Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-10
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
February 11, 1987
Information Notice No. 87-10: POTENTIAL FOR WATER HAMMER DURING
RESTART OF RESIDUAL HEAT REMOVAL PUMPS
Addressees:
All boiling water reactor (BWR) facilities holding an operating license or a
construction permit.
Purpose:
This information notice is to alert addressees of the potential for water
hammer in the residual heat removal (RHR) system of BWRs during a design
basis loss of coolant accident (LOCA) coincident with a loss of offsite
power (LOOP) if the RHR system is aligned to suppression pool cooling.
Recipients are expected to review the information for applicability to their
facilities and consider actions, if appropriate, to preclude similar
problems occurring at their facilities. However, suggestions contained in
this information notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
On December 11, 1986, the Susquehanna nuclear power plant reported that
based on results of an ongoing review of the potential effects of water
hammer events, the RHR system could be susceptible to water hammer loads
that would exceed the allowable stresses in the RHR system and piping. The
specific condition of concern involves a design-basis LOCA coincident with a
LOOP, while one or one RHR loops are in the suppression pool cooling mode.
During the power loss and subsequent valve realignment, portions of the RHR
system will void because of the drain down to the suppression pool as a
result of elevation differences. A water hammer may occur in those RHR loops
that were in the suppression pool cooling mode when the RHR pumps restart
after the diesel generators reenergize the buses.
The core spray system also may be subject to such a water hammer if it is
lined up in the suppression pool mixing mode full flow test.
The Susquehanna design basis for LOCA/LOOP assumes that the suppression pool
cooling flow path valves are initially closed in the standby lineup. The
potential duration factor used in the consideration of the coincident
LOCA/LOOP with the RHR in suppression pool cooling mode was one percent, or
roughly 90 hours per year.
8702100126
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IN 87-10
February 11, 1987
Page 2 of 2
Contrary to the design basis assumption, a licensee review of operating
history found that the worst case RHR system usage factor approached 25%
during cycles in which significant safety relief valve weeping was
experienced.
For interim corrective action, the licensee has modified operating
procedures to allow, at a time, only one loop of RHR to operate in
suppression pool cooling. In addition, the licensee will revise plant
procedures to address the restart of an RHR pump if it trips while operating
in the suppression pool cooling mode. The core spray system is currently
prohibited from being operated in the suppression pool mixing mode, except
for required surveillance testing.
Discussion:
The NRC discussed the potential for this general type of event in
Engineering Evaluation No. AEOD/E309, "The Potential for Water Hammer During
the Restart of RHR Pumps at BWR Nuclear Power Plants," dated April 1983.
In the type of scenario discussed in AEOD/E309, the line most likely to
drain and experience a water hammer is the drywell spray line because it has
the largest elevation difference between it and the suppression pool. RHR
system pipes less than 33 feet above the suppression pool will not usually
drain because atmospheric pressure will support a column of water that high.
A water hammer in the drywell spray line could endanger RHR system
integrity, and thus jeopardize all modes of RHR including low-pressure
coolant injection.
The analysis performed by the licensee of the Susquehanna nuclear power
plant goes beyond AEOD/E309 in that detailed site-specific computer modeling
was performed which shows that piping system integrity could be challenged.
Besides Susquehanna, other plants may have high usage factors for
suppression pool cooling mode and large elevations differences in the RHR
system, making those plants potentially subject to water hammer in the RHR
system.
No specific action or written response is required by this information
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Eric Weiss, IE
(301) 492-9005
George Lanik, IE
(301) 492-9007
Attachment:
List of Recently Issued IE Information Notices
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