ML25062A069

Draft Safety Evaluation of Topical Report 007834 Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology Revision 2 EPID L-2024-TOP-0017

ML25062A069
Person / Time
Site:	99902071
Issue date:	05/06/2025
From:	Stephen Philpott NRC/NRR/DANU/UNPL
To:	Kalinousky D X-Energy
McGovern D
References
EPID L-2024-TOP-0017
Download: ML25062A069 (1)

Text

Mr. Douglas Kalinousky Licensing Manager, X Energy, LLC 530 Gaither Road, Suite 700 Rockville, MD 20850

SUBJECT:

U.S. NUCLEAR REGULATORY COMMISSIONS DRAFT SAFETY EVALUATION FOR X ENERGY LLCS XE-100 LICENSING TOPICAL REPORT TRANSIENT AND SAFETY ANALYSIS METHODOLOGY (EPID NO: L-2024-TOP-0017)

Dear Mr. Kalinousky:

By letter dated March 14, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25077A288), X Energy, LLC., (X-energy) submitted Revision 2 of its Xe-100 Licensing Topical Report (TR) Transient and Safety Analysis Methodology to the U.S. Nuclear Regulatory Commission (NRC) staff for review. This TR describes the transient and safety analysis evaluation model development for use in the preliminary analysis of design basis accidents for the Xe-100 reactor.

The enclosed draft safety evaluation for the aforementioned TR is being provided to the Advisory Committee for Reactor Safeguards (ACRS) to support the upcoming ACRS Subcommittee meeting, scheduled for June 3, 2025.

If you have any questions, please contact Denise McGovern at (301) 415-0681 or via email at Denise.McGovern@nrc.gov.

Sincerely, Stephen Philpott, Acting Chief Advanced Reactor Licensing Branch 2 Division of Advanced Reactors and Non-Power Production and Utilization Facilities Office of Nuclear Reactor Regulation Project No. 99902071

Enclosure:

As stated cc: Distribution via X-Energy Xe-100 GovDelivery May 6, 2025 Signed by Philpott, Stephen on 05/06/25

ML25062A069 NRR-043 OFFICE NRR/DANU/UAL2:PM NRR/DANU/UAL2:LA NRR/DANU/UTB1:BC NAME DMcGovern CSmith TTate DATE 3/11/2025 03/12/2025 4/29/2025 OFFICE OGC/NLO NRR/DANU/UAL2:BC NAME JEzell SPhilpott DATE 04/25/2025 5/6/2025

Enclosure X-ENERGY - DRAFT SAFETY EVALUATION OF TOPICAL REPORT 007834 XE-100 LICENSING TOPICAL REPORT TRANSIENT AND SAFETY ANALYSIS METHODOLOGY, REVISION 2 (EPID L-2024-TOP-0017)

SPONSOR AND SUBMITTAL INFORMATION Sponsor:

X Energy, LLC. (X-energy)

Sponsor Address:

X Energy, LLC.

530 Gaither Road, Suite 700 Rockville, MD, 20850 Project No.:

99902071 Submittal Date: March 14, 2025 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML25077A288 (package)

Supplement and RAI response letter Date(s) and ADAMS Accession No(s): N/A Brief Description of the Topical Report:

On April 30, 2024, X-energy, LLC (X-energy) submitted topical report (TR) 007834, Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology, Revision 1, (hereafter referred to as TSAM) for review by the U.S. Nuclear Regulatory Commission (NRC) staff (ADAMS Accession No. ML24121A285 (package)). X-energy submitted Revision 2 of this TR on March 14, 2025 (ML25077A288 (package)). The TR describes the transient and safety analysis evaluation model (EM) development for use in the preliminary analysis of design basis accidents1 (DBAs) for the Xe-100 reactor described in Section 3 of the TR.2 Specifically, TSAM follows the Evaluation Model Development and Assessment Process (EMDAP) described in Regulatory Guide (RG) 1.203, Transient and Accident Analysis Methods, ((ML053500170) to develop an EM for DBAs.

1 DBA, as used in TSAM, is defined in NEI 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, Revision 1 (ML19241A472) which is endorsed by RG 1.233, Guidance for a Technology Inclusive, Risk-Informed, and Performance-Based, Methodology and Content of Applications for Licenses, Certifications, and Approvals for Non-Light Water Reactors (ML20091L698).

2 This SE does not evaluate or approve the Xe-100 design information provided in the TSAM TR.

TSAM uses the Flownex and GOTHIC codes in accordance with TR 008585, GOTHIC and Flownex Analysis Codes Qualification, Revision 3 (ML25076A053)3 to analyze the dynamic thermal-fluid response of the Xe-100 under transient conditions. Reactor physics input for use in TSAM is provided by the reactor core design and analysis methods described in TR 006889, Reactor Core Design Methods and Analysis, Revision 2 (ML25071A399). Output from Flownex and GOTHIC analyses are used in radiological consequence evaluations performed in accordance with TR 000632, (ML25073A093). On August 29, 2024, the NRC staff transmitted an audit plan to X-energy (ML24236A768), and subsequently conducted an audit of materials related to these four TRs (i.e. TSAM, GOTHIC and Flownex, reactor core design and analysis methods, and mechanistic source term).

1 REGULATORY EVALUATION 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. This evaluation must determine that:

An individual located at any point on the exclusion area boundary (EAB) for any 2-hour period following the onset of the postulated fission product release, would not receive a radiation dose in excess of 25 rem total effective dose equivalent (TEDE).

An individual located at any point on the outer boundary of the low population zone, who is exposed to the radioactive cloud resulting from the postulated fission product release (during the entire period of its passage) would not receive a radiation dose in excess of 25 rem TEDE.

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. These analyses are associated with the principal design criteria (PDC) and associated SSC design bases, which are required by 10 CFR 50.34(a)(3). Revision 3 of the Xe-100 Principal Design Criteria topical report (PDC TR) has been determined to be suitable for referencing in future licensing applications for the Xe-100 design, subject to the limitations documented in the NRC staffs safety evaluation (ML24319A155). Based on the Xe-100 design description, provided in TSAM section 3, Xe-100 Plant Structures, Systems, and Components Overview, the NRC staff identified the following 3 While the TSAM TR references older revisions of TR 008585, TR 006889, and TR 000632, the NRC staff references the most recent revisions of these TRs submitted to the NRC for review in this SE. The TR contains a limitation and condition that requires the use of the NRC staff-approved revisions of TR 008585, TR 006889, and TR 000632, which the staff discusses in section 2.5 of this SE.

PDC, including required functional design criteria (RFDC),4, as applicable to the analysis of DBAs for the Xe-1005:

Xe-100 PDC 10, Reactor design, requires that the reactor system and associated heat removal, control, and protection systems be designed with appropriate margin such that specified acceptable system radionuclide release design limits (SARRDLs) are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences. Demonstrating adequate reactor design generally includes, in part, the use of safety analysis methodologies (potentially including source term and consequence analysis methodologies).

Xe-100 PDC RFDC 11, Reactor inherent protection, requires that the reactor core and associated systems be designed with sufficient negative reactivity feedback characteristics such that, in the power operating range, the net effect compensates for a rapid increase in reactivity, adequately controls heat generation, and ensures fuel performance and radionuclide release limits are not exceeded during design basis events (DBEs) or DBAs. The NRC staffs SE for the Xe-100 PDC TR (ML24284A012) includes condition 2 which requires that applicable safety analyses cover the full scope of operating range, DBE, and DBA conditions applicable to the design (i.e., safety analyses are not limited to the power operating range). Demonstrating reactor inherent protection generally includes, in part, the use of safety analysis methodologies (potentially including source term and consequence analysis methodologies).

Xe-100 PDC RFDC 16, Functional containment design, requires that the design of the reactor fuel particles and pebbles provide barriers as part of the reactor functional containment to control the release of radioactivity to the environment to ensure that the functional containment design limit is not exceeded during DBEs and DBAs.

Demonstrating the functional containment design generally includes, in part, the use of safety analysis methodologies (potentially including source term and consequence analysis methodologies).

Xe-100 PDC 19, Control room, requires, in part, that adequate radiation protection be provided to permit access to and occupancy of the control room during anticipated operational occurrences (AOOs) and DBEs without personnel receiving radiation exposures in excess of 5 rem TEDE. Demonstrating this criterion generally involves the use of transient and safety analysis methodologies (including source term and radiological consequence analysis).

4 RFDC, as used in the Xe-100 PDC TR, is defined in NEI 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development, Revision 1 (ML19241A472) which is endorsed by RG 1.233, Guidance for a Technology Inclusive, Risk-Informed, and Performance-Based, Methodology and Content of Applications for Licenses, Certifications, and Approvals for Non-Light Water Reactors, (ML20091L698).

5 The Xe-100 PDC TR also includes complementary design criteria (CDC) which are defined in NEI 21-07, Technology Inclusive Guidance for Non-Light Water Reactors, (ML21250A378). CDC are not identified as part of the TSAM regulatory basis because the scope of TSAM is DBA analyses, which rely on safety-related SSCs, and CDCs are associated with non-safety related with special treatment (NSRST) SSCs.

Xe-100 PDC 20, Protection system functions, requires, in part, that the protection system be designed to sense conditions and initiate the operation of necessary systems and components to perform required safety functions. Demonstrating that this criterion is met generally involves the use of transient analysis methodologies.

Xe-100 PDC RFDC 26, Reactivity control systems, requires that the reactor be designed to include movable poisons that can insert and maintain safe shutdown during DBEs and DBAs. Demonstrating that this criterion is met generally involves the use of transient analysis methodologies.

Xe-100 PDC 28, Reactivity limits, requires that the reactor core, including the reactivity control systems, be designed with appropriate limits on the potential amount and rate of reactivity increase to ensure that DBEs and DBAs can neither: (1) result in damage to the reactor helium pressure boundary greater than limited local yielding, nor (2) sufficiently disturb the core, its support structures, or other reactor vessel internals to impair significantly the capability to cool the core. Demonstrating that this criterion is met generally involves the use of transient analysis methodologies.

Xe-100 PDC RFDC 30, Integrity of reactor helium pressure boundary, requires that the reactor be designed to detect moisture ingress within the helium pressure boundary and automatically isolate the source of moisture ingress during DBEs and DBAs.

Demonstrating that this criterion is met generally involves the use of transient analysis methodologies.

Xe-100 PDC RFDC 34, Residual heat removal, requires that a passive means to remove residual heat be designed to provide effective heat removal to ensure that fuel and radionuclide release limits are not exceeded during DBEs and DBAs. Demonstrating this criterion generally involves the use of transient and safety analysis methodologies (including source term and radiological consequence analysis).

The NRC staffs SE regarding the Xe-100 PDC TR includes a condition that applications referencing the TR must confirm that the PDCs remain appropriate for the design (ML24284A012). The NRC staff determined that the list of PDC identified above must be confirmed to ensure conformance with TR Xe-100 PDC. Accordingly, the NRC staff imposed condition 1 requiring an applicant referencing TSAM to confirm or update the regulatory basis relevant to the use of TSAM methods (i.e., relevant PDC and requirements to evaluate offsite radiological consequences).

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).

Under 10 CFR 50.43(e)(1)(iii) (for applications for an operating license, design certification, combined license, standard design approval, or manufacturing license which differ significantly from light-water reactor designs that were licensed before 1997) sufficient data must exist on the safety features of the design to assess the analytical tools used for safety analyses over a sufficient range of normal operating conditions, transient conditions, and specified accident sequences, including equilibrium core conditions. As described in 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, (ML23269A222):

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 EMs.

RG 1.203, Transient and Accident Analysis Methods, (ML053500170) describes a process that the NRC staff considers acceptable for use in developing and accessing EMs that may be used to analyze transient and accident behavior that is within the design basis of a nuclear power plant. Specifically, RG 1.203 provides a 20 step process, organized into four elements, that the NRC staff finds to be an acceptable means of developing and assessing EMs for use in the safety analysis of a nuclear power plant. TSAM section 1.2, Scope, states that the report, provides a high-level description of how X-energy plans to approach performing production safety analyses and how X-energy plans to address the methodology elements described in

[RG 1.203].

2 TECHNICAL EVALUATION TSAM section 1.2, Scope, clarifies that EMDAP, as described in RG 1.203, is used to develop the transient and safety analysis methodology for the Xe-100. Accordingly, the NRC staffs review of TSAM follows the steps outlined in RG 1.203, with EMDAP elements addressed in the following sections of this SE:

SE section 2.3 evaluates TSAM against EMDAP Element 1, Establish Requirements for Evaluation Model Capability (steps 1-4) of RG 1.203.

SE section 2.4 evaluates TSAM against EMDAP Element 2, Development Assessment Base, (steps 5-9) of RG 1.203.

SE section 2.5 evaluates TSAM against EMDAP Element 3, Development Evaluation Model, (steps 10-12) of RG 1.203.

SE section 2.6 evaluates TSAM against EMDAP Element 4, Assess Evaluation Model Adequacy, (steps 13-20) of RG 1.203.

SE section 2.7 evaluates the TSAM disposition of applicable limitations and conditions.

Specifically, X-energy previously submitted TR 000714, Transient and Safety Analysis Methodologies Framework, (ML21288A173) which was reviewed by the NRC staff in its SE dated October 8, 2021 (ML23086C082). The NRC staff SE includes four limitations that are discussed in TSAM section 2.5. The NRC staffs evaluation of the disposition of those limitations is provided in SE section 2.7.

SE section 2.8 discusses an additional consideration associated with the limitations on TSAM for the evaluation of radiological consequences.

2.1 Introduction and Outline TSAM is divided into 14 sections that the NRC staff evaluated as follows:

TSAM section 1, Introduction, provides an introduction to the TR and clarifies the scope for which the NRC staffs approval is being requested. The NRC staff makes no determinations on the information in TSAM section 1.

TSAM section 2, Overview of Regulatory Requirements and Guidance, identifies a wide spectrum of regulatory requirements (including all of 10 CFR 50.34(a) for CPs, all of 10 CFR 50.34(b) for operating licenses, and certain regulations in 10 CFR Part 52).

TSAM section 2 also identifies RG 1.203, Transient and Accident Analysis Methods, as applicable guidance and outlines how TSAM addresses the EMDAP elements described in RG 1.203 (TSAM figure 2 highlights how EMDAP is applied to the Xe-100). The NRC staff identified the regulatory basis in section Regulatory Basis, above and focused on the requirements applicable to the transient and safety analysis for a CP application (including relevant PDCs). The NRC staff focused on the requirements applicable to a CP because TSAM section 1.5, Outcome Objectives, states that: X-energy is requesting NRC review and approval [] to support preliminary analysis and evaluation of the Xe-100. The NRC staff considers the information in TSAM section 2 throughout the technical evaluation in SE section 2 but makes no determinations on the information in TSAM section 2. Additionally, for the reasons described in SE section 2.1 and section 2.3, step 1, the NRC staff imposed limitation 1 to limit the applicability of TSAM to the preliminary safety analysis of the Xe-100 as part of a CP application described in section 3 of the TSAM.

TSAM section 3, Xe-100 Plant Structures, Systems, and Components Overview, provides an overview of the Xe-100 design. The NRC staff considers this information throughout the technical evaluation in SE section 2 but makes no determinations on the information in TSAM section 3.

TSAM section 4, Transient and Safety Analysis Methodology Overview, and its associated appendices provide general information regarding the analysis of DBAs (e.g.,

initial conditions, decay heat modeling). The NRC staff considers this information in SE section 2.6 but makes no determinations on the information in TSAM section 4.

TSAM section 5, Xe-100 Transient and Safety Analysis Code Development, provides an overview of the analysis tools used in the EM (Flownex, GOTHIC, XSTERM) and also discusses the screening criteria applied to the thermal-hydraulic analyses that are used to estimate dose consequences. The NRC staff evaluates the overview of the analysis tools in SE section 2.5 and the screening criteria in SE section 2.6.

TSAM section 6, Code Verification and Validation Plans and Scaling, provides a high-level discussion of EM assessment plans and a description of the scaling approach that X-energy plans to use, in part, to justify the assessment data applicability for the Xe-100 design described in Section 3 of the TR. The NRC staff evaluates this information in SE section 2.4.

TSAM section 7, Xe-100 Figure of Merit, describes the figures of merit (FOMs) that are applicable to the Xe-100 transient and safety analysis. This information is evaluated in SE section 2.3.

TSAM section 8, Xe-100 Phenomena Identification and Ranking Tables, and its associated appendix summarizes the phenomena identification and ranking tables (PIRTs) completed by X-energy to-date. The NRC staff evaluates this information in SE section 2.3.

TSAM section 9, Transient and Safety Analysis Uncertainty Methodology, describes the process to address analysis uncertainty. The NRC staff evaluates this information in SE section 2.6.

TSAM section 10, Xe-100 Licensing Basis Event (LBE) Evaluation Model, of TSAM expands upon the discussion provided in section 4, Transient and Safety Analysis Methodology Overview, to further describe the initial conditions and explains the separation of safety analyses into plant transient and long-term phases. The NRC staff evaluates the initial conditions in SE section 2.5.

TSAM section 11, Specific Transient and Accident LBE Methodologies, and its appendices further expand upon the information in section 4, Transient and Safety Analysis Methodology Overview, and section 10, Xe-100 LBE Evaluation Model, of TSAM by discussing common assumptions applied to all initiating events, and demonstrating the EM execution for the control rod withdrawal, depressurized loss of forced circulation, and steam generator tube leak and rupture events. The NRC staff considers this information in SE section 2.3 and SE section 2.6.

TSAM section 12, Quality Assurance for Transient and Safety Analysis, 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 the NRC staff (ML24218A128). The NRC staff considers this information in SE section 2.5.

TSAM section 13, Conclusions and Limitations, restates that TSAM was developed in accordance with RG 1.203 and that the EM assessment will be provided in future submittals. Additionally, TSAM section 13 provides the following limitations that are considered by the NRC staff throughout the technical evaluation provided in SE section 2:

o The EM described in TSAM shall be used in combination with the NRC approved versions of: (1) 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.

o The completion of the EM assessment is needed to support a Final Safety Analysis Report (FSAR) (i.e., for an operating license application).

TSAM section 14, Cross References and References, contains the reference list. The NRC staff makes no determinations on the information in TSAM section 14.

2.2 Gaps in EM Development and Associated Requirements Under 10 CFR 50.34(a)(8)

The NRC staffs review of TSAM, provided in the following sections, identified individual EMDAP steps that are not fully addressed as outlined in RG 1.203. Under 10 CFR 50.34(a)(8) a CP applicant must identify and describe the research and development programs that will be conducted to resolve any safety questions associated with SSCs. Accordingly, the NRC staff identified conditions where an applicant that references TSAM in its application needs to describe how the EMDAP step, as applicable, will be addressed. The NRC staff notes that, in accordance with 10 CFR 50.35, Issuance of construction permits, the Commission may issue a CP if the Commission finds, in part, that there is reasonable assurance that: (i) such safety questions will be satisfactorily resolved at or before the latest date stated for the completion of the construction of the proposed facility, and (ii) taking into consideration the site criteria contained in 10 CFR 100, Reactor site criteria, the facility can be constructed and operated at the proposed location without undue risk to the health and safety of the public.

2.3 EMDAP Element 1: Establish Requirements for Evaluation Model Capability Step 1: Specify analysis purpose, transient class and power plant class TSAM section 2.3.1.1, Element 1: Establish Requirements for Evaluation Model Capability, states that: (1) TSAM is applicable to the Xe-100 reactor as described in TSAM section 3, (2) the primary purpose of the DBA EM is to calculate the offsite doses, (3) that analyses and transient classes (e.g., AOO, DBE, DBA) are identified using the NEI 18-04 methodology (ML19241A472), and (4) events included in the DBA EM are listed in TSAM section 11.2 and were identified using the Xe-100 full power probabilistic risk assessment (PRA). TSAM section 11.2 lists the following events:

1.

Turbine trip 2.

Reactor trip 3.

Partial loss of primary flow (single circulator trip) 4.

Loss of primary flow 5.

Loss of main feedwater flow 6.

Loss of offsite power 7.

Control rod withdrawal 8.

Small helium pressure boundary breach 9.

Medium helium pressure boundary breach

10. Large helium pressure boundary breach

11. Steam generator tube faults (small leaks and moderate leaks)

12. Partial loss of nuclear island cooling water

13. Loss of nuclear island cooling water system

14. Feedwater line break and main steam line breaks within the nuclear island auxiliary building

15. Feedwater line and main steam line leaks and breaks inside the reactor building

16. Main steam line large break in the steam generator citadel with induced large helium pressure boundary depressurization

17. Loss of condensate flow to deaerator

18. Loss of instrument air

19. Loss of nuclear island module and reactor building heating, ventilation, and air conditioning system

20. Fuel handling system line break The NRC staff recognizes that the list of DBA events identified by X-energy is preliminary because: (1) several events identified as DBAs (e.g., turbine trip, reactor trip, partial loss of primary flow, control rod withdrawal) are generally expected to occur on the frequency of an AOO and thus are not expected to be categorized as a DBAs, (2) TSAM contains no information regarding the evaluation of several events in TSAM section 11.2 (e.g., loss of condensate flow to deaerator, loss of instrument air, fuel handling system line break), and (3) DBA analyses are expected to be performed over the full range of applicable conditions and the list of identified events is derived from a full power PRA. Accordingly, the NRC staff imposed limitation 1 to limit the applicability of TSAM to the preliminary safety analyses of the Xe-100 design described in section 3 of TSAM, and condition 2 requiring an applicant referencing TSAM to justify how the DBA events identified in the preliminary safety analysis report are addressed by TSAM.

Step 2: Specify FOMs TSAM section 7, Xe-100 Figure of Merit, states that the FOM for the Xe-100 safety analysis is radiation dose at the EAB and references the acceptance criteria under 10 CFR 50.34(a)(1)(ii)(D) (see Regulatory Basis, above). In addition to the information provided in TSAM section 7:

TSAM section 8.2, Safety Analysis Figures of Merit, identifies additional FOMs that support the PDCs. Specifically, the FOMs identified in TSAM section 8.2 affect the design bases for SSCs that are credited to address PDCs (e.g., reactor fuel temperature is an FOM that directly affects SARRDLs from Xe-100 PDC 10 and maintaining adequate heat removal to limit fuel temperature impacts the design basis for SSCs credited to address Xe-100 PDC RFDC 34);

TSAM section 8.5.1, Figures of Merit, reidentifies dose consequences as a FOM and identifies controlled state as a separate FOM. The TSAM description of the controlled state FOM appears to describe a subset of attributes associated with the transient response (e.g., fuel temperature, fuel time at temperature, core reactivity and total power); and TSAM section 9.3.5, Steam Generator Tube Rupture Phenomena and Key Parameter Identification and Ranking Table (PK-PIRT), states that flammable gas production and offsite dose are the FOMs for the steam generator tube rupture event.

The NRC staff determined that TSAM partially addresses EMDAP step 2 because it identifies radiological dose as the FOM and identifies acceptance criteria that are consistent with NRC requirements. However, the NRC staff are unable to evaluate the information in TSAM section 8.2, section 8.5.1, and section 9.3.5 with respect to FOMs because: (1) the information provided is not consistent, (2) quantitative standards of acceptance are not provided (in accordance with RG 1.203), and (3) TSAM does not clearly describe how this information is used in the EM development process. Additionally, the NRC staff recognizes that surrogate FOMs may be necessary to assess the top level FOM (radiological dose) because the EM structure includes analysis tools that do not directly evaluate radiological dose (e.g., Flownex, GOTHIC). To evaluate EMDAP step 2, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 3 requiring an applicant referencing TSAM to describe how surrogate FOMs will be assessed as part of a relevant research and development program or justify that they are not needed to assess the top level FOM.

Step 3: Identify systems, components, phases, geometries, fields, and processes that must be modeled X-energy stated that EMDAP step 3 is addressed in TSAM section 8. However, the NRC staff did not identify information in TSAM that clearly addressed EMDAP step 3. As part of an audit, the NRC staff noted that X-energy addressed EMDAP step 3 through the PIRT process and that importance rankings associated with the highest level of a system breakdown were maintained across the system (i.e., the applicant identified and applied importance rankings associated with the highest level of a system to all SSCs with the system). Based on this information, the NRC staff are unable to determine that TSAM adequately addresses EMDAP step 3 because: (1)

TSAM does not contain information addressing EMDAP step 3, and (2) X-energy departed from RG 1.203 by not performing a hierarchal system decomposition. Performing the hierarchal system decomposition is important because phenomena at different hierarchical levels are likely to be different or have different importance or knowledge levels. To support a finding that a relevant research and development program will address EMDAP step 3, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 4 requiring an applicant referencing TSAM to identify the systems, components, phases, geometries, fields, and processes that must be modeled as part of a relevant research and development program.

Step 4: Identify and Rank Key Phenomena and Processes TSAM section 8, Xe-100 Phenomena Identification and Ranking Tables, TSAM section 9, Transient and Safety Analysis Uncertainty Methodology, TSAM appendix C, In-Core FOM PIRTs, and TSAM appendix D, Xe-100 Transient and Safety Analysis PK-PIRTs, summarize PIRTs that were performed and discusses how PIRT information is used to inform EM calculation uncertainty through the PK-PIRT process. Specifically:

TSAM section 8.1, Xe-100 PIRT Methodology, identifies the seven steps that were taken to complete the PIRTs. The NRC staff compared the steps outlined in TSAM against the nine step process used to develop PIRTs for the Next Generation Nuclear Plant (NGNP) as documented in NUREG/CR-6944, Next Generator Nuclear Plant PIRTs - Volume 1 Main Report, (ML081140459), section 1.2, The Phenomena Identification and Ranking Table (PIRT), and found the following differences:

o NGNP PIRT Methodology step 1 - define the issue that is driving the need for a PIRT is missing from the Xe-100 PIRT Methodology; o

NGNP PIRT Methodology step 2 - define the specific objectives for the PIRT that is missing from the Xe-100 PIRT Methodology; o

NGNP PIRT Methodology step 5 - identify, compile, and review the current knowledge base that is missing from the Xe-100 PIRT Methodology; and o

NGNP PIRT Methodology step 8 - assess the knowledge level for the phenomena that is missing from the Xe-100 PIRT Methodology.

TSAM section 8.5, In-Core PIRTs, states that the PIRTs were performed to address the following events:

o Control rod withdrawal o

Loss of secondary cooling o

Steam generator tube rupture o

Depressurization o

Pressurized loss of forced cooling o

Seismic o

Normal operating conditions o

Fuel mechanical performance TSAM appendix C provides a summary of all the phenomena that were considered by the PIRT panel that were considered for the events identified above.

Based on the information described above, the NRC staff determined that TSAM partially addresses EMDAP step 4 because TSAM identifies the importance levels for the phenomena associated with particular events. However, the NRC staff are unable to determine that TSAM fully addressed EMDAP step 4 because: (1) EMDAP steps 1 through 3 are not fully addressed (see above) and these steps inform the PIRT process, (2) the Xe-100 PIRT Methodology, as described in TSAM, does not describe the knowledge base informing the PIRT panel so the NRC staff are unable to determine that the PIRT panel was informed by sufficient or suitable information, (3) the Xe-100 PIRT Methodology, as described in TSAM, does not appear to assess the knowledge level of the related phenomena or the associated bases for that knowledge level which is an essential element for informing the EM assessment base, and (4)

TSAM section 10.4 clarifies that the transient and safety analyses are addressed using two characteristic time periods (see SE section 2.5, step 11) but this division into characteristic time periods does not appear to be captured in the PIRT process in accordance with RG 1.203 section 1.1.4, Step 4 Identify and Rank Key Phenomena and Processes, (ML053500170). To support a finding that a relevant research and development program will address EMDAP step 4, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 5 requiring an applicant referencing TSAM to 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 research and development program.

2.4 EMDAP Element 2: Development of Assessment Base Step 5: Specify objectives for assessment base X-energy stated that EMDAP step 5 is addressed in TSAM section 6. TSAM section 6.1 through TSAM section 6.4 provides high-level discussions of verification and validation plans for VSOP99/05, Flownex, XSTERM, and GOTHIC analysis codes. Specifically:

TSAM section 6.1, VSOP99/05 Code Verification and Validation Plans, provides no assessment objectives for VSOP99/05; TSAM section 6.2, Flow Code Verification and Validation Plan, highlights FOMs relevant to Flownex analyses (see SE section 2.3, step 2) but assessment objectives (e.g., important phenomena to be addressed through separate effects tests, assessment of system interactions and global code capability through integral effects tests) are not provided; TSAM section 6.3, Mechanistic Source Term Code Verification and Validation, contains no information regarding the assessment of XSTERM but refers to TR 000632, Mechanistic Source Term Approach (ML25073A093); and TSAM section 6.4, GOTHIC Code Verification and Validation, identifies the situations considered in the GOTHIC validation plan and highlights relevant FOMs (see SE section 2.3, step 2) but assessment objectives (e.g., important phenomena to be addressed through separate effects tests, assessment of system interactions and global code capability through integral effects tests) are not provided.

Based on the information discussed above and the information provided in SE section 2.3, the NRC staff are unable to determine that TSAM addressed EMDAP step 5 because: (1) TSAM does not clearly state the objectives of the assessment base, and (2) the NRC staff are unable to determine that the steps under EMDAP Element 1, Establish Requirements for Evaluation Model Capability, were addressed and as stated in RG 1.203, The selection of the database is a direct result of the requirements established in Element 1, (ML053500170). To support a finding that a relevant research and development program will address EMDAP step 5, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 6 requiring an applicant referencing TSAM to clearly state the objectives of the EM assessment base as part of a relevant research and development program.

Step 6: Perform scaling analysis and identify similarity criteria TSAM section 6.5.1 describes a scaling methodology that is consistent with the hierarchical two-tiered scaling (H2TS) method described in Zuber, et. al. [1] and referenced in RG 1.203. This method includes top-down and bottom-up analyses for assessing the adequacy of the experimental data being used to validate the EM and is supported by the PIRT with the identification of critical phenomena (see SE section 2.3, step 4). TSAM section 6.5.2.1, Step 1:

Identify Relevant and Critical Phenomena, states that scaling will be addressed for all high and medium ranked phenomena.

TSAM section 6.5.1.1, Description of the Hierarchical Two-Tiered Scaling (H2TS) Method, describes the H2TS method as including four elements: system decomposition, scale identification, top-down/system scaling analysis, and bottom-up/process scaling analysis.

The first element is the decomposition of the system into a hierarchical set of subsystems. The second element is the selection of the appropriate level for the scaling to be done for each critical phenomenon. The third element is the top-down scaling where characteristic time ratios are derived, and dimensionless groups are developed for each critical phenomenon. Non-dimensional coefficients generated from these groups are used to generate a similarity ratio, a ratio of test coefficient to reactor coefficient. The fourth element is the bottom-up scaling where detailed scaling analysis is performed for important processes.

TSAM section 6.5.2.4, Determining Adequacy of a V&V to Reactor Conditions, states that code accuracy calculated from validation activities needs to be addressed on a case-by-case basis and TSAM sections 6.4.2.5 through 6.4.2.7 discusses scenarios for various similarity ratios. The NRC staff agrees that the code accuracy should be addressed on a case-by-case basis but are unable to make any determination on the approaches because similarity ratios are generally not sufficient to demonstrate EM applicability or quantify EM uncertainty. Specifically, RG 1.203, section 1.2.2, Step 6: Perform Scaling Analysis and Identify Similar Criteria, (ML053500170) clarifies that, the need [for scaling analysis] is to demonstrate that the experimental database is sufficiently diverse [such] that the expected plant-specific response is bounded and the EM calculations are comparable to the corresponding tests in non-dimensional space. Therefore, the NRC staff expects that the experimental database encompass a range of experimental data whose non-dimensional numbers bound the values applicable to the specific plant. To support a finding that a relevant research and development program will address EMDAP step 6, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 7 requiring an applicant referencing TSAM to include assessing 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.

Based on the use of the H2TS scaling, and subject to condition 7, the NRC staff determined that the general scaling approach is acceptable because H2TS is an established scaling approach that is referenced in RG 1.203. However, TSAM does not contain Xe-100 specific scaling analyses to enable the NRC staff to determine data applicability to the Xe-100. Accordingly, the NRC staff imposed limitation 2, which limits approval to the general use of the H2TS methodology.

Step 7: Identify existing data and/or perform Integral Effects Tests (IETs) and Separate Effects Tests (SETs) to complete the database X-energy stated that EMDAP step 7 is addressed in TSAM section 6. As described in SE section 2.4, step 5, TSAM provides general information regarding scenarios considered for validation and highlights FOMs. However, the NRC staff did not identify information in TSAM identifying existing data or assessing the need for IETs or SETs. Based on the information provided in TSAM, the NRC staff are unable to determine that TSAM addressed EMDAP step 7 because TSAM does not contain the relevant information needed. To support a determination that a relevant research and development program will address EMDAP step 7, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 8 requiring an applicant referencing TSAM to identify existing data and assess the need for IETs and SETs, either directly or incorporated by reference, to complete the assessment database as part of a relevant research and development program.

Step 8: Evaluate the effects of IET distortions and SET scaleup capability X-energy stated that EMDAP step 8 is addressed in TSAM section 6. TSAM does provide a general description of scaling using the H2TS approach (see SE section 2.4, step 6). As discussed in SE section 2.4, step 6, test adequacy is assessed on a case-by-case basis and the NRC staff are unable to make a determination regarding the adequacy of the approaches discussed in TSAM sections 6.4.2.5 through 6.4.2.7 (which discussed the use of scaling ratio to assess test adequacy). Additionally, the NRC staff did not identify any information in TSAM that addresses SET scaleup capability. Based on the information discussed in this section, the NRC staff are unable to determine that TSAM addresses EMDAP step 8 because TSAM does not contain the relevant information needed to address this step.

To support a finding that a relevant research and development program will address EMDAP step 8, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 9 requiring an applicant referencing TSAM to include evaluation of effects of IET distortion and SET scaleup capability.

Step 9: Determine experimental uncertainties as appropriate X-energy stated that EMDAP step 9 is addressed in TSAM section 6. As stated in SE section 2.4, step 7, the NRC staff did not identify information in TSAM identifying existing data or assessing the need for IETs or SETs. Therefore, the NRC staff did not identify information in TSAM describing the associated experimental uncertainties. TSAM section 9.1 does state that calculation uncertainty accounts for bias and uncertainty related to how accurately the computer code selected matches benchmark experiments. This determination of bias and uncertainty falls under EMDAP Element 4 which is assessed in SE section 2.6. Based on the information provided, the NRC staff are unable to determine that TSAM addresses EMDAP step 9 because TSAM does not contain the relevant information needed. To support a finding that a relevant research and development program will address EMDAP step 9, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 10 requiring an applicant referencing TSAM to include evaluation of experimental uncertainties associated with EM assessment data.

2.5 EMDAP Element 3: Development of Evaluation Model Step 10: Establish EM development plan X-energy stated that EMDAP step 10 is addressed in TSAM section 5 and TSAM section 12.

TSAM section 5, Xe-100 Transient and Safety Analysis Code Development, summarizes the codes that are used as part of the EM. The NRC staff identifies the information in TSAM section 5 as generally associated with EMDAP step 11 and assesses this information in SE section 2.5, step 11.

TSAM section 12, Quality Assurance for Transient and Safety Analysis, 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 the NRC staff (ML24218A128).

Based on the information described in TSAM section 12, the NRC staff concludes that TSAM addressed EMDAP step 10 because: (1) EM development is being performed under an approved quality assurance program, and (2) items identified in RG 1.203, section 1.3.1, Step 10: Establish an Evaluation Model Development Plan, are expected to be addressed by the quality assurance program (e.g., documentation requirements, programming standards and procedures, configuration control procedures).

Step 11: Establish EM structure X-energy stated that EMDAP step 11 is addressed in TSAM section 10. TSAM section 10 states that there is one EM for modeling all licensing basis events, including DBAs, that uses Flownex, GOTHIC, and XSTERM. TSAM figure 22, Xe-100 Transient Safety Analysis Flowchart, in TSAM section 10 shows the EM interfaces. In addition to this information, TSAM section 5, Xe-100 Transient and Safety Analysis Code Development, summarizes the codes that are used as part of the EM and discusses a simplified approach for performing source term evaluations.

Specifically:

TSAM section 5.1, Xe-100 Transient and Safety Analysis Chart, provides an example high-level structure showing how calculations and data may interact.

TSAM section 5.2, Neutronics Code, states that reactor physics input to the EM is provided in accordance with TR 006889, Reactor Core Design Methods and Analysis, (ML25071A399).

TSAM section 5.3,1 Flownex SE, describes the Flownex Simulation Environment system code that: (1) is based on one-dimensional conservation of mass, momentum, and energy equations that are discretized using finite volume, (2) includes an integrated point kinetics model for reactor physics, and (3) includes specialized models for modeling high-temperature gas reactors.

TSAM section 5.4.2, GOTHIC, describes GOTHIC as a versatile, general-purpose, thermal hydraulics software package that solves conservation of mass, momentum, and energy equations in lumped parameter or multi-dimensional spaces.

TSAM section 5.4, Mechanistic Source Term, states that: (1) a software code, XSTERM, was developed for quantification of the Xe-100 source terms and dose calculations, and (2) XSTERM is described in TR 000632, Mechanistic Source Term Approach (ML25073A093).

TSAM section 5.5, Flownex Screening Criteria, clarifies that response surfaces, referred to as Flownex Screening Criteria (FSC), are developed using XSTERM that intends to provide conservative dose estimates using Flownex calculated pebble temperatures as input. The NRC staff notes that the FSC, as described in TSAM does not provide actual screening criteria (i.e., there are no acceptance criteria described in TSAM that could determine whether XSTERM evaluations are needed).

TSAM section 10 also provides the following information:

TSAM section 10.1, Initial Conditions Methodology, restates that reactor physics input to the EM is provided in accordance with TR 006889, Reactor Core Design Methods and Analysis, (ML25071A399), Flownex calculates steady-state thermal-fluid conditions, and FSC/XSTERM calculated stead-state isotope inventory, fission product release from fuel, radioactivity in helium heat transport system. Additionally, TSAM section 10.1 provides a subset of preliminary initial conditions that appears to correspond to nominal full power operation.

TSAM section 10.2, Boundary Conditions Methodology, states that the final set of conditions and interfaces will be outlined in future submittals for the NRC staffs review, and also states that Flownex calculates the thermal and flow boundary conditions across the reactor used in VSOP, FSC, and GOTHIC simulations.

TSAM section 10.3, States for Safety Analysis, states that the examples provided in TSAM are focused on a nominal full power operating state, but future work will include finalizing the methodology for running cases to cover all operating modes and states (and considering uncertainties in the key parameters that may affect the analysis outcome). TSAM section 10.3 further states that the example analyses are provided for the purpose of preliminary SSC performance evaluation. Additionally, TSAM section 10.3 discusses operating states which ranges from Mode 1, Depressurized Shutdown, to Mode 5, Power Operation. The NRC staff notes that the mode definitions selected by X-energy are generally opposite from standard convention.

Specifically, reactor designs licensed, certified, or permitted to-date generally define Mode 1 as Power Operation with increasing mode numbers corresponding to lower power, shutdown, and refueling states (e.g., NUREG-1433, Standard Technical Specifications - General Electric Plants (BWR/4): Specifications, Revision 5 (ML21272A357), NUREG-1431, Standard Technical Specifications - Westinghouse Plants: Specifications, Revision 5 (ML21259A155), Generic Technical Specifications -

NuScale Nuclear Power Plants Volume 1: Specifications, (ML20224A514), Hermes Non-Power Reactor Preliminary Safety Analysis Report, Chapter 14 Technical Specifications, (ML21272A378)).

TSAM section 10.4, Xe-100 Transient and Safety Analysis Process, states that transient and safety analyses fall into two portions (i.e., characteristic time periods) of plant transient and passive cooling. The NRC staff notes that this separation of transients into separate characteristic time periods does not appear to have been considered as part of the PIRT process as described in RG 1.203, section 1.1.4, Step 4:

Identify and Rank Key Phenomena and Processes, (see SE section 2.3, step 4). TSAM section 10.4.1, Plant Transient, states that this time period: (1) corresponds to the portion of the scenario where plant parameters are evolving on the order of minutes or seconds in response to the initiating event, (2) is analyzed using the Flownex code, and (3) is screened for the need to execute XSTERM using FSC (as discussed in SE section 2.4, the FSC as described in TSAM do not include screening criteria). TSAM section 10.4.2, Long-Term, states that a long-term passive cooling scenario: (1) is entered once the core is simply conducting and radiating heat away, (2) is expected that most fission product release will occur during this time period, and (3) is analyzed using GOTHIC and XSTERM long-term pressurized loss of forced cooling, depressurized loss of forced cooling, steam generator tube rupture, or if the Flownex analysis does not meet the FSC.

TSAM section 10.5, Use of Flownex in the Evaluation Model, and TSAM section 10.6, Use of GOTHIC in the Evaluation Model, clarifies that: (1) the Flownex base model is intended for short-term transient analysis of the Xe-100 licensing basis events that occur on the time scale of approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, (2) GOTHIC is used to simulate the Xe-100 reactor building response to licensing basis events, and (3) further details regarding Flownex and GOTHIC are provided in TR 008585, GOTHIC and Flownex Analysis Codes Qualification, (ML25076A053).

Based on the information described in this section:

The NRC staff are unable to determine that the codes in the EM are suitable for preliminary analysis because the details of the codes are not provided in TSAM (the details are provided in separate TRs). However, TSAM section 13.2, Limitations, clarifies that the EM described in TSAM shall be used in combination with NRC approved versions of: (1) 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. The NRC staffs review of those methodologies is documented in the safety evaluations associated with those TRs.

The NRC staff agrees that the use of FSC, as described in TSAM section 5.5, appears reasonable for estimating dose associated with diffusion driven phenomena from the fuel because diffusion of radionuclides is a strong function of temperature. However, TSAM does not describe what radionuclide release mechanisms are accounted for by the FSC and no justification is provided for its applicability. Therefore, the NRC staff is unable to assess the application of FSC to specific events because: (1) TSAM does not provide any specific FSC, (2) the FSC do not appear to address all radionuclide release mechanisms, (3) TSAM does not describe the specific events where FSC would be applied, and (4) TSAM does not describe screening criteria (e.g., objective criteria indicating when FSC can be used in lieu of XSTERM analysis). To assess whether the use of FSC are acceptable for analyses of the Xe-100 (preliminary and final), the NRC staff imposed condition 11, requiring an applicant referencing TSAM to provide justification for the use of FSC on a case-by-case basis and condition 12 requiring an applicant referencing TSAM to 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: Develop or incorporate closure models X-energy stated that EMDAP step 12 is addressed in TSAM section 5. However, the NRC staff did not identify information in TSAM describing the development or incorporation of closure models. During a regulatory audit, the NRC staff further noted X-energy statements that there are few closure models being developed specifically for X-energys EM and that the models are still under development and/or are being validated. The NRC staff were not able to identify information in TSAM describing what models are being developed or the basis for that development. Based on the information described in this section, the NRC staff is unable to determine that TSAM addressed EMDAP step 12 because information relevant to EMDAP step 12 is not provided in TSAM. To support a finding that a relevant research and development program will address EMDAP step 12, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 13 requiring an applicant referencing TSAM to describe how closure models will be developed or incorporated into the EM, either directly or incorporated by reference.

2.6 EMDAP Element 4: Assess Evaluation Model Adequacy X-energy stated that EMDAP Element 4 (steps 13-20) is not addressed by TSAM. Additionally, TSAM section 13.2, Limitations, states that the EM described in TSAM are unable to be used to support final safety analysis report until validation and verification of analysis codes within the EM have been approved by the NRC staff. The NRC staff confirmed that the steps under EMDAP Element 4 are not addressed in TSAM, but the NRC staff did identify information relevant to EMDAP step 20, Determine Evaluation Model Biases and Uncertainties.

Specifically, TSAM section 9, Transient and Safety Analysis Uncertainty Methodology, contains information that is associated with EMDAP step 20. TSAM section 9 provides the following information:

TSAM section 9.1, Uncertainty Analysis Methodology, states that all key uncertainties will be identified and accounted for using either conservative or best-estimate plus uncertainty approaches.

TSAM section 9.2, Uncertainty Analysis Input Methodology, states that: (1) uncertainty on a FOM is quantified either by propagating input uncertainties through the code to the output parameters or from other sources of validation where the uncertainty is directly applied to the final calculated output parameter, (2) uncertainty distributions and associated range (typically 2-sigma is applied to relevant input parameters) are applied, (3) uncertainty distributions may not be well known and would need to be justified, (4) if no basis for the uncertainty bandwidth can be found, it is assumed that large uncertainty bandwidth of +/- 10 percent of the nominal value would be sufficient to envelope the +/-

2-sigma value, and (5) the assumed +/- 10 percent uncertainty will be revisited if the parameter is found to have a significant impact on the safety analysis FOM.

TSAM section 9.3, Xe-100 PK-PIRT Methodology, states that: (1) phenomena characterizing parameters are decomposed into more fundamental quantities using code specific theory and associated documentation, and (2) the set of decomposed parameters or sub-parameters constitute the set of uncertainty parameters. TSAM section 9.3 summarizes the PK-PIRT process into 17 steps. Particular aspects of the PK-PIRT process noted by the NRC staff include:

o PK-PIRT steps 3 and 4 appear to restrict the number of phenomena considered without justification; o

PK-PIRT step 9 sets a goal of two parameters per model and states that medium and low parameters can be dropped without justification; o

TSAM equation 12 appears to consider the impact of the perturbations calculated in PK-PIRT steps 12-17 and treats each parameter as independent, but the perturbation process maintains extreme values of the preceding propagation (e.g.,

PK-PIRT step 15), so the associated sensitivities may have a dependency on the order of perturbation; and o

TSAM section 9.3.1, Acceptance Criteria, provides acceptance criteria that restricts the number of parameters that are considered in the uncertainty propagation without justification.

TSAM section 9.3.2, PK-PIRT Panels, provides the general composition of a PK-PIRT panel.

TSAM section 9.3.3 through section 9.3.6 summarizes PK-PIRTs for specific event scenarios and highlights event specific FOMs.

Based on the information described in this section:

The NRC staff are unable to determine that TSAM addresses EMDAP Element 4 because the associated steps are not addressed in TSAM. To determine whether a relevant research and development program will address EMDAP Element 4, in accordance with 10 CFR 50.34(a)(8) (see SE section 2.2), the NRC staff imposed condition 14 requiring an applicant referencing TSAM to address EM assessment, either directly or incorporated by reference.

The NRC staff are unable to determine that the uncertainty analysis methodology is appropriate for the following reasons.

1.

The assumption on uncertainty distributions (i.e., +/- 10 percent is assumed to envelope +/- 2 sigma when no basis for bandwidth can be found) is not justified and does not appear to be sufficiently large because several inputs are expected to have uncertainties that exceed 10 percent (e.g., uncertainty in heat transfer coefficients for the pebble bed generally exceeds 20 percent [2], uncertainty in pressure drop in the pebble bed is expected to exceed 10 percent [3], NRC staff expected the uncertainty in irradiated graphite thermal-conductivity to exceed 10 percent).

2.

The PK-PIRT process as described in TSAM appears to restrict the number of phenomena considered without justification and the NRC staff expects that all high and medium importance ranked phenomena be either incorporated into the EM or otherwise justified.

3.

The PK-PIRT process as describe in TSAM appears to restrict the number of parameters that can be considered per model without justification and the NRC staff are unable to determine that two parameters would be adequate to capture input parameter uncertainty for several models (e.g., a heat transfer coefficient that is a function of Reynolds and Prandtl numbers would decompose to six parameters).

4.

Use of TSAM equation 12 to perform uncertainty propagation requires further justification because it is unclear whether the extreme value perturbation process, described in PK-PIRT steps 12-17, results in independent or conservative sensitivities.

5.

The acceptance criteria imposed on the PK-PIRT process restricts the number of parameters that are considered in the uncertainty propagation without justification and the NRC staff expects that all high and medium importance phenomena (and associated parameters) be either incorporated into the EM or otherwise justified.

The NRC staff did not impose conditions specific to uncertainty propagation because uncertainty propagation and overall EM uncertainty is associated with EMDAP step 20 and is addressed by condition 14.

2.7 Disposition of Relevant Limitations and Conditions TSAM section 2.5, X-energy NRC SER for Xe-100 Topical Report: Transient and Safety Analysis Methodologies Framework, Revision 1, describes how TSAM addresses the four limitations provided in the NRC staffs SE, dated October 8, 2021 (ML23086C082), for the previously reviewed TR 000714, Transient and Safety Analysis Methodologies Framework, (ML21288A173).

Limitation1 from the NRC staffs SE states:

An appropriate set of FOMs, justified for the full spectrum of events to be modeled for the X-energy design will need to be provided. This is likely to require sufficient design finalization such that scenario classes and operational regimes can be identified or bounded.

To address this limitation, TSAM section 2.5 references the information provided in TSAM section 7 and TSAM section 8. The NRC staffs review of the FOMs is provided in section 2.3, step 2 of this SE and determined that TSAM generally addresses EMDAP step 2 but also imposed condition 3 (see SE section 2.3, step 2).

Limitation 2 from the NRC staffs SE states:

A final PIRT (or functionally similar tool) and list of relevant SSCs and phenomena to be modeled, along with relevant assumptions, should be made available as part of a future submittal related to the TR in order to facilitate review of the final EM.

To address this limitation, TSAM section 2.5 references the information provided in TSAM section 8, TSAM section 9, and the associated appendices. The NRC staffs review of the PIRTs is provided in SE section 2.3, step 4 and determined that TSAM does not fully address EMDAP step 4 and imposed condition 5 (see SE section 2.3, step 4).

Limitation 3 from the NRC staffs SE states:

Adequate verification and validation assessment information should be made available to the NRC staff as part of future submittals supporting the codes that make up the EM. This verification and validation information should be justified to reasonably bound the operational envelope for the design for any applicant referencing the TR. To the extent that the computer codes are coupled to one another, the verification and validation of the coupled configuration should also have adequate verification and validation associated with the configuration exercised as used for the safety analysis.

To address this limitation, TSAM section 2.5 references the information provided in TSAM section 6. The NRC staffs review of EM assessment is provided in SE section 2.6 and determined that EMDAP Element 4 is generally not addressed by TSAM and imposed condition 14 (see SE section 2.6).

Limitation 4 from the NRC staffs SE states:

In order to link the transient and accident analysis methodology to the design itself, an applicant utilizing the TR needs to justify the use of the model for the design. This justification must discuss the capability of the model in the context of what is needed to appropriately represent the design and discuss how the model is applicable to the design, including consideration of system interactions occurring in the design, system conditions (which may affect the applicability of models or validation data). Uncertainties associated with the EM and the validation data should be discussed in accordance with RG 1.203.

To address this limitation, TSAM section 2.5 references the information provided in TSAM section 9 through section 11 and TSAM appendix B. EM applicability is generally considered throughout the EMDAP process as described in RG 1.203 (ML053500170) with particular emphasis placed on EMDAP Element 4. The NRC staffs evaluation of TSAM against the EMDAP elements is provided in SE section 2.3 through section 2.6. Based on that evaluation, the NRC staff was not able to determine that the EMDAP elements were fully addressed because the NRC staff were not able to determine that steps within each element were fully addressed and imposed condition 2 through condition 14 to address these steps.

Based on the information described in this section, the NRC staff is unable to determine that all the limitations from the NRC staffs SE, dated October 8, 2021 (ML23086C082), were fully addressed because the required information is not provided in TSAM. However, for an application that references this TR, this SE identifies limitations and conditions that supersede the limitations identified in the NRC staffs SE dated October 8, 2021.

2.8 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 (FSC as described in TSAM section 5 and section 10 is associated with assessing radiological consequences using fuel thermal conditions and is reviewed in SE section 2.5, step 11). Specifically, the NRC staff did not identify information in TSAM addressing radionuclide production, transport, dispersion, and conversion to radiological dose.

Additionally, TSAM figure 22, Xe-100 Transient Safety Analysis Flowchart, shows that the radiological consequence analysis code (XSTERM) uses thermal-fluid information from Flownex, and GOTHIC. Accordingly, the NRC staff imposed limitation 3, which limits the approval of TSAM to the thermal-fluid inputs used for radiological consequence assessment and general use of FSC.

3 LIMITATIONS AND CONDITIONS In addition to the limitations described in TSAM section 13.2, Limitations, the NRC staffs conclusions regarding TSAM are subject to the following limitations and conditions:

Limitation 1 Application of this TR is limited to the preliminary safety analysis of the Xe-100 design as part of CP application as described in section 3 of TSAM. SE section 2.1 and section 2.3, step 1 describe the basis for this limitation.

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. SE section 2.4, step 6 describes the basis for this limitation.

Limitation 3 The NRC staffs approval of this TR for radiological consequence analysis is limited to the thermal-fluid inputs use in radiological consequence analysis and the general use of FSC. SE section 2.8 describes the basis for this limitation.

Condition 1 A CP application referencing this TR must confirm or update the regulatory basis relevant to the use of TSAM methods (i.e., relevant PDC and requirements to evaluate offsite radiological consequences). SE section 1 describes the basis for this condition.

Condition 2 A CP application referencing this TR must: (1) identify the preliminary set of DBAs identified for the design and (2) justify how those DBAs are addressed by this TR. SE section 2.3, step 1 describes the basis for this condition.

Condition 3 A CP application referencing this TR must describe how surrogate FOMs will be assessed as part of a relevant research and development program or justify that they are not needed. SE section 2.3, step 2 describes the basis for this condition.

Condition 4 A CP application referencing this TR 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. SE section 2.3, step 3 describes the basis for this condition.

Condition 5 A CP application referencing this TR 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. SE section 2.3, step 4 describes the basis for this condition.

Condition 6 A CP application referencing this TR must clearly state the objectives of the EM assessment base as part of a relevant research and development program. SE section 2.4, step 5 describes the basis for this condition.

Condition 7 A CP application referencing this TR 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. SE section 2.4, step 6 describes the basis for this condition.

Condition 8 A CP application referencing this TR must identify existing data and assess the need for IETs and SETs, either directly or incorporated by reference, to complete the assessment database as part of a relevant research and development program. SE section 2.4, step 7 describes the basis for this condition.

Condition 9 A CP application referencing this TR must include evaluation of effects of IET distortion and SET scaleup capability as part of a relevant research and development program. SE section 2.4, step 8 describes the basis for this condition.

Condition 10 A CP application referencing this TR must include evaluation of experimental uncertainties associated with EM assessment data as part of a relevant research and development program. SE section 2.4, step 9 describes the basis for this condition.

Condition 11 A CP application applying FSC in accordance with this TR must provide justification for the use of FSC on a case-by-case basis. SE section 2.5, step 11 describes the basis for this condition.

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. SE section 2.5, step 11 describes the basis for this condition.

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. SE section 2.5, step 12 describes the basis for this condition.

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. SE section 2.6 describes the basis for this condition.

4 CONCLUSION The NRC staff approves the use of TR 007834, Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology, Revision 2, for the preliminary safety analysis of DBAs for the Xe-100 subject to the limitations and conditions identified in SE section 3. This conclusion is based on the following:

The use of the EM as described in TSAM is acceptable for informing radiological consequence evaluations to address Xe-100 PDC 19 and 10 CFR 50.34(a)(1)(ii)(D) because, pursuant to the limitations described in TSAM section 13.2, condition 11, and condition 12, sufficient justification would be provided in a CP application or during the associated safety review to ensure that the codes used within 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 safety analysis of the design and performance of Xe-100 SSCs in accordance with 10 CFR 50.34(a)(4) and, pursuant to condition 1, demonstrating, in part, preliminary compliance with Xe-100 PDC 10, Xe-100 PDC RFDC 11, Xe-100 PDC RFDC 16, Xe-100 PDC 19, Xe-100 PDC 20, Xe-100 PDC RFDC 26, Xe-100 PDC 28, Xe-100 PDC RFDC 30, and Xe-100 PDC RFDC 34 because: (1) CP applications are not required to provide evaluations of the safety margins using approved EMs (see SE section 1), and (2) pursuant to the limitations provided in TSAM section 13.2, condition 11, and condition 12, sufficient justification would be provided in a CP application or during the associated safety review to ensure that the codes used within 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 requirement under 10 CFR 50.34(a)(8) to describe a research plan to resolve safety questions regarding EM applicability to the Xe-100 because addressing those conditions would result in an EM that complies with the approved guidance provided in RG 1.203, Transient and Accident Analysis Methods, (ML053500170).

For an application that references this TR, the limitations and conditions identified in SE section 3 supersede the limitations identified in the NRC staffs SE dated October 8, 2021 (see SE section 2.7).

5 REFERENCES 1.

N. Zuber et al., An Integrated Structure and Scaling Methodology for Severe Accident Technical Issue Resolution: Development of Methodology, Nuclear Engineering and Design, 186 (pp. 1-21), issued 1998.

2.

KTA 3102.2, Reactor Core Design of High-Temperature Gas-Cooled Reactors, Part 2: Heat Transfer in Spherical Fuel Elements, Safety Standards of the Nuclear Safety Standards Commission (KTA), issued June 1983.

3.

KTA 3102.3, Reactor Core Design of High-Temperature Gas-Cooled Reactors, Part 3: Loss of Pressure through Friction in Pebble Bed Cores, Safety Standards of the Nuclear Safety Standards Commission (KTA), issued March 1981.

Principal Contributors:

Tim Drzewiecki, NRR/DANU/UTB1 Tracy Radel, NRR/DANU/UTB2 Pravin Sawant, NRR/DANU/UTB1 Santosh Bhatt, NRR/DANU/UTB3 Date: May 6, 2025