Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
October 30, 1996
NRC INFORMATION NOTICE 96-59: POTENTIAL DEGRADATION OF POST LOSS-OF-
COOLANT RECIRCULATION CAPABILITY AS A
RESULT OF DEBRIS
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 that the suppression pool and associated components of two boiling-water
reactors (BWRs) have been found to contain foreign objects that could have impaired
successful operation of emergency safety systems using water from the suppression pool. 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
Niagara Mohawk Power Corporation, the licensee for Nine Mile Point Unit 2, reported on
October 17, 1996, that a significant amount of debris was found during inspection of the
drywell-to-suppression chamber downcomers. Most downcomers were clean or contained
minimal debris. However, 17 downcomers contained debris, and 7 of the 8 downcomers
located directly under the reactor vessel had cleanliness covers installed over the downcomer
opening. Some debris was floating on the water inside the downcomers and consisted of
foam rubber cleanliness covers, plastic bags, Tygon tubing, hard hats, and so on. The
suppression pool had been cleaned during the previous refueling outage.
Commonwealth Edison Company reported on October 16, 1996, that during the first thorough
cleaning of the LaSalle Unit 2 suppression pool, a significant amount of foreign material had
been found under a layer of sludge. Sludge is a generic term for rust particles from the
carbon steel piping connected to the suppression pool. Foreign material was also found in
several downcomers. The foreign material included a rubber mat, a sheet of gasket material,
a nylon bag, and a substantial amount of sludge. The licensee concluded that sufficient
material was present to challenge the clogging limit for multiple emergency core cooling
system (ECCS) strainers. The Unit 2 pool had been inspected previously to remove visible
debris, and the strainers had been cleaned.
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October 30, 1996
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Discussion
Section 50.46 of Title 10 of the Code of Federal Regulations (10 CFR 50.46)
requires that licensees design their ECCSs so that the calculated cooling performance
following a loss-of-coolant accident (LOCA) meets five criteria, one of which is to provide
long-term cooling capability of sufficient duration following a successful system initiation so
that the core temperature shall be maintained at an acceptably low value and decay heat
shall be removed for the extended period required by the long-lived radioactivity remaining in
the core.
On October 17, 1995, the NRC issued Bulletin 95-02, "Unexpected Clogging of a Residual
Heat Removal (RHR) Pump Strainer While Operating in Suppression Pool Cooling Mode,"
which requested BWR licensees to review the operability of their ECCS and other pumps that
draw suction from the suppression pool while performing their safety function. The
addressees' evaluations were to be based on suppression pool cleanliness, suction strainer
cleanliness, and the effectiveness of their foreign material exclusion practices. In addition,
licensees were to implement appropriate procedural modifications and other actions (e.g.,
suppression pool cleaning), as necessary, to minimize foreign material in the suppression
pool, the drywell, and systems that interface with the suppression pool. Licensees were to
verify their operability evaluation through appropriate testing and inspection.
The actions of both licensees were a consequence of the requested actions of Bulletin 95-02.
The LaSalle Unit 2 suppression pool was being thoroughly cleaned as requested by the
bulletin, and the Nine Mile Point Unit 2 suppression pool was being reinspected as part of the
enhanced surveillance requested by the bulletin.
The NRC has issued a number of generic communications to describe aspects of the
potential for loss of recirculation capability as a result of strainer clogging and debris
blockage. While the past generic communications contain examples that focus on specific
considerations that are most applicable to either pressurized-water reactors (PWRs) or
BWRs, the basic safety concern applies to both BWRs and PWRs. These events as well as
those in previous generic communications demonstrate the need for a thorough cleaning of
all areas of PWRs and BWRs that may contain materials which could adversely affect LOCA
recirculation. Visual inspection and spot cleaning cannot ensure that all undesirable and
unanticipated foreign material will be eliminated.
Related Generic Communications
Recent instances of problems with strainer clogging are described in the
following generic communications:
. NRC Generic Letter 85-22: "Potential for Loss of Post LOCA Recirculation Capability
Due to Insulation Debris Blockage," dated November 22, 1985.
. NRC Information Notice 89-77: "Debris in Containment Emergency Sumps and
Incorrect Screen Configuration," dated November 21, 1989.
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October 30, 1996
Page 3 of 3
ú NRC Information Notice 92-71: "Partial Plugging of Suppression Pool
Strainers at a Foreign BWR," dated September 30, 1992.
ú NRC Information Notice 92-85: "Potential Failures of Emergency Core
Cooling Systems Caused by Foreign Material Blockage," dated December 23, 1992.
ú NRC Bulletin 93-02 and Supplement 1: "Debris Plugging of Emergency Core
Cooling Suction Strainers," dated May 11, 1993 and February 18, 1994.
ú NRC Information Notice 93-34 and Supplement 1: "Potential for Loss of
Emergency Core Cooling Function Due to a Combination of Operational and
Post-LOCA Debris in Containment," dated April 26, 1993 and May 6, 1993.
. NRC Information Notice 95-06: "Potential Blockage of Safety-Related
Strainers by Material Brought Inside Containment," dated January 25, 1995.
. NRC INFORMATION NOTICE 95-47: "Unexpected Opening of a Safety/Relief
Valve and Complications Involving Suppression Pool Cooling Strainer
Blockage," dated October 4, 1995.
. NRC Bulletin 95-02: "Unexpected Clogging of a Residual Heat Removal (RHR) Pump
Strainer While Operating in Suppression Pool Cooling Mode, dated October 13, 1995.
. NRC Bulletin 96-03: "Potential Plugging of Emergency Core Cooling Suction
Strainers By Debris in Boiling-Water Reactors," dated May 6, 1996.
This information notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
signed by
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Richard Lobel, NRR
(301) 415-2865
Email: rml@nrc.gov
Jerry Carter, NRR
(301) 415-1153
Email: tjc@nrc.gov
