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
                           WASHINGTON, D.C.   20555

                                August 16, 1993

                               A LOSS-OF-COOLANT ACCIDENT IN PRESSURIZED       
                               WATER REACTORS


All holders of operating licenses or construction permits for pressurized
water reactors. 


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.


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 


                                                            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

List of Recently Issued NRC Information Notices


Page Last Reviewed/Updated Wednesday, March 24, 2021