ML25076A692

Draft Safety Evaluation of Topical Report Xe-100 Licensing Topical Report Gothic and Flownex Analysis Codes Qualification Rev. 3

ML25076A692
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?0021
Download: ML25076A692 (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 XE100 LICENSING TOPICAL REPORT GOTHIC AND FLOWNEX ANALYSIS CODES QUALIFICATION, REVISION 3, (EPID NO.: L2024TOP0021)

Dear Mr. Kalinousky:

By letter dated March 13, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25076A053), X Energy, LLC., (Xenergy) submitted Revision 3 of its Xe100 Licensing Topical Report (TR) GOTHIC and Flownex Analysis Codes Qualification, to the U.S. Nuclear Regulatory Commission (NRC) staff for review. This TR describes the GOTHIC and Flownex models and computer codes that represent the thermal-hydraulic phenomena associated with the transient and safety analysis evaluation model for the Xe100 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) 4150681 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 XEnergy Xe100 GovDelivery May 6, 2025 Signed by Philpott, Stephen on 05/06/25

ML25076A692 NRR043 OFFICE NRR/DANU/UAL2:PM NRR/DANU/UAL2:LA NRR/DANU/UTB1:BC NAME DMcGovern CSmith TTate DATE 3/26/2025 3/26/2025 4/29/2025 OFFICE OGC/NLO NRR/DANU/UAL2:BC NAME JEzell SPhilpott DATE 4/29/2025 5/6/2025

Enclosure XENERGY-DRAFT SAFETY EVALUATION OF TOPICAL REPORT XE-100 LICENSING TOPICAL REPORT GOTHIC AND FLOWNEX ANALYSIS CODES QUALIFICATION, REVISION 3 (EPID L2024TOP0021)

SPONSOR AND SUBMITTAL INFORMATION Sponsor:

X Energy, LLC. (Xenergy)

Sponsor Address:

X Energy, LLC.

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

99902071 Submittal Date:

March 13, 2025 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.:

ML25076A053 (package)

Brief Description of the Topical Report:

On May 22, 2024, Xenergy, LLC (Xenergy) submitted topical report (TR), Xe-100 Licensing Topical Report GOTHIC and Flownex Analysis Codes Qualification, Revision 2 (ML24143A192 (package)) for U.S. Nuclear Regulatory Commission (NRC) staff review and approval. On March 13, 2025, Xenergy submitted Revision 3 of this TR (ML25076A053 (package)) (hereafter referred to as GFQ). This TR describes the GOTHIC and Flownex analysis codes and associated models used to perform transient analysis evaluations for the Xe100 plant design.1 Additionally, TR 007834, Xe-100 Licensing Topical Report Transient and Safety Analysis Methodology, Revision 2 (ML25077A288) (hereafter referred to as TSAM) which describes the overall safety analysis methodology developed to perform preliminary analysis and evaluation of Design Basis Accidents (DBAs) for the Xe100 design, is dependent upon the use of the GOTHIC and Flownex codes as described in the GFQ TR in support of the safety analysis methodology. On August 29, 2024, the NRC staff transmitted an audit plan to Xenergy (ML24236A768), and subsequently conducted an audit of materials related to the TR.

The GFQ TR: (1) provides an overview of the GOTHIC and Flownex computer codes, (2) describes the preliminary Xe100 models developed for DBA analysis, (3) summarizes the completed and planned verification and validation (V&V) activities, and (4) describes the quality assurance approach for the codes. The Flownex computer code is used to analyze short-term transient response while the GOTHIC computer code is used to analyze the long-term transient response. Section 3.1, Introduction, of the GFQ TR states: The short-term transient is defined as the period at which [systems, structures, and components (SSCs)] are actively responding to an initiating event, forced cooling remains available or the primary system is actively depressurizing. The long-term transient is defined as the period at which passive heat transfer 1 This SE does not evaluate or approve the Xe-100 design.

begins and no additional active plant responses to the initiating event are considered. Xenergy requests the NRC staffs review and approval of the GOTHIC and Flownex models and codes in the GFQ TR to perform the preliminary analysis of DBAs for the Xe100 design.

REGULATORY EVALUATION Regulatory Basis:

Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(4) requires, in part, an applicant for a construction permit (CP) to 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 margins of safety during normal operations and transient conditions. The GOTHIC and Flownex models presented in this TR support preliminary analysis of the Xe100 design described in TSAM Section 3 for a CP application.

Regulatory Guide (RG) 1.233, Revision 0, Guidance for Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors (ML20091L698),

provides the NRC staffs guidance on using a technology-inclusive, risk-informed, and performance-based methodology to inform the licensing basis and content of applications for non-light water reactors (LWRs). It endorses NEI 18-04, Revision 1, Risk-Informed Performance-Based Technology-Inclusive Guidance for Non-Light Water Reactors, (ML19241A472) with clarifications as one acceptable method for informing the licensing basis and determining the appropriate scope and level of detail for parts of applications for licenses, certifications, and approvals for non-LWRs.

NEI1804 states that the codes and models used in DBA analysis are expected to satisfy RG 1.203, Transient and Accident Analysis Methods, (ML053500170) requirements for evaluation models (EMs). RG 1.203 describes a process that the NRC staff considers acceptable for use in developing and assessing 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, Revision 0, provides a 20-step process called Evaluation Model Development and Assessment Process (EMDAP), organized into four elements that the NRC staff determined to be an acceptable means of developing and assessing EMs for use in the safety analysis of a nuclear power plant. The EMDAP is followed in the development of the overall safety analysis methodology and the GFQ TR addresses portions of EMDAP. Accordingly, the NRC staffs review of the GFQ TR follows the steps in RG 1.203 that are applicable to the GFQ TR.

TECHNICAL EVALUATION The scope of the NRC staffs review documented in this safety evaluation (SE) includes:

The review of the GOTHIC and Flownex models and qualifications (validations) presented in the GFQ TR against the guidance in applicable EMDAP Steps, and The evaluation of the adequacy of GOTHIC and Flownex Xe100 input models for the preliminary analysis of DBAs in the Xe100 design.

The overview of the Xe100 design information provided for this TR is available in TSAM Section 3, Xe-100 Plant Structures, Systems, and Components Overview, (ML25077A285). As summarized in Table 1, EMDAP roadmap for the Xe-100 DBA EM, of this SE, TSAM also addresses several steps in EMDAP. EMDAP Element 1, Establish Requirements for Evaluation Model Capability, for Xe-100 DBA EM is addressed in TSAM. The phenomena identification and ranking table (PIRT) presented in Appendix C of TSAM is referenced in the GFQ TR to support the development of GOTHIC-Flownex models and their qualification plans.

Furthermore, as summarized in Table 1 of this SE and discussed in the NRC staffs SE on TSAM (ML25062A069), some of the EMDAP Steps under Element 2, Develop Assessment Base, and Element 3, Develop Evaluation Model, are partially addressed in TSAM. EMDAP Element 4, Assess Evaluation Model Adequacy, is only partially addressed in TSAM (i.e.,

TSAM partially addresses uncertainty analysis); therefore, the NRC staff placed a condition as discussed in the NRC staffs SE on TSAM (ML25062A069) to address EMDAP Element 4, which is consistent with RG 1.203.

SE Section 2, EMDAP Element 3: Develop Evaluation Model, evaluates the Flownex and GOTHIC computer codes and Xe100 input models against the EMDAP Steps 11 and 12.

Specifically, SE Section 2.1, Flownex Code and Input Model, evaluates the Flownex code and associated Xe100 input model, and SE Section 2.2, GOTHIC Code and Input Model, evaluates the GOTHIC code and associated Xe100 input model.

SE Section 3, EMDAP Element 2: Develop Assessment Base, evaluates the completed and planned validation of Flownex and GOTHIC against EMDAP Steps 5 and 7. Specifically, SE Section 3.1, Flownex Qualifications, and Section 3.2, GOTHIC Qualifications, evaluate the completed and planned validation of Flownex and GOTHIC, respectively.

SE Section 4, Flownex and GOTHIC Documentation, Configuration Control, and Quality Assurance, evaluates the Flownex and GOTHIC code documentation, configuration control, and quality assurance against the EMDAP Step 10.

Table 1: EMDAP Roadmap for the Xe100 DBA EM EMDAP Steps TSAM/GFQ TR SE Section Element 1: Establish Requirements for Evaluation Model Capability

1. Specify Analysis Purpose, Transient Class, and Power Plant Class TSAM Section 2.3 of TSAM SE

2. Specify Figures of Merit TSAM Section 2.3 of TSAM SE

3. Identify Systems, Components, Phases, Geometries, Fields, and Processes That Must Be Modeled TSAM Section 2.3 of TSAM SE

4. Identify and Rank Key Phenomena and Processes TSAM Section 2.3 of TSAM SE EMDAP Steps TSAM/GFQ TR SE Section Element 2: Develop Assessment Base

5. Specify Objectives for Assessment Base GFQ Section 3.1 (Flownex)

Section 3.2 (GOTHIC)

6. Perform Scaling Analysis and Identify Similarity Criteria TSAM (Partially), not addressed in GFQ Section 2.4 of TSAM SE

7. Identify Existing Data and/or Perform Integral Effects Tests (IETs) and Separate Effects Tests (SETs) To Complete the Database GFQ Section 3.1 (Flownex)

Section 3.2 (GOTHIC)

8. Evaluate the Effects of IET Distortions and SET Scaleup Capability Not addressed Section 2.4 of TSAM SE

9. Determine Experimental Uncertainties as Appropriate TSAM (Partially), not addressed in GFQ Section 2.4 of TSAM SE Element 3: Develop Evaluation Model

10. Establish an Evaluation Model Development Plan TSAM (Partially) and GFQ Section 2.5 of TSAM SE Section 4 (Flownex and GOTHIC)

11. Establish Evaluation Model Structure TSAM (Partially) and GFQ Section 2.5 of TSAM SE Section 2.1 (Flownex)

Section 2.2 (GOTHIC)

12. Develop or Incorporate Closure Models GFQ Section 2.1 (Flownex)

Section 2.2 (GOTHIC)

Element 4: Assess Evaluation Model Adequacy Steps 1320 TSAM (Partially), not addressed in GFQ See Section 2.6 of TSAM SE 1.0 Topical Report Overview The GFQ TR consists of the following major sections and Appendixes:

Section 1, Introduction, provides a brief description of the purpose, scope and objectives of the TR. The NRC staff considers the information in Section 1 of the GFQ TR throughout the technical evaluation in this SE, but does not make any determinations on the information in TR Section 1.

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 10CFR 50.34(b) for operating licenses, and rules under 10 CFR 52, Licenses, Certifications, and Approvals for Nuclear Power Plants). Section 2 also identifies RG 1.203, RG 1.233, and RG 1.253, 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, as applicable.

The NRC staff identified the regulatory basis in Regulatory Basis, above and focused on requirements applicable to a CP because Section 1.5, Outcome Objectives, of the GFQ TR states that Xenergy is requesting NRC review and approval [] to support preliminary analysis and evaluation of the Xe100. The NRC staff considers the information in Section 2 of the GFQ TR throughout the technical evaluation in this SE, but does not make any determinations on the information in TR Section 2.

Section 3, Overview of Safety Analysis Code Models, provides an overview of the Flownex and GOTHIC code models. This overview includes a brief introduction to the Flownex and GOTHIC codes and a brief description of components in the Flownex and GOTHIC system models. The NRC staffs review of the Flownex and GOTHIC codes and input models is found in SE Sections 2.1 and 2.2, respectively.

Section 4, Flownex Code Manuals and Qualifications, summarizes the contents of the Flownex User Manual, Library Manual and Theory Manual followed by the status of the Flownex V&V efforts. The review of Flownex manuals is considered in SE Section 2.1.

The NRC staffs review of the completed and planned Flownex validations (qualifications) is provided in Section 3.1.

Section 5, GOTHIC Code Manuals and Qualifications, provides a brief listing of the contents of the GOTHIC User Manual and Technical Manual followed by the status of the GOTHIC V&V efforts. The NRC staffs review of GOTHIC manuals is considered in SE Section 2.2. The review of the completed and planned GOTHIC validations (qualifications) is provided in Section 3.2.

Section 6, Quality Assurance, provides a brief description of the Xenergy Quality Assurance Program as it applies to the activities described in the GFQ TR. The NRC staffs review of this information is provided in SE Section 4.

Section 7, Conclusions and Limitations, concludes that Flownex and GOTHIC can support the Xe100 DBA evaluation model. Section 7 also provides the limitation that until the review and approval by the NRC staff is complete, the codes described in the TR cannot be used to support a Final Safety Analysis Report (FSAR) (e.g., for an operating license application). However, as stated in Section 1.5 of the GFQ TR, Outcome Objectives, the applicability of the GFQ TR is limited to support the preliminary analysis and evaluation of the Xe100 (i.e., for a construction permit application).

Appendix A, Flownex Model Theory Overview, provides more detailed background regarding the Flownex code theory. The NRC staff considers this information in SE Section 2.1, but does not make any determinations on the information in TR Appendix A.

Appendix B, GOTHIC Model Theory Overview, provides more detailed background regarding the GOTHIC code theory. The NRC staff considers this information in SE Section 2.2, but does not make any determinations on the information in TR Appendix B.

Appendix C, Flownex Model, provides more detailed information of the Flownex input model for the Xe100 design. The NRC staff considers this information in SE Section 2.1, but does not make any determinations on the information in TR Appendix C.

Appendix D, GOTHIC Model, provides more detailed information of the GOTHIC input model for the Xe100 design. The NRC staff considers this information in SE Section 2.2, but does not make any determinations on the information in TR Appendix D.

2.0 EMDAP Element 3: Develop Evaluation Model EMDAP Steps under Element 3 include:

Step 10, Establish an Evaluation Model Development Plan (see SE Section 4 for the evaluation of EMDAP Step 10)

Step 11, Establish Evaluation Model Structure Step 12, Develop or Incorporate Closure Models As requested in Section 1.5, Outcome Objectives, of the GFQ TR, the NRC staff reviewed and evaluated the Flownex and GOTHIC codes and Xe100 input models to determine their adequacy to support the DBA analyses needed for the Preliminary Safety Analysis Report (PSAR) for a construction permit application, which is consistent with 10 CFR 50.34(a)(4).

Therefore, the scope of review in this section, associated with EMDAP Steps 11 and 12, is to assess if:

Flownex and GOTHIC codes are able to model important transient phenomena for the Xe100 design, and, The Flownex and GOTHIC input models represent the plant geometric input, nodalization, boundary conditions, initial plant state conditions and controls commensurate with the preliminary nature of the Xe100 design.

2.1 Flownex Code and Input Model 2.1.1 Summary of Flownex Code The Flownex computer software serves as the short-term transient thermal-hydraulic calculational model for the Xe100. Section 3.1 of the GFQ TR defines the short-term transient as, the period at which SSCs are actively responding to an initiating event, forced cooling remains available or the primary system is actively depressurizing. The Flownex computer code is described briefly in the main body of the GFQ TR in the following sections:

Section 3.1, Introduction Section 3.2, Flownex - Xe-100 Transient and Safety Analysis Section 3.2.1, Flownex Theory Section 4.1, Flownex Code User Manuals Additionally, details of Flownex modeling capabilities are provided in GFQ TR Appendix A, Flownex Model Theory Overview. Each of the sections listed above and the associated appendix are evaluated against the guidance in RG 1.203, Steps 11 and 12. The NRC staff also examined the information in the Flownex Theory Manual, Flownex Simulation Environment General User Manual, and Flownex Library Manual during a regulatory audit and noted that the summaries provided in the GFQ TR capture the content of the manuals for the sections relevant to the Xe100 DBA EM (ML25078A162).

Section 3 and Appendix A, Flownex Model Theory Overview, of the GFQ TR describes Flownex as follows. Flownex is based upon the solution of multicomponent integral conservation equations for mass, momentum and energy. Translation into differential forms of the conservation equations including mass, momentum, and energy transfer were formulated in one-dimensional form. Where multidimensional effects are important to the Xe100, the theory was extended to two-dimensional axisymmetric finite difference formulations, permitting the modeling of radial, as well as axial mass, momentum and energy flows. Fluid properties for gases and single and two-phase water provide some of the closure relationships, which also include various single and two-phase friction correlations, various flow boiling, nucleate boiling, film boiling, and condensation correlations, conduction, convection and radiation heat transfer correlations in Flownex.

2.1.2 Summary of Flownex Base Input Model for Xe100 Design Section 3.2.3, Deaerator, through Section 3.2.12, Nodes, and Appendix C, Flownex Model, of the GFQ TR describes the Xe100 components represented in the Xe100 Flownex model as follows:

Flownex represents the Xe100 components with: (1) elements consisting of pipes, tanks, pumps, valves, compressors, or heat exchangers, and (2) nodes which are the endpoints of elements and represent the one-dimensional flow between elements.

The Flownex reactor model includes the pressure vessel, pebble bed core, fuel elements, core barrel structures, graphite reflectors, defueling chute, helium inlet and risers connected to the steam generator through the annular hot gas duct. The pressure vessel, pebble bed core, core barrel and graphite reflectors are represented by a matrix of two-dimensional axial and radial elements. Junctions represent axial and radial helium flow between the fluid elements in the pebble bed core using the 2D Flownex formulation.

Figure 19 in Appendix C of the GFQ TR depicts cross-sections of the steam generator.

The hot helium upper plenum, helium steam generator shroud in the tube region, cold helium lower plenum, cold helium annular riser, helium circulator pumps and cold helium return annulus are represented by 1D elements and junctions. Figure 21 of the GFQ TR depicts helium side nodalization of the steam generator. Similarly, the steam generator water side uses 1D elements and junctions to represent the feedwater inlets, feedwater distribution manifold, helical coil steam generator tubes, steam collection manifold and main steam piping to the turbine. The reactor cavity cooling system (RCCS) design is undergoing development and is currently represented as a boundary condition.

2.1.3 Conclusions on Flownex Code and Input Model Based on the information provided in TR Section 3, TR Section 4, and their associated Appendixes (as described in SE Sections 2.1.1 and 2.1.2), the NRC staff determined that the GFQ TR: (1) describes the systems and components, constituents and phases, field equations, closure relations, and numerics consistent with Section 1.3.2, Step 11: Establish Evaluation Model Structure, of RG 1.203 and (2) the preliminary model contains the major Xe100 SSCs necessary to perform preliminary steady state and DBA analysis. Additionally, the NRC staff determined that the GFQ TR describes closure models available within Flownex that, based on the NRC staffs judgment, are capable of supporting preliminary analysis for the Xe100 reactor design. The NRC staffs assessment of the scalability of closure models within Flownex to the Xe100 reactor design will be evaluated as part of considerations of EMDAP Element 4 which is not addressed by the GFQ TR (see Table 1 of this SE).

The NRC staffs conclusions regarding the Flownex codes consistency with EMDAP Steps 11 and 12 for preliminary analysis is limited to the applicability of Flownex and general modeling capabilities but does not extend to or approve individual input parameters in the Xe100 Flownex model. The NRC staffs review of the input parameters will be performed as part of a CP application review. Accordingly, the NRC staff imposed Limitation 1 limiting, in part, the NRC staffs approval to the applicability of Flownex, in accordance with the modeling features described in the GFQ TR, for preliminary analysis of the Xe100.

2.2 GOTHIC Code and Input Model 2.2.1 Summary of GOTHIC Code The GOTHIC computer software serves as the long-term transient thermal-hydraulic calculational model for the Xe100. Section 3.1 of the GFQ TR defines the long-term transient as, the period at which passive heat transfer begins and no additional active plant responses to the initiating event are considered. The GOTHIC computer code is described briefly in the main body of the GFQ TR in the following sections:

Section 3.1, Introduction Section 3.3, GOTHIC Xe-100 Transient and Safety Analysis Section 5.1, GOTHIC Code Manuals Additionally, details of GOTHIC modeling capabilities are provided in GFQ TR Appendix B, GOTHIC Model Theory Overview. Each of the sections listed above and the associated appendix are evaluated against the guidance in RG 1.203, Steps 11 and 12.

Section 3 and Appendix B of the GFQ TR describes GOTHIC as a hybrid code offering the combined capabilities of lumped parameter and three-dimensional representations. The GOTHIC code is based on integral forms of the conservation equations for mass, momentum and energy solved for volume and surface integrals. GOTHIC uses conductors to represent structures for heat transfer which may be represented as flat plates, hollow tubes, solid cylinders (rods) and hollow or solid spheres with conduction in one or two dimensions. Thermal radiation heat exchange among conductor surfaces can also be modeled in GOTHIC.

Additionally, GOTHIC is used to support safety analyses in several NRC approved methodologies (e.g., DPC-NE3004, Mass & Energy Release & Containment Response Methodology, (ML19311B759); WCAP16608, Westinghouse Containment Analysis Methodology, (ML090230441); and WCAP17065, Westinghouse ABWR Subcompartment Analysis Using GOTHIC, (ML120520101)).

2.2.2 Summary of the GOTHIC Base Input Model for Xe100 Design Section 3.3.1, Volumes, through Section 3.3.8, Components, and Appendix D, GOTHIC Model, of the GFQ TR describes the Xe100 components represented in the Xe100 Flownex model as follows:

The Xe100 reactor system GOTHIC model includes representations for primary and secondary systems including the reactor system, cross-over pipe, and steam generator.

The GOTHIC reactor system model includes helium fluid control volumes for the reactor vessel including the helium riser, upper head, core, outlet plenum, lower head, defueling chute, core bypass helium flow and control rod voids. The pebble bed core region is represented by a cylinder using a 2D axial-radial mesh with a number of axial levels and a number of radial regions consistent with the modeling in the fuel performance code.

The core outlet plenum is also represented by a cylinder with a number of radial regions equal to the pebble bed core.

The RCCS is modeled as a temperature dependent heat flux on the reactor pressure vessel outer surface based on the results from a stand-alone GOTHIC RCCS model.

The steam generator helium fluid control volumes of the primary system are nodalized to include the cross-over pipe and hot elbow, helium flow distributor, steam generator hood, helium side of the helical coil heat exchanger tubes, lower plenum, gap between the shroud and vessel wall, helium manifold and outlet plenum. The GOTHIC steam generator water/steam system model includes the water fluid control volumes on the secondary side of the steam generator, including intact and potentially faulted feedwater lines, intact and broken helical coils, and steam header.

Thermal conductors are used to model convective heat transfer between solid surfaces and the fluid, radiative heat transfer between solid structure surfaces, and conduction heat transfer through solid structures separating fluid volumes, and heat sources associated with the structures. Conduction is one-dimensional perpendicular to the surfaces in a variety of geometries including slab, cylindrical, and spherical. The Biot number represents the ratio of the thermal conduction resistance to the thermal convective resistance and is used to estimate the thickness of a layer in composite thermal conductors. The pebble bed fuel conductors are represented as a single, spherical conductor with the average power of the fuel pebbles in an axial and radial sub-volume, while the surface area is represented by the total surface area of pebbles in the sub-volume.

2.2.3 Conclusions Regarding the GOTHIC Code and Input Model Based on the information provided in TR Section 3, TR Section 5, and their associated Appendixes (as described in SE Sections 2.2.1 and 2.2.2), the NRC staff determined that the GFQ TR: (1) describes the systems and components, constituents and phases, field equations, closure relations, and numerics consistent with Section 1.3.2 of RG 1.203, Step 11: Establish Evaluation Model Structure, and (2) the preliminary model contains the major Xe100 SSCs necessary to perform preliminary steady state and DBA analysis. Additionally, the NRC staff determined that the GFQ TR closure models available within GOTHIC that, based on the NRC staffs judgment, are capable of supporting preliminary analysis for the Xe100 reactor design.

The NRC staffs assessment of the scalability of closure models within GOTHIC to the Xe100 reactor design will be evaluated as part of the considerations of EMDAP Element 4 which is not addressed by the GFQ TR (see Table 1 of this SE).

The NRC staffs conclusions regarding the GOTHIC codes consistency with EMDAP Steps 11 and 12 for preliminary analysis is limited to the applicability of GOTHIC and general modeling capabilities but does not extend to or approve individual input parameters in the Xe100 GOTHIC model. The NRC staffs review of the input parameters will be performed as part of a CP application review. Accordingly, the NRC staff imposed Limitation 1 limiting, in part, the NRC staffs approval to the applicability of GOTHIC in accordance with the modeling features described in the GFQ TR, for the preliminary analysis of the Xe-100 3.0 EMDAP Element 2: Develop Assessment Base The EMDAP Steps under Element 2 include:

Step 5, Specify Objectives for Assessment Base Step 6, Perform Scaling Analysis and Identify Similarity Criteria Step 7, Identify Existing Data and/or Perform Integral Effects Tests (IETs) and Separate Effects Tests (SETs) To Complete the Database Step 8, Evaluate Effects of IET Distortions and SET Scaleup Capability Step 9, Determine Experimental Uncertainties as Appropriate Sections 4 and 5 of the GFQ TR address EMDAP Steps 5 and 7. The scaling analysis, distortion analysis, and determination of experimental uncertainties, required by EMDAP Steps 6, 8, and 9, are not addressed by the GFQ TR but are evaluated in the NRC staffs SE on TSAM (ML25062A069). Therefore, the scope of the review in this section is limited to the EMDAP Steps 5 and 7.

The GFQ TR Sections 4 and 5 provide discussions regarding the existing SETs, IETs, and fundamental assessments (or analytical validations) selected for the validation (or qualification) of the Flownex and GOTHIC codes. The GFQ TR also identifies planned validations. Table 1 of the GFQ TR, Validation Basis, contains: (1) information from the PIRT process for different DBA event categories, and (2) the validation matrix for the Flownex and GOTHIC codes. The SETs, IETs, and analytical solutions selected to validate the codes for the prediction of different high-ranked phenomena, are identified. Some of the validations have been performed or will be performed by Xenergy. Many of the validations are part of the GOTHIC and Flownex developmental assessments and are documented predominantly in the GOTHIC Qualification Report and the Flownex Validation Plan, respectively. All the selected SET and IET data are legacy data (i.e., existing data) except the planned validation of GOTHIC against the Argonne National Laboratory Natural Convection Shutdown Heat Removal Test Facility (NSTF) data.

3.1 Flownex Qualifications Table 1 of the GFQ TR shows that the Flownex validation matrix includes nine SETs and four IETs. The validation matrix shows coverage for the high-ranked phenomena except for the following three:

Outlet plenum flow distribution Flow reversal in core bringing hot core coolant into the inlet plenum Distribution and concentration of moisture in primary system The GFQ TR does not identify SETs or IETs to validate the code for the prediction of these three phenomena. Table 1 of the GFQ TR identifies the outlet plenum flow distribution as high importance only for the normal operating condition while the flow reversal in core and moisture distribution or concentration in primary system are identified as important for multiple event categories.

Section 4.3.2, Flownex Fuel Temperature Validation Exercise Report, through Section 4.3.13, Existing Validation Against Analytical Solutions, of the GFQ TR provide further discussions regarding existing Flownex validations and Section 4.4, Planned Flownex Code Verification and Validation, of the GFQ TR summarizes the planned Flownex validations. Table 2, Summary of Existing and Planned Flownex Validations, below summarizes the scope and key insights gained from these validations.

Table 2: Summary of Existing and Planned Flownex Validations Flownex Validation Summary Fuel Temperature Validation (Analytical validation) *

• Compared the Flownex predicted pebble temperatures against a mathematical solution for a stylized problem with the pebble geometry similar to the Xe100 fuel pebble design described in TSAM Section 3.

• Performed sensitivity studies for the impact of nodalization.

• Provided verification for the Flownex conduction heat transfer solution in pebble geometry.

• Showed that the discretization of pebble geometry strongly influences bias in code prediction.

Flownex Validation Summary

• Also showed that the prediction bias is a strong function of fuel power and for the low power conditions expected after the reactor trip, the bias in fuel temperature prediction is expected to be less significant.

HTR10 benchmark steady state conditions

• Compared Flownex predicted core temperatures distribution against HTR10 steady state data.

• Provided the validation of thermal conduction, convective heat transfer and radiation heat transfer through the Flownex capability to predict the steady state HTR10 reactor core temperature distribution within a +/-15 percent prediction error.

• Examined nominal power conditions and fractional power conditions with discretization sensitivity estimated at approximately 9 C pebble centerline underprediction for the coarse node model.

HTR10 benchmark reactor power transients**

• Comparison of the Flownex predicted core temperatures distribution against HTR10 transient test data will be performed to validate transient thermal conduction, convection and radiation phenomena.

• HTR10 experiments have been conducted to simulate the loss of forced circulation without scram and single control rod withdrawal without scram at a partial load of 30 percent of full power.

• Transient simulations will be performed to provide the validation of transient fluid resistance, core specific heat, thermal conduction, convective heat transfer and radiation heat transfer and to determine the discretization needed for temporal and spatial grid independence.

• Flownex predicted powers will be compared with the HTR10 test data. The mean bias and variation in bias will be calculated.

SANA validation of pebble bed temperature prediction

• Compared the Flownex predicted graphite pebble surface temperatures in natural circulation to the SANA (Selbstttige Abfuhr der Nachwre) test rig experimental data to validate conductive and radiative heat transfer in a pebble bed.

• Steady state and a step increase in the power transient were examined spanning the temperature range from 60 C to 1200 C, exceeding temperatures expected for the Depressurized Loss of Forced Circulation (DLOFC) event to validate fluid resistance, core thermal conductivity and heat transfer models in Flownex.

• Generally, Flownex slightly underpredicted the steady state temperature data across the pebble bed radius at 35 kW.

• The validation results examined during the audit (ML25078A162) indicate that, for the step increase in power from 10 kW to 25 kW, Flownex overpredicted the initial temperature increase and slightly overpredicted the equilibrium temperature.

Blowdown pressure validation

• Compared the Flownex predicted pressures for pressurization and depressurization to data from the three tanks in the volume blowdown test rig using helium and nitrogen gases.

• Blowdown tests performed at low pressures were justified as applicable to the Xe100.

• In addition to validating Flownex models for fluid resistance through pressure comparisons, testing with helium and nitrogen allowed for the assessment of mixing behavior. Flow through sharp-edged orifices validated the critical flow and break discharge flow.

Flownex Validation Summary

• The validation results examined during the audit (ML25078A162) indicate that Flownex slightly overpredicted pressures in depressurization cases and slightly underpredicted pressures in pressurization cases.

Pebble Bed Micro Model (PBMM) validation of temperature and pressure

• Compared the Flownex predicted steady state pressures and temperatures to the data from the Brayton cycle PBMM test facility using nitrogen gas.

• Temperatures varied from approximately 25 C to 600 C and pressures varied from approximately 110 kPa to 300 kPa through the cycle.

• Forced flow fluid resistance, thermal conductance, and convective heat transfer were validated in each step of the cycle through gas pressure and temperature comparisons.

• Examination of the results during the audit (ML25078A162) and available publicly in Reference 1 indicate that the Flownex simulation followed the experimental data very closely with only minor deviations in the temperature and pressure comparisons. Minimum and maximum values, and the bias and variation in the bias were determined.

PBMM system startup validation of temperature and pressure

• Compared the Flownex predicted transient pressures and temperatures to the startup data from the Brayton cycle PBMM test facility using nitrogen gas.

• The bootstrap temperature (self-sustaining temperature) was calculated by Flownex as a function of heater outlet temperature and compared to the measured data.

• Transient forced flow, fluid resistance, thermal conductance and convective heat transfer were validated through comparisons of the inlet temperatures, which were the most important parameters with respect to defining of the bootstrap point because the energy that the turbines can deliver depends on the inlet gas temperatures.

• The bootstrap temperature of approximately 590 C was slightly underpredicted by Flownex (approximately 580 C). Pressure trends for the low-pressure compressor suction pressure and the recuperator high pressure side pressure were well represented although Flownex overpredicted the low-pressure compressor pressure and overpredicted the recuperator high pressure side pressure as the system neared the bootstrap point [Reference 2].

PBMM nitrogen injection pressure validation

• Compared the Flownex predicted steady state and transient pressures to data from the Brayton cycle PBMM test facility when nitrogen gas was injected into the test facility.

• The injection of nitrogen resulted in an increase in the mass of nitrogen in the system which corresponded to an increase in the power output of the high pressure turbine.

• Forced flow fluid resistance and energy balance were validated through comparisons of pressures at the low-pressure compressor suction and recuperator high pressure side.

• Validation exercise demonstrated that Flownex has the capability to correctly predict the experimental transient trends for increasing power output.

Flownex Validation Summary Branched piping network pressure validation

• Compared Flownex predicted transient pressures to data from the Branched Network Test Section of the Volume Blowdown Test Rig when compressible gas was released from a gas reservoir tank into the test section.

• The release of compressible gas into the network test section resulted in a wave of increasing pressure to validate the transient pressure response in a piping network.

• Fluid resistance model in Flownex was validated by predicting the transient pressure trends, although the initial pressure increases were slightly overpredicted.

• The validation results examined during the audit (ML25078A162) demonstrated that Flownex has the capability to correctly predict the experimental transient trends for increasing pressures throughout the network. Minimum and maximum values, the bias and variation in the bias were determined.

Piping network compressible gas mass flow validation

• Compared Flownex predicted steady state mass flow rates to data from the Network Balancing Experiment facility where air was pumped through up to six tanks interconnected to pipes with valves which could be opened and closed for different network configurations.

• Mass flow rate validation of Flownex was performed for various network flow in the steady state.

• The validation results of the compressible gas in a piping network experiment, examined during the audit (ML25078A162), indicated that Flownex simulations followed the mass flow rate data very closely.

• Validation of fluid resistance was supported by demonstrating that Flownex has the capability to correctly predict the steady state experimental mass flow rate results throughout the various network configurations.

• Although tests were performed at low, ambient temperatures, applicability to the Xe100 described in TSAM Section 3 was justified because the gas will continue to approximate an ideal gas at the higher temperatures predicted in the safety analysis.

Heat exchanger transient temperature validation

• Compared the Flownex predicted transient temperatures to data from a pipe in counter flow heat exchanger using water as the working media.

• Validated the thermal conduction and convective heat transfer in Flownex.

• Examination of the results during the audit (ML25078A162) and available publicly in Reference 3 indicated that normalized heat exchanger maximum temperature point differences of 20.8, 0.64, 0.4 and 2 percent for the primary side inlet, primary side outlet, secondary side inlet and secondary side outlet, respectively were observed. The Euclidean difference for the primary side inlet, however, was only 5 percent for the transient.

• Validation of thermal conduction and convective heat transfer was supported by demonstrating that Flownex has the capability to correctly predict the transient experimental temperature response.

Flownex Validation Summary Existing analytical solutions validation*

• Cited comparison of the Flownex predicted node temperature change and pressure change to analytical solutions to the following cases:

o step change in inlet temperature to a fixed adiabatic volume with constant inlet and outlet helium mass flowrates, o step change in heat transfer to a fixed volume with constant inlet and outlet helium mass flowrates, o fixed volume with constant helium mass inlet flow rate and no outlet flow, and o sudden mass injection into a tank.

Planned Flownex Simulations

• Flownex simulation of the fuel temperature will be performed with a higher fuel conductivity and compared to the analytic solution.

• Validation of thermal conductivity with accurate temperature simulations.

• Higher thermal conductivities will be representative of the values in the Xe100 safety analyses. Higher thermal conductivity is expected to result in a better simulation of analytical solution.

Planned bias and bias variation calculation validation

• Mean bias and the variation in bias will be calculated for the validation case noted above.

• This analytical validation case is not identified in Table 1 of the GFQ TR

• • This is a planned validation.

Based on the information described in this section, the NRC staff determined that the GFQ TR:

(1) addresses EMDAP Step 5 for the preliminary analysis with Flownex because it identifies the data needs required to assess Flownex, and (2) addresses EMDAP Step 7 for preliminary Flownex analysis because it identifies SETs, IETs, and analytical assessments needed for the validation of Flownex. As described above, EMDAP Steps 6, 8, and 9, are not addressed by the GFQ TR but are evaluated in the NRC staffs SE on TSAM (ML25062A069). Additionally, applicability of the assessment data to the analysis of the Xe100 reactor design will be evaluated as part of the considerations of EMDAP Element 4 which is not addressed by the GFQ TR (see Table 1 of this SE).

3.2 GOTHIC Qualifications Table 1, Validation Basis, in Section 4.2.1, V&V Scope, of the GFQ TR shows that the GOTHIC portion of the validation matrix includes 14 fundamental assessments (or analytical solutions), 22 SETs, and 10 IETs. Similar to the Flownex validation matrix, the GOTHIC validation matrix also shows coverage for the high importance ranked phenomena except for three phenomena discussed earlier in SE Section 3.1.

Table 2, GOTHIC Validation Test Description, in Section 5.3, Existing GOTHIC Code Verification and Validation, of the GFQ TR provides a brief description of the selected SETs, IETs, and analytical solutions. Table 3, Summary of Key GOTHIC Validations, below, summarizes the scope and key insights gained from some of the validations identified in Table 2 in Section 5.3 of the GFQ TR.

Section 5.3.1, Additional GOTHIC Applications, of the GFQ TR provides a brief discussion of additional GOTHIC use cases for the High Temperature Gas-Cooled Reactor (HTGR) evaluations. Xenergy noted that these additional GOTHIC cases are not part of the GOTHIC Assessment Base for the Xe100 DBA EM.

Section 3.3.4, Reactor Building, noted that the Reactor Building is not modeled for Xe100 DBA analyses with GOTHIC. However, the GOTHIC validations for the phenomena in reactor buildings discussed in Section 5.3.2, Reactor Building Validation, of the GFQ TR provide validation of fundamental phenomena such as mixing, thermal conduction, natural convection, etc., and were considered on that basis.

Section 5.4, Planned GOTHIC Code Verification and Validation, of the GFQ TR describes the scope of the planned GOTHIC validations against the IET data. These include:

SANA test for the pebble bed temperature prediction which will validate the GOTHIC model for pebble bed temperatures similar to the planned Flownex validation with SANA.

HTR10 test for reactor temperature and power prediction similar to the planned Flownex validation with HTR10.

NSTF tests for the RCCS phenomena will validate the performance of the planned water-cooled RCCS, which is important in the long-term transient analysis considerations with GOTHIC.

Table 3: Summary of Key GOTHIC Validations GOTHIC Validation Summary Thermally Driven Cavity

• Compared the GOTHIC predicted natural circulation velocity and temperature profile to data in an air-filled cavity between a hot wall and a parallel cold wall.

• Provided validation of the GOTHIC thermal convection.

• Showed good comparisons for velocity and temperature at the mid-plane for a 20x20 mesh grid.

Natural Convection Through Horizontal and Vertical Openings

• Compared the GOTHIC predicted natural convection between a hot compartment and a cold compartment for various sized horizontally oriented and vertical openings between the compartments.

• Provided validation of the GOTHIC fluid resistance, buoyancy and mixing and convective heat transfer.

• While GOTHIC tended to overpredict the natural convection through both the horizontal and vertical openings, the comparison was within the +/-20 percent uncertainty.

Natural Convection Heat Transfer from a Horizontal Cylinder

• Compared the GOTHIC predicted heat transfer in air from a heated horizontal cylinder to cooled rectangular walls.

• Provided validation of the GOTHIC fluid resistance, buoyancy and convective heat transfer.

• GOTHIC predicted that the heat transfer was 10 percent higher than the data, but within the unstated uncertainty.

Fluid Thermal Diffusion

• Compared the GOTHIC simulations of 1D fluid conduction through either air or liquid with the application of heat to both sides of the surface to the theoretical analytic solution.

GOTHIC Validation Summary

• Provided validation of the GOTHIC thermal conduction and effective thermal conductivity to helium in the core.

• GOTHIC provided a very good prediction of the analytical results with differences attributed to differences in thermal properties and thermal expansion.

Aerosol Deposition

• Compared the GOTHIC simulations of the gravity driven droplet settling velocity to the theoretical analytic solution for a selected correlation set.

• Provided validation of the GOTHIC hydrodynamic drag and droplet momentum balance.

• GOTHIC provided a very good prediction of the analytical results as a function of droplet diameter.

Drop Heat and Mass Transfer

• Compared the GOTHIC simulations of droplet evaporation to Spillman test results and the Vesala model.

• Provided validation of the GOTHIC hydrodynamic drag and droplet momentum balance.

• GOTHIC and the Vesala model overpredicted the Spillman data, but the prediction is reasonable considering the scatter in the data.

Drop Heat and Mass Transfer

• Compared the GOTHIC simulations of droplet evaporation and condensation to the IRSN CARAIDAS experiments.

• Provided validation of the GOTHIC hydrodynamic drag and droplet momentum balance.

• Although GOTHIC tended to overpredict the mass transfer, there was good agreement for the droplet size as a function of the distance from the injection point.

BFMC Test 6

• Compared the GOTHIC predicted hydrogen concentrations and temperatures in two compartments of the Battelle-Frankfurt Model Containment (BFMC) test facility as hydrogen was injected into the lower compartment.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC well predicted the hydrogen concentration in the lower, R1, compartment and reasonably predicted in the upper, R2, compartment.

BFMC Test 12

• Compared the GOTHIC predicted hydrogen concentrations and temperatures in six compartments of the BFMC test facility as hydrogen was injected into a lower compartment, R1, with all compartments initially at the same temperature.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC reasonably but underpredicted the hydrogen concentration in all the compartments at the end of the test which was attributed to the failure to consider the containment pressure in the injection rates.

• From BFMC Test 6 and Test 12, it was found that temperature stratification had a large influence on hydrogen concentrations.

BFMC Test 20

• Compared the GOTHIC predicted hydrogen concentrations and temperatures in six compartments of the BFMC test facility as GOTHIC Validation Summary hydrogen was injected into a lower compartment, R6, with the upper compartments approximately 20 C higher in temperature in the lower compartments.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• Excluding the results for compartment R6, GOTHIC reasonably predicted the hydrogen concentrations.

HEDL HM 5

• Compared the GOTHIC predicted temperatures and hydrogen concentrations to data from the Containment Systems Test Facility (CSTF) at Hanford Engineering Development Laboratory (HEDL) for vertical injection of helium and steam into an air containment environment.

• Provided validation of the GOTHIC fluid forced flow, resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC slightly exceeded the hydrogen concentration at the lowest circumferential position and decreased to slightly above at the highest circumferential position.

HEDL HM 6

• Compared the GOTHIC predicted temperatures and hydrogen concentrations to data from the CSTF at the HEDL for horizontal injection of hydrogen and steam into a nitrogen containment environment.

• Provided validation of the GOTHIC fluid forced flow, resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC underpredicted the vertical temperature variations before and after the steam break. Helium concentration data during and after the steam break showed greater vertical variation than was calculated by GOTHIC.

NUPEC Tests

• Compared the GOTHIC predicted pressure, vapor temperature and helium concentrations in a 1/4 -scale model of a multicompartment dry containment for various tests varying initial pressures, temperatures, steam flow rates, helium flow rates, different injection compartments and with or without containment spray operation.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and flow distribution.

• Generally, GOTHIC provided reasonably good predictions for the pressure, vapor temperature and helium concentrations although the dome compartment and injection compartments tended to provide larger deviations.

CVTR

• Compared the GOTHIC predicted pressures, temperatures and heat transfer coefficients to the results from a lumped parameter and 3D model to the tests in the Carolina Virginia Tube Reactor (CVTR) representation of a large dry containment resulting from steam injection.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• The GOTHIC lumped model overpredicted the peak pressure while the 3D model only slightly overpredicted the peak pressure and GOTHIC Validation Summary well predicted the transient pressure in the upper and lower data envelopes. The lumped parameter temperatures tended to be underpredicted while the 3D model temperatures were reasonably well predicted. The lumped parameter heat transfer coefficients were underpredicted while the 3D model provided better predictions.

TOSQAN

• Compared the GOTHIC predicted pressures, steam concentrations, helium concentrations and temperatures in four steady states to the results from the TOSQAN enclosure tests resulting from steam injection and in in two of the four tests helium injection as well.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC well predicted the steady state end condition resulting from the long transient injection of various time lengths.

THAI

• Compared the GOTHIC predicted pressure, temperatures, and relative humidity to the results from the THAI multiroom test vessel resulting from a sequence of helium injection, steam injection in upper rooms and steam injection in lower rooms.

• Provided validation of the GOTHIC fluid resistance, buoyancy, mixing, and plenum flow distribution.

• GOTHIC well predicted the pressure transient. Temperatures at various locations in the steel vessel were also reasonably well predicted. Relative humidity trends were reasonably well predicted at various locations, but the data variations resulting from the transitions from the helium to upper steam and upper steam to lower steam injections were not captured.

Based on the information described in this section, the NRC staff determined that the GFQ TR:

(1) addresses EMDAP Step 5 for preliminary GOTHIC analysis because it identifies that the data needs required to assess GOTHIC, and (2) addresses EMDAP Step 7 for preliminary GOTHIC analysis because it identifies SETs, IETs, and analytical assessments needed for the validation of GOTHIC. As described above, EMDAP Steps 6, 8, and 9, are not addressed by GFQ TR but are evaluated in the NRC staffs SE on TSAM (ML25062A069). Additionally, applicability of the assessment data to the analysis of the Xe100 reactor design will be evaluated as part of the considerations of EMDAP Element 4 which is not addressed by the GFQ TR (see Table 1 of this SE).

4.0 Flownex and GOTHIC Documentation, Configuration Control, and Quality Assurance This section provides the NRC staffs evaluation of EMDAP Step 10, Establish an Evaluation Model Development Plan, which, per RG 1.203, addresses development standards and procedures for the following areas:

design specifications for the calculational device documentation requirements (Regulatory Position 3) programming standards and procedures transportability requirements quality assurance procedures (Regulatory Position 2) configuration control procedures Section 6, Quality Assurance, of the GFQ TR states that all activities described in the GFQ TR are subject to and conducted in accordance with the provisions of the Xenergy Quality Assurance Program Description which has been reviewed and approved by the NRC staff (ML24218A128). Section 6.1, QAP 3.1, Control of Design & Development Procedure, through Section 6.7, QAP 3.14, Software Configuration and Change Control Procedure, of the GFQ TR provide brief descriptions of selected Xenergy quality assurance procedures related to the activities described in GFQ TR. These procedures include:

Control of design and development procedure Technical analysis procedure Software procedure Computer program technical evaluation and acceptance procedure Software V&V for design and safety analysis procedure Software problem reporting and resolution procedure Software configuration and change control procedure Additionally, the NRC staff compared the information provided in TSAM and the GFQ TR to the guidance provided in RG 1.203, Section 3, Documentation, in Table 4, GOTHIC and Flownex Documentation, of this SE. Based on this comparison, the NRC staff determined that: (1) the documentation maintained by Xenergy is consistent with the guidance provided in RG 1.203 for the work that has been performed because the NRC staff identified documentation for those items, and (2) it is reasonable for some documentation to not be available at the preliminary analysis stage because the EM assessment has not been completed.

Table 4: GOTHIC and Flownex Documentation Document Flownex GOTHIC EM requirements Element 1 of EMDAP is addressed in the TSAM TR EM Methodology TSAM and GFQ TRs TSAM and GFQ TRs Code description manuals Flownex Theory Manual, Flownex Library Manual EPRI, GOTHIC Thermal-Hydraulic Analysis Package Technical Manual, Version 8.4(QA)

User manuals and guidelines Flownex General User Manual EPRI, GOTHIC Thermal-Hydraulic Analysis Package User Manual, Version 8.4(QA)

Scaling reports Not provided Assessment reports Final assessment report not provided. However, validation matrix and brief discussion on the existing validations against SET, IET, and analytical solutions are discussed in the GFQ TR (See Section 3)

Uncertainty analysis report Not provided Based on the information described in this section, the NRC staff determined that the GFQ TR satisfies EMDAP Step 10 for Flownex and GOTHIC analysis because: (1) activities described in GFQ TR are performed in accordance with an approved quality assurance program, and (2) documentation is either available or its absence is reasonable for an EM supporting preliminary analysis for a CP application.

LIMITATIONS AND CONDITIONS The NRC staffs conclusions regarding the GFQ TR are subject to the following limitation:

Limitation 1 The NRC staffs approval of the GFQ TR is limited to the applicability of the Flownex and GOTHIC codes, in accordance with the modeling features described in the GFQ TR, for preliminary analysis of the Xe-100. The review of the input parameters into these models is expected to be performed as part of the review of a CP application. SE Sections 2.1.3 and 2.2.3 describe the basis for this limitation.

In addition to this limitation, the NRC staff imposed conditions on the use of TSAM in the SE associated with the Xenergy TSAM TR (ML25062A069) which also impact the use of Flownex and GOTHIC as part of the overall Xe-100 safety analysis methodology.

CONCLUSION The NRC staff approves the use of TR 008585, Xe-100 Licensing Topical Report GOTHIC and Flownex Analysis Codes Qualification, Revision 3, for the preliminary analysis of Xe100 DBAs required under 10 CFR 50.34(a)(4), subject to the limitation identified in this SE.

The NRC staff performed this review following the guidance in RG 1.203 EMDAP Steps 5, 7, 10, 11, and 12. The NRC staffs conclusion is based on the following:

The review of Flownex and GOTHIC codes against RG 1.203, Steps 11 and 12, show that these codes are capable of modeling the phenomena of high importance to support the Xe100 DBA EM safety analysis commensurate with the expectations for the preliminary analysis in a PSAR for a CP application.

Consistent with RG 1.203, Steps 11 and 12, the Flownex and GOTHIC input models as described in the GFQ TR indicate that all the major Xe100 SSCs are represented with sufficient level of detail to support the generation of a steady state simulation necessary for preliminary DBA analysis for a CP application. The nodalization, modeling assumptions, boundary conditions, initial plant state conditions, and controls are commensurate with the preliminary nature of the Xe100 design described in TSAM Section 3 (ML25077A285).

Consistent with RG 1.203, Steps 5 and 7, the GFQ TR identifies SETs, IETs, and analytical assessments needed for the validation of Flownex and GOTHIC codes. The preliminary validation results provide confidence that Flownex and GOTHIC can be used to perform preliminary Xe100 DBA analysis in support of a CP application.

Consistent with RG 1.203, Step 10, the NRC staff determined that there is adequate documentation of the EM development plan that follows established software quality standards and configuration control procedures.

REFERENCES 1.

J. P. van Ravenswaay, et al., Verification and validation of the HTGR systems CFD code Flownex, Nuclear Engineering and Design 236 (2006) 491-501.

2.

W. A. Landman, et al., Flownex Nuclear Architecture, Implementation and Verification &

Validation, International Congress on Advanced Nuclear Power Plants (ICAPP),

May 4 - 7, 2003, Cordoba, Spain.

3.

J. P. van Ravenswaay, et al., Verification and validation of the HTGR systems CFD code Flownex, 2nd International Topical Meeting on High Temperature Reactor Technology, September 22 - 24, 2004, Beijing, CHINA.

Principal Contributors: Pravin Sawant, NRR/DANU/UTB1 Tim Drzewiecki NRR/DANU/UTB1 Ashley Smith, NRR/DANU/UTB1 Date: May 6, 2025