United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-47: Unexpected Opening of a Safety/Relief Valve and Complications Involving Suppression Pool Cooling Strainer Blockage

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                                October 4, 1995

                               COOLING STRAINER BLOCKAGE


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent failure of a safety/relief valve (SRV)
to remain closed during steady-state reactor operation and the attendant
complications involving suppression pool cooling including strainer blockage. 
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

Description of Circumstances

On September 11, 1995, the Limerick Unit 1 plant was being operated at 
100 percent power when control room personnel observed alarms and other
indications that one SRV ("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].  Before the SRV opened, the
licensee was running the "A" loop of suppression pool cooling to remove heat
being released into the pool by leaking SRVs.  

The licensee has 2-stage vertical discharge SRVs manufactured by Target Rock
Corporation.  This particular valve design, which is oriented such that
condensate collects on the main stage valve seat, is believed to be the cause
of the continual problems with main stage leakage and is unique to the
Limerick units.  Other licensees use Target Rock valves that have a similar 
2-stage design, but that are configured to discharge horizontally without
condensate collecting on the valve seat.  Even though the licensee had
modified the valve bodies to promote drainage of condensate buildup, these
leaking valves were assumed to still have the same leakage problem through the
main stage valve seat.  

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 

9510030107.                                                            IN 95-47
                                                            October 4, 1995
                                                            Page 2 of 4

the SRV and slow the cooldown of the reactor vessel.  Approximately 30 minutes
later, fluctuating motor current and flow were observed on the Unit 1 "A"
suppression pool cooling loop.  Cavitation was believed to be the cause and
the loop was secured.  

After checking out the pump, the  "A" pump was restarted, but at only 8 kL/m
[2000 gpm], a small fraction of the original flow.  No problems were observed
so the flow rate was gradually increased to 32 kL/m [8500 gpm].  No problems
were observed so the licensee continued to operate the pump 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 strainer 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 and there were no further


Safety Relief Valve:

Shortly after the licensee started up following a refueling outage in March
1994, three SRVs ("F," "M," and "S") were leaking, as determined by tailpipe
temperatures which ranged from 79 �C [175 �F] to 104 �C [220 �F].  These
valves had been refurbished and reset before the restart and they were not
leaking when installed.  However, SRVs "M" and "S" had been stroked during a
3550 kPa [500-psi] automatic depressurization system operability test. 
Reactor operation continued from March 1994 until September 1995, except for
two short mini-outages.  Prior to the recent opening of the "M" SRV, SRVs "D"
and "L" were also observed to be leaking.  Tailpipe temperatures of the five
leaking SRVs were reported as ranging between 102 �C [215 �F] to 141 �C 
[285 �F] and, because of prior experience, the leakage was believed to be past
the main stage valve seat.  Tailpipes at Limerick are uninsulated.  

After the September 11, 1995 shutdown, the leaking SRVs were removed and the
"M" and "S" SRVs sent offsite for inspection to determine the root cause for
the "M" SRV opening.  The "M" SRV was found to have been leaking through the
pilot valve; the other four valves were leaking through the main valve. 
Disassembly of the "M" SRV disclosed that the pilot valve disk was badly
eroded; the nose of the disk had been steam cut 360 degrees around the disk
and had separated from the rest of the disk.  The interior of the disk and the
push rod also showed evidence of erosion.  The pilot valve seat was eroded,
but to a lesser degree.  The reason for the initial leakage is not known.  The
pilot valve seat and disk were fabricated of Stellite 6 and Stellite 6B,
respectively, and were not expected to erode so severely.  

The licensee has replaced the five leaking SRVs and has resumed operation. 
The licensee is investigating techniques for identifying whether the leakage
is through the pilot valve or main stage valve.  Until such a technique is.                                                            IN 95-47
                                                            October 4, 1995
                                                            Page 3 of 4

found, the licensee will assume that all leakage is through the pilot valve
and will monitor tailpipe temperatures so that a plan for valve replacement
will be developed by the time the tailpipe temperature exceeds 121 �C 
[250 �F]; a tailpipe temperature of 135 �C [275 �F] will cause an immediate
reactor shutdown.  A steam leakage through the pilot valve of about 450 kg/hr
[1000 lb/hr] is estimated to cause a tailpipe temperature of 121 �C [250 �F]. 

Testing has demonstrated that this amount of leakage will not cause either the
pilot valve or the main stage valve to open.  Actual experience at Limerick
Unit 1 showed that the "M" SRV operated for more than a year with a tailpipe
temperature in excess of 121 �C [250 �F] before it failed.  The last recorded
temperature of the uninsulated tailpipe before the SRV opened was 141 �C 
[285 �F]; the temperature was recorded as 146 �C [295 �F] a week earlier. 

Suppression Pool:

Limerick Unit 1 has been in commercial operation since 1986 without having had
the suppression pool cleaned; cleaning was scheduled for the 1996 refueling
outage.  The pool of Unit 2 was cleaned during the 1995 refueling outage.

After a plant cooldown following the blowdown event, a diver was sent into the
Unit 1 suppression pool to observe the condition of the strainers and 
general pool cleanliness.  Each strainer is a "T" arrangement with two
truncated cones fabricated from perforated plate; the entire cone surface is
covered by a 12x12 316 L stainless steel wire mesh.  The suction strainer in
the "A" loop of suppression pool cooling was found to be covered with a thin
"mat" of material, consisting of fibers and sludge.  The "B" strainer had a
similar covering, but to a lesser extent.  These are the two loops that had
been used for suppression pool cooling necessitated by the leaking SRVs.  The
other strainers in the pool were covered with a dusting of sludge.  Debris was
subsequently brushed off the surface of the strainers, and the suppression
pool floor and water were cleaned by use of a temporary filtration system.  

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.  Whether
the blowdown caused by the SRV opening increased the rate of accumulation on
the strainer is not known.  The licensee removed about 635 kg [1400 lb] 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 were not inherent
with the suppression pool.  There was no trace of either fiberglass or
asbestos fibers..                                                            IN 95-47
                                                            October 4, 1995
                                                            Page 4 of 4

Related Generic Communications
�     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"
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 project manager.

                                    /s/'d by DMCrutchfield

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

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

                     Jerry Carter, NRR
                     (301) 415-1153
Page Last Reviewed/Updated Monday, November 18, 2013