ML25065A161
| ML25065A161 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 03/06/2025 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25065A159 | List: |
| References | |
| LO-180130 | |
| Download: ML25065A161 (1) | |
Text
Response to SDAA Audit Question Question Number: A-15.9-1 Receipt Date: 06/10/2024 Question:
In Section 15.9.3.4.3 please provide results for a 50% feedwater flow reduction as described in the methodology in TR-0516-49417.
Response
Note that there is no Final Safety Analysis Report (FSAR) Section 15.9.3.4.3. There is an FSAR Section 15.9.3.4, Increase in Reactor Coolant Inventory, but it has no subsections. The audit question describes a feedwater flow reduction. The feedwater flow reduction is described in FSAR Section 15.9.3.2.
The TR-0516-49417-P-A, Revision 1, Evaluation Methodology for Stability Analysis of the NuScale Power Module, evaluates stability during a feedwater flow reduction in Section 8.2.2.
As stated in Section 8.2.2, a complete loss of feedwater is not considered because it would result in actuation of the MPS and a reactor trip. A 50 percent reduction is considered with the additional explanation provided in Section 8.2.2.1 that [t]his magnitude of change is selected to determine the acceptability of a partial loss of feedwater and a successful runback that avoids a reactor trip.
The NRC review and approval of the feedwater flow reduction in TR-0516-49417-P-A, Revision 1, is documented in the Safety Evaluation Report (SER) Section 3.4.3.2 as indicated below, with emphasis added:
Section 8.2.2 of the Stability TR addresses AOOs that decrease heat removal by the secondary system. The applicant analyzed a reduction in feedwater flow that results in a decrease in heat removal but is a sufficiently mild perturbation that the reactor does not trip. In determining this feedwater flow reduction, the applicant NuScale Nonproprietary NuScale Nonproprietary
considered a 50-percent flow reduction, which would, operationally, correspond to a partial loss of feedwater and a successful runback to avoid a reactor trip.
The US600 Design Certification Application (DCA) FSAR Section 15.9.3.2 analysis that implements TR-0516-49417-P-A, Revision 1, considers a 10 percent feedwater flow reduction rather than 50 percent. As identified in DCA FSAR Section 15.9.3.2.2, [t]his magnitude of change is chosen to determine the acceptability of a partial loss of feedwater flow. While this magnitude of change would normally cause a reactor trip, this trip is not simulated and, thus, it conservatively bounds smaller changes to feedwater flow that would not result in a reactor trip.
In the Final Safety Evaluation Report (FSER) for the US600 DCA, the NRC review and approval of the feedwater flow reduction was documented in FSER Section 15.9.3.4.3 as indicated below, with emphasis added:
The staff reviewed NPM stability AOOs that decrease heat removal by the secondary system in DCA Part 2, Tier 2, Section 15.9.3.2. For stability analyses, conditions that produce the largest reduction in secondary side heat removal are not limiting because they would likely lead to a prompt, automatic reactor trip due to an increase in PZR pressure. The most adverse event from a stability perspective would be an AOO that maximizes the potential for the riser to void while avoiding the high PZR pressure trip.
To establish a conservative, bounding analysis method, TR-0516-49417 assumes a 50-percent feedwater flow reduction. This reduction is large enough to cause a reactor trip due to high PZR pressure, which is not credited in the TR-0516-49417 analysis.
Therefore, TR-0516-49417 defined a methodology that, if followed, conservatively bounds feedwater flow reduction transients.
However, the analysis in DCA Part 2, Tier 2, Section 15.9.3.2, departs from the methodology in TR-0516-49417 since the feedwater flow is reduced by only 10 percent in the DCA compared to the 50-percent reduction prescribed by TR-0516-49417.
According to the applicants analyses, and its description in DCA Part 2, Tier 2, Section 15.9.3.1.2, the 10 percent feedwater flow reduction will not actuate a high PZR pressure trip but will eventually actuate a high hot-leg temperature trip, which protects subcooling margin. To demonstrate long term stability, the applicant presents results in figures 15.9-8 and 15.9-11 from analyses where the high hot-leg temperature trip is ignored. These results show core flow and power oscillations that rapidly decay in time over intervals of 2000 to 3000 seconds, which are greater than 10 times the period of the reactor and NuScale Nonproprietary NuScale Nonproprietary
sufficiently long to characterize long term stability. Since the applicant departed from the methodology and did not use the prescribed 50-percent feedwater flow reduction, the staff evaluated the margin available in the analysis over a wide spectrum of flow reductions and noted the stability margin is insensitive to the feedwater reduction level.
Therefore, the DCA Part 2, Tier 2, Section 15.9.4.4.3 analysis demonstrates that the long-term stability solution meets the requirement of GDC 12 since the riser voiding is mitigated following a decrease in secondary heat removal which prevents instabilities that could challenge SAFDLs.
The applicants analyses for feedwater flow reduction demonstrate that the RCS heats up and that a reactor trip protecting the riser subcooling margin is eventually initiated. Based on these analyses, the staff finds that this event progression is generally applicable and occurs regardless of the magnitude of the feedwater flow reduction. The staff also finds that the long-term stability solution is effective in preventing the reactor from reaching an unstable condition by initiating a reactor trip before such an instability would occur and therefore meets the requirement of GDC 12.
The analysis presented in DCA Part 2, Tier 2, Section 15.9.3.2, is a departure from, and nonconservative with respect to, the stability analysis methodology presented in TR-0516-49417. Therefore, the staff finds that DCA Part 2, Tier 2, Section 15.9.3.2, safety conclusions shall not be construed as approval of the departure or as tacit acceptance of a change in the stability evaluation methodology presented in TR 0516-49417.
Nevertheless, for the reasons indicated above, the staff notes that for the NuScale NPM, the long-term stability solution is effective in preventing instability during AOOs that cause a decrease in secondary side heat removal.
NuScale disagrees with the FSER characterization that analyzing a 10 percent feedwater flow reduction in the DCA FSAR is a departure from TR-0516-49417-P-A, Revision 1. Nevertheless, NuScale agrees with the FSER conclusion that larger feedwater flow reductions result in trips that protect against instabilities that could challenge specified acceptable fuel design limits (SAFDLs) and therefore General Design Criterion (GDC) 12 is met.
For the US460 Standard Design Approval Application (SDAA), NuScale elected to take the same approach as the DCA. Rather than performing a stability analysis for a feedwater flow reduction of 50 percent per Section 8.2.2 per TR-0516-49417-P-A, Revision 1, NuScale NuScale Nonproprietary NuScale Nonproprietary
considered a feedwater flow reduction that is large and would result in trip, but the trip is not simulated in the PIM analysis. ((2(a),(c) the stability analyses in the SDAA use a 30 percent feedwater flow reduction as described in FSAR Section 15.9.3.2.2. The 30 percent reduction meets the intent of the TR-0516-49417-P-A, Revision 1 methodology, to analyze a partial loss of feedwater that does not result in reactor trip. No new analyses for a 50 percent feedwater flow reduction are added to FSAR Section 15.9.3.2.2. The considerations that led to NRC approval of the DCA with a feedwater flow reduction different than 50 percent are applicable to the SDAA. No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}