ML25150A337
| ML25150A337 | |
| Person / Time | |
|---|---|
| Issue date: | 06/03/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML25150A337 (16) | |
Text
Xe-100 GOTHIC And Flownex Code Qualifications (GFQ)
NRC Staff Review ACRS Meeting of the X-Energy Design Center Subcommittee June 3, 2025 https://www.nrc.gov/reactors/new-reactors/advanced.html
Outline
- Background
- Regulatory Basis
- Safety Evaluation Approach
- Evaluation of GOTHIC and Flownex codes and input models
- Evaluation Model Development and Assessment Process (EMDAP) Element 3:
Develop Evaluation Model (Steps 11 & 12)
- Evaluation of GOTHIC and Flownex qualifications
- EMDAP Element 2: Develop Assessment Base (Steps 5 & 7)
- Documentation, Configuration Control, and Quality Assurance (QA)
- EMDAP Element 3: Develop Evaluation Model (Step 10)
- Limitation and Condition
- Conclusion 2
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Background===
Xe-100 Topical Report: GOTHIC and Flownex Analysis Codes Qualification, Revision 2 was submitted on May 22, 2024 (ML24143A192) with updated submittal of Revision 3 on March 13, 2025 (ML25076A053)
Provides overview of GOTHIC and Flownex computer codes Describes preliminary Xe-100 input models developed for preliminary Design Basis Accident (DBA) analysis Summarizes completed and planned verification and validation activities, and Describes quality assurance approach for the codes GFQ TR supports Transient and Safety Analysis Methodology (TSAM) to perform preliminary analysis and evaluation of DBAs for the Xe-100 design
- Flownex analyzes short-term transient response
- Short-term transient period: systems, structures, and components (SSCs) are actively responding, forced cooling is available and primary system is actively depressurizing
- GOTHIC analyzes long-term transient response Long-term transient period: passive heat transfer begins and no additional active plant responses to the initiating event are considered X-energy requested review and approval of GOTHIC and Flownex codes and models to perform preliminary DBA analysis for Xe-100 design 5
Regulatory Basis Under 10 CFR 50.34(a)(4) an applicant for a Construction Permit (CP) must perform a preliminary analysis and evaluation of the design and performance of 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.
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) endorses NEI 18-04, Revision 1, Risk-Informed Performance-Based Technology-Inclusive Guidance for Non-Light Water Reactors, (ML19241A472) as one acceptable method to inform licenses, certifications, and approvals for non-light water reactors (LWRs).
NEI-18-04 states that 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).
Review of GFQ TR follows RG 1.203 Evaluation Model Development and Assessment Process 4
Safety Evaluation Approach
- Many EMDAP Steps are addressed in TSAM TR
- EMDAP Element 1, Establish Requirements for Evaluation Model Capability (PIRT) addressed in TSAM (partially)
- EMDAP Elements 2, Develop Assessment Base and Element 3, Develop Evaluation Model steps partially addressed in TSAM
- GFQ TR addresses primarily steps under EMDAP Elements 2 and 3
- Scope for GFQ TR Review
- Review of Flownex and GOTHIC codes and Xe-100 input models against EMDAP Element 3 Steps 11 and 12
- Review of documentation, configuration control, and quality assurance approach for Flownex and GOTHIC codes against EMDAP Element 3 Step 10
- Review of Flownex and GOTHIC qualifications (validations) against applicable steps under EMDAP Element 2 (Steps 5 and 7)
- Review focuses on evaluating adequacy of Flownex and GOTHIC codes, input models, and validations for preliminary analysis of DBAs in Xe-100 design 5
Evaluation of GOTHIC & Flownex Codes & Input Models
- Confirm adequacy of GOTHIC and Flownex codes and input model to support preliminary safety analysis of Xe-100 design consistent with EMDAP Steps 11 and 12
- GOTHIC and Flownex codes are capable of modeling important transient phenomena for the Xe-100 design, and,
- GOTHIC and Flownex input models represent the plant geometric input, nodalization, boundary conditions, initial plant state conditions and controls commensurate with the preliminary nature of the Xe-100 design.
6 EMDAP Element 3 Develop Evaluation Model 10.
Establish EM development plan TSAM and GFQ (partially) 11.
Establish EM structure TSAM (partially) and GFQ (partially) 12.
Develop or incorporate closure models GFQ
Flownex Code and Input Model Flownex Code Performs steady state and transient thermal-fluid simulation of plant with integrated reactor kinetics and control system model 1-D finite difference equations for mass, momentum, and energy (multicomponent, compressible, homogeneous mixture model) 1-D framework extended to 2-D axisymmetric representation of porous pebble bed core with 1-D conduction model for heat transfer in pebbles Closure models include fluid properties, single-and two-phase friction including porous media pressure drop, two-phase flow related closures (e.g., boiling), heat transfer correlations such pebble to coolant and pebble to pebble effective conduction, porosity variation, etc.
Xe-100 Flownex Input Model Xe-100 components are represented with network of nodes (one-D control volumes) and elements (models) (GFQ TR Appendix C Figure 13,19)
Primary system, secondary system and power conversion cycle, control system modeled with Flownex 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 Pressure vessel, pebble bed core, core barrel and graphite reflectors modeled with 2-D 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.
Reactor cavity cooling system (RCCS) represented as boundary condition 7
GOTHIC Code and Input Model GOTHIC Code General purpose, thermal-hydraulic system analysis code, used extensively in nuclear safety analysis and in NRC approved methodologies (e.g., LWR LOCA and Containment Analysis)
Wide range of capabilities from lumped parameter approach to three-dimensional representations (multicomponent, compressible and multi-fields for liquid and gas)
Closing relations unique pebble bed application: pressure drop correlation (to be included), pebble-to-coolant convective heat transfer, effective thermal conductivity for pebble-to-pebble and pebble to reflector heat transfer GOTHIC Xe-100 Input Model Primary and secondary systems including the reactor system, cross-over pipe, and steam generator represented with multiple control volumes, flow paths and heat structures (Figure 27 of GFQ TR)
Reactor model includes 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 multiple axial levels and radial regions. The core outlet plenum is also represented by a cylinder with the same number of radial regions as the pebble bed core. Conical core sections neglected.
RCCS is modeled as a temperature dependent heat flux on reactor pressure vessel outer surface based on results from a stand-alone GOTHIC RCCS model Detailed model for helical coil steam generator (SG) primary and secondary sides with ability to model SG tube rupture accident Pebble bed fuel conductors represented as a single, spherical conductor with the average power of fuel pebbles in an axial and radial sub-volume, while the surface area represented by total surface area of pebbles in sub-volume.
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Determination on GOTHIC & Flownex Codes and Input Models Systems and components, constituents and phases, field equations, closure relations, and numerics for GOTHIC and Flownex are adequately described consistent with EMDAP Steps 11 and 12 for preliminary DBA analysis of Xe-100 design Major Xe-100 SSCs necessary to perform preliminary steady-state and DBA analysis are represented in plant input models consistent with EMDAP Step 11 Closure models available within GOTHIC and Flownex, based on the NRC staffs high-level review and judgment, are adequate to support preliminary analysis of Xe-100 design consistent with EMDAP Step 12 Evaluation of scalability and applicability of closure models to Xe-100 reactor design will be performed in future as part of review of EMDAP Element 4 Current review for preliminary analysis is limited to the applicability of codes and general modeling capabilities and does not extend to or approve individual input parameters in the Xe-100 in models.
Staff imposed Limitation 1 that limits approval of codes and input models, in accordance with modeling features described in GFQ TR, for preliminary analysis of Xe-100 9
Evaluation of GOTHIC & Flownex Qualifications
- Review Scope Limited to EMDAP Steps 5 & 7
- Scaling analysis, distortion analysis, and determination of experimental uncertainties, required by EMDAP Steps 6, 8, and 9, are not addressed
- Table 1 of GFQ TR Validation Basis provides (1) High importance PIRT phenomena for different DBA event categories, and (2) Validation matrix for Flownex and GOTHIC codes
- Separate effects test (SET), integral effects test (IET), and analytical validations selected
- Majority validations are part of GOTHIC and Flownex developmental assessments and use legacy data (i.e., existing data) except for planned validation of GOTHIC against future applicable Argonne National Laboratory (ANL) Natural Convection Shutdown Heat Removal Test Facility (NSTF) data 10 Element 2 Develop Assessment Base 5.Specify objectives for assessment base GFQ 6.Perform scaling analysis and identify similarity criteria TSAM (partially) 7.Identify existing data and/or perform IETs and SETs to complete data/base GFQ 8.Evaluate the effects of IET distortions and SET scaleup capability TSAM (partially) 9.Determine experimental uncertainties Not addressed
Flownex Qualifications Flownex validation matrix includes nine SETs, four IETs, and two analytical benchmarks
- 10 MW High Temperature Gas-Cooled Reactor Test Module (HTR-10) steady state conditions (IET) (Chinese 10 MW reactor)
- SANA pebble bed temperature (SET) (German facility, heated pebble bed)
- Pebble bed micro model (PBMM)
Shows coverage for all PIRT high-ranked phenomena except
- Outlet plenum flow distribution (only for normal operation)
- Flow reversal in core bringing hot core coolant into the inlet plenum
- Distribution and concentration of moisture in primary system Planned validation
- HTR-10 reactor power transient benchmark
- Flownex analytical simulations EMDAP Steps 5 & 7 addressed for preliminary analysis with Flownex Applicability of assessment data to analysis of the Xe-100 reactor design will be evaluated as part of the considerations of EMDAP Element 4 11
GOTHIC Qualifications
- GOTHIC validation matrix includes 22 SETs, 10 IETs, and 14 analytical benchmarks
- All are legacy validations
- Shows coverage for all PIRT high-ranked phenomena except
- Outlet plenum flow distribution (only for normal operation)
- Flow reversal in core bringing hot core coolant into the inlet plenum
- Distribution and concentration of moisture in primary system
- Planned validation
- HTR-10: steady state, LOFC without scram, and control rod withdrawal
- SANA for pebble bed heat transfer
- ANL NSTF data to validate performance of RCCS (important for long-term cooling)
- EMDAP Steps 5 & 7 addressed for preliminary analysis with GOTHIC
- Applicability of assessment data to the analysis of the Xe-100 reactor design will be evaluated as part of the considerations of EMDAP Element 4 12
Documentation, Configuration Control, and QA
- GOTHIC and Flownex EM development is performed under approved X-energy Quality Assurance Program (ML24218A128)
- Section 6 of GFQ TR provides description of selected X-energy quality assurance procedures related to software development
- Documentation of EM requirements, EM methodology, code theory and user manuals was provided. Scaling report, uncertainty analysis and final assessment report are not available.
- EMDAP Step 10 satisfied for preliminary analysis because
- Activities described in GFQ TR are performed in accordance with an approved quality assurance program
- Documentation is either available or its absence is reasonable for an EM supporting preliminary analysis for a CP application 13
Limitation and Condition
- 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.
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Conclusion Review of Flownex and GOTHIC against EMDAP Steps 11 and 12 show that these codes have models to represent high importance phenomena to support preliminary Xe-100 DBA safety analysis in a Preliminary Safety Analysis Report for a CP application.
Consistent with EMDAP Steps 11 and 12, Flownex and GOTHIC input models as described in GFQ TR indicate that all major Xe-100 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 Xe-100 design described in TSAM Section 3 (ML25077A285).
Consistent with EMDAP Steps 5 and 7, the GFQ TR identifies SETs, IETs, and analytical assessments needed for validation of Flownex and GOTHIC codes. Preliminary validation results provide confidence that Flownex and GOTHIC can be used to perform preliminary Xe-100 DBA analysis in support of a CP application.
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16 Acronyms ANL Argonne National Laboratory CP Construction Permit DBA Design Basis Accident EM Evaluation Model EMDAP Evaluation Model Development and Assessment Process GFQ GOTHIC And Flownex Code Qualifications HTR-10 10 MW High Temperature Gas-Cooled Reactor Test Module IET Integral Effects Test LWR Light Water Reactors NSTF Natural Convection Shutdown Heat Removal Test Facility PIRT Phenomena Identification and Ranking Table QA Quality Assurance RCCS Reactor Cavity Cooling System SANA Selbstttige Abfuhr der Nachwre SG Steam Generator TSAM Transient and Safety Analysis Methodology