United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-57 - IN 94-57: Debris in Containment and the Residual Heat Removal System


August 12, 1994



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 the problem associated with debris recently
discovered in the containment and the residual heat removal (RHR) system at
some BWR sites.  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 are instances in which debris were found in BWR suppression

LaSalle County Station, Unit 1

On April 26 and May 11, 1994, while in a refueling outage, the licensee made
two dives into the Mark II design suppression pool to clean the emergency core
cooling system (ECCS) suction strainers of a small amount of debris which
caused less than 1 percent clogging.  The diver found that the strainers had
experienced no apparent damage or deformation of the strainer faces.  However,
while in the pool on both occasions, the divers found and removed an
assortment of operational debris.  On the first dive, the diver removed a
hardhat, a pair of anti-contamination coveralls, a 15.2 meter (m) [50 ft]
length of Tygon tubing, 3 nuts, and a 4.6 m [15 ft] length of black duct tape.
On the second dive, the diver removed four lengths of 1.9 cm [3/4 in] hose
ranging in length about 8 m [25 ft] to about 46 m [150 ft], three lengths of
Tygon tubing ranging in length from 6 m [20 ft] to 15 m [50 ft], a short
length of 5 cm by 10 cm [2 in by 4 in] wood, and a flashlight.

The diver also noted that a sediment had formed on the suppression pool floor
ranging in thickness from 0.3 cm to 5 cm [1/8 in to 2 in].  The suppression
pool floor is a level floor with raised ridges in a waffle pattern.  The
5 cm [2 in] accumulations of sediment were found in the raised corners of

9408080111.IN 94-57
August 12, 1994
Page 2 of 4

the waffle pattern.  The licensee took a sample of the sediment and had it
analyzed.  The analysis results showed that the filterable solid consisted of
over 99 percent iron oxide, with trace amounts of nickel, copper, and chrome.
The filtrate contained 120 parts per billion (ppb) dissolved nickel.  No
organic constituents were found.  Radiologically, the sample consisted of
75 percent cobalt-60 with small contributions from manganese-54, cobalt-58,
and iron-59.  The licensee concluded that the sample contained normal system
corrosion products with no contaminants such as hydraulic fluid or solvents.
Before startup, the licensee cleaned all ECCS strainers of debris and removed
all objects from the pool.

River Bend Nuclear Station

On June 13, 1994, while the plant was in a refueling outage, the licensee
conducted an inspection of the ECCS suction strainers and the Mark III design
suppression pool.  During the inspection, 16 objects were located in the
suppression pool.  One of these objects, a plastic bag, was removed from the
residual heat removal system "A" suction strainer.  The other objects that
were removed from the pool included a hammer, grinding wheel, slugging wrench,
socket, hose clamp, bolt, nut, step-off-pad, two ink pens, antenna, scaffold
knuckle,  short length of rope, and used tape.  Most of these items were not
listed in the station suppression pool lost item log.  These findings prompted
the licensee to take the following corrective actions:  (1) remove all items
from the suppression pool, (2) inspect all accessible areas for additional
debris, (3) verify the strainers for all ECCS pumps to be clean, and (4)
increase surveillance of the suppression pool work area to minimize additional
objects dropped into the pool.  The licensee is reviewing its policies and
practices regarding loose objects in and around the grating areas in the
containment to determine their adequacy.

The licensee also found sediment in the suppression pool.  During the previous
refueling outage, that ended in September 1992, the licensee drained and
cleaned the pool.  However, the licensee was unable to completely clean the
pool.  After draining the pool, there was still about 0.3 m [1 ft] of water
inside the weir wall that was "mucky."  During the current refueling outage,
the licensee used a portable cleanup system to clean the water in the pool.
By the end of the outage, water clarity in the pool significantly improved.
However, a layer of sediment still remains on the pool floor.  The licensee is
planning to install a permanent pool cleanup system two outages from now;
however, the planned system will not be able to remove the sediment inside the
weir wall.

The following is an instance in which debris was found in the RHR system.

Quad Cities Unit 1

On July 14, 1994, during a post-maintenance test run of the "A" loop of the
RHR system, test data indicated that the RHR torus cooling/test return valve,
valve 1001-36A, was plugged.  When the 36A valve was opened for inspection,
the remains of a plastic bag were found shredded and caught within the .IN
94-57 August 12, 1994
Page 3 of 4

anti-cavitation trim which was installed during the recent outage.  Some of
the material appeared to have travelled the entire way through the
anti-cavitation trim.  The majority of the material was found lodged on the
suction side of the valve trim.  A few small pieces of plastic were found in
the Mark I design suppression pool and removed.

Subsequent to the July 14 event, the licensee observed reduced flow from the
"C" RHR pump and initiated further investigation.  On July 23, 1994, licensee
maintenance personnel removed a drain plug on the volute of the "C" RHR pump
and used a boroscope to inspect the pump internals.  A 10 cm [4-inch] diameter
wire brush wheel and a piece of metal were found wrapped around a vane of the
pump.  The licensee had opened the RHR system during the outage to work on the
RHR 7D valve and removed a butterfly valve on the common suction line (valve
RHR 6B).  The licensee retrieved the wire wheel brush, the metal  and two
washers from the pump.


The events described above illustrate the potential for adverse effects on
emergency core cooling system performance due to debris.  The debris resulted
from inadequate control of foreign material inside the containment or resulted
from inadequate inspection after maintenance activities were performed on a
safety system (the RHR system).

Previous NRC generic communications have noted that ECCS strainer clogging
represents a potential threat to the reliable functioning of the ECCS pumps
throughout a design basis accident.  These previous NRC communications have
dealt with the potential to clog ECCS strainers with debris generated during
plant work activities, debris from a loss-of-coolant accident (LOCA), or a
combination of these.  The types of debris described herein are further
examples of debris licensees have found in their suppression pools or in the
RHR system.  Although the licensees in the LaSalle and River Bend cases stated
that the debris found in their suppression pools would have been insufficient
to clog strainers, these debris in combination with the debris generated
during a LOCA could accelerate a loss of net positive suction head for the
ECCS pumps or cause other types of damage to the strainers.  For example,
corrosion sediment in the pool could clog the strainers if debris such as the
anti-contamination clothing were already drawn to the strainer surface where
the clothing could filter the finer particles of sediment out of the water.
In addition, damage to strainers might occur if some of the heavier tools were
to strike the strainers during the early stages of a LOCA blowdown.

Previous NRC generic communications also have addressed plant events where
debris was found in safety systems, namely the auxiliary feedwater system and
the safety injection system, and resulted in reduced flow during testing of
the systems.  The Quad Cities event discussed above involved debris in the RHR
system.  The debris in this case could potentially cause a transient, result
in failure to mitigate a transient or accident, or result in damage to
.IN 94-57
August 12, 1994
Page 4 of 4

Related Generic Communications

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"

NRC Information Notice 88-87:  "Pump Wear and Foreign Objects in Plant
Piping Systems"

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

This information notice requires no specific action or written response.  If
you have any questions regarding 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.


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

Technical contacts:  Robert B. Elliott, NRR
   (301) 504-1397

   Amy E. Cubbage, NRR
   (301) 504-2875

List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Friday, November 15, 2013