Information Notice No. 91-12: Potential Loss of Net Positive Suction Head (NPSH) of Standby Liquid Control System Pumps

                                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 


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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
.