminary analysis and evaluation of the design and performance of structures, systems, and components (SSCs) with the objective of assessing the risk to public health and safety resulting from the operation of the facility and including the determination of margin of safety during normal operations and transient conditions anticipated during the life of the facility.
- Staff identified relevant Principal Design Criteria (PDC): Xe-100 PDC 10, RFDC 11, RFDC 16, PDC 19, PDC 20, RFDC 26, PDC 28, RFDC 30, RFDC 34
- Condition 1: A CP application referencing TSAM must confirm or update the regulatory basis relevant to the use of TSAM 4
Regulatory Basis Under 10 CFR 50.34(a)(8) an applicant for a CP must identify the systems, structures or components of the facility, if any, which require research and development to confirm the adequacy of their design and describe the research program that will be conducted to resolve any safety questions associated with such systems, structures, or components. Such research and development may include obtaining sufficient data regarding the safety features of the design to assess the analytical tools used for safety analysis in accordance with 10 CFR 50.43(e)(1)(iii).
Section C.4 of Regulatory Guide (RG) 1.253, Revision 0, Guidance for a Technology-Inclusive Content-of-Application Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors,
- It is also important to note that for CP applicants, the requirements of 10 CFR 50.43(e)(1)(iii) to ensure that sufficient data exist on the safety features of the design to assess the analytical tools used for safety analyses do not apply. Accordingly, CP applicants are not required to provide evaluations of the safety margins using approved [Evaluation Models].
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Review Approach Review follows the steps outlined in RG 1.203, Transient and Accident Analysis Methods TSAM sections 1.1 and 1.2 clarify that RG 1.203 is used to develop the methodology RG 1.203 describes an evaluation model (EM) development and assessment process (EMDAP) 20 steps divided into 4 elements Technical Evaluation in Safety Evaluation (SE) maps relevant content of TSAM into the EMDAP steps SE section 2.3 <- EMDAP Element 1 SE section 2.4 <- EMDAP Element 2 SE section 2.5 <- EMDAP Element 3 SE section 2.6 <- EMDAP Element 4 Other portions of Technical Evaluation:
SE section 2.1: Introduction and Outline (Map TSAM content to SE)
SE section 2.2: Requirements under 10 CFR 50.34(a)(8) (Primary basis for conditions identified throughout the SE)
SE section 2.7: Limitations from Transient and Safety Analysis Methodologies Framework SE section 2.8: Additional considerations on radiological assessment 6
Summary of Technical Review - EMDAP Element 1 Step 1:
TSAM is applicable to the Xe-100 reactor design as described in TSAM section 3 DBA EM addresses events listed in TSAM section 11.2 DBA list is derived from a full power probabilistic risk assessment and appears to be preliminary (e.g., DBA list in TSAM is different than DBA list in Long Mott Generating Station CPA)
Limitation 1: TSAM applicability is limited to preliminary safety analyses (i.e., construction permit)
Condition 2: An applicant referencing TSAM must justify how DBA events identified in preliminary safety analysis report are addressed by TSAM Step 2:
TSAM identifies radiological dose and as the figure of merit (FOM) and acceptance criteria that are consistent with NRC requirements (10 CFR 50.34(a)(1)(ii)(D)).
Other information in TSAM regarding FOMs is not consistent, quantitative standards of acceptance are not provided, TSAM does not clearly describe how FOMs are used in the EM development process (e.g., use in the PIRT process),and surrogate FOMs are not identified (e.g., Flownex and GOTHIC do not directly evaluate radiological consequences).
Condition 3: A CP application referencing TSAM must describe how surrogate FOMs will be assessed as part of a relevant research and development program or justify that they are not needed.
7 Element 1 Establish Requirements for Evaluation Model Capability 1.
Specify analysis purpose, transient class and power plant class 2.
Specify figures of merit 3.
Identify systems, components, phases, geometries, fields and processes that should be modeled 4.
Identify and rank phenomena and processes Fully addressed Mostly addressed Partially addressed Not addressed
Summary of Technical Review - EMDAP Element 1 Step 3:
EMDAP step 3 partially addressed through PIRT process TSAM does not contain information addressing step 3 TSAM appears to depart from RG 1.203 by not performing a hierarchal system decomposition Condition 4: A CP application referencing TSAM must clearly identify the systems, components, phases, geometries, fields, and processes that must be modeled in their EM as part of a relevant research and development program.
Step 4:
TSAM identifies importance levels for phenomena associated with particular events (TSAM section 8.5 identifies eight PIRTs including control rod withdrawal, loss of secondary cooling, steam generator tube rupture, )
TSAM section 8.1 describes the seven steps that were taken to complete the PIRTs. NRC staff compared this against the nine-step process used to develop the PIRTs for the Next Generation Nuclear (NUREG/CR-6944) and noted several differences.
TSAM steps 1-3 are not fully addressed and these are input to the PIRT Knowledge base is not described and knowledge levels are not provided TSAM addresses transient and safety analyses using two characteristic time periods. These characteristic time periods do not appear to be reflected in the PIRTs.
Condition 5: A CP application referencing TSAM must describe how the PIRT methodology will incorporate a suitable knowledge base, assess knowledge levels of associated phenomena, and address characteristic time periods as part of a relevant research and development program.
8 Element 1 Establish Requirements for Evaluation Model Capability 1.
Specify analysis purpose, transient class and power plant class 2.
Specify figures of merit 3.
Identify systems, components, phases, geometries, fields and processes that should be modeled 4.
Identify and rank phenomena and processes Fully addressed Mostly addressed Partially addressed Not addressed
Summary of Technical Review - EMDAP Element 2 Step 5:
TSAM section 6 provides high-level discussions on verification and validation plans for VSOP90/95, Flownex, XSTERM, and GOTHIC but objectives are not provided (e.g., phenomena to be assessed, system interactions and global code capability assessment).
GOTHIC/Flownex TR partially addressing this step Condition 6: A CP application referencing TSAM must clearly state the objectives of the EM assessment base as part of a relevant research and development program.
Step 6:
TSAM section 6.5.1 describes use a scaling methodology that is consistent with the hierarchical two-tired scaling (H2TS) method that is referenced in RG 1.203.
TSAM section 6.5.2.4 clarifies that code accuracy from validation activities needs to be addressed on a case-by-case basis.
TSAM sections 6.5.2.5 through 6.5.2.7 discusses verification and validation applicability based on similarity ratios NRC staff are unable to make any determination on these approaches because similarity ratios are not sufficient to demonstrate EM applicability or quantify EM uncertainty Condition 7: A CP application referencing TSAM must assess the EM using a range of experimental data whose non-dimensional numbers bound the values applicable to the Xe-100 or provide alternative justification as part of a relevant research and development program.
Limitation 2: The NRC staffs approval of the scaling approach described in this TR is limited to the general application of the H2TS scaling methodology. No approval of Xe-100 specific scaling or associated data applicability is provided.
9 Fully addressed Mostly addressed Partially addressed Not addressed Element 2 Develop Assessment Base 5.
Specify objectives for assessment base (Partially addressed in GOTHIC/Flownex TR) 6.
Perform scaling analysis and identify similarity criteria 7.
Identify existing data and/or perform IETs and SETs to complete data/base (Partially addressed in GOTHIC/Flownex TR) 8.
Evaluate the effects of IET distortions and SET scaleup capability 9.
Determine experimental uncertainties
Summary of Technical Review - EMDAP Element 2 Step 7:
NRC staff did not identify information in TSAM identifying existing data or assessing the need for integral effects tests (IETs) or separate effects tests (SETs).
GOTHIC/Flownex TR partially addressed this step Step 8:
TSAM provide general description of H2TS scaling methodology (see Step 6)
No findings made on distortion analysis (see Step 6)
NRC staff did not identify information on SET scaleup capability Condition 9: A CP application referencing TSAM must include evaluation of effects of IET distortion and SET scaleup capability as part of a relevant research and development program.
Step 9:
NRC staff did not identify information in TSAM addressing experimental uncertainties Condition 10: A CP application referencing TSAM must include evaluation of experimental uncertainties associated with EM assessment data as part of a relevant research and development program.
10 Fully addressed Mostly addressed Partially addressed Not addressed Element 2 Develop Assessment Base 5.
Specify objectives for assessment base (Partially addressed in GOTHIC/Flownex TR) 6.
Perform scaling analysis and identify similarity criteria 7.
Identify existing data and/or perform IETs and SETs to complete data/base (Partially addressed in GOTHIC/Flownex TR) 8.
Evaluate the effects of IET distortions and SET scaleup capability 9.
Determine experimental uncertainties
Summary of Technical Review - EMDAP Element 3 Step 10:
TSAM section 12 clarifies that activities described in TSAM are performed in accordance with the X-energy Quality Assurance Program Description which has been reviewed and approved by NRC staff.
Item identified in RG 1.203 for EMDAP step 10 are expected to be addressed by the quality assurance program (e.g., documentation requirements, programming standards and procedures, configuration control procedures)
Step 11:
TSAM Figure 22 shows EM interfaces and TSAM section 5 summarizes the codes and methods that are used in the EM (e.g., Reactor Core Design methodology, Mechanistic Source Term methodology, Flownex, GOTHIC, Flownex Screening Criteria (FSC))
TSAM section 10.2 states that the final sets of conditions and interfaces will be outlined in future submittals for NRC staff review TSAM section 13.2 clarifies that the EM described in TSAM shall be used in combination with NRC approved versions of TR 008585, GOTHIC and Flownex Analysis Codes Qualification, (2) TR 000632, Mechanistic Source Term Approach, and (3) TR 006889, Reactor Core Design Methods and Analysis. GOTHIC and Flownex Analysis Codes.
NRC staff review of these methodologies is performed outside of TSAM review TSAM section 10.4 clarifies that analyses fall into two characteristic time periods:
Plant transient - evaluated with Flownex Long-term passive cooling - evaluated with GOTHIC and XSTERM 11 Fully addressed Mostly addressed Partially addressed Not addressed Element 3 Develop Evaluation Model
- 10. Establish EM development plan
- 11. Establish EM structure
- 12. Develop or incorporate closure models (Partially addressed in GOTHIC/Flownex TR)
Summary of Technical Review - EMDAP Element 3 Step 11 (cont):
TSAM section 5.5 describes FSC as a response surface that converts Flownex calculated pebble temperatures into a conservative dose estimate NRC staff agree that FSC appears reasonable for estimating dose associated with diffusion driven phenomena from the fuel, but TSAM does not describe what radionuclide release mechanisms are accounted for by the FSC NRC staff did not identify screening criteria in TSAM Condition 11: A CP application applying FSC in accordance with TSAM must provide justification for the use of FSC on a case-by-case basis.
Condition 12: A CP application applying FSC in accordance with this TR must provide screening criteria for justifying the use of FSC in lieu of performing XSTERM evaluations.
Subject to SE conditions 11 and 12 and the limitations and conditions provided in TSAM section 13.2, the NRC staff concludes that TSAM addresses EMDAP step 11 because the codes used in the EM have been identified and initial interfaces are described.
Step 12:
NRC staff did not identify information in TSAM addressing EMDAP step 12.
During a regulatory audit, the NRC staff noted that X-energy statements that there are few closure models still under development and/or are being validated.
Partially addressed in GOTHIC/Flownex TR Condition 13: A CP application referencing this TR must describe how closure models will be developed or incorporated into the EM, either directly or incorporated by reference, as part of a relevant research and development program.
12 Fully addressed Mostly addressed Partially addressed Not addressed Element 3 Develop Evaluation Model
- 10. Establish EM development plan
- 11. Establish EM structure
- 12. Develop or incorporate closure models (Partially addressed in GOTHIC/Flownex TR)
Summary of Technical Review - EMDAP Element 4 Element 4:
- X-energy states that EMDAP Element 4 (steps 13-20) is not addressed by TSAM (or any other TR)
- TSAM section 13.2 states that the EM described in TSAM cannot be used to support a final safety analysis until validation and verification of the codes within the EM have been approved by the NRC.
- TSAM contains some information relevant to EMDAP step 20
- TSAM section 9 describes an uncertainty analysis methodology that includes a novel Phenomena and Key (PK)-
PIRT process
- NRC staff are unable to determine that the uncertainty analysis methodology is appropriate (reasons specified in SE)
- Condition 14: A CP application referencing this TR must describe how EM assessment will be addressed, either directly or incorporated by reference, as part of a relevant research and development program.
13 Fully addressed Mostly addressed Partially addressed Not addressed Element 4 Assess Evaluation Model Adequacy 13 - 15 Closure Relations (Bottom-up) 16 - 19 Integrated EM (Top-down) 20.
Determine EM biases and uncertainties
Limitations from Referenced Topical Report
- Xe-100 Topical Report: Transient and Safety Analysis Methodologies Framework, Revision 1 was submitted October 2021 (ML21288A173) and SE issued March 2023 (ML23086C082).
- Limitations identified to address needs for (1) identification of figures of merit, (2) a finalized PIRT, (3) verification and validation, and (4) justification of the models used.
- TSAM section 2.5 describes how TSAM addressed the four limitations provided in the NRC staff SE, dated March 29, 2023.
- NRC staff determined that TSAM does not fully address the limitations in the NRC staff SE dated March 29, 2023
- For an application that references TSAM, the limitations and conditions identified for TSAM supersede the limitations identified in the NRC staffs SE dated March 29, 2023.
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Additional Considerations Regarding Radiological Consequence Assessment
- TSAM information associated with the evaluation of offsite radiological consequences is high-level and, with the exception of FSC, in TSAM section 5 and section 10, is generally limited to thermal-fluid assessment.
- NRC did not identify information in TSAM addressing radionuclide production, transport, dispersion, and conversion to radiological dose.
- Limitation 3: The NRC staffs approval of this TR for radiological consequence analysis is limited to the thermal-fluid inputs used in radiological consequence analysis and the general use of FSC.
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Conclusions NRC staff approves use of TR 007834, Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology, Revision 2 for the preliminary analysis of DBAs for the Xe-100 subject to 3 Limitations and 14 Conditions (provided in section 3 of the SE)
The use of the EM as described in TSAM is acceptable for informing radiological consequence analysis evaluations to address Xe-100 PDC 19 and 10 CFR 50.34(a)(1)(ii)(D) because, pursuant to limitations described in TSAM section 13.2, SE condition 11, and SE condition 12, sufficient justification would be provided in a CP application or during the associated safety review to ensure that the codes used in the EM are reasonably capable of analyzing DBAs for the Xe-100.
The use of the EM as described in TSAM is acceptable for the preliminary analysis of the design and performance of Xe-100 SSCs in accordance with 10 CFR 50.34(a)(4) and demonstrating, in part, preliminary compliance with relevant PDC because (1) CP applications are not required to provide evaluations of safety margins using approved EMs, and (2) pursuant to limitations described in TSAM section 13.2, SE condition 11, and SE condition 12, sufficient justification would be provided in a CP application or during the associated safety review to ensure that the codes used in the EM are reasonably capable of analyzing DBAs for the Xe-100.
The use of TSAM, pursuant to condition 2 through condition 14, is capable of addressing the requirements under 10 CFR 50.34(a)(8) to describe a research plan to resolve safety questions associated with EM applicability to the Xe-100 because addressing those conditions would result in an EM that complies with the approved guidance in RG 1.203, Transient and Analysis Methods.
NRC staff expects that the approval of TR007834, "Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology, Revision 2 for the preliminary analysis of DBAs for the Xe-100 subject to 3 Limitations and 14 Conditions" will provide efficiencies for licensing applications that incorporate it by reference.
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EMDAP Status 17 Element 1 Establish Requirements for Evaluation Model Capability
- 1. Specify analysis purpose, transient class and power plant class
- 2. Specify figures of merit
- 3. Identify systems, components, phases, geometries, fields and processes that should be modeled
- 4. Identify and rank phenomena and processes Element 4 Assess Evaluation Model Adequacy 13 - 15 Closure Relations (Bottom-up) 16 - 19 Integrated EM (Top-down) 20.
Determine EM biases and uncertainties Element 2 Develop Assessment Base
- 5. Specify objectives for assessment base (Partially addressed in GOTHIC/Flownex TR)
- 6. Perform scaling analysis and identify similarity criteria
- 7. Identify existing data and/or perform IETs and SETs to complete data/base (Partially addressed in GOTHIC/Flownex TR)
- 8. Evaluate the effects of IET distortions and SET scaleup capability
- 9. Determine experimental uncertainties Element 3 Develop Evaluation Model
- 10. Establish EM development plan
- 11. Establish EM structure
- 12. Develop or incorporate closure models (Partially addressed in GOTHIC/Flownex TR)
Fully addressed Mostly addressed Partially addressed Not addressed
18 Acronyms CP Construction Permit DBA Design Basis Accidents EM Evaluation Model EMDAP Evaluation Model Development and Assessment Process FOM Figure of Merit FSC Flownex Screening Criteria IET Integral Effects Tests PDC Principal Design Criteria PIRT Phenomena Identification and Ranking Table PK Phenomena and Key PRA Probabilistic Risk Assessment PSAR Preliminary Safety Analysis Report QAPD Quality Assurance Program Description SET Separate Effects Tests TR Topical Report TSAM Transient and Safety Analysis Methodology