Information Notice No. 87-63: Inadequate Net Positive Suction Head in Low Pressure Safety Systems

                                                         IN 87-63

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

                                December 9, 1987

                                   IN LOW PRESSURE SAFETY SYSTEMS


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to problems 
which could result in inadequate net positive suction head (NPSH) at the inlet 
to the low pressure pumps 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:

A Nuclear Regulatory Commission review has identified several similar reported 
problems related to excessive flow rates in low pressure safety systems that 
could occur following a LOCA.  Higher than expected flow rates in the pump 
discharge lines can lead to lower than calculated suction line pressures and 
consequently to inadequate pump NPSH.  

On May 19, 1987, Turkey Point, Unit 3, personnel reported that during a 
reexamination of the containment spray (CS) system the hydraulic resistance of 
the CS system was found to be less than that assumed in the design 
calculations.  Furthermore, they reported that adequate NPSH for the CS pumps 
could not be assured during the injection phase following a LOCA when the CS 
system was drawing water from the refueling water storage tank.  This 
deficiency was caused by missing flow orifices in the pump discharge lines 
that were assumed in the system design but never installed.  The licensee 
corrected the problem by installing the missing flow orifices and reported the 
event in Licensee Event Report 250-87-014.  

On December 12, 1986, while designing modifications for the facility residual 
heat removal (RHR) system, the Haddam Neck licensee discovered that pump 
cavitation could occur in the low pressure pumps during the recirculation 

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following a LOCA (in a narrow range of break sizes).  A combination of break 
flow, a system arrangement where the high pressure pumps are downstream of the 
low pressure pumps, and a certain volume of water in the containment sump at 
the time of switchover from the injection to the recirculation phase all 
contributed to the scenario in which inadequate NPSH would exist at the low 
pressure pump inlet.  The licensee corrected the problem by throttling the RHR 
system control valves to balance the flowpaths and effectively increase the 
system hydraulic resistance on the low pressure pump discharge line while 
maintaining minimum flow requirements.  

On March 31, 1986, the Trojan licensee discovered that there would be inade-
quate NPSH at the inlet to the low pressure pumps of the emergency core 
cooling system (ECCS) at Trojan under certain accident conditions.  During the 
recirculation phase following a LOCA, two low pressure pumps feed the charging 
pumps, the safety injection pumps, and the cold leg injection paths.  If only 
one low pressure pump is operating (accounting for a single failure) and the 
cross-tie between ECCS trains is open, the low pressure pump NPSH would be 
deficient by about 10 feet.  The licensee corrected the problem by modifying 
procedures to (1) isolate the cross-tie between trains prior to entering the 
recirculation phase and (2) secure all other pumps in the same train as the 
inoperable low pressure pump.  

In July 1977, the Farley, Unit 1, licensee reported that the residual heat 
removal (RHR) pump flow rate at that facility would be significantly above the 
expected flow rate during the cold leg recirculation mode of operation follow-
ing a LOCA.  Investigations by the licensee revealed that the high flow rates 
were due to lower than expected hydraulic resistances in the RHR pump 
discharge piping.  The actual roughness of the installed piping was less than 
the standard commercial steel roughness assumed in the calculations, and the 
hydraulic resistance of installed check valves was less than estimated.  
Consequently, the licensee installed orifices in the pump discharge piping to 
reduce the flow to a rate that would provide adequate NPSH at the low pressure 
pumps under all post-LOCA conditions.  


Inadequate NPSH can cause pump cavitation and lead to pump unavailability.  
Identification of deficiencies in system design, installation, or operation 
that could result in inadequate NPSH can occur in any type of pump system 
arrangement and may require more than a review of the original design calcula-
tions, as noted by the above mentioned events.  In general, pump availability 
may also be affected by sudden suction pressure oscillations during pump 
starts that may cause unexpected pump trips.  This is discussed in NRC 
Information Notice 87-53, "Auxiliary Feedwater Pump Trips Resulting From Low 
Suction Pressure".
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                                                            December 9, 1987
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 

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

Technical Contact:  Sanford Israel, AEOD
                    (301) 492-4437

Attachment:  List of Recently Issued NRC Information Notices

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