Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
IN 87-63
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 9, 1987
Information Notice No. 87-63: INADEQUATE NET POSITIVE SUCTION HEAD
IN LOW PRESSURE SAFETY SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
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
phase
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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.
Discussion:
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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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
office.
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
