ML18199A191

2018/07/18 Nuscale SMR DC RAI - Request for Additional Information No. 495 Erai No. 9565 (3.9.6)

ML18199A191
Person / Time
Site:	NuScale
Issue date:	07/18/2018
From:	NRC
To:	NRC/NRO/DLSE/LB1
References
Download: ML18199A191 (4)

Text

The NRC regulations in Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50 specify principal design criteria to establish the necessary design, fabrication, construction, testing, and performance requirements for structures, systems, and components (SSCs) important to safety; that is, SSCs that provide reasonable assurance that the facility can be operated without undue risk to the health and safety of the public. With respect to containment isolation valves (CIVs), General Design Criterion (GDC) 54, "Piping systems penetrating containment," in 10 CFR Part 50, Appendix A, requires that piping systems penetrating primary reactor containment shall be provided with leak detection, isolation, and containment capabilities having redundancy, reliability, and performance capabilities which reflect the importance to safety of isolating these piping systems. GDC 54 also requires that such piping systems shall be designed with a capability to test periodically the operability of the isolation valves and associated apparatus and to determine if valve leakage is within acceptable limits. GDC 55, "Reactor coolant pressure boundary penetrating containment," in 10 CFR Part 50, Appendix A, requires that each line that is part of the reactor coolant pressure boundary (RCPB) and that penetrates primary reactor containment shall be provided with CIVs as specified in this GDC, unless it can be demonstrated that the containment isolation provisions for a specific class of lines, such as instrument lines, are acceptable on some other defined basis. GDC 56, "Primary containment isolation," in 10 CFR Part 50, Appendix A, requires that each line that connects directly to the containment atmosphere and penetrates primary reactor containment shall be provided with CIVs as specified in this GDC, unless it can be demonstrated that the containment isolation provisions for a specific class of lines, such as instrument lines, are acceptable on some other defined basis. GDC 57, "Closed system isolation valves," in 10 CFR Part 50, Appendix A, requires that each line that penetrates primary reactor containment and is neither part of the RCPB nor connected directly to the containment atmosphere shall have at least one CIV which shall be either automatic, or locked closed, or capable of remote manual operation. NuScale FSAR (Revision 1) Tier 2, Section 6.2.4.2.2, "Component Design," indicates the primary system containment isolation valves (PSCIVs) consist of a single body with two ball valves in series. Section 6.2.4.2.2 states that the PSCIV ball valve operation is based on a hemispherical design that rotates such that the ball presses into the valve seat to close the valve. Section 6.2.4.2.2 indicates that the secondary system containment isolation valves (SSCIVs) consist of a single ball valve design with the ball positioned off-center to provide for a tight seal on both the upstream and downstream metal seats. Similarly, Section 6.2.6.3, "Containment Isolation Valve Leakage Rate Test," states that the CIVs are a wedged, quarter-turn ball valve type. Figure 6.2-7, "Containment Isolation Valve Actuator Hydraulic Schematic," indicates a rack and pinion arrangement for the operation of the CIVs by a gas bottle actuator. Section 20.1.2.2, "Applicable Structures, Systems, and Components," in Chapter 20, "Mitigation of Beyond-Design-Basis Events," states that the CIVs fail-safe to their closed position using stored energy, and references Section 6.2.4 for details of the CIV design and function. During discussions regarding the NuScale reactor response to a long-term loss of ac power, NuScale personnel stated that the CIVs are assumed to remain closed for their design-basis and beyond-design-basis functions following closure. The NRC staff notes that the rack and pinion arrangement for operation of the CIVs might retain residual torque that could reopen or unseal the valve following depletion of the gas bottle actuator. Therefore, the NRC staff requests that NuScale revise FSAR Tier 2, Section 6.2.4, to specify that each ball valve in the PSCIVs and SSCIVs will be designed and qualified for torque closure using the gas bottle actuator to provide sufficient wedging and sealing to prevent reopening and unsealing of each ball valve following depletion of the gas bottle actuator for the extended time period for the design-basis and beyond-design-basis functions assumed for each individual ball valve. The staff also requests that NuScale confirm that the different descriptions of the ball valves and their operation for the PSCIVs and SSCIVs in FSAR Tier 2, Section 6.2.4.2.2, are not intended to indicate any differences in the PSCIV and SSCIV design.