United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 93-02: Supplement 1, Debris Plugging of Emergency Core Cooling Suction Strainers

                                UNITED STATES              OMB No.: 3150-0011
                        NUCLEAR REGULATORY COMMISSION      NRCB 93-02, Supp. 1
                           WASHINGTON, D.C. 20555

                              February 18, 1994

                                  SUCTION STRAINERS

For Action: 

All holders of operating licenses or construction permits for boiling-water

For Information:

All holders of operating licenses or construction permits for pressurized-
water reactors. 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this bulletin

(1)  to inform Action and Information addressees about the vulnerability of
     emergency core cooling system (ECCS) suction strainers in boiling-water
     reactors (BWRs) and containment sumps in pressurized-water reactors
     (PWRs) to clogging during the recirculation phase of a loss-of-coolant
     accident (LOCA). 

(2)  to request that Action addressees take the appropriate actions to ensure
     reliability of the ECCS in view of the information discussed in this
     bulletin supplement regarding the vulnerability of the ECCS strainers to

(3)  to require that Action addressees report to the NRC whether and to what
     extent the requested actions will be taken and to notify the NRC when
     actions associated with this bulletin supplement are complete.  


The NRC staff concerns related to the potential loss of post-LOCA
recirculation capability due to insulation debris were discussed in Generic
Letter 85-22 (December 3, 1985), "Potential for Loss of Post-LOCA
Recirculation Capability due to Insulation Debris Blockage" which documented
the NRC's resolution of Unresolved Safety Issue (USI) A-43, "Containment
Emergency Sump Performance."  Although the staff concluded at that time that
no new requirements would be imposed on licensees and construction permit
holders, the staff did recommend that Regulatory Guide 1.82, Revision 1,
"Water Sources for Long-Term 

9402180174.                                                NRCB 93-02, Supp. 1
                                                February 18, 1994, Page 2 of 7
Recirculation Cooling Following a Loss-of-Coolant Accident," be used as
guidance for the conduct of 10 CFR 50.59 reviews dealing with the modification
of thermal insulation installed on primary coolant system piping and
components.  NUREG-0897, Revision 1, "Containment Emergency Sump Performance"
(October 1985), contained technical findings related to USI A-43, and was the
principal reference for developing the revised regulatory guide.

Since 1985, the NRC staff has issued several information notices on this
subject because of new developments.

On May 19, 1988, the NRC issued Information Notice 88-28, "Potential for Loss
of Post-LOCA Recirculation Capability due to Insulation Debris Blockage,"
which discussed an item reported under 10 CFR Part 21 concerning the
deterioration of drywell insulation and the potential for the aluminum foil
coating of the insulation to block ECCS strainers during a LOCA.

On January 30, 1990, the NRC issued Information Notice 90-07, "New Information
Regarding Insulation Material Performance and Debris Blockage of PWR
Containment Sumps," which discussed experiments exposing NUKON insulation to
high temperatures and alkaline conditions.  The results of these tests
indicated that head loss across the insulation material increases
significantly after about 24 hours due to a reduction in flow area caused by
compaction of the fiberglass material at elevated pH conditions.

On September 30, 1992, the NRC issued Information Notice 92-71, "Partial
Plugging of Suppression Pool Strainers at a Foreign BWR," which reported on
the plugging of two ECCS strainers at the Barseb�ck Unit 2 BWR in Sweden on
July 28, 1992.  The strainers were plugged by mineral wool insulation that had
been dislodged by steam from a pilot-operated relief valve that opened while
the reactor was at 3100 kPa [435 psig].  Two of the five strainers on the
suction side of the containment spray pumps were in service and became
partially plugged with mineral wool.  Following an indication of high
differential pressure across both suction strainers 70 minutes into the event,
the operators shut down the reactor and backflushed the strainers.

Information Notices 93-34 and 93-34 Supplement 1, "Potential for Loss of
Emergency Cooling Function due to a Combination of Operational and Post-LOCA
Debris in Containment," were issued on April 26, 1993, and May 6, 1993,
respectively.  They described several instances of clogging of ECCS pump
strainers including two that occurred at the Perry Nuclear Plant, a domestic 
BWR 6.  The first Perry event entailed clogging of residual heat removal
strainers by operational debris.  The second Perry event involved the
deposition of filter fibers on residual heat removal strainers.  The debris
consisted of glass fibers that had been inadvertently dropped into the
suppression pool from temporary drywell cooling filters, and corrosion
products that had been filtered from the pool by the glass fibers adhering to
the surface of the strainer.  On May 11, 1993, in response to this event, the
staff issued NRC Bulletin 93-02, "Debris Plugging of Emergency Core Cooling
Suction Strainers," which requested that both PWR and BWR licensees (1)
identify fibrous air filters and other temporary sources of fibrous material
in containment not designed to withstand a LOCA and (2) take prompt action to
remove the material and ensure the 

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                                                          Page 3 of 7

functional capability of the ECCS.  All licensees have responded to the
bulletin and the staff has reviewed their responses.  Discussions are
continuing with several licensees.  Resolution of the bulletin is anticipated
for all licensees in the near future.

In response to these two events, the Perry Nuclear Plant licensee took several
actions.  These included an increase in the suction strainer area from 
1.9 to 3.9 m2 [20 to 42 ft2], provisions for a suction strainer backflush
capability, and improved measures to maintain a high level of cleanliness in
the suppression pool.

Description of Circumstances

After learning of the Barseb�ck event, the staff performed approximate
calculations for all domestic BWRs, based partly on information obtained from
resident inspectors on the design of each BWR and partly on approximations
based on general BWR features (such as the close proximity of steam lines
leaving the drywell).  These calculations showed the potential for loss of net
positive suction head (NPSH) of ECCS pumps in some large-break LOCA scenarios
in U.S. BWRs.  

In the meantime, the regulatory authorities of Sweden and other northern and
central European countries have viewed the Barseb�ck incident as a precursor
event related to potential loss of ECCS cooling due to LOCA-generated debris. 
They initiated a safety reanalysis effort, coupled with experiments directed
at estimating the following: (1) amount of insulation destroyed by the steam
jet, (2) resulting composition of debris, (3) amount of debris transported to
the suppression pool, (4) extent of insulation debris buildup on strainers,
and (5) resultant pressure drop across the blocked strainer under the
postulated conditions.  The staff compared the recently obtained results of
this work with information in NUREG/CR-2982, Revision 1, "Buoyancy, Transport,
and Head Loss of Fibrous Reactor Insulation" (July 1983), which was developed
as part of the resolution of USI A-43.  This comparison showed that the method
of fragmenting insulation materials used in U.S. experiments done in support
of USI A-43 may not be representative of the scenario following a large LOCA,
and that the extent of debris generation due to the jet resulting from a
postulated pipe break as reported in NUREG-0897 (1985) was underestimated. 
Also, the second event at the Perry Nuclear Plant described in IN 93-34
demonstrated that small particles, in combination with debris fibers,
significantly increased the pressure drop across the strainers.

Upon completion of the approximate calculations, the staff contracted for a
plant-specific study using a BWR 4 as a model to more accurately quantify the
effect of LOCA-generated debris on available NPSH.  A draft report,
"Parametric Study of the Potential for BWR ECCS Strainer Blockage Due to LOCA
Generated Debris," was published on January 20, 1994.  Initial results from
this study indicated that the available NPSH margin for the ECCS pumps may be
inadequate following dislodging of insulation caused by a LOCA and transport
of insulation debris to the suction strainers.  This study presently does not
consider (1) the effect of corrosion products and other particulates (which
were demonstrated in the second Perry event and in experiments to
significantly increase the pressure drop across the strainers), (2) the dense
packing of debris on the surface of the .                                                          NRCB 93-02, Supp. 1
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                                                          Page 4 of 7

strainers (which was observed in the Barseb�ck incident and in experiments
performed in Europe), and (3) the loss of insulation from any pipe but the
pipe postulated to break (that is, loss of insulation due to impact of the
steam jet on adjacent piping).  While the modeling of the transport of debris
to the strainers may be conservative, and no credit was taken for accident
containment pressure in calculating the available NPSH margin, the staff would
expect the analysis results to yield a net reduction in the available NPSH had
all these effects been included.  Work is now being done to include these
effects in the BWR 4 model.

Members of the NRC staff and representatives of domestic BWR licensees
attended an OECD/NEA workshop on the Barseb�ck incident held in Stockholm,
Sweden, on January 26 and 27, 1994.  Representatives from other countries at
this conference discussed actions taken or planned to prevent or mitigate the
consequences of BWR strainer blockage.  These actions including changes in
insulation materials in containment from mineral wool to fiberglass or a
reflective metallic insulation, increasing the (total) area of BWR strainers
from 5 m2 to more than 100 m2, providing instrumentation to monitor the
differential pressure across the strainers during an accident, providing or
improving the capability to backflush the strainers, and providing guidance in
the emergency operating procedures concerning the correct response to this

The NRC staff has met on two occasions with the Boiling Water Reactor Owners'
Group (BWROG) on this issue.  These meetings have been productive and both
sides have expressed their intent to continue a discussion of the resolution
of this issue using the meeting format.  Further meetings are anticipated soon
and the BWROG has informed the staff of interim actions it is planning to
address this issue.  In addition, the BWROG has presented two analyses to the
NRC staff which form the basis for continued operation while this issue is
being resolved.  These are discussed further below.  


10 CFR 50.46 requires that each BWR and PWR must have an ECCS that is designed
so that the calculated cooling performance following a postulated LOCA
conforms to the acceptance criteria set forth in the regulation.  These
calculations are done assuming a single failure.  Experience from recent
operating events, as discussed above, demonstrates that excessive buildup of
debris from thermal insulation, corrosion products, and other particulates on
ECCS pump strainers has the potential to cause a common-mode failure of the
ECCS.  The staff presently considers these concerns plant specific because
there is such variability of insulations installed, strainer or debris screen
sizes and NPSH margins available.  

Based on the operating events at Barseb�ck and Perry and information from 
on-going domestic and foreign programs, the staff considers it important to
address the issue of strainer blockage.  In particular, the results of the
recent plant-specific BWR 4 analysis done for the NRC staff and information
from the recently completed OECD/NEA conference in Stockholm, discussed above,
indicate that immediate interim actions are warranted and prudent until the
NRC staff and the BWROG complete studies of the recently identified phenomena
and until appropriate actions, based on these studies, can be implemented. 
The recently 
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identified phenomena include the increased pressure drop across the ECCS
strainers due to corrosion products and other particulate matter, and the
effect of compaction of debris on the strainers to a greater extent than
measured in the work leading to resolution of USI A-43.

Because of the larger surface area of the screens surrounding PWR sumps, the
staff considers it acceptable to wait until further studies are performed 
before determining the need for further action at PWRs.

The NRC considers the interim actions given below to be adequate based on the
low probability of the initiating event.  
Actions Requested 

The NRC requests that pending final resolution of this issue, Action
addressees take the following interim actions to enhance the capability to
prevent or mitigate loss of the ECCS following a LOCA due to strainer

   -   Provide training and briefings to apprise operators and other
       appropriate emergency response personnel of the information contained
       herein and in the referenced information notices regarding the
       potential for suppression pool strainer clogging.

   -   Assure that the emergency operating procedures make the operator aware
       of possible indications of ECCS strainer clogging and provide guidance
       on mitigation.

   -   Institute procedures and other measures to provide compensatory actions
       to prevent, delay, or mitigate a loss of available NPSH margin under
       LOCA conditions.  Such measures should be consistent with providing the
       design basis emergency system functions for core and containment
       cooling.   Actions to assure sufficient core and containment cooling
       may include:

       -  Reduction of flow (consistent with delivering the required ECCS
          flow) through the strainers to reduce head loss and extend the time
          for debris deposition

       -  Operator realignment of existing systems to allow backflushing of
          clogged strainers

       -  Operator realignment of existing systems to allow injection to the
          core from water sources other than the suppression pool

       -  Intermittent operation of the containment sprays, when possible, to
          reduce the transport of debris to the strainers

       -  Other plant-specific measures which assure availability of
          sufficient core and containment cooling to meet the design basis of
          the plant.                                                          NRCB 93-02, Supp. 1
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                                                          Page 6 of 7

Action addressees should complete these requested interim actions within 90
days of the date of this bulletin supplement.

Action addressees are encouraged to work with the BWROG to obtain a final
resolution of this issue. 

Reporting Requirements

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

(1)  Within 60 days of the date of this bulletin supplement, a report
     indicating whether or not the addressee intends to comply with the
     actions requested above, description of planned actions, and the schedule
     for completing them.  If an addressee chooses not to take the requested
     actions, the report shall contain a description of a proposed alternative
     course of action, the schedule for completing this alternative course of
     action, and a justification for any deviations from the requested

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

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

Adequate flow from the ECCS is required to meet a condition of a plant
operating license and the requirements of 10 CFR 50.46.  The actions requested
by this bulletin supplement represent a new staff position and are necessary
to ensure that licensees are in compliance with existing NRC rules and
regulations where these conditions are applicable.  Therefore, this bulletin
supplement is being issued as a compliance backfit under the terms of

Paperwork Reduction Act Statement  

The information collections contained in this request were approved by the
Office of Management and Budget, clearance number 3150-0011, which expires
June 30, 1994.  The public reporting burden for this collection of information
is estimated to average 200 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 these collections of information, including suggestions for reducing this
burden to the Desk Officer, Office of Information and Regulatory Affairs,
NEOB-3019, (3150-0011), Office of Management and Budget, Washington D.C.
20503, and to the U.S. Nuclear Regulatory Commission, Information and Records
Management Branch, (MNBB-7714), Washington, D.C. 20555.

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

(1)  the licensee staff time and costs to perform requested procedure 
     reviews and implementation of changes;

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

(3)  the additional short-term costs incurred to address the changes, such as
     the costs of the corrective actions or the costs of down time; and

(4)  an estimate of the additional long-term costs that will be incurred as a
     result of implementation commitments.

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

                                      ORIGINAL SIGNED BY

                                      Steven A. Varga
                                      Acting Associate Director for Projects
                                      Office of Nuclear Reactor Regulation

Technical contacts:  Rob Elliott, NRR
                     (301) 504-1397 

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


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