Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 15, 1991
Information Notice No. 91-12: POTENTIAL LOSS OF NET POSITIVE SUCTION
HEAD (NPSH) OF STANDBY LIQUID CONTROL
SYSTEM PUMPS
Addressees:
All holders of operating licenses or construction permits for boiling water
reactors (BWRs).
Purpose:
This information notice is intended to alert licensees to potential problems
with the adequacy of Net Positive Suction Head (NPSH) for Standby Liquid
Control System (SLCS) pumps. It is expected that recipients will review
this 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:
On February 11, 1991, a special test of the Standby Liquid Control System
was performed at Quad Cities Unit 1, to determine if NPSH would be
maintained with the conditions of minimum SLCS tank level and maximum sodium
pentaborate solution temperature in the tank. The test was terminated after
about 20 minutes following indications of pump cavitation due to loss of
NPSH. Following the test, the licensee determined that with the flow rate
achieved during the test, the amount of sodium pentaborate necessary to
bring the plant to cold shutdown would not be pumped to the reactor vessel
with one or both SLCS pumps within the 20 minute period. In light of the
test results at Quad Cities, the licensee declared the SLCS at Quad Cities
Unit 2, and the operating sister units, Dresden Units 2 and 3, inoperable.
Following further evaluation of the test results and engineering analysis,
the licensee increased the administratively controlled limit on SLCS tank
level and reduced the administratively controlled limit on solution
temperature in the SLCS tank at Quad Cities to ensure sufficient NPSH for
the SLCS pumps; verified that current limits at Dresden allowed for
sufficient NPSH; and declared the pumps at the Quad Cities and Dresden units
operable.
The SLCS system is required to provide liquid poison in the event that
control rod insertion is inadequate to achieve and maintain a cold shutdown
condition. For low probability Anticipated Transient Without Scram (ATWS)
events, SLCS pump failure to inject a sufficient amount of solution to
shutdown the reactor
9102190010
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IN 91-12
February 15, 1991
Page 2 of 2
could result in a serious threat to containment integrity. The total volume
of poison solution required to assure subcriticality at cold, xenon free
conditions in most plants results in a near depletion of the poison storage
tank. During the course of poison injection, the tank level decreases
resulting in a decrease of available NPSH. Insufficient NPSH can lead to
reduced pumping capability or pump failure. While lesser amounts of poison
can still maintain the reactor shutdown at hot conditions, there is a poison
level below which the reactor would still be producing power. Under such
conditions containment integrity can be threatened in the short term.
The basic SLCS design is common to all BWRs and nearly all use similar
positive displacement pumps. Consequently, there is a question as to
whether adequate testing has been performed at other BWRs to assure that
throughout the required range of operating conditions the SLCS pumps will
have sufficient NPSH to perform their intended function.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Mark A. Caruso, NRR
(301) 492-3235
Attachment: List of Recently Issued NRC Information Notices
.
