United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-71: Diversion of the Residual Heat Removal Pump Seal Cooling Water Flow During Recirculation Operation Following a Loss-Of-Coolant Accident

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              October 19, 1989

                                   PUMP SEAL COOLING WATER FLOW DURING
                                   RECIRCULATION OPERATION FOLLOWING A 
                                   LOSS-OF-COOLANT ACCIDENT


All holders of operating licenses or construction permits for nuclear power


This information notice is intended to alert addressees to a potential 
problem resulting from a design deficiency that may affect the operability 
of the residual heat removal (RHR) pumps during recirculation operation 
following a loss-of-coolant accident (LOCA).  The problem, which can occur 
as a result of a single failure can cause a diversion of the cooling water 
flow from the RHR pump seal coolers, resulting in inadequate seal cooling 
and the potential for ultimate failure of the RHR pumps.  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 

Description of Circumstances:

During a probabilistic risk assessment (PRA) study of the emergency core 
cooling system at Haddam Neck, the licensee discovered that a single failure 
in response to a LOCA could disable both RHR pumps.  The single failure 
would cause the diversion of the cooling water flow from the RHR pump seal 
coolers resulting in inadequate seal cooling and possible pump failure.

As indicated in Attachment 1, cooling water for the RHR pump seal coolers 
comes from the cooling water inlet lines to each RHR heat exchanger.  During 
normal operation, the component cooling water system provides cooling to the 
RHR heat exchangers and pump seal coolers.  However, during a LOCA 
condition, component cooling water is isolated, and service water is used to 
provide cooling to the RHR components.  Because the RHR pump seal cooler 
water supply line is cross-connected to both the component cooling and 
service water systems, a failure of one of the service water motor-operated 
valves to open following a LOCA would result in only one branch of service 
water being available to provide cooling to both RHR heat exchangers and the 
seal water coolers.  


                                                            IN 89-71
                                                            October 19, 1989
                                                            Page 2 of 2

Under these conditions, the RHR seal water coolers would receive inadequate 
flow, with the potential for ultimately causing the failure of both RHR 
pumps.  The licensee corrected this problem by installing check valves in 
the seal water cooling piping (shown in Attachment 2) to preclude one branch 
of service water from feeding two RHR heat exchangers.

Discussion of Safety Significance:

The design deficiency described above was identified in a plant that 
switches cooling water systems during a LOCA.  However, the problem could 
occur in any auxiliary cooling water system that provides sizably different 
cooling needs for different redundant components such as RHR heat exchangers 
and seal coolers.  The important feature in the system is the 
interconnecting piping between the auxiliary cooling water system branches 
and the piping to the individual components.  

In the situation described in this notice, a single failure of a valve to 
open would cause one branch of auxiliary cooling water to service multiple 
redundant components, thereby reducing flow below design requirements.  
Licensees may wish to review their auxiliary cooling water system designs 
for similar deficiencies.  Several other flow design problems that affect 
operation while in the recirculation mode following a LOCA have been 
previously identified in IN 87-63, "Inadequate Net Positive Suction Head in 
Low Pressure Safety Systems," and IN 88-74, "Potentially Inadequate 
Performance of ECCS in PWRs During Recirculation Operation Following a 
LOCA."  It is important to note that a flow balance analysis would determine 
the impact of asymmetric flow balances caused by single failures in these 
systems.  A simple review of piping diagrams may be sufficient to identify 
systems with potential flow problems, but hydraulic analyses are necessary 
to confirm suspected problems.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 

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

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

                     D. Prochnow, NRR
                     (301) 492-1166

1.  Diagram of Haddam Neck's Original RHR Cooling Water System
2.  Diagram of Haddam Neck's Modified RHR Cooling Water System
3.  List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013