United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-71: Partial Plugging of Suppression Pool Strainers at a Foreign BWR

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                              September 30, 1992


NRC INFORMATION NOTICE 92-71:  PARTIAL PLUGGING OF SUPPRESSION POOL           
                               STRAINERS AT A FOREIGN BWR


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for loss of emergency core cooling
systems (ECCS) that was identified as a result of an event at a Swedish
boiling water reactor (BWR).  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 are not NRC requirements; therefore, no specific
action or written response is required.

Description of Circumstances:

On July 28, 1992, Sydkraft AB, the utility licensed by the Swedish regulatory
agency for the Barseb�ck nuclear power plant, was restarting Unit 2 after the
annual outage.  While the reactor was at 1 or 2 percent power and at 435 psi,
a leaking pilot valve caused a safety valve for the reactor vessel to open. 
This valve discharges directly to the drywell.  Reactor scram, high pressure
safety injection, core spray, and containment spray systems initiated
automatically in response to the event.  Steam from the open safety valve,
impinging on thermally-insulated equipment, dislodged 440 pounds of metal-
jacketed, mineral wool.  An estimated 220 pounds of insulating material was
washed into the suppression pool.  Two of five strainers on the suction side
of the ECCS pumps were in service and became partially plugged with mineral
wool.  Plugging caused pressure to decrease significantly across the strainers
and caused indications of cavitation in one pump in about an hour after the
event began.  The operators successfully back flushed the strainers and shut
down the reactor without additional problems.  

Discussion

In the safety analysis for Unit 2 of the Barseb�ck plant, the licensee
concluded that the strainers would not require back flushing during the first
10 hours after a loss-of-coolant accident.  Because of the event of July 28,
the Swedish regulatory agency required Barseb�ck Unit 2 and four other units
of similar design to remain shut down until this problem is resolved.  

9209290014.

                                                            IN 92-71
                                                            September 30, 1992
                                                            Page 2 of 2


This event at Barseb�ck is directly applicable to an earlier study conducted
by the NRC.  That study evaluated the potential for displaced thermal
insulation to form debris and to be drawn to the sump screen.  The results of
the study are documented in "Containment Sump Performance," NUREG-0897,
Revision 1, dated March 1984.  The NRC concluded that the risk associated with
the sump clogging concern did not warrant backfitting of domestic plants. 
However, based on the Barseb�ck event, this conclusion is being reevaluated.

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 Office of Nuclear
Reactor Regulation (NRR) project manager.


                                       ORIGINAL SIGNED BY


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


Technical contact:  Robert L. Dennig, NRR
                    (301) 504-1156


Attachment:  List of Recently Issued NRC Information Notices
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