ML24215A243
| ML24215A243 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24215A000 | List:
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| References | |
| LO-169995 | |
| Download: ML24215A243 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-14 Receipt Date: 12/04/2023 Question:
Provide TR markups specifying whether RETRAN-3D calculations described in TR section 5.3.4 use point kinetics or spatial kinetics. TR section 5.3.4 states that a code-to-code benchmark was performed to compare the NRELAP5 and RETRAN-3D responses to a range of reactivity insertion conditions in an NPM. Without specifying whether RETRAN-3D calculations used point kinetics or spatial kinetics, it is unclear whether the code-to-code benchmark provides evidence that point kinetics models are appropriate for these transients in the NPM-20. Because of the increased power in the NPM-20, the spatial dynamic nuclear response (multi-group spectrum effects, particularly near the core periphery and core locations of large neutron flux gradients) to the various reactivity events (e.g., control rod mis-operation event), may prevent a point kinetics model from determining the changing power and core kinetics. If the comparison is based on a RETRAN-3D point kinetics model, provide evidence that power and reactivity response are adequately modeled for transients described in TR Section 5.3.4.
Response
The purpose of the non-LOCA topical report Section 5.3.4 is to benchmark the NRELAP5 point kinetics model to RETRAN-3D during reactivity transient events. A point kinetics model is used in both codes as already stated in Section 5.3.4.7 of the topical report: This is important because the main purpose of the benchmark is to compare the point kinetics model response between NRELAP5 and RETRAN-3D. Therefore, no topical report markups are required.
The code-to-code benchmarking was previously reviewed by the NRC as indicated in Section 3.5.3.4 of the safety evaluation report (SER) for the non-LOCA topical report, TR-0516-49416-P-A, Revision 3. ((2(a),(c) the benchmark is not intended to be a validation of the NuScale Power Module (NPM) response itself, but one of many independent forms of evidence of the adequacy NuScale Nonproprietary NuScale Nonproprietary
of the point kinetics model in NRELAP5. Therefore, the benchmarking analysis was not re-performed for the NPM-20 design. As stated in the SER, the NRC staff finds that the comparisons between NRELAP5 and RETRAN-3D provide confidence in the ability of the NRELAP5 point kinetics model to acceptably predict reactivity feedback. For these reasons, non-LOCA topical report Section 5.3.4, and its subsections, were not revised in the Revision 4 submitted to support the standard design approval application (SDAA) and currently under review. In addition, the point kinetics model used in NRELAP5 uses kinetics input calculated from the more detailed SIMULATE5 code as described in Chapter 4 of the final safety analysis report (FSAR). The kinetics inputs are then conservatively applied in NRELAP5 to ensure the overall conservatism of the NRELAP5 calculated power and reactivity responses. For more severe reactivity insertion events (i.e., rod ejections), the non-LOCA methodology is not used. In general, the limiting cases for reactivity events are driven by the module protection system (MPS) limits rather than by the worst-case combination of kinetics conditions. The increased power in the NPM-20 does not significantly change the power distributions. In an NPM, like existing PWRs, core power gradients exists in all assemblies in the core, most acutely for assemblies on the periphery. The gradients are due to geometric buckling (i.e., radial leakage). Such gradients occur regardless of core design, though lower leakage cores will tend to exhibit lower power gradients than higher leakage cores. In the overall Chapter 15 safety analysis methodology, including the non-LOCA methodology, each unique core design must be specifically analyzed and compared to the bounding limits developed for the NPM. Thus, bounding evaluations of all power gradients, which inherently includes the corresponding depletion history, are evaluated in each application of this method. Because SIMULATE5 has been shown to reliability predict critical conditions, power distributions, and depletion, it reliably predicts the spatial dynamic nuclear response, including from important phenomena such as power and depletion gradients. Because the NPM-20 design does not significantly change the power distributions, the NRELAP5 validation discussed in non-LOCA topical report Section 5.3.4 applies to the NPM-20 as well. No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}