ML24346A272
| ML24346A272 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 12/11/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24346A130 | List:
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| References | |
| LO-175762 | |
| Download: ML24346A272 (1) | |
Text
Response to SDAA Audit Question Question Number: A-15.1.2-3 Receipt Date: 05/13/2024 Question:
FSAR Section 15.1.2.2 states that in order to bound the possible feedwater flow increase scenarios, a spectrum of feedwater flow increases are analyzed to demonstrate that limiting conditions for MCHFR are reached. However, because the FSAR does not describe the largest increase in feedwater flow analyzed (e.g. up to three operating feedwater pumps increasing to their maximum operating speed), it is not clear how the spectrum of analyzed cases bound the possible causes of increase in feedwater flow. Provide FSAR markups stating the largest increase in feedwater flow analyzed.
Response
Final Safety Analysis Report (FSAR) Section 15.1.2.2 is revised as shown in the attached markups to add the maximum feedwater flow rate considered in the supporting analysis (EC-0000-8328, Revision 0, Increase in Feedwater Flow Analysis, - previously provided in the Chapter 15 electronic reading room).
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Increase in Heat Removal by the Secondary System NuScale US460 SDAA 15.1-6 Draft Revision 2 15.1.2.2 Sequence of Events and Systems Operation The sequence of events for an increase in FW flow event is provided in Table 15.1-4 for the limiting MCHFR case and Table 15.1-5 for the limiting SG overfill case.
Unless specified below, the analysis of an increase in FW flow event assumes the plant control systems and the ESFs perform as designed, with allowances for instrument inaccuracy. No operator action is credited to mitigate the effects of an increase in FW event.
Audit Question A-15.1.2-3 The FWS could malfunction and increase FW flow by increasing the speed of normally operating FW pumps, turning on a FW pump, opening a FW regulating valve (FWRV), or opening a DHRS valve at low RCS power. The inadvertent opening of a DHRS valve at low RCS power is addressed by the analysis in Section 15.2.9. In order to bound the possible FW flow increase scenarios, a spectrum of FW flow increases, up to the FW flow associated with three FW pumps operating at maximum speed, are analyzed to demonstrate that limiting conditions for MCHFR are reached. For the limiting MCHFR case, the steam outlet is modeled as a constant steam pressure, allowing the steam flow to increase providing an increase in SG heat transfer in response to the increase in FW flow.
An increase in FW flow can lead to overfilling the SG. For the SG overfill scenario, the maximum FW pump flow curve is used, as this results in the greatest increase in secondary inventory and the highest SG levels. Increased SG levels degrade DHRS condensers by increasing DHRS condenser static head liquid levels and causing a reduction in heat transfer surface area. The single failure of an FWIV leads to the most limiting SG overfill condition.
Operator action is not credited for regulating control rod movement or increasing boron concentration, which ensures the maximum reactivity insertion is reached as the control system attempts to maintain RCS temperature by pulling the regulating control rods from the core.
The MPS is credited to protect the plant in the event of an increase in FW flow. If the FW flow increases to a level that causes a high enough power excursion, the MPS high power signal trips the reactor, preventing the reactor from reaching a power level at which the acceptance criteria could be challenged. The following MPS signals protect the plant during an increase in FW flow:
high power high main steam superheat high main steam pressure low main steam superheat high pressurizer (PZR) pressure