Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 19, 1989
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
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
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
required.
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.
8910130014
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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
manager.
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
Attachments:
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
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