ML24346A310
| ML24346A310 | |
| 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: ML24346A310 (1) | |
Text
Response to SDAA Audit Question Question Number: A-19.1-52 Receipt Date: 11/20/2023 Question:
The containment isolation function supports the passive core cooling and heat removal key safety functions by ensuring sufficient coolant inventory in the reactor pressure vessel and the containment vessel. Table 19.-6, Success Criteria per Top Event, states that the ECCS provides passive cooling without additional inventory or closure of normally open CIVs and the Success Criteria Notebook indicates that containment isolation is irrelevant for sequences that do not involve a break outside containment or SGTF, and is therefore equivalent to a thermal-hydraulic simulation that reflects success of containment isolation. The containment isolation function is accordingly not questioned in any of the Level 1 event trees except for the CVCS pipe breaks outside containment and the SGTF.
In the staffs DCA FSER, staff made a finding on the Level 1 model and assumptions related to the containment isolation function (based on RAI 8840). Given the power uprate, the redesign of the CES, and the reduced pool level including changes to TS 3.5.3, describe any simulations and their results that include failure of containment isolation to verify that passive core cooling and heat removal are not dependent on containment isolation for LOCAs inside containment to support a similar staff finding for the SDAA. The description of the simulations should include a discussion of NRELAP5 modeling assumptions, including the initial reactor pool temperature, credit for active pool cooling systems, inventory addition, and ambient heat loss from the pool. If such simulations have not been performed, provide the detailed technical justification that passive core cooling and heat removal are not dependent on containment isolation for LOCAs inside containment, including the basis for not performing relevant simulations for the SDAA.
Response
In NuScales Design Certification Application (DCA) for the US600 design, the containment evacuation system (CES) is the only low-pressure system open to the containment during NuScale Nonproprietary NuScale Nonproprietary
normal operation. If the containment isolation valves (CIVs) on the CES line fail to close during a loss-of-coolant accident or emergency core cooling system (ECCS) actuation, components of the CES outside of containment could be exposed to pressures exceeding their pressure rating, potentially resulting in a failure of the pressure boundary and a loss of coolant from the NuScale Power Module (NPM).
During the DCA review, NuScales response to Request for Additional Information (RAI) 8840 described simulations considering ECCS actuation with a failure of the CIVs on the CES line to close and an induced failure of the CES piping outside of containment. These simulations demonstrate that, despite the failure of the pressure boundary, sufficient coolant remains in the NPM to maintain the liquid level above active fuel and provide passive core cooling for more than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. On this basis, closure of the CIVs on the CES line is not necessary for the success of passive core cooling for the US600 design.
A design change rendered a similar analysis as the one described in RAI 8840 unnecessary for the US460 design. The pressure rating of the portion of the CES between the CIVs and the next isolation valve, which is the vacuum pump suction valve, was raised to the containment design pressure; this effectively extends the containment design pressure boundary to the vacuum pump suction valves. ((2(a),(c) Assumption 2.2.1 of ER-102067, Accident Sequence Analysis Notebook, which is available in the SDAA Audit Section 19.1-19.3 eRR, provides the justification for why containment isolation is not modeled as a requirement for ECCS passive cooling: 2.2.1 Emergency Core Cooling without Containment System Isolation The ECCS provides passive cooling without additional inventory or closure of normally open containment isolation valves (CIVs). Basis/Rationale - Engineering judgement. As described in the Containment System Probabilistic Risk Assessment Notebook, ER-102074 (Reference 1.5.4), normally open CIVs are located on the reactor component cooling water system (RCCWS) lines, steam generator (SG) system lines, chemical and volume control system (CVCS) lines, and the containment evacuation system (CES) line. The RCCWS and SG lines are closed systems that are not open to the reactor coolant system (RCS) or containment vessel (CNV), so there is no pathway for losing RCS coolant from the NPM if those CIVs fail to close. The CVCS is designed to operate at primary system pressure, so there is no mechanism that would breach the CVCS boundary. The CES line outside containment is rated to the CNV design pressure up to the CES vacuum pump suction valves. In the NuScale Nonproprietary NuScale Nonproprietary
event the CES CIVs fail to close, the CES vacuum pump would trip and the pump suction and discharge valves would close. (( }}2(a),(c) NuScale revised Note 2 to FSAR Table 19.1-6, Success Criteria per Top Event, to include the CES piping design feature. Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-102 Draft Revision 2 Audit Issue A-19.1-51, Audit Question A-19.1-52 Table 19.1-6: Success Criteria per Top Event Top Event Mitigating System1 Description CFDS-T01 CFDS In sequences with a continued loss of RCS inventory (e.g., unisolated pipe break) and success of ECCS, the CFDS can provide RCS makeup inventory. Actuation requires an operator action that includes unisolating containment and activating a CFDS pump (CFDS--HFE-0001C-FOP-N). CVCS-T02 CVCS isolation Following an injection line break outside of containment, closure of either CIV in the injection line isolates the line and minimizes the loss of RCS inventory. CVCS-T03 CVCS isolation Following a discharge line break outside of containment, closure of either CIV in the discharge line isolates the line and minimizes the loss of RCS inventory. CVCS-T01 CVCS makeup The CVCS can provide RCS makeup via the injection or pressurizer spray line. CVCS-T04 CVCS makeup Following a CVCS injection or spray line break inside containment, RCS makeup can be provided via the alternate line; the injection line following a spray line break, or the spray line following an injection line break. Actuation requires an operator action to unisolate containment and activate a CVCS makeup pump (CVCS--HFE-0001C-FOP-N). The BAS and the DWS provide inventory to support the PRA mission. DHRS-T01 DHRS The DHRS provides fuel assembly heat removal. The DHRS is a passive cooling system that removes fuel assembly heat by circulating coolant through the SGs and DHRS condensers that transfer heat to the UHS. One of two trains is required and each requires opening an actuation valve and closing the respective secondary system CIVs or backup valves in the MSS and the FWS. DHRS-T02 DHRS Following an SGTF, the DHRS in the unaffected SG provides fuel assembly heat removal. ECCS-T01 ECCS The ECCS provides fuel assembly heat removal and control of RCS inventory without the need for makeup inventory or containment isolation2. Success of the ECCS requires the opening of one RVV and one RRV. The system passively circulates coolant by removing heat from the reactor core through the CNV to the UHS. The main ECCS RRVs include a passive opening feature. If a valve fails to open because of a failure in the hydraulic actuator (i.e., closed trip valve or closed IAB), the valve passively opens when the spring force overcomes the differential pressure force across the valve disc3. Thermal-hydraulic simulations were performed to confirm the effectiveness of the low differential pressure opening mechanism, including the timing of opening of the valves. Only passive opening of the RRVs is credited in the PRA4. Thermal-hydraulic simulations demonstrate that the negative reactivity provided by the ESB function reduces the potential for power oscillations in ATWS scenarios, in particular at end-of-cycle conditions when reactor coolant boron concentration is low. However, the ESB function is not required to prevent core damage. Actuation signals include low RCS level, high-high RCS pressure, high-high RCS average temperature, the reactor trip 8-hour timer, and the low AC voltage 24-hour timer. Loss of two or more EDAS buses also deenergizes the solenoids to open the ECCS valves. ECCS-T02 ECCS Following an unisolated break outside containment, the opening of both RVVs and both RRVs can provide RCS heat removal and control of RCS inventory without the need for makeup inventory. An operator action to actuate the ECCS in cases where automatic initiation fails is considered (ECCS--HFE-0001C-FTO-N).
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-103 Draft Revision 2 ECCS-T03 ECCS Following normal post-trip response with the RTS and the DHRS, bypassing the reactor trip 8-hour timer (after confirming shutdown margin) precludes an ECCS actuation. An operator action to bypass the timer is considered (ECCS--HFE-0002C-FTB-N). BPSS-T01 Electric Power Backup power via a BDG precludes an ECCS actuation and allow for RCS makeup (i.e., CVCS, CFDS) if needed. Alignment of a BDG requires operator action (BPSS--HFE-0001C-FTS-N). EHVS-T01 Electric Power Recovery of the EHVS via offsite power within 24 hours (following bypass of the reactor trip 8-hour timer) precludes an ECCS actuation. EHVS-T02 Electric Power Recovery of the EHVS via offsite power within 8 hours (following failure to bypass the reactor trip 8-hour timer) will preclude an ECCS actuation. RCS-T01 RSV opens The RSVs provide pressure relief. Although a cycling RSV can serve as a backup to DHRS heat removal, when the RCS level reaches the low level setpoint, the ECCS is actuated. RCS-T04 SGTF isolation Following an SGTF, closure of the CIVs or backup isolation valves in the affected MSS and FWS lines minimizes the loss of RCS inventory and isolates the faulted SG. RCS-T05 RSV not demanded This event accounts for the possibility that primary pressure does not increase to the point of reaching the RSV setpoint (e.g., one train of the DHRS may remove heat quickly enough to prevent an RSV demand to open). RCS-T06 RSV closes Closure of the RSV after opening re-establishes RCS integrity. This top event is only considered in scenarios with DHRS success where there is a single RSV cycle. In scenarios with DHRS failure or an ATWS, the RSV cycles many times. When RCS level reaches the low level setpoint, the ECCS is actuated, regardless of RSV closure. RTS-T01 RTS The RTS provides reactivity control. Notes: 1.The PCS is not considered as a mitigating system in the PRA. Because almost any unplanned transient results in actuation of the DHRS, the PCS is isolated (i.e., MSS and FWS lines). 2.The ECCS provides passive cooling without additional inventory. or closure of normally open CIVs. Containment isolation of the CES is achieved by redundant CIVs and an additional diverse valve on the CES vacuum pump suction. The CES piping downstream of the CIVs, up to and including isolation valves, is rated for containment design pressure. Failure of both CIVs coincident with failure of the CES vacuum pump suction valve (which includes diversity in the closure signal) is judged to be not credible, and hence not modeled. 3.The main valve control chamber can be depressurized through the internal orifice located in the body of the valve disk (i.e., passive opening); as differential pressure lowers, the main valve spring assisted by reactor coolant pressure opens the valve. 4.The PRA NRELAP5 runs show the low differential pressure across the RVVs (i.e., passive opening) is not reached in time to prevent core damage. Table 19.1-6: Success Criteria per Top Event (Continued) Top Event Mitigating System1 Description}}