ML24348A027

Response to NuScale Topical Report Audit Question Number A-NonLOCA.LTR-23

ML24348A027
Person / Time
Site:	05200050
Issue date:	12/13/2024
From:	NuScale
To:	Office of Nuclear Reactor Regulation
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Download: ML24348A027 (1)

Text

Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-23 Receipt Date: 04/01/2024 Question:

Provide a methodology and acceptance criteria for identifying the limiting cases for MCHFR.

The TR states: "After the system transient analysis calculations are performed and assessed, for events that require subchannel analysis, a number of cases are identified as limiting for MCHFR. However, it is not clear how the cases are identified and what are the criteria for selecting the cases as limiting for MCHFR. Please provide proposed markups to the TR.

Response

The version of the non-LOCA topical report previously approved by the NRC (TR-0516-49416-P-A, Revision 3) contains the same quotation in Section 4.3.5 and also did not provide further details regarding selection of cases from NRELAP5 to analyze in the downstream subchannel (VIPRE-01) analyses. As stated in the safety evaluation report (SER), Section 3.4.3.5, The NRC staff finds that the limiting MCHFR cases for downstream subchannel analysis will be appropriately identified using the non-LOCA EM because the MCHFR predicted using NRELAP5 follows the same trend as VIPRE-01. Further, when in doubt, an analyst will pass several potentially limiting cases to VIPRE-01. In other words, a more detailed explanation than that already provided in Section 4.3.5 of the topical report was not required during the NRCs previous approval of the non-LOCA topical report. However, some additional information is provided below.

In other audit requests, the NRC has requested the NRELAP5 and VIPRE-01 calculation documents for multiple Chapter 15 events. By comparison of the NRELAP5 and VIPRE-01 calculations documents for a specific Chapter 15 event, it can be determined the extent of cases selected for analysis of minimum critical heat flux ratio (MCHFR) in VIPRE-01. Examples are provided below.

NuScale Nonproprietary NuScale Nonproprietary

Audit question A-15.2.7-3 requested the calculations for the loss of feedwater flow (LOFW) analysis in Final Safety Analysis Report (FSAR) Section 15.2.7. In response, NuScale provided EC-116269 Revision 0 (for NRELAP5) and EC-117457 Revision 2 (for VIPRE-01). ((2(a),(c) The overall limiting MCHFR from VIPRE-01 is 2.03 compared to the analysis limit of 1.45 (i.e., approximately 40 percent margin).

Audit question A-15.4.2-1 requested the calculations for the uncontrolled control rod assembly bank withdrawal (UCBW) analysis in FSAR Section 15.4.2. In response, NuScale provided EC-0000-8672 Revision 0 (for NRELAP5) and EC-102745 Revision 2 (for VIPRE-01). (( }}2(a),(c) The overall limiting MCHFR from VIPRE-01 is 1.58 compared to the analysis limit of 1.45 (i.e., approximately 10 percent margin). ((

}}2(a),(c)

Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary

Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2024 by NuScale Power, LLC 44 limiting initialization for a given transient progression to ensure the maximum power and core inlet fluid temperature are reached prior to reactor trip system actuation. For example, in the case of a heatup event, the RCS increases in temperature, causing a pressurizer insurge and subsequent increase in pressure. The limiting CHF scenario is the transient progression that results in the highest core outlet temperature at the time of reactor trip on high pressure, which is generally the faster heatups where the pressurizer initialization is biased to delay the high pressure trip. For some transients, a spectrum of cases is analyzed from the limiting initialization. (( Audit Question A-NonLOCA.LTR-23 }}2(a),(c) After the system transient analysis calculations are performed and assessed, for events that require subchannel analysis, a number of cases are identified as limiting for MCHFR. For the limiting cases selected, the required system transient parameters are tabulated as a function of time for input to the downstream subchannel analysis calculations, to calculate margin to CHF. The system transient parameters are provided for subchannel analysis for sufficient time for the subchannel analyses to demonstrate that the MCHFR has occurred, typically 10-15 seconds following reactor trip.}}