United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 95-02: Unexpected Clogging of a Residual Heat Removal (RHR) Pump Strainer While Operating in Suppression Pool Cooling Mode

                                                            OMB No. 3150-0011
                                                            NRCB 95-02


                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               October 17, 1995


NRC BULLETIN 95-02:  UNEXPECTED CLOGGING OF A RESIDUAL HEAT REMOVAL (RHR) PUMP
                     STRAINER WHILE OPERATING IN SUPPRESSION POOL COOLING MODE


Addressees

All holders of boiling-water reactor (BWR) operating licenses or construction
permits for nuclear power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this bulletin to
accomplish the following:

(1)   Alert addressees to complications experienced during a recent event in
      which a licensee initiated suppression pool cooling in response to a
      stuck-open safety relief valve (SRV) and subsequently experienced
      clogging of one RHR pump suction strainer.

(2)   Request addressees to review the operability of their emergency core
      cooling system (ECCS) and other pumps which draw suction from the
      suppression pool while performing their safety function.  The
      addressee's evaluation should be based on suppression pool cleanliness,
      suction strainer cleanliness, and the effectiveness of their foreign
      material exclusion (FME) practices.  In addition, addressees are
      requested to implement appropriate procedural modifications and other
      actions (e.g., suppression pool cleaning), as necessary, to minimize
      foreign material in the suppression pool, drywell and containment. 
      Addressees are requested to verify their operability evaluation through
      appropriate testing and inspection.

(3)   Require that addressees report to the NRC whether and to what extent
      they have complied with the requested actions.  In addition, require a
      second report indicating completion of confirmatory test(s) and
      inspection(s) and providing the test results by addressees that have
      complied with the requested actions, or indicating completion of any
      proposed alternative course of action by addressees that have not
      complied with the requested actions.


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Background

On September 11, 1995, Limerick Unit 1 was being operated at 100 percent power
when control room personnel observed alarms and other indications that one
safety relief valve ("M") was open.  Emergency procedures were implemented. 
Attempts to close the valve were unsuccessful, and within 2 minutes a manual
reactor scram was initiated.  The main steam isolation valves were closed to
reduce the cooldown rate of the reactor vessel.  The maximum cooldown rate was
54� C/hr [130� F/hr].  

Prior to the opening of the SRV, the licensee was running the "A" loop of
suppression pool cooling to remove heat being released into the pool by
leaking SRVs.  Shortly after the manual scram, and with the SRV still open,
the "B" loop of suppression pool cooling was started.  Operators continued
working to close the SRV and reduce the cooldown of the reactor vessel. 
Approximately 30 minutes later, fluctuating motor current and flow was
observed on the "A" loop.  Cavitation was believed to be the cause, and the
loop was secured.  After it was checked the "A" pump was restarted, but at a
reduced flowrate of 8kl/m [2,000 gpm].  No problems were observed, so the flow
rate was gradually increased back to  32kl/m [8,500 gpm], the full flowrate
for the RHR pumps when operating in suppression pool cooling mode.  Again, no
problems were observed, so the pump continued to be operated at a constant
flow.  A pressure gauge located on the pump suction was observed to have a
gradually lower reading, which was believed to be indicative of an increased
pressure drop across the pump suction strainers located in the suppression
pool.  After about 30 minutes of additional operation, the suction pressure
remained constant.

The rest of the reactor shutdown was routine, with no further complications.  

Discussion

Limerick Unit 1 has been in commercial operation since 1986 without its
suppression pool ever being cleaned. Cleaning was scheduled for the upcoming
1996 refueling outage.  The pool of Unit 2 was cleaned during the last
refueling outage in 1995.

At Limerick, each pump suction inlet is constructed in a "T" arrangement with
two truncated cone-type strainers.  The strainers are constructed of 0.95 cm
[3/8 inch] thick perforated 304L stainless steel plate with 1.6 cm [5/8 inch] 
holes on 2.2 cm [7/8 inch] centers.  All strainer surfaces are covered by a
12x12 316L stainless steel wire mesh.  Because of the leaking SRVs, the "A"
and "B" loops of RHR had typically been used for suppression pool cooling
during the last few months before the event.  Originally, the licensee only
used the "A" loop for suppression pool cooling.  Approximately 3 months before
the event, the licensee changed its practice so that use of the "A" and "B"
loops could be alternated.  

After cooldown following the blowdown event, a diver was sent into the
suppression pool at Unit 1 to inspect the condition of the strainers and the
general cleanliness of the pool.  Both suction strainers in the "A" loop of
suppression pool cooling were found to be almost entirely covered with a thin
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                                                            Page 3 of 7


"mat" of material, consisting mostly of fibers and sludge. The "B" loop
suction strainers had a similar covering, but to a lesser extent.  One of the
"B" loop suction strainers was approximately 75% covered by the mat.  The
other had only limited coverage.  The other strainers in the pool were covered
with a dusting of corrosion products (sludge).  Debris was subsequently
removed from the strainers and the suppression pool floor, and the water was
cleaned by use of a temporary filtration system.  The strainers were easily
cleaned by brushing the material off the surface.

It is believed that during operation of the suppression pool cooling system,  
the strainer filtered out fibers that were in the pool water.  The resulting
mat of fibers improved the filtering action of the strainers, thereby
collecting sludge and other material on the surface of the strainer.  The
licensee has concluded that the blowdown caused by the SRV opening did not
significantly increase the rate of debris accumulation on the strainer. 
Following the event, the licensee removed about 635kg [1400 pounds] of debris
from the pool of Unit 1.  A similar amount of material had previously been
removed from the Unit 2 pool.

Analysis showed that the sludge was primarily iron oxides and the fibers were
of a polymeric nature.  The source of the fibers has not been positively
identified, but the licensee has determined that the fibers  did not 
originate within the suppression pool.  There was no trace of either
fiberglass or asbestos fibers.  In addition, other foreign material was found
in the pool, such as pieces of wood, nails, and hose.  In light of these
findings, the licensee decided to modify their FME procedures to specifically
address material control in the suppression pool and drywell.

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 of time required by the long-lived radioactivity remaining in
the core.  The ECCS is designed to meet this criterion, assuming the worst
single active failure and only partially obstructed flow through the strainer. 
Experience gained from the Limerick event demonstrates that inadequate
suppression pool cleanliness can lead to unacceptable buildup of foreign
material, debris and corrosion products on the strainers during normal
operation, which could prevent the ECCS from providing long-term cooling
following a LOCA.  The staff concludes, therefore, that licensees should take
the actions discussed below to ensure that debris which is located in the
suppression pool, or will accumulate in the suppression pool during normal
operation, does not adversely impact ECCS capability during normal or
transient operations or following a LOCA.  

Prior to the Limerick event, the staff had issued a draft bulletin for public
comment entitled, "Potential Plugging of Emergency Core Cooling Suction
Strainers by Debris in Boiling Water Reactors."  The draft bulletin and
associated draft regulatory guide provide the staff's proposed resolution to 
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                                                            Page 4 of 7

the generic BWR strainer clogging issue.  The issue covered by the draft 
bulletin, however, differs from the issue covered in this bulletin because the
draft bulletin focuses on the potential for ECCS strainers to be clogged by
debris generated by a LOCA.  This bulletin has been issued to resolve a
related issue, highlighted by the Limerick event, of the potential for ECCS
suction strainers to be clogged during normal operations by debris which is
presently in the suppression pool, or may accumulate in the suppression pool
during normal operation.  The draft bulletin was published in the Federal
Register on July 31, 1995.  The public comment period ended on October 2,
1995.  The staff is currently involved in the review and disposition of the
public comments as well as in resolving the open issues identified in the
federal register notice.  

Requested Actions

To ensure that unacceptable buildup of debris that could clog strainers does
not occur during normal operation, all addressees are requested to take the
following actions:

1)    Verify the operability of all pumps which draw suction from the
      suppression pool when performing their safety functions (e.g., ECCS,
      containment spray, etc.), based on an evaluation of suppression pool and
      suction strainer cleanliness conditions.  This evaluation should be
      based on the pool and strainer conditions during the last inspection or
      cleaning and an assessment of the potential for the introduction of 
      debris or other materials that could clog the strainers since the pool
      was last cleaned.

2)    The operability evaluation in requested action 1 above should be
      confirmed through appropriate test(s) and strainer inspection(s) within
      120 days of the date of this bulletin.

3)    Schedule a suppression pool cleaning.  The schedule for cleaning the
      pool should be consistent with the operability evaluation in requested
      action 1 above.  In addition, a program for periodic cleaning of the
      suppression pool should be established, including procedures for the
      cleaning of the pool, criteria for determining the appropriate cleaning
      frequency, and criteria for evaluating the adequacy of the pool
      cleanliness.  

4)    Review FME procedures and their implementation to determine whether
      adequate control of materials in the drywell, suppression pool, and 
      systems that interface with the suppression pool exists.  This review
      should determine if comprehensive FME controls have been established to
      prevent materials that could potentially impact ECCS operation from
      being introduced into the suppression pool, and whether workers are
      sufficiently aware of their responsibilities regarding FME.  Any
      identified weaknesses should be corrected.  In addition, the
      effectiveness of the FME controls since the last time the suppression
      pool was cleaned and the ECCS strainers inspected, and the impact that
      any weaknesses noted may have on the operability of the ECCS should be
      assessed.

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                                                            October 17, 1995
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5)    Consider additional measures such as suppression pool water sampling and
      trending of pump suction pressure to detect clogging of ECCS suction
      strainers. 

By letter dated September 29, 1995, (serial BWROG-95083), the BWR Owners Group
(BWROG) Executive Oversight Committee (EOC) provided to the BWROG Executive
Committee their recommended utility interim actions in response to the recent
ECCS suction strainer plugging event at Limerick, Unit 1.  The letter also
provides additional guidance on the BWROG recommended method for evaluating
pool cleanliness and on demonstrating adequate pool cleanliness.


Required Response

All addressees are required to submit the following written reports:

(1)   Within 30 days of the date of this bulletin, a report indicating to what
      extent the licensee intends to comply with the requested actions in this
      bulletin.  In the report, licensees that intend to comply should provide
      a detailed description of their actions, the results of their
      evaluations, any corrective actions they have taken, and a description
      of the licensee's planned test(s) and inspection(s) for confirming their
      operability evaluation.  In addition, licensees should include their
      schedule for pool cleaning, the basis for the cleaning schedule, and a
      summary of any additional measures taken to detect and prevent clogging
      of the ECCS strainers.  If a licensee does not intend to comply with
      these requested actions, its report should contain a detailed
      description of any proposed alternative course of action, its schedule
      for completing this alternative course of action, and the safety basis
      for its having determined the acceptability of the planned alternative
      course of action.

(2)   If not addressed in the report discussed above by licensees that intend
      to comply with the requested actions, within 10 days of the completion
      of confirmatory tests and inspections or completion of proposed
      alternative actions, a second report confirming the completion of all
      pump operability testing and inspection and providing a description of
      the test/inspection results.  Licensees who do not intend to comply with
      the requested actions should provide a second report indicating the
      completion of any proposed alternative actions within 10 days of
      completing the alternative actions.

Address the required written reports to the U.S. Nuclear Regulatory
Commission, ATTN:  Document Control Desk, Washington, D.C. 20555-0001, under
oath or affirmation under the provisions of Section 182a, the Atomic Energy
Act of 1954, as amended, and 10 CFR 50.54(f).  In addition, submit a copy of
the reports to the appropriate regional administrator.  
.                                                            NRCB 95-02
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                                                            Page 6 of 7


Related Generic Communications

Recent instances of problems with strainer clogging are described in the
following generic communications:

.     NRC Information Notice 95-47:  "Unexpected Opening of a Safety/Relief
      Valve and Complications Involving Suppression Pool Cooling Strainer
      Blockage"

.     NRC Information Notice 95-06:  "Potential Blockage of Safety-Related
      Strainers by Material Brought Inside Containment"

�     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"

�     NRC Bulletin 93-02 and Supplement 1:  "Debris Plugging of Emergency Core
      Cooling Suction Strainers"

�     NRC Information Notice 92-85:  "Potential Failures of Emergency Core
      Cooling Systems caused by Foreign Material Blockage"

�     NRC Information Notice 92-71:  "Partial Plugging of Suppression Pool
      Strainers at a Foreign BWR"

Backfit Discussion

The actions requested by this bulletin, if required, would be backfits in
accordance with NRC procedures and are necessary to ensure that licensees are
in compliance with existing NRC rules and regulations.  Specifically, 
10 CFR 50.46 requires that the ECCS be designed so that it is calculated to
provide adequate flow capability to maintain the core temperature at an
acceptably low value and to remove decay heat for the extended period of time
required by the long-lived radioactivity remaining in the core following a
LOCA.  The Limerick event has demonstrated that suppression pool cleanliness
can adversely impact ECCS performance and could prevent the ECCS from
performing its safety function of long-term decay heat removal following a
LOCA.  Therefore, this bulletin is being issued as if the requested actions
were compliance backfits under the terms of 10 CFR 50.109(a)(4)(i).  A full
backfit analysis was not performed.  An evaluation was performed in accordance
with NRC procedures.  A statement of the objectives of and the reasons for the
requested actions and the basis for invoking the compliance exception if the
requested actions were to be required, has been included.  A copy of this
evaluation will be made available in the NRC Public Document Room.

Paperwork Reduction Act Statement

This Bulletin contains information collections that are subject to the
Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).  These information
collections were approved by the Office of Management and Budget, approval
number 3150-0011, which expires July 31, 1997.

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The public reporting burden for this collection of information is estimated to
average 240 hours per response, including the time for reviewing instructions,
searching existing date sources, gathering and maintaining the data needed,
and completing and reviewing the collection of information.  The U.S. Nuclear
Regulatory Commission is seeking public comment on the potential impact of the
collection of information contained in the (Bulletin, etc.) and on the
following issues:

1.    Is the proposed collection of information necessary for the proper
      performance of the functions of the NRC, including whether the
      information will have practical utility?

2.    Is the estimate of burden accurate?

3.    Is there a way to enhance the quality, utility, and clarity of the
      information to be collected?

4.    How can the burden of the collection of information be minimized,
      including the use of automated collection techniques?

Send comments on any aspect of this collection of information, including
suggestions for reducing the burden, to the Information and Records
Management Branch (T-6 F33), U.S. Nuclear Regulatory Commission, Washington,
DC  20555-0001, and to the Desk Officer, Office of Information and Regulatory
Affairs, NEOB-10202, (3150-0011), Office of Management and Budget, Washington,
DC  20503.

The NRC may not conduct or sponsor, and a person is not required to respond
to, a collection of information unless it displays a currently valid OMB
control number.

If you have any questions about this matter, please contact the technical
contact listed below or the appropriate Office of Nuclear Reactor Regulation
(NRR) project manager.

                                    /s/'d by DMCrutchfield


                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contact:  Robert Elliott, NRR
                    (301) 415-1397

Lead project manager:  Robert M. Latta, NRR
                       (301) 415-1314

 

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