United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-34 – IN 93-34: Potential for Loss of Emergency Cooling Function due to a Combination of Operational and Post-LOCA Debris in Containment

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

                               April 26, 1993


NRC INFORMATION NOTICE 93-34:  POTENTIAL FOR LOSS OF EMERGENCY COOLING
                               FUNCTION DUE TO A COMBINATION OF
                               OPERATIONAL AND POST-LOCA DEBRIS IN CONTAINMENT


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 potential problems regarding operational and
post-accident debris that could block emergency core cooling pump strainers in
a boiling water reactor (BWR) or containment emergency sump screens in a
pressurized water reactor (PWR).  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

The following paragraphs discuss instances that involve either the actual
clogging of emergency core cooling (ECC) or residual heat removal (RHR) pump
suction strainers or the potential for clogging of suction strainers in both
BWRs and PWRs.     

Perry (BWR-6)

On May 22, 1992, during refueling outage 3 at the Cleveland Electric
Illuminating Company, Perry Nuclear Plant, the licensee performed an
inspection of the suppression pool floor and all suction strainers in the
suppression pool using an underwater video camera mounted on a robotic
submarine.  The licensee found debris on the suppression pool floor and on RHR
"A" and "B" suction strainers.  The debris consisted of general maintenance-
type material and a coating of fine dirt that covered most of the surface of
the strainers and the pool floor.  As a corrective action, the licensee
vacuumed the suppression pool and cleaned the strainers during a mid-cycle
outage in January 1993.  After cleaning the strainers, it became evident that
the RHR "A" and "B" strainers were deformed.  The strainers are conical shaped
devices made of 18 gauge stainless steel perforated plate with 0.18 cm 


9304260085.

                                                            IN 93-34
                                                            April 26, 1993
                                                            Page 2 of 4


[0.07 inch] diameter holes at 0.287 cm [0.113 inch] centers with internal
cruciform shaped stiffener plates for support.  The deformation consisted of
a buckling of the stainless steel plate between the stiffeners and 3 small
cracks were observed on one of the strainers.  The licensee replaced the
deformed strainers in February 1993, prior to startup from the mid-cycle
outage.  When the licensee reviewed the historical data on RHR "A" and "B"
pump suction pressure and strainer differential pressure, it found no
significant trend in pump suction pressure.  

After the licensee cleaned the suppression pool and replaced the strainers, an
event occurred at the plant in March 1993, during which several safety relief
valves (SRVs) were manually lifted and RHR was then used for suppression pool
cooling.  The licensee inspected the strainers to assess their condition after
use and found that the RHR "B" strainer was again coated with debris.  A 
test was run on the RHR "B" pump with the strainer in the as-found condition
to determine pump operability and was terminated after 10 hours at the
direction of the system engineer when pump suction pressure dropped from an
initial reading of 44.1 kPa gauge [6.4 psig] (after pump start) to 
0 kPa gauge [0.0 psig].  A second test that used improved suction pressure
instrumentation was run on the same loop with similar results (pump suction
pressure dropped to 0 kPa gauge [0.0 psig] after 18 hours).  The licensee
continued to run that pump for an additional 8 hours during the second test,
and observed no further decrease in pump suction pressure.  Also, in both
tests, no change in system flow rates or pump motor amperage was observed. 

Grand Gulf (BWR-6)

Entergy Operations' Grand Gulf Nuclear Station experienced similar problems
with RHR suction strainers.  On March 18, 1988 and again on July 2, 1989, the
RHR "A" pump before-start suction pressure fell below the inservice inspection
(ISI) acceptance criteria of 17.2 kPa gauge [2.5 psig].  The licensee
determined that the low suction pressure was caused by a clogged strainer that
takes suction from the suppression pool.  The licensee developed more
stringent suppression pool cleanliness requirements and more restrictive pump
suction pressure limits to ensure that the strainers are cleaned when pump
after-start pressures reach the new limits.  After an initial cleaning
including hydrolazing the walls and floor, the licensee also established a
requirement for vacuum cleaning the suppression pool at the end of every
refueling outage.  Since the July 1989 problem occurred, Grand Gulf has not
observed any additional instances of before-start or after-start suction
pressures falling below the minimum requirement of 17.2 kPa gauge [2.5 psig].

North Anna (Westinghouse, PWR)

Virginia Power Company's North Anna, Unit 1 personnel removed the mirror
insulation from the steam generators (SGs) as part of their SG replacement
program and discovered that most of the unqualified silicon aluminum paint
covering the SGs had come loose from the SG exterior surface and was only
being supported by the insulation jacketing.  The pieces of paint ranged in
size from sheets 0.61 m [2 feet] wide to dust particles.  The same paint had
also been used on the pressurizer and was also loose.  The quantity of this .

                                                            IN 93-34
                                                            April 26, 1993
                                                            Page 3 of 4


coating is significant, approximately 1,087 square meters [11,700 square feet]
in containment.  Although the loose paint is held in place by the insulation
during normal operation, it could be exposed during a design basis accident,
if there was a pipe or component breach in the vicinity of the SGs or
pressurizer causing the insulation jacketing to be removed.  Paint fragments
could potentially reach the containment sump and reduce the net positive
suction head (NPSH) of ECC system pumps that take suction from the sump. 

Discussion

It is important that emergency coolant is provided to maintain the reactor at
safe temperature levels during all postulated design basis accident
conditions.  This function is performed by the ECC systems.  In the long-term
cooling, suction for these systems is either the containment sump (for PWRs)
or the suppression pool (for BWRs).  In addition, the RHR system provides
suppression pool cooling for BWRs during normal operation and transients.

As a result, it is important that adequate NPSH be provided to the pumps
throughout the long-term cooling phase.  One threat to NPSH is the clogging of
the strainers on the suction inlet.  Excessive deposits of foreign material on
the surfaces of strainers in the suppression pools of BWRs or debris screens
in the containments of PWRs can lead to challenges to pump operability.  These
deposits can reduce the hydraulic head at the suction nozzles of emergency
core cooling pumps to less than the net positive suction head required to
ensure that the pumps operate without cavitation.  If cavitation occurs, the
pumps may fail to deliver adequate flow to maintain the integrity of the fuel
cladding and the containment pressure boundary.

These recent events, as well as the stuck open relief valve event referenced
in NRC Information Notice 92-71, "Partial Plugging of Suppression Pool
Strainers at a Foreign BWR," have revealed that debris can be created during
the LOCA event as well as during normal operation.  Therefore, the complete
evaluation of the effects of debris on the performance of safety related
systems would consider the combination of both sources.

Related Generic Communications

(1)  NRC INFORMATION NOTICE 92-71:  "Partial Plugging of Suppression Pool
       Strainers at a Foreign BWR"

(2)  NRC INFORMATION NOTICE 88-28:  "Potential for Loss of Post-LOCA  
       Recirculation Capability Due to Insulation Debris Blockage"
.

                                                            IN 93-34
                                                            April 26, 1993
                                                            Page 4 of 4


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.


                                       ORIGINAL SIGNED BY


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  B. Wetzel, NRR 
                     (301) 504-1355

                     J. Kudrick, NRR 
                     (301) 504-2871

Attachment:
List of Recently Issued NRC Information Notices

.
Page Last Reviewed/Updated Tuesday, November 12, 2013