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PrintHome > NRC Library > Document Collections > Generic Communications > Information Notices > 1993 > IN 93-66 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.Page Last Reviewed/Updated Tuesday, November 12, 2013Home- News   Releases
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