ML25150A338
| ML25150A338 | |
| Person / Time | |
|---|---|
| Issue date: | 06/03/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML25150A338 (14) | |
Text
NRC Staff Review of Topical Report (TR) 000632 Xe-100 Licensing Topical Report Mechanistic Source Term Approach (MST)
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
- Scope of NRC Staff Review
- Summary of Technical Review
- Conclusions 2
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Background===
- MST TR originally submitted on May 10, 2024 (ML24131A146) with updated submittal on March 14, 2025 (ML25073A093)
- Updated submittal includes updates identified during regulatory audit
- Updates to MST TR sections 1.5 and 7.1 clarify that applicability is limited to preliminary analyses
- Correction of typos
- Updates to Appendix H showing MST model interfaces (next slide) 3
Background - Preliminary MST Model Interfaces
- Model interfaces, as described in the MST TR, appear to be preliminary and subject to change (e.g., Reactor Kinetics and Tritium models are described in MST TR but were removed from the interface chart in updated MST TR) 4
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Background===
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Regulatory Basis Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(1)(ii)(D) requires, in part, that an applicant for a construction permit (CP) perform an evaluation and analysis of a postulated fission product release to evaluate the offsite radiological consequences.
Under 10 CFR 50.34(a)(4) an applicant for a CP must perform a preliminary analysis and evaluation of the design and performance of structures, systems, and components with the objective of assessing the risk to public health and safety resulting from the operation of the facility and including the determination of margin of safety during normal operations and transient conditions anticipated during the life of the facility.
- Staff identified relevant Principal Design Criteria (PDC): Xe-100 PDC 10, RFDC 16, PDC 19 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).
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Scope of NRC Staff Review
- MST TR section 4.2 describes that MST models, implemented in the XSTERM code, are used to calculate dose consequences for licensing basis events, including the deterministic evaluation of design basis accidents
- NRC staff reviewed MST modeling approach to address radionuclide transport phenomena to support preliminary analysis of the Xe-100
- Review is limited to and focused on high-level physical phenomena of interest and whether the analysis approach and methods can reasonably support future licensing actions
- Design is preliminary
- Development and assessment of methods are in progress or planned
- Evaluation of models within XSTERM for acceptability will be conducted during the review of an application that relies on the results of XSTERM evaluations 7
Barriers to Radionuclide Release Fuel
- Fuel particle kernel (Uranium Oxycarbide (UCO)) within the TRISO fuel particles
- Silicon Carbide and Pyrolytic Carbon coatings applied to the fuel kernel
- Fuel matrix and fuel free zone of the fuel pebble Helium Pressure Boundary Reactor Building (Not credited) 8
XSTERM Models
- Thermodynamics Calculation Model (THM) calculates detailed temperature distributions in fuel pebbles, TRISO-coated fuel particles, and all core components (reflectors, core barrel, and reactor pressure vessel)
- NRC staff identifies this model to be of high importance because radionuclide release is expected to be diffusion dominant (temperature-dependent).
- Use of THM for analyses supporting a Xe-100 licensing application requires justification by the applicant.
- Point Kinetics Core Simulation Model (KSIM) is described as a modeling 2D axisymmetric geometry to simulate the transient behavior of the Xe-100 core.
- TR section 5.1.9 further states that the point kinetics approach is applied to each cell individually (there are many cells in the model) and that the flux profile between timesteps can be reshaped using a diffusion kernel.
- The description of point kinetics appears to be different than standard point kinetics approaches (i.e., 0D, single eigenvalue, lack diffusion coupling). Use of KSIM for analyses supporting a Xe-100 CP application requires justification by the applicant.
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XSTERM Models MST TR describes nine models in XSTERM (cont):
- Tritium Production and Transport Model calculates the overall tritium mass balance in the Xe-100. MST TR section 5.1.8 clarifies that this model is under development.
- Remaining models:
- TRISO Particle Failure Probability Model (FPM) calculates TRISO-coated particle failures probabilities
- Solids Product Transport Calculations Model (SOLM) calculates the production, decay, transmutation, transport and leakage of gaseous and solid fission products from fuel pebbles to the helium pressure boundary
- Steady-State Gaseous Fission Products Transport Calculations Model (GASM) calculates gaseous fractional release-to-birth ratios of fission product release from TRISO-coated fuel particles and pebbles into the primary circuit coolant gas.
- Dust Production Rate Calculations Model (DUSTM) calculates the graphite and metallic dust production rates for the Xe-100.
- Helium Pressure Boundary Model (HPBM) calculates the deposition and resuspension of fission products on helium pressure boundary surfaces and the release of fission products into the reactor building.
- Core Corrosion Model (CORRM) simulates the mass transport and chemical reaction aspects of the core corrosion phenomena encountered during water or air ingress into the core of the Xe-100.
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XSTERM Models MST TR describes nine models in XSTERM (cont):
- The remaining models (FPM, SOLM, GASM, DUSTM, HPBM, CORRM) address phenomena needed to predict MST to support preliminary analysis:
- Models rely on previous modeling and operational experience from gas-cooled reactor such as Arbeitsgemeinschaft Versuchs Reaktor (AVR)
- Based on the NRC staffs experience with light water reactor (LWR) and non-LWR source term analysis, the NRC staff did not identify significant gaps in the MST models.
- NRC staff did not perform a detailed technical review for the models described in MST TR
- NRC make no conclusions regarding the acceptability of these models 11
Assessment Plans (Verification and Validation (V&V))
1.
V&V effort is underway to ensure that XSTERM is qualified to support final safety analyses 2.
Validation plans are developed to cover high and medium ranked phenomena that are identified through a Phenomena Identification and Ranking Table (PIRT) process 3.
The phenomena modeled by XSTERM were extracted from an earlier version of the PIRT
- NRC staff determined that the assessment process is acceptable because the identification of code assessment requirements through the PIRT process is an established approach (see RG 1.203)
- NRC staff are unable to assess the adequacy of the V&V plan:
- Validation plan is not based on the latest PIRT information
- MST TR does not contain information describing the knowledge level of the phenomena identified in the PIRT
- The plan is preliminary and subject to change 12
Conclusions The NRC staff concludes that X-energys TR 000632, Xe-100 Licensing Topical Report Mechanistic Source Term Approach, Revision 3, provides a reasonable plan for the development of the MST methodology.
- The FPM, SOLM, GASM, DUSTM, HPBM, CORRM models in XSTERM appear to cover the phenomena needed to predict the MST to support the preliminary analysis and evaluation of the Xe-100 design
- The TR describes an acceptable approach to V&V NRC staff make no conclusions regarding the acceptability of the models in XSTERM for the MST analyses of the Xe-100 because:
Models within XSTERM are still under development
- A detailed technical review of the individual models was not completed
- Details regarding key phenomena identification and associated knowledge levels are not provided in MST TR
- The models and associated validation plans are preliminary and subject to change The NRC staff expects that a detailed technical review of XSTERM model applicability to the Xe-100 reactor will be addressed as part of the review of a licensing application that references MST TR.
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14 Acronyms AVR Arbeitsgemeinschaft Versuchs Reaktor CORRM Core Corrosion Model CP Construction Permit DUSTM Dust Production Rate Calculations Model FPM Failure Probability Model GASM Steady-State Gaseous Fission Products Transport Calculations Model HPBM Helium Pressure Boundary Model KSIM Point Kinetics Core Simulation Model LWR Light Water Reactor MST Mechanistic Source Term PDC Principal Design Criteria PIRT Phenomena Identification and Ranking Table SOLM Solids Product Transport Calculations Model THM Thermodynamics Calculation Model TR Topical Report TRISO Triple Coated Isotropic Particle UCO Uranium Oxycarbide V&V Verification and Validation VSOP Very Superior Old Programs