ML25035A247
| ML25035A247 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 02/04/2025 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25035A245 | List: |
| References | |
| LO-179124 | |
| Download: ML25035A247 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-20 Receipt Date: 04/01/2024 Question:
(( 2(a),(c) The SG-DHRS loop inventory is determined when the SG-DHRS loop is isolated for DHRS operation. In various Chapter 15 events, the loop isolation timing varies depending on the event progression and plant response. Provide a bounding SG-DHRS loop inventory analysis for applicable Chapter 15 events to ensure sufficient DHRS heat removal capacity.
Response
((
}}2(a),(c) The results show that there is significant DHRS heat transfer capability across a wide range of loop inventory. The results also show that the DHRS behavior for the SDAA is consistent with the DCA. (( }}2(a),(c)
NuScale Nonproprietary NuScale Nonproprietary
((
}}2(a),(c) The loop inventory during a transient event is a function of the initial inventory in the SG tubes, relevant single failures, the isolation timing, and, importantly, the transient boundary conditions modeled for the condensate feedwater (FW) pump response. Figure 1 of the file shows the condensate FW pump design limits and NRELAP5 model implementation for Chapter 15 events where a detailed FW pump response is modeled. The Chapter 15 events that result in a single train of DHRS conservatively model the FW pump response and also model conservative isolation times as specifically described in the Final Safety Analysis Report (FSAR) for those events. Sufficient DHRS heat removal capacity is demonstrated in Chapter 15 events by showing that specified acceptable fuel design limits (SAFDLs) are met and a safe, stabilized condition is achieved.
Section 15.1.2 of the FSAR addresses the anticipated operational occurrence (AOO) consisting of an increase in FW flow. The analysis of this event explicitly considers the potential for loop inventory to affect DHRS performance. Tables 15.1-4 and 15.1-5 show the sequence of events for the limiting minimum critical heat flux ratio (MCHFR) and SG overfill cases, respectively. For MCHFR cases, Section 15.1.2.2 states that, In order to bound the possible FW flow increase scenarios, a spectrum of FW flow increases are analyzed to demonstrate that limiting conditions for MCHFR are reached. Figure 15.1-8 through Figure 15.1-16 of SDAA show results for the limiting MCHFR case. The limiting FW flow increase is a near instantaneous 15-percent increase of normal FW flow. Section 15.1.2.3.3 concludes that, Passive DHRS cooling is established, and the transient is terminated with the NPM in a safe, stable condition. For SG overfill cases, Section 15.1.2.2 identifies that the increase in FW flow could overfill the SG that increased SG levels can degrade DHRS performance (as shown in Slide 11). Therefore, Section 15.1.2.3.3 describes the conservative assumptions to increase SG level and possibly reduce DHRS heat removal. For the limiting SG overfill case, slow closure (i.e., maximum closing time) of secondary isolation valves is modeled and single failure of a feedwater isolation valve is modeled, allowing FW flow into the affected loop to continue until the feedwater regulating valve closes. The results show that the SG does not overfill as shown in Figure 15.1-18 and that the [e]valuation of cases biased for SG overfill demonstrate NuScale Nonproprietary NuScale Nonproprietary
adequate DHRS heat removal capability is available under these conditions as shown in Figure 15.1-17. Section 15.1.5 of the FSAR addresses the postulated accident (PA) consisting of a main steam line break (MSLB). The analysis of the MSLB event explicitly considers the potential for DHRS performance to be affected. Figures 15.1-29 through 15.1-43 show the MSLB results for various cases. Some of the cases result in disabling an entire train of DHRS, including the limiting reactor coolant system (RCS) pressure case as described in Section 15.1.5.3.3. The RCS average temperature response for the limiting RCS pressure case in Figure 15.1-32 demonstrate there is adequate DHRS heat removal capability even with only a single train of DHRS. Note that the supporting calculations for FSAR Section 15.1.5 have been provided in the eRR for Chapter 15 with the response to audit question A-15.1.1-5. The effect of loop inventory on DHRS performance has been considered in Chapter 15 based on the conservative assumptions used for single failure, isolation times, and FW pump response in relevant events. The results demonstrate that DHRS performance is adequate for the predicted range of DHRS loop inventories during those events. In summary, each aspect of the audit question is addressed by providing the text of the question in indented, italicized text, followed by the NuScale response in regular text. The SG-DHRS loop inventory is determined when the SG-DHRS loop is isolated for DHRS operation. NuScale agrees. In various Chapter 15 events, the loop isolation timing varies depending on the event progression and plant response. NuScale agrees. Provide a bounding SG-DHRS loop inventory analysis for applicable Chapter 15 events to ensure sufficient DHRS heat removal capacity. Relevant Chapter 15 events where DHRS inventory could be negatively impacted due to loop inventory (e.g., increase in FW flow in Section 15.1.2) are already analyzed to explicitly account for the effect of loop inventory during these events by the assumed single failure, modeled isolation times, and FW pump response. Adequate DHRS heat removal capacity is demonstrated in Chapter 15 for the relevant events. NuScale Nonproprietary NuScale Nonproprietary
No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}