United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 93-02: Debris Plugging of Emergency Core Cooling Suction Strainers

                                                            OMB No.: 3150-0011
                                                            NRCB 93-02

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C. 20555

                                 May 11, 1993



For Action:

All holders of operating licenses for nuclear power reactors.

For Information:

All holders of construction permits for nuclear power reactors.


This bulletin notifies licensees of a previously unrecognized contributor to
the potential loss of net positive suction head (NPSH) for the Emergency Core
Cooling Systems (ECCS) for Light Water Reactors during the recirculation phase
of a loss-of-coolant accident (LOCA).  All operating reactor licensees are
requested to take the recommended actions, and are required to provide the
U.S. Nuclear Regulatory Commission (NRC) with a written response describing
the actions taken associated with this bulletin.


On December 3, 1985, the NRC issued Generic Letter 85-22, "Potential for Loss
of Post-LOCA Recirculation Capability Due to Insulation Debris Blockage,"
which recommended that Regulatory Guide 1.82, Revision 1, "Water Sources for
Long-Term Recirculation Cooling Following a Loss of Coolant Accident," be used
as guidance for 10 CFR 50.59 reviews dealing with modification of thermal
insulation.  Regulatory Guide 1.82, Rev. 1 discussed, among other things, the
blockage of strainers due to fibrous debris.  On September 30, 1992, the NRC
issued Information Notice (IN) 92-71, "Partial Plugging of Suppression Pool
Strainers at a Foreign BWR," which provided information on the plugging of two
ECCS suction strainers by mineral wool insulating material that had been
dislodged by steam from an open safety valve.  On April 26, 1993, the NRC
issued Information Notice 93-34, "Potential for Loss of Emergency Cooling
Function Due to a Combination of Operational and Post-LOCA Debris in
Containment," which described two incidents of debris identified in
suppression pools associated with damaged and clogged strainers, and the
identification of a significant source of material in a PWR with the potential
to restrict the flow through the sump debris screen.


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                                                            May 11, 1993
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Description of Circumstances

Information Notice 93-34 and Supplement 1 to IN 93-34 issued on May 6, 1993,
described clogging of ECCS pump suction strainers that occurred at the Perry
Nuclear Plant, a BWR-6.  The event involving deposition of filter fibers on
residual heat removal strainers occurred in March 1993, two months after the
strainers at Perry had been replaced and the suppression pool had been
thoroughly cleaned.  Subsequent to issuance of IN 93-34, the licensee
chemically analyzed the debris on the strainer.  The debris consisted of glass
fibers from temporary drywell cooling filters that had been inadvertently
dropped into the suppression pool, and corrosion products that had been
filtered from the pool by the glass fibers adhering to the surface of the
strainer.  A small amount of the fibrous filter material was also found in the
suppression pool near the weir wall.

The licensee used the fibrous material as a filter in the drywell in three
cooling air return ducts that each have a surface area of about 5.6 square
meters [60 square feet].  In addition, there were six similar air filters in
containment.  The purpose of the filters is to maintain cleanliness in
containment and the drywell during reactor outages.  It has been the
licensee's practice to replace the filter material at the end of each outage
and to leave the material in the drywell and containment during operation of
the plant at power.  As a result of the March 1993 event, the licensee will
remove the filters from the drywell and containment prior to startup.  They
will also remove fibrous insulation from ducting in the pool-swell area of the
containment.  In addition, Perry has thoroughly cleaned the drywell,
containment, and suppression pool to reduce dirt, dust, and foreign material
which could contribute to the clogging of the strainers.  They have also
initiated a program to periodically monitor RHR strainer differential
pressure.  They have replaced their strainers, significantly increasing the
flow area.  In addition, a procedure has been developed to use existing
equipment to backflush the strainers in the event of clogging.


Section 50.46 of Title 10 of the Code of Federal Regulations (10 CFR 50.46)
requires that each operating nuclear power plant have an ECCS that provides,
among other characteristics, long-term cooling capability.  For the LOCA
sequence, this long-term cooling capability is provided through recirculation
of the coolant from the Boiling Water Reactor (BWR) suppression pool or the
Pressurized Water Reactor (PWR) sump back to the reactor vessel.  Appendix A
to 10 CFR Part 50, General Design Criterion (GDC) 35, "Emergency Core
Cooling," GDC 36, "Inspection of Emergency Core Cooling System," and GDC 37,
"Testing of Emergency Core Cooling System," require appropriate design,
inspectability and testability of the Emergency Core Cooling Systems.  This
includes the ability of the ECCS to provide long-term core cooling.

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                                                            May 11, 1993
                                                            Page 3 of 5

NUREG-0897, Rev. 1, "Containment Emergency Sump Performance," which was
written in conjunction with resolution of Unresolved Safety Issue (USI) A-43,
addressed transport of fibrous thermal insulation from the containment to the
strainers during a LOCA.  Resolution of USI A-43 was based in part on strainer
head loss tests with fibrous thermal insulation obstructing flow.  The
consequences on head loss of the filtering action of the fibrous material on
the strainer was beyond the scope of USI A-43.  The staff has in place a
program to systematically evaluate the larger implications of the Perry
experience and the aforementioned foreign reactor event.  This will include
consideration of strainer area, containment housekeeping, pool cleanliness,
and measures to cope with clogged strainers.

The Perry event showed that filtering of corrosion products, dust, and other
debris from the drywell, as occurred at Perry, may cause an unexpectedly rapid
loss of net positive suction head for the ECCS pumps when they are needed to
perform their intended function.  Fibrous air filters and other temporary
material appear to be likely sources of such fibrous material.  This bulletin
deals with the presence of such material in reactor containments.

Requested Actions

All holders of operating licenses for nuclear power reactors, immediately upon
receiving this bulletin, are requested to take the following actions:

Identify fibrous air filters or other temporary sources of fibrous material,
not designed to withstand a LOCA, which are installed or stored in your
primary containment.  Take any immediate compensatory measures which may be
required to assure the functional capability of the ECCS.  Take prompt action
to remove any such material.  Because of the low probability of a LOCA event,
the staff considers removal of this material at the next shutdown, or within
120 days, whichever comes first, to be sufficiently prompt.  If the facility
is currently in a shutdown, you are requested to remove such material prior to

Reporting Requirements

All action addressees are required to submit the following written reports:

1.    Within 30 days of the date of this bulletin, a written response stating
      whether the actions requested above have been or will be performed.  If
      the use of such material is identified, this written response shall also
      include the locations and quantity of use, any immediate compensatory
      measures taken, and the current schedule for removal of the material.  

2.    Within 30 days of completion of the requested actions, a report
      confirming completion.

3.    If an addressee proposes not to take the actions requested in this
      bulletin, provide to the NRC staff, within 30 days of the date of this
      bulletin, your proposed alternative course of action and a justification
      for any deviations from the requested actions.


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                                                            May 11, 1993
                                                            Page 4 of 5

Address the required written reports to the U.S. Nuclear Regulatory
Commission, ATTN:  Document Control Desk, Washington, D.C. 20555, under oath
or affirmation under the provisions of Section 182a, Atomic Energy Act of
1954, as amended, and 10 CFR 50.54(f).  In addition, submit a copy to the
appropriate regional administrator.

Backfit Discussion

The operability of the ECCS suction for recirculation is required to meet a
condition of a plant operating license and the requirements of Section 50.46
of 10 CFR.  The actions requested by this bulletin represent a new staff
position and are considered necessary to ensure that licensees are in
compliance with existing NRC rules and regulations where these conditions are
applicable.  Therefore, this bulletin is being issued as a compliance backfit
under the terms of 50.109(a)(4), and is being issued as an immediately
effective action [10 CFR 50.109(a)(6)].

Paperwork Reduction Act Statement

This bulletin contains information collection requirements that are subject to
the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).  These
requirements were approved by the Office of Management and Budget, approval
number 3150-0011.

The public reporting burden for this collection of information is estimated to
average 60 hours per response, including the time for reviewing instructions,
searching existing data sources, gathering and maintaining the data needed,
and completing and reviewing the collection of information.  Send comments
regarding this burden estimate or any other aspect of this collection of
information, including suggestions for further reducing reporting burden, to
the Information and Records Management Branch (MNBB-7714), U.S. Nuclear
Regulatory Commission, Washington, D.C. 20555; and to the Desk Officer, Office
of Information and Regulatory Affairs, NEOB-3019, (3150-0011), Office of
Management and Budget, Washington, D.C. 20503.

Compliance with the following request for information is purely voluntary. 
The information would assist NRC in evaluating the cost of complying with this

(1)   the licensee staff time and costs to perform requested inspections,
      corrective actions, and associated testing;

(2)   the licensee staff time and costs to prepare the requested reports and

(3)   the additional short-term costs incurred as a result of the inspection
      findings such as the costs of the corrective actions or the costs of
      down time;

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                                                            May 11, 1993
                                                            Page 5 of 5

(4)   an estimate of the additional long-term costs which will be incurred in
      the future as a result of implementing commitments such as the estimated
      costs of conducting future inspections or increased maintenance.

If you should have any questions about this matter, please contact one of the
technical contacts listed below or the appropriate NRR project manager.

                                       ORIGINAL SIGNED BY

                                    James G. Partlow 
                                    Associate Director for Projects
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Roger Woodruff, NRR
                     (301) 504-2917

                     John B. Hickman, NRR
                     (301) 504-3017

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