ML24086A100
| ML24086A100 | |
| Person / Time | |
|---|---|
| Site: | 99902071, 99902117 |
| Issue date: | 03/26/2024 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML24086A100 (1) | |
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Enclosure 1 : Technical Areas of Focus Related to the Xe-100 Approach to Qualify Graphite Core Components The NRC staff identified several areas that would benefit from X-energy providing additional information as part of the construction permit application or through other submittals, such as topical reports. Those are as follows:
The NRC staff understands that X-energy intends to apply American Society of Mechanical Engineers (ASME)Section XI Division 2 Reliability and Integrity Management (RIM) to graphite components. The version of ASME Section XI Division 2 endorsed by NRC in Regulatory Guide (RG) 1.246, Acceptability of ASME Code,Section XI, Division 2, `Requirements for Reliability and Integrity Management (RIM) Programs for Nuclear Power Plants,' for Non-Light Water Reactors, does not include language or guidance for graphite (although Code language for graphite is under development).
o X-energy should provide further information about how it will determine that in-service inspection or monitoring programs (specifically, the location, frequency and techniques) will ensure that the degradation of graphite components is detected before they lose the capacity to perform their intended functions.
The NRC staff understands that X-energys safety analysis may rely, in part, on the use of data from graphite tests which it did not, itself, perform, and for which there may be limited quality assurance information available.
o X-energy should provide further information about how it will determine that it has accounted for uncertainties arising from the use of data with limited quality assurance information, such as in performing modeling or simulations.
o X-energy may consider whether use of Nonmandatory appendix to ASME NQA-1, Quality Assurance Requirements for Nuclear Facility Applications, Subpart 4.2.3, Guidance on Qualification of Existing Data, is relevant for its intended use of existing data. This subpart provides several possible approaches and has been used previously. For example, see the NRC staffs safety evaluation regarding the Argonne National Laboratory Quality Assurance Program Plan for Sodium Fast Reactor Metallic Fuel Data Qualification (ML20106F242).
The NRC staff understands that X-energy plans to use existing data sets from certain grades of graphite to develop structural integrity models for different grades as part of the Xe-100 reactor safety analysis.
o X-energy should provide further information about how it will determine that it has accounted for uncertainties arising from the comparison of different graphite grades, such as in performing modeling or simulations. This may include consideration of experimental data scatter and the approach to manage interpolation and extrapolation of data.
o X-energy should provide further information about its approach to verification and validation of the graphite structural integrity models that involve assumptions about the performance of a certain grade of graphite based on experimental data from a different grade.
The NRC staff understands that X-energy intends to acquire data from additional testing to support its safety analysis.
o X-energy should provide further information about the timing and scope of additional testing, as well as the intended use of data acquired from that testing.
o X-energy should provide further information about what Xe-100 operating conditions (e.g.,
fluence, temperature) are not bounded by existing test data, such that additional testing is
required, and how it will determine that the parameters for additional tests are adequate to support the safety analysis.
The NRC staff understands that X-energy will establish limits on the extent to which graphite components would be affected by degradation, for instance with respect to the density of cracks or the amount of dimensional change, at which point they would be treated as though they are unable to perform their intended function without mitigation. There would be margin between these limits and the point at which the components are unable to perform their intended functions because of their actual condition.
o X-energy should provide further information on how it will determine any such limits and how it will demonstrate that there is sufficient margin between the state of graphite components at those limits and the state at which the components are actually unable to perform their intended functions.
o X-energy should provide further information about how uncertainties in the modeling of graphite behavior, such as could be attributed to limited available test data, would affect the determination of any such limits.
The NRC staff understands that X-energy will use models intended to predict the creep behavior of graphite as part of its safety analysis.
o X-energy should provide further information about how it will demonstrate that those models represent the behavior of graphite for the full operational temperature range, including how it will determine that test data used for those models is representative of the Xe-100 design and operating conditions.
o X-energy should provide further information about how it will demonstrate that its models have accounted for uncertainties arising from limited test data and phenomenological changes in creep behavior across the operational temperature range.
The NRC staff understands that X-energy will assess the effects of other degradation mechanisms in addition to creep as part of their safety analysis. These include fatigue, abrasion and erosion caused by the flow of the fuel pebbles, and oxidation under normal and licensing basis event conditions.
o To the extent that the analyses of these or any other degradation mechanisms will rely on conformance with American Society of Mechanical Engineers Code Section III, Division 5, X-energy should provide further information on how it will determine whether any deviations will be needed and how those would be justified.
o X-energy should provide further information about how it will demonstrate that modeling assumptions (e.g., the use of X-energys indicated method of determining fatigue life) are justified, for instance with reference to experimental test data.
o To the extent that the analyses of these or any other degradation mechanisms will rely on reference to operating experience from other reactors with graphite components, X-energy should provide further information on how it will determine that this is applicable to the Xe-100 design.