ML24346A270
| ML24346A270 | |
| 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: ML24346A270 (1) | |
Text
Response to SDAA Audit Question Question Number: A-15.1.2-1 Receipt Date: 04/15/2024 Question:
FSAR Section 15.1.2 evaluates increases in feedwater flow due to failures in the feedwater system, such as the opening of a feedwater regulating valve (FWRV). In order to ensure that adequate decay heat removal is available following an increase in feedwater flow, the applicant evaluates scenarios that maximize the steam generator liquid level (referred to as SG overfill cases).
For these cases, single failure of a feedwater isolation valve (FWIV) is considered as it increases steam generator level by prolonging feedwater flow. FSAR Table 15.0-8 indicates that FWRVs are nonsafety systems credited in this transient. FSAR Table 15.1-5 states that SSI valves are fully closed at 18 seconds. This appears to be inconsistent with other transient evaluations that credit closure of the FWRV in lieu of the FWIV. For example, the loss of external load transient evaluation credits FWRV closure within 30 seconds as stated in FSAR Section 15.2.1.2.
The applicant provided EC-0000-8328 as one of the calculation documents to support FSAR Section 15.1.2. (( 2(a),(c) Clarify what FWRV and FWIV closure times (including sensing delays, actuation delays, stroke times, and any other delays) are modeled in analysis of steam generator overfill due to increase in feedwater flow and provide the basis. Address potential inconsistencies with assumptions used in other transients. Revise the FSAR to describe how FWRVs are credited in this transient and to state the FWRV closure time and basis. Provide revised EC-0000-8328, Appendix C NuScale Nonproprietary NuScale Nonproprietary
figures that allow review of the period preceding peak SG level for NRC staff audit. Additionally, explain why the FWRV can be credited to mitigate the increase in feedwater event while the FWRV failure is one of the potential initiators of this transient.
Response
EC-0000-8328, Revision 0, NPM-20 Increase in Feedwater Flow Analysis, (available in the Chapter 15 electronic reading room) and the applicable references used as input to this calculation describe the modeling of feedwater regulating valves (FWRVs) and feedwater isolation valves (FWIVs) for the increase in feedwater flow analysis. As explained in Section 2 of EC-0000-8328, the non-loss-of-coolant accident (LOCA) model includes several inputs and assumptions, documented in EC-0000-8507, Revision 0, NPM-20 NRELAP5 Non-LOCA Model, and EC-0000-7782, Revision 1, NPM-20 NRELAP5 Model, basemodels. Section 2 of EC-0000-8328 also describes additional inputs and assumptions beyond those in the referenced NRELAP5 basemodels. As described in these documents: (( }}2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
(( }}2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
((
}}2(a),(c)
The closure time of the FWRVs in the SG overfill case is consistent with the modeling assumptions described in EC-0000-8328 and applicable NRELAP5 modeling references. This modeling of the FWRV is consistent with other transient analyses that use the same modeling assumptions. No FSAR markups are provided. Crediting the FWRV as a backup to the failed open FWIV is shown in FSAR Table 15.0-8. Descriptions of FWRV operation during the SG overfill event are consistent with those provided in the NRC-approved Design Certification Application associated with the NuScale US600 power plant design. Figures that allow review of the period preceding peak SG level for case sgl-16 are provided in the Chapter 15 electronic reading room for NRC staff audit (file name Results for sgl-16 from EC-0000-8328 Rev. 0). The audit question also asks for an explanation of why the FWRV can be credited to mitigate the increase in feedwater event while the FWRV failure is one of the potential initiators of this transient. Note that FWRV failure is not described as an initiator in Section 15.1.2.2, rather FWRV opening is an identified initiator. The opening of the valve does not prevent its later closure. As described in FSAR Section 10.4.6.2.2, normal control of the FWRVs is through the module control system. In off-normal conditions the module protection system (MPS) overrides normal control of the valves and may force closure. Each FWRV is designed to close on loss of power or control signal of decay heat removal system actuation and secondary system isolation NuScale Nonproprietary NuScale Nonproprietary
regardless of the operating mode, and performs a feedwater isolation function as a backup to the FWIV. Operability of the FWRVs is also a requirement per Technical Specification Limiting Condition for Operation 3.7.2. Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Other Features of Steam and Power Conversion System NuScale US460 SDAA 10.4-13 Draft Revision 2 The FWS incorporates considerations to prevent erosion and corrosion. These considerations include material selection, limits on flow velocity, inspection programs, and limits on water chemistry to reduce FAC, erosion, and corrosion of piping and piping components. Section 10.3.6 discusses FAC. 10.4.6.2.5 System Operation The turbine provides extraction steam to the shell side of the FWHs, raising feedwater temperature as feedwater flow through the tube side of the heater increases. There are three feedwater pumps and three condensate pumps. During normal operations, all feedwater pumps and condensate pumps are operational. The FWS is able to accommodate the step load changes from programmed SG water level or a major effect on the feedwater system. The FWS has the capability to accommodate changes in feedwater flow to the SG with the steam pressure increase resulting from a 100 percent load rejection. The PSS, described in Section 9.3.2, provides the capability to collect and analyze FWS samples. Condensate pumps are configured to provide redundancy to minimize adverse impact to plant operation in the event of a pump failure or trip. Loss of a single feedwater pump does not result in a turbine generator or reactor trip. Loss of normal alternating current power results in a loss of feedwater to the SG. Section 15.2.7 discusses loss of normal feedwater. Audit Question A-15.1.2-1 An excessive feedwater flow malfunction causes an increase in feedwater flow resulting in a reduction of steam superheat, increased SG inventory, and reduction in outlet temperature. Section 15.1.2 discusses an increase in feedwater flow. To limit the potential for overfilling the SG during an increase in FW flow, the condensate and feedwater system pump design capacity is required to be within the feedwater pump curve assumed in Table 15.1-19. Loss of feedwater heating malfunction causes a decrease in feedwater temperature that increases heat removal from the RCS and lowers the RCS temperature. Section 15.1.1 discusses a loss of feedwater heating. A feedwater line break outside of containment is isolated by the FWIVs. The FWRVs provide a backup isolation to the FWIVs. Section 15.2.8 discusses feedwater line breaks. Inadvertent DHRS actuation and secondary system isolation causes closure of the main steam isolation valve and main feedwater isolation valve on the
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. Audit Question A-15.1.2-1 An increase in FW flow can lead to overfilling the SG. For the SG overfill scenario, the maximum FWcondensate and feedwater system pump flow curve shown in Table 15.1-19 is assumedused, as this results in the greatest increase in secondary inventory and the highest SG levels. Flow into the SG is terminated by the closure of either the FWIV or FWRV, or when SG pressure reaches the FW pump cutoff pressure. 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
NuScale Final Safety Analysis Report Increase in Heat Removal by the Secondary System NuScale US460 SDAA 15.1-46 Draft Revision 2 Audit Question A-15.1.2-1 Table 15.1-19: Condensate and Feedwater System Pump Curve Assumed in Steam Generator Overfill Analysis FWS Pump Head (psi) Maximum FW Flow Rate (lbm/s) 0.0 365.0 320.0 365.0 500.0 325.0 800.0 245.0 1100.0 70.0 1120.0 0.0}}