United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-74: Potentially Inadequate Performance of ECCS in PWRs During Recirculation Operation Following a LOCA

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                               September 14, 1988

                                   ECCS IN PWRs DURING RECIRCULATION OPERATION
                                   FOLLOWING A LOCA 


All holders of operating licenses or construction permits for Westinghouse (W)
and Babcock and Wilcox (B&W)-designed nuclear power reactors.


This information notice is being provided to alert addressees to potential 
problems that could result in inadequate performance of the emergency core 
cooling system (ECCS) during the recirculation phase of operation following a 
loss-of-coolant accident (LOCA).  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 do not constitute NRC requirements; therefore, no 
specific action or written response is required.

Description of Circumstances:
The NRC has received two licensee event reports on potential deficiencies in 
performance of the ECCS during the recirculation phase of operation following 
a small-break LOCA.  These events were reported by the licensees of the Oconee
and Turkey Point facilities after engineering analysis for each facility had 
identified the potential problems.

Oconee Nuclear Station

During a technical review of nuclear safety-related mechanical calculations, 
the licensee discovered that the design analysis had not been adequately per-
formed for the recirculation phase of operation following a small-break LOCA. 
Subsequent engineering analysis indicated the following design deficiencies:

(1)  Under certain small-break LOCA scenarios, when the borated water storage 
     tank (BWST) is depleted and the reactor coolant system (RCS) pressure 
     remains high (greater than the low-pressure injection (LPI) pump shut-off 
     head), a piggyback-type operation is required to ensure core cooling.  
     This operation entails taking suction from the reactor building sump, 
     through the LPI pumps, through the high-pressure injection (HPI) pumps, 

.                                                            IN 88-74 
                                                            September 14, 1988
                                                            Page 2 of 3 

     and then to the RCS.  However, because of (1) the high flow resistance in
     the cross-connect piping between the HPI and LPI pumps, (2) the potential
     for partially closed LPI discharge throttle valves, and (3) the high flow
     rate downstream of the LPI pumps, there may not have been adequate net 
     positive suction head (NPSH) at the suction inlets of the HPI pumps.  
     Inadequate NPSH would render the HPI system inoperable.  The high flow 
     rate would have occurred when the reactor building spray pumps and HPI 
     pumps take suction simultaneously from the LPI pump discharge.

     The licensee has corrected the problem by revising the operating pro-
     cedures to provide guidance for maintaining adequate NPSH to the HPI 
     pumps operating in the piggyback mode.  The revised procedures provide 
     for (1) HPI flow limitations when the HPI and reactor building spray 
     pumps are piggybacked off the LPI pumps, (2) precautions on the use of 
     reactor building spray, and (3) precautions on the throttling of the LPI 
     discharge valves LP-12 and LP-14. (A simplified Oconee ECCS and reactor 
     building spray system are shown in Attachment 1.)

(2)  A single failure of the "TD" ECCS switchgear during loss of offsite power
     would have prevented the remote alignment of the LPI system to the HPI 
     system in the piggyback mode because (1) valves LP-9, LP-15, and LPI pump
     B are powered by ECCS switchgear bus "TD" but their backup power supplies
     are classified as load shed power supplies, and (2) valves LP-9 and LP-15
     were in the normally closed position. 
     The licensee has corrected the problem by (1) changing valve LP-9 to a 
     normally open position so that it will not be required to operate when 
     the LPI system is being aligned with the HPI system in the piggyback mode
     of operation, and (2) revising the emergency operating procedure to 
     ensure time is available for operators to locally, manually open valves 
     LP-15 and LP-16 (if the valves cannot be opened remotely) before LPI 
     suction must be swapped from the BWST to the reactor building sump.

Turkey Point Nuclear Station

As part of its ongoing design-basis reconstitution program, the licensee dis-
covered a condition of potentially inadequate NPSH for the containment spray 
and safety injection (SI) pumps during the recirculation mode of operation 
following a LOCA.  Under these conditions, the low head residual heat removal 
(RHR) pumps are providing flow to the containment spray pumps and the SI 
pumps.  A throttle valve in each RHR discharge line is normally throttled to 
30 percent.  However, an analysis to support this valve throttling position 
had not been performed.  To resolve this discrepancy, the subject valves were 
locked in the fully open position until an engineering evaluation showed that 
the original configuration (30 percent open) was acceptable.

.                                                            IN 88-74 
                                                            September 14, 1988
                                                            Page 3 of 3 


Following a small-break LOCA,  the piggyback mode of operation may be required
(depending on the size and location of the break) to sustain injection flow 
when the BWST (B&W plant) or refueling water storage tank (Westinghouse plant)
has been depleted and RCS pressure remains above the shutoff head of the low 
head pumps.  As previously described, two mechanisms could prevent this in-
tended piggyback mode from operating:  (1) insufficient NPSH at HPI (or SI) 
pump suction inlets, or (2) inadequate operating procedures combined with 
system design deficiencies for a loss-of-offsite-power condition.  Although 
the low head pumps provide relatively high discharge pressure, the intervening
cross-connect piping between the high head and low head systems and/or 
partially closed low head pump discharge valves could result in excessive 
hydraulic resistance and cause insufficient NPSH at the HPI (or SI) pump 
suction inlets.  Insufficient NPSH could also be caused by running multiple 
pumps off the dis-charge of a single pump that may increase the flow above the 
design flow rate and consequently reduce the available pressure at the low 
head pump discharge.  These are safety concerns because of the reliance on HPI 
(or SI) for mitigating small-break LOCAs, and in some cases, the 
piggyback-type operation is used for hot leg recirculation even for 
large-break LOCAs.

There are no redundant systems to perform the function of the HPI (or SI) 
system in the piggyback mode.  Although the reactor vessel high-point vents 
and the power-operated relief valve(s) could be used to depressurize the RCS 
to low head pump operating pressures, the related safety analysis had never 
been performed - thus, their effectiveness was not ensured.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the 
technical contacts listed below or the Regional Administrator of the 
appropriate regional office.

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts: S. Israel, AEOD
                    (301) 492-4437

                    J. Thompson, NRR
                    (301) 492-1175 

                    P. Wen, NRR 
                    (301) 492-1172 

Attachments:  1.    Figure 1, Simplified Oconee Emergency Core Cooling 
                      and Reactor Building Spray Systems
              2.    List of Recently Issued NRC Information Notices 
.                                                            Attachment 2 
                                                            IN 88-74 
                                                            September 14, 1988
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-73          Direction-Dependent Leak      9/8/88         All holders of OLs
               Characteristics of                           or CPs for nuclear
               Containment Purge Valves                     power reactors. 

88-72          Inadequacies in the Design    9/2/88         All holders of OLs
               of dc Motor-Operated Valves                  or CPs for nuclear
                                                            power reactors. 

88-71          Possible Environmental        9/1/88         All holders of OLs
               Effect of the Reentry                        or CPs for nuclear
               of COSMOS 1900 and                           power reactors, 
               Request for Collection                       fuel cycle 
               of Licensee Radioactivity                    licensees, and 
               Measurements Attributed                      Priority 1 
               to That Event                                material 

88-70          Check Valve Inservice         8/29/88        All holders of OLs
               Testing Program                              or CPs for nuclear
               Deficiencies                                 power reactors. 

88-69          Movable Contact Finger        8/19/88        All holders of OLs 
               Binding in HFA Relays                        or CPs for nuclear
               Manufactured by General                      power reactors. 
               Electric (GE) 

88-48,         Licensee Report of Defective  8/24/88        All holders of OLs
Supplement 1   Refurbished Valves                           or CPs for nuclear
                                                            power reactors. 

88-68          Setpoint Testing of Pres-     8/22/88        All holders of OLs
               surizer Safety Valves with                   or CPs for nuclear
               Filled Loop Seals Using                      power reactors. 
               Hydraulic Assist Devices 

88-67          PWR Auxiliary Feedwater Pump  8/22/88        All holders of OLs
               Turbine Overspeed Trip                       or CPs for nuclear
               Failure                                      power reactors. 

88-66          Industrial Radiography        8/22/88        All NRC industrial
               Inspection and Enforcement                   radiography 

OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015