ML25014A277
| ML25014A277 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 01/13/2025 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25013A204 | List: |
| References | |
| LO-178078, LO-177590 | |
| Download: ML25014A277 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-66 Receipt Date: 06/28/2024 Question:
TR Table 7-22 indicates MSIV single failure and FWIV single failure are potentially bounding for mass releases for break locations outside the CNV if mass release is calculated for use in the downstream radiological analysis. This implies that MSIV single failure and FWIV single failure are not bounding for some scenarios. Please clarify when MSIV single failure and FWIV single failure are not bounding.
Please provide proposed markups to the TR.
Response
TR-0516-49416-P, Non-Loss-of-Coolant Accident Analysis Methodology, Revision 4, Table 7-22 provides the acceptance criteria, single active failure, and loss of power scenarios for the steam line break event. Table 7-22 identifies that the single active failures of the NuScale Power Module (NPM) designs that could affect the steam line break event progression do not affect the limiting minimum critical heat flux (MCHFR) scenarios. The limiting MCHFR condition occurs around the time of reactor trip, at or before secondary side isolation or decay heat removal system (DHRS) actuation.
In the NRCs Safety Evaluation Report (SER) for TR-0516-49416-P-A, Revision 3, a limitation and condition was included that stated that [a]n applicant or licensee seeking to apply this methodology to a design must receive a separate approval through that design review for the event-specific electrical power assumptions (AC/DC), single failures, and the need for operator actions necessary to mitigate non-LOCA design basis events, with emphasis added.
Under the assumption that the same limitation and condition would be imposed for TR-0516-49416-P, Revision 4, the single failures in the tables in Section 7.2 of TR-0516-49416-P, NuScale Nonproprietary NuScale Nonproprietary
Revision 4, are those that are typically limiting. The specific single failures that are limiting for each event are identified in the design-specific Final Safety Analysis Report (FSAR).
For the steam line break, Table 7-22 provides a discussion of why the single failures typically do not impact MCHFR as discussed above. Single failures do impact the mass release. However, the TR-0516-49416-P, Revision 4, methodology identifies that the radiological consequence analyses can be performed using bounding input that does not require determination of a specific release quantity. In the case where the bounding input is used, the single failure assumptions associated with mass release are irrelevant. In the case where the bounding input is not used, the single failures that are typically limiting for mass release are identified in Table 7-22: main steam isolation valve (MSIV) for breaks outside containment and feedwater isolation valve (FWIV) for breaks inside containment.
Consistent with the previous SER limitation and condition, the FSAR for the US460 design identifies the limiting single failure for each event. The steam line break is described in FSAR Section 15.1.5. The discussion in FSAR Section 15.1.5.2 addresses possible single failures and identifies the single failures that are typically limiting, consistent with Table 7-22 of TR-0516-49416-P, Revision 4. The subsequent discussion in FSAR Section 15.1.5.3.2 identifies the specific single failure limiting for each case. Finally, the discussion in FSAR Section 15.1.5.3.3 identifies the impact of the assumed single failure on the results for each case. The single failures for each case are also identified in some of the sequence of events tables, such as FSAR Tables 15.1-11 and 15.1-12. The FSAR Section 15.1.5 discussion does not include mass release cases because the bounding approach is used for the US460 radiological consequence analysis of the steam line break (FSAR Section 15.0.3.7.3). Therefore, single failures related to mass release are not applicable.
The FSAR section 15.1.5.3.2 discussion of the limiting MCHFR case identifies that this case is a break outside containment and single failure of an MSIV to close is modeled. Table 15.1-13 shows the sequence of events for this case. The high power signal actuates reactor trip and MCHFR is reached at 34 seconds. The low pressurizer pressure signal actuates secondary side isolation at 85 seconds. The modeled single active failure of an MSIV to close occurs well after the time of MCHFR. Failure of an MSIV to close in this case does affect the available DHRS capacity because the break is located between the MSIV and secondary MSIV as described in FSAR 15.1.5.3.2. Therefore, the modeled failure of the MSIV on the broken loop to close disables one train of DHRS in this case, similar to the limiting reactor coolant system pressure case, but has no impact on the figure of merit for this case (i.e., MCHFR).
NuScale Nonproprietary NuScale Nonproprietary
In summary, the discussion of the typical limiting single failures in Table 7-22 is consistent with the FSAR Section 15.1.5 discussion of the typical limiting single failures. The identification of the specific limiting single failures is outside the approved scope of TR-0516-49416-P, Revision 4, and is addressed in the FSAR instead. However, TR-0516-49416-P, Revision 4, is revised as shown in the attached markup to clarify the wording for consistency with this response and other similar tables in the report.
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 557 Audit Question A-NonLOCA.LTR-66 7.2.4.2 Acceptance Criteria Evaluation of the most challenging case relative to the acceptance criteria is presented in Table 7-23.
Table 7-22 Acceptance criteria, single active failure, loss of power scenarios - steam line break Acceptance Criteria / Single Active Failure / Loss of Power Scenarios of Interest Discussion MCHFR Radiological consequences Critical heat flux is potentially challenged for this overcooling event. (Reactivity insertion rates from the overcooling event are insufficient to challenge fuel centerline temperature.)
A postulated break in the main steam line is evaluated for radiological consequences.
Failure of one MSIV to close on the train with break Failure of one FWIV to close on the train with the break MSIV single failure typically has no effect on MCHFR since DHRS actuation is not before the time when MCHFR occurs. MSIV single failure is typically limitingpotentially bounding for mass releases for break locations outside the CNV if mass release is calculated for use in the downstream radiological analysis.
FWIV single failure typically has no effect on MCHFR since DHRS actuation is not before the time when MCHFR occurs. FWIV single failure is typically limitingpotentially bounding for mass releases for break locations inside the CNV if mass release is calculated for use in the downstream radiological analysis.
No loss of power Loss of power scenarios typically terminate feedwater and/or trip the reactor, thus mitigating the overcooling event.
Table 7-23 Acceptance criteria - steam line break Acceptance Criteria Discussion Primary pressure Due to the depressurizing nature of this cooldown event, primary pressure remains below the acceptance criterion for peak primary pressure.
Secondary pressure Due to the depressurizing nature of this cooldown event, secondary pressure remains below the acceptance criterion for peak secondary pressure.
Critical heat flux ratio This criterion is evaluated by downstream subchannel analysis. Sensitivity cases are performed to support the follow-on MCHFR evaluation.