ML25056A215
| ML25056A215 | |
| Person / Time | |
|---|---|
| Site: | 99902094 |
| Issue date: | 02/25/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML25056A213 | List: |
| References | |
| Download: ML25056A215 (1) | |
Text
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION
SUMMARY
REPORT FOR THE REGULATORY AUDIT RELATED TO THE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN FUEL QUALIFICATION METHODOLODY TOPICAL REPORT July 2024 - January 2025
1.0 BACKGROUND
AND PURPOSE By letter dated March 11, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24071A237), the University of Illinois at Urbana-Champaign (UIUC) submitted to the U.S. Nuclear Regulatory Commission (NRC) Release 1 of the topical report (TR), University of Illinois Urbana-Champaign High-Temperature Gas-Cooled Research Reactor: Fuel Qualification Methodology. The TR provides a methodology for qualification of the Fully Ceramic Micro-encapsulated (FCM) fuel that would be used in the Ultra Safe Nuclear Corporation (USNC) Micro Modular Reactor (MMRTM) that UIUC plans to construct on its campus. By email dated April 4, 2024 (ML24093A219), the U.S. Nuclear Regulatory Commission (NRC) staff accepted the UIUC Fuel Qualification Methodology (FQM) TR for review. UIUC submitted a Release 2 of the TR to the NRC by letter dated December 12, 2024 (ML24347A211).
The NRC and the Canadian Nuclear Safety Commission (CNSC) collaborated on portions of this FQM TR audit under the NRC/CNSC Memorandum of Cooperation dated August 2019 (ML19275D578) to promote and achieve efficiencies for planned licensing applications in both the United States and Canada through early regulatory engagements by familiarizing both regulators with the fuel design for USNC MMRs in both countries. However, the audit collaboration does not affect the proposed limited applicability of the UIUC FQM TR to use by UIUC for the MMR it proposes to construct on its campus. The Office for Nuclear Regulation (ONR), the United Kingdoms independent nuclear regulator, also participated in some audit activities as an observer.
This audit enabled the NRC staff to gain a better understanding of technical topics related to UIUCs TR through review and discussion of underlying supporting documentation. Enhanced understanding and communications supported effective and efficient resolution of technical issues.
2.0 AUDIT REGULATORY BASES The basis for the audit was the regulations in Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Domestic Licensing of Production and Utilization Facilities, Section 50.34, Contents of applications; technical information, which delineate the information that is required to be included in a preliminary safety analysis report or final safety analysis report supporting a construction permit or operating license application, respectively. UIUC may reference approved topical reports to support future licensing applications related to the planned MMR.
3.0 AUDIT OBJECTIVES The primary objective of the audit was for the NRC staff to seek clarification, gain understanding, and verify information regarding technical topics relevant to the review of the FQM TR. Gaining access to underlying documentation and engaging in audit discussions about
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION the FQM facilitated the NRC staffs understanding of the TR and aided in assessing the safety of the planned UIUC MMR. The audit improved communication and provided detailed information for the NRC staff.
4.0 SCOPE OF THE AUDIT AND AUDIT ACTIVITIES The audit was conducted from July 2024 to January 2025, via teleconference and the electronic reading room (Blackberry Workspaces). The audit was conducted in accordance with the Office of Nuclear Reactor Regulation (NRR) Office Instruction NRR-LIC-111, Revision 1, Regulatory Audits (ML19226A274).
Members of the audit team, listed below, were selected based on their detailed knowledge of the audit subject matter. Audit team members included:
- 1. Paulette Torres, NRC/NRR (Project Manager)
- 2. Edward Helvenston, NRC/NRR (Project Manager)
- 3. Dan Beacon, NRC/NRR (Technical Reviewer)
- 4. Ben Adams, NRC/NRR (Technical Reviewer)
- 5. Ayesha Athar, NRC/NRR (Technical Reviewer)
- 6. Gregory Oberson, NRC/NRR (Branch Chief, Technical)
- 7. Holly Cruz, NRC/NRR (Branch Chief, Licensing)
- 8. Michael Nicolas, CNSC (Project Manager)
- 9. Peter Devitt, CNSC (Project Manager)
- 10. Kelly Conlon, CNSC (Subject Matter Expert)
- 11. Beth Vary, CNSC (Director of Advanced Reactor Assessment Division)
- 12. Tim Parkes, ONR (Head of Regulation for Advanced Nuclear Technologies) (Observer)
- 13. Ivaylo Stanev, ONR (Lead of High-Temperature Gas Reactor Project) (Observer)
Prior to the audit, the audit team reviewed the TR and provided UIUC with an audit plan dated June 14, 2024 (ML24166A223). The following table documents dates that the staff transmitted audit questions and when audit meetings were held:
Audit Questions (ADAMS Accession No.)
Audit Meetings August 7, 2024 (ML24220A062)
July 8, 2024 (entrance meeting)
July 18, 2024 August 12, 2024 September 11, 2024 October 15, 2024 November 20, 2024 January 29, 2025 (exit meeting)
The staff reviewed the following documents via the electronic reading room:
Written responses that UIUC prepared for audit questions to address the questions and/or facilitate discussion with NRC and CNSC staff TR changes proposed by UIUC in response to various audit questions Slides for USNC presentation USNC Fuel Qualification Methodology Joint Review presented during July 18, 2024, audit meeting
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION 5.0
SUMMARY
OF AUDIT OUTCOME The audit focused on the review of supporting documents associated with questions provided to UIUC during the audit. The NRC staff reviewed information through the Blackberry Workspaces portal and held discussions with UIUC and USNC staff to understand and resolve questions. In some cases, UIUC updated the TR to resolve items discussed in the audit. The table below replicates the specific audit questions transmitted by email to UIUC as listed above, and summarizes the resolution of the audit questions. A table also lists and summarizes the resolution of some additional topics covered as part of this audit beyond the scope of the specific audit questions.
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION Resolution of Audit Questions Question Number Question Resolution 1
Clarify the differences (manufacturing, final product properties, dimensions, etc.) between the test specimens that will be tested and the as-designed Fully Ceramic Micro-encapsulated (FCM) fuel pellet.
- a. Specifically, TR Table 6.1 and Figure 6.1 appear contradictory. In the table, the outer diameter of the surrogate pellet is listed to be smaller, while the inner diameter is larger (the surrogate is a thinner walled annular cylinder). However, the depiction in Figure 6.1 doesnt appear to reflect the larger inner diameter of the test specimen.
- b. How will these differences be accounted/adjusted for during data acquisition, determination of mechanical properties, and/or modelling, if necessary? If differences exist that are not necessary to model/disposition, explain the supporting rationale.
- i.
Will the thinner-walled test specimen generate internal stresses in a comparable (or conservative) manner with respect to the as-designed FCM pellets? How will this be shown/justified?
ii.
How will the effective silicon carbide (SiC) matrix densities vary between the test specimens and as-designed pellets? (Specifically, this question Regarding item a. of this question, UIUC revised the TR by letter dated December 12, 2024 (ML24347A211), to ((
].
Regarding item b.i. of this question, in information provided for audit and during audit discussions, UIUC reiterated information from the TR regarding the testing plan and conditions. Specifically, UIUC described how [
)).
Regarding item b.ii. of this question, in information provided for audit and during audit discussions, UIUC described ((
)).
Regarding this question generally, in information provided for audit and during audit discussions, UIUC also stated that test specimens would be manufactured in largely the same fashion as the as-designed FCM fuel. Most differences would be minor and primarily related to [
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION was formed from the context that SiC density resulting from the chemical vapor infiltration process is dependent on penetration depth, and the surrogate pellets are thinner (and therefore have a lower penetration depth on average.)
)).
2
((
))
In information provided for audit and during audit discussions, UIUC provided further explanation regarding ((
))
UIUC revised TR section 6.1 by letter dated December 12, 2024 (ML24347A211), to [
)).
3 Will pellet-to-pellet vibration/wear and/or pellet-graphite vibration/wear in the MMR core over the period of operation be considered when assessing outer dense surface layer (ODSL) performance? If the intent will be to justify that vibration/wear will not meaningfully impact pellet performance or radionuclide diffusion/transport phenomena, how will this be justified?
In information provided for audit and during audit discussions, UIUC confirmed that vibration/wear will be assessed through testing and will be considered in safety analyses supporting MMR FCM fuel qualification (e.g., in a future TR submittal) but this topic is outside the scope of the FQM TR.
UIUC also revised TR sections 1.2.2 and 1.2.3 by letter dated December 12, 2024 (ML24347A211), to clarify the scope of the FQM TR, including to indicate that vibration/wear are not within the scope of the
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION FQM TR.
4 High levels of surface roughness and high levels of adhesion between individual tristructural isotropic (TRISO) layers, and/or at the TRISO-particle-to-pellet-matrix interfaces, could contribute to TRISO particle failures if the pellet matrix material fractures.
Specifically, potential exists for pellet fractures to propagate through strongly-bonded TRISO particles, causing the particles to fail. [
]
UIUC revised TR sections 4.4.2 and 6.1 by letter dated December 12, 2024 (ML24347A211), to [
].
5 Is FCM pellet fracture considered or expected during MMR operating or accident conditions, or will it be precluded by associated testing and analysis and establishment of appropriate operating limits?
In information provided for audit and during audit discussions, UIUC discussed how testing that bounds actual expected MMR conditions will be used to demonstrate margin to pellet failure/fracture in the MMR core, such that failures/fractures should not be expected to occur for normal operation or transient conditions. However, UIUC noted that during MMR operations, circulating activity in the core will be monitored, and operational actions (such as reactor shutdown) taken if needed, to ensure that regulatory dose limits would not be exceeded due to any unexpected FCM pellet failures/fractures that may occur.
6 TR Section 4.3 explains that SiC matrix and ODSL materials may exhibit higher swelling rates and swelling saturation levels at lower temperatures.
- a. Will the effects of irradiation at low temperatures be quantified during testing?
Regarding item a. of this question, in information provided for audit and during audit discussions, UIUC clarified that testing over a range of temperatures will be included as part of the steady-stated irradiation testing that is planned to be conducted at the High
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION
- b. Will temperature (and temperature gradient) be varied/controlled/monitored during the short-duration irradiation testing described in TR Section 6.2.5?
- c. The following sub-questions may require completion of testing, development of an evaluation model, and/or performance of analyses to be fully answered/bounded, and are therefore not expected/required to be fully addressed at this time for this TR review, but are included below to provide context for the above questions and for general consideration and discussion.
- i.
Could irradiation at low temperatures be challenging/limiting with respect to internal stresses created in FCM pellets due to expansion of SiC and/or TRISO particle materials?
ii.
Could initial MMR start-up/heat-up from cold temperatures challenge pellet integrity? (i.e., Given the thermal mass of core constituents (graphite, etc.) that would need to be heated and the amount of fission heat/irradiation necessary to do so, could steep thermal gradients develop in pellets during startup and result in differential thermal expansion and irradiation swelling rates within the pellets?) Will a start-up rate limit or required pre-heating approach to start-up be considered or required, operationally?
iii.
How will time/temperature/thermal Flux Reactor in the Netherlands (see TR section 6.2.4).
Regarding item b. of this question, in information provided for audit and during audit discussions, UIUC confirmed and discussed how temperatures and temperature gradients will be varied, controlled, and monitored during the short-term irradiation testing discussed in TR section 6.2.5.
Regarding item c. of this question, in information provided for audit and during audit discussions, UIUC discussed how points raised in items c.i. through c.iii.
will be considered in planned FCM pellet irradiation testing and in approaches for UIUC MMR startup and operation.
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION gradient differences between actual UIUC MMR start-up/operation and the steady state irradiation testing conditions be modelled/dispositioned when justifying initial MMR startup?
7 Will high-temperature safety testing include specimens that are damaged/fractured during the irradiation tests (for instance, from short-duration irradiation testing)? (i.e., if pellet fracture/failure is predicted to occur during postulated accident conditions, how will the resultant fission product release rate quantification method be supported by the safety testing data obtained?)
UIUC revised TR section 6.3 by letter dated December 12, 2024 (ML24347A211), to provide clarifying information. ((
]
8 How much time (decay time) will pass between completion of irradiation testing and performance of high-temperature safety testing? Will any meaningful short-lived radioactive isotopes effectively decay away prior to the safety testing/measurement? If so, how will they be accounted for when assessing dose consequences of postulated accident conditions?
In information provided for audit and during audit discussions, UIUC acknowledged that cooling time will have to pass prior to completion of safety testing, and that some short-lived isotopes would decay off in that period. However, surrogate isotopes will be measured, and UIUC expects that information for the short-lived isotopes can be derived from those results.
9 How will testing and/or modelling be used to justify the predicted core operational period ((
)),
specifically, with respect to phenomena that occur slowly and/or may not necessarily be challenging/limiting during the planned time-accelerated experimental testing? For example:
- a. Chemical attack of the ODSL will likely occur
((
In information provided for audit and during audit discussions, UIUC discussed various fuel failure mechanisms with potentially increasing probability over time. UIUC explained how these can generally be ruled out as significant concerns for the FCM TRISO fuel (and the TRISOs Outer Dense Surface Layer (ODSL) coating) in the MMR based on, for example, MMR conditions and prior experience from the U.S.
Department of Energy Advanced Gas Reactor (AGR)
Program.
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION
)). Will the reaction rate be quantified and will the potential for long-term ((
))
cumulative effects be examined to determine whether this phenomenon may meaningfully challenge ODSL performance near the fuels end-of-life?
- b. What other time-at-temperature effects were considered and/or dispositioned (and how were they dispositioned?) with the long-term timeframe in mind?
UIUC revised the TR to add new section 6.5.3 by letter dated December 12, 2024 (ML24347A211). TR section 6.5.3 provides information on potential long-term fuel failure mechanisms (including chemical attack of silicon carbide (SiC) in TRISO and ODSL; TR section 6.5.3 notes that chemical attack of SiC was the leading cause of SiC failure in AGR fuel) and how the mechanisms are dispositioned for the UIUC MMR.
10 Which of the parameters in TR Table 2.2 are the limits in Table 2.2 referred to in TR Section 6.7 (TR Table 6.3, first criterion)? Do the limits include all values with an equivalency expression (e.g., <, >,
etc.), or a subset? If they include nominal values without equivalency expressions, how are tolerances defined?
UIUC revised TR table 6.3 (in TR section 6.7) by letter dated December 12, 2024 (ML24347A211), to indicate the six defect fractions that are the limits in TR table 2.2 being referred to in TR table 6.3 (the NRC staff notes the TR table 2.2 limits for all six of these defect fractions include equivalency expressions).
Additionally, in information provided for audit, UIUC clarified that these six defect fractions are measured on samples of as-manufactured TRISO particles, and statistics are used to determine upper limits for the defect fractions of the samples at a 95 percent confidence level; a defect fraction specification is considered to be met if the upper limit determined for a sample is below the TR table 2.2 value. UIUC also revised TR table 6.3 by letter dated December 12, 2024 (ML24347A211), to add a footnote clarifying that the defect fraction limits listed in TR table 2.2 (and referenced in TR table 6.3) are maximum values for the upper limits, determined at a 95 percent confidence level, of defect fractions for samples of as-fabricated FCM TRISO particles.
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION 11 How will the thickness of FCM pellet ODSLs be confirmed during manufacturing? How will hermeticity be tested during manufacturing? Will every FCM pellet be tested/confirmed, or will a statistical sampling be used to estimate that FCM pellets are being manufactured to the appropriate thickness and that no defective ODSLs exist?
UIUC revised TR section 3.4 by letter dated December 12, 2024 (ML24347A211), to add information on
((
)).
[
]
12 Were other potential fuel acceptance criteria considered for inclusion to TR Tables 2.5 and 2.6 (and by reference, TR section 6.7)? Specifically, in Advanced Gas Reactor (AGR) program documentation (Electric Power Research Institute (EPRI), Uranium Oxycarbide (UCO) Tristructural Isotropic (TRISO)-Coated Particle Fuel Performance, Topical Report EPRI-AR-1(NP)-A, 2020, ML20336A052), envelopes are described based on temperature, burnup, and fluence (akin to the parameters specified in the TR tables), but also power density and particle packing fraction (which are not included in the acceptance criteria in the TR).
Explain why power density and particle packing fraction are not included/needed to adequately define fuel acceptance criteria.
Regarding packing fraction, in information provided for audit, UIUC stated that TRISO particle packing fraction is not a limiting factor for MMR FCM pellet fuel, due to the manufacturing process of the pellets which is different than the process for AGR fuel. Specifically, FCM fuel is manufactured without a compacting process. The chemical vapor infiltration process that is used for FCM pellets eliminates the need for pressing and does not yield any stress that could damage the TRISO particles.
Regarding power density, in information provided for audit, UIUC indicated that ((
)). The
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION NRC staff additionally considered that temperature acceptance criteria should cover power-related aspects of the pellets, and that they are more direct acceptance criteria since temperature directly influences radionuclide diffusion rates.
Also related to power density and the rationale for not including it as an acceptance criterion for the FCM fuel, UIUC additionally noted in information provided for audit that ((
].
13 Sections 1.2.3 and 7.2 of the TR state that meeting the acceptance criteria in Section 6.7 of the TR qualifies the fuel for use. It is not expected that a final determination regarding the fuels qualification for use would be made until after the data/testing/modelling are complete. Should these statements be revised to not imply that this TR will grant final fuel qualification?
UIUC revised TR sections 1.2.3 and 7.2 by letter dated December 12, 2024 (ML24347A211), to provide clarification regarding when UIUC would consider MMR fuel to be fully qualified for use.
Resolution of Additional Topics Beyond the Scope of the Specific Audit Questions Topic Question Resolution Testing and Characterization of Fuel (related to TR section 6.1)
Clarify what techniques/experiments will be used to obtain material properties listed in TR Section 6.1.
For example, whether the techniques/experiments have been determined and if they're standard testing approaches, etc.
((
In information provided for audit and during audit discussions, UIUC stated that these material properties are reasonably obtainable using typical material testing techniques previously used in the nuclear industry and that generally-accepted testing practices and techniques such as those specified by the American Society for Testing and Measurement Standards would be used.
UIUC provided some example industry standards to demonstrate that supportable and accepted practices will be used (although it noted it is not committing to specific standards at this time).
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION
))
Do the referenced data and experiments meet UIUC/USNC's QA process/requirements?
UIUC stated that [
))
UIUC stated that a verification and validation program will be used for the FCM models. The FCM model development process would also be consistent with the NRC-approved UIUC quality assurance program description TR (ML23167C137) as well as USNC quality assurance processes.
Fuel Pellet Irradiation Testing (related to TR section 6.2)
The NRC staff notes that test specimens will be
((
)) enriched, but qualification is sought for
((
)) enriched fuel. One statement in the TR (Table 6.2, note a) appears to indicate ((
)), but another statement (Above Table 6.1) appears to indicate that preliminary studies show that ((
)). Please clarify the apparent discrepancy.
UIUC revised TR section 6.2.2 by letter dated December 12, 2024 (ML24347A211), to ((
)).
OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY AND EXPORT-CONTROLLED INFORMATION 6.0 EXIT BRIEFING The staff conducted an audit closeout meeting on January 29, 2025. At the exit briefing the staff reiterated the purpose of the audit and summarized the audit activities. Additionally, the staff stated that they did not identify areas where further additional information would be necessary to support the review.
There were no deviations from the audit plan.
7.0 ADDITIONAL INFORMATION RESULTING FROM AUDIT No requests for additional information were generated as a result of topics discussed in this audit. However, UIUC updated the TR on its own initiative as noted above to address several items discussed during the audit.
8.0 OPEN ITEMS AND PROPOSED CLOSURE PATHS Not applicable.