United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-10: Potential for Water Hammer during Restart of Residual Heat Removal

                                                    SSINS No.: 6835 
                                                         IN 87-10 

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              February 11, 1987

Information Notice No. 87-10:   POTENTIAL FOR WATER HAMMER DURING 
                                   RESTART OF RESIDUAL HEAT REMOVAL PUMPS 


All boiling water reactor (BWR) facilities holding an operating license or a 
construction permit. 


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. 


                                                       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 

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. 


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 

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

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