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Information Notice No. 93-66: Switchover to Hot-Leg Injection Following a Loss-Of-Coolant Accident in Pressurized Water Reactors
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 August 16, 1993 NRC INFORMATION NOTICE 93-66: SWITCHOVER TO HOT-LEG INJECTION FOLLOWING A LOSS-OF-COOLANT ACCIDENT IN PRESSURIZED WATER REACTORS Addressees All holders of operating licenses or construction permits for pressurized water reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a potential for inadequate core flow as a result of the switchover to the hot-leg injection mode following a loss-of-coolant accident (LOCA) in the event of a single failure. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On January 8, 1993, Westinghouse issued a formal written report pursuant to Part 21 of Title 10 of the Code of Federal Regulations (10 CFR), on the potential for inadequate core flow as a result of the isolation of the residual heat removal/low-pressure safety injection (RHR/LPSI) pumps from the reactor coolant system (RCS) when aligning them to the RCS hot legs for the hot-leg recirculation mode following a LOCA. A single failure of a valve could prevent the alignment of these pumps to the RCS hot legs. Westinghouse has issued a formal letter to the affected licensees advising them that plant- specific emergency operating procedures (EOPs) may have to be revised to reflect the need to realign the RHR/LPSI pumps to the RCS cold legs to mitigate this situation. Discussion General Design Criterion 35 of Appendix A to 10 CFR Part 50, requires a system to provide abundant emergency core cooling following any LOCA. Section 50.46(b)(5) of 10 CFR Part 50 requires long-term core cooling following a 9308100006. IN 93-66 August 16, 1993 Page 2 of 3 calculated successful initial operation of the emergency core cooling system (ECCS). The Standard Review Plan, NUREG-0800, Section 6.3 states: The criteria, supporting analyses, plant design provisions, and operator actions will be reviewed to ensure that there will not be unacceptably high concentrations of boric acid in the core region (resulting in precipitation of a solid phase) during the long-term cooling phase following a postulated LOCA. A common means of preventing a high concentration of boric acid is to initiate hot-leg injection following a large-break LOCA to flush the core before the boron concentration becomes a concern. On January 8, 1993, Westinghouse issued a 10 CFR Part 21 report that identified the switchover to hot-leg injection as potentially vulnerable to a single failure for several pressurized water reactors (PWRs) with a Westinghouse-designed nuclear steam supply system. The potential single failure vulnerability for the switchover to hot-leg injection for a medium to large hot-leg LOCA, when high-pressure safety injection (HPSI) flow is being injected into the RCS hot legs and LPSI flow is being injected into the RCS cold legs is described below. (1) Switchover to LPSI is initiated by closing the valves in the pipes that provide LPSI flow to the RCS cold legs while the HPSI flow continues to be injected into the RCS hot legs. However, the HPSI flow into the RCS hot legs may not provide adequate long-term core flow is spilling from a hot-leg break. (2)In some PWRs, switchover to LPSI into the RCS hot legs is completed by opening a single valve. Single failure of this valve could result in failure to increase the core cooling flow into the RCS hot legs, thereby allowing inadequate long-term core cooling to continue for a hot-leg break. (3)Licensee EOPs may not address this situation. The single failure criterion is defined in 10 CFR Part 50, Appendix A, Definitions and Explanations and is further specified in plant-specific licensing bases. This issue may also be applicable to Combustion Engineering and Babcock & Wilcox designed PWRs. Therefore, this information notice is addressed to all PWR licensees and holders of PWR construction permits.. IN 93-66 August 16, 1993 Page 3 of 3 This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. /s/'d by CIGrimes/for Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contact: Warren C. Lyon, NRR (301) 504-3892 Attachment: List of Recently Issued NRC Information Notices .
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