ML24348A038
| ML24348A038 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 12/13/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24348A006 | List:
|
| References | |
| LO-176318 | |
| Download: ML24348A038 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-28 Receipt Date: 04/15/2024 Question:
Provide a methodology for establishing the effect of pressure on critical heat flux (CHF).
The TR states that the effect of pressure on critical heat flux (CHF) will be established on a case-by-case basis to determine the appropriate direction for biasing. The TR however was revised to remove discussion of various pressure effects on CHF as well as a method for biasing pressure to conservatively estimate MCHFR.
Please provide proposed markups to the TR to include the methodology for determining effect of pressure on CHF including discussion of the relative importance of various effects.
Response
The non-loss-of-coolant accident (non-LOCA) methodology topical report, TR-0516-49416-P-A, Revision 3, was previously reviewed and approved by the NRC. Section 4.3.5 of that revision identified the conservative bias direction for determining minimum critical heat flux ratio (MCHFR) for the following four parameters: reactor power, core exit pressure, core inlet temperature, and system flow rate. The conservative bias directions for reactor power (high),
core inlet temperature (high), and system flow rate (low) were all intuitive and did not require more detailed explanation. ((2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
In Section 4.3.5 of the non-LOCA methodology topical report currently under NRC review, TR-0516-49416-P, Revision 4, the conservative bias directions for reactor power, core inlet temperature, and system flow rate, and associated rationales, have not changed from the previously approved version. However, the discussion of pressure biasing has been greatly simplified. Rather than justifying why one bias direction is always conservative, NuScale has taken a more conservative approach and identified that effect of pressure on MCHFR is to be determined on a case by case basis. In other words, there is no a priori assumption of the effect of pressure on MCHFR. Instead, cases with both high pressure and low pressure are used to determine which is more limiting for MCHFR. This approach ensures that the limiting MCHFR will be determined and is therefore conservative. With this background, each aspect of the NRC audit question is addressed by providing the text of the question in indented, italicized text, followed by the NuScale response in non-indented, regular text. Provide a methodology for establishing the effect of pressure on critical heat flux (CHF). Section 4.3.5 of TR-0516-49416-P, Revision 4, already provides a methodology: cases with different pressures are run and the most limiting MCHFR is selected from the different cases. The TR states that the effect of pressure on critical heat flux (CHF) will be established on a case-by-case basis to determine the appropriate direction for biasing. Correct. The TR however was revised to remove discussion of various pressure effects on CHF as well as a method for biasing pressure to conservatively estimate MCHFR. The removed text provided a justification for biasing pressure in one direction only. Because NuScale is no longer biasing pressure in only one direction, the text was unnecessary. Please provide proposed markups to the TR to include the methodology for determining effect of pressure on CHF including discussion of the relative importance of various effects. Section 4.3.5 of TR-0516-49416-P, Revision 4, already provides a methodology: cases with different pressures are run and the most limiting MCHFR is selected from the different cases. NuScale Nonproprietary NuScale Nonproprietary
Additional discussion is not necessary since the limiting MCHFR is selected from the different cases regardless of the relative importance of various effects of pressure on MCHFR. No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}