Information Notice No. 90-33: Sources of Unexpected Occupational Radiation Exposures at Spent Fuel Storage Pools
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
May 9, 1990
Information Notice No. 90-33: SOURCES OF UNEXPECTED OCCUPATIONAL
RADIATION EXPOSURES AT SPENT FUEL STORAGE
All holders of operating licenses or construction permits for nuclear power
This information notice is intended to alert addressees to potential sources
of unexpected occupational radiation exposures at spent fuel storage (SFS)
pools. 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 do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances:
SFS pools provide a water-shielded location for the storage of spent fuel
and other highly radioactive materials that are potential sources of high
radiation exposures. SFS pools also may be contaminated with highly
radioactive particles having activities of tens of millicuries (mCi) or
more. This information notice identifies a number of events in which
sources of unexpected occupational radiation exposures were encountered in
the activities associated with SFS pools.
Events Involving Highly Radioactive Objects That Floated in SFS Pools
In June 1989, several individuals involved in SFS pool cleanup activities at
the James A. FitzPatrick Nuclear Power Plant received unexpected radiation
exposures (within NRC limits) from an object floating near the surface of
the SFS pool near their work location. Subsequent radiation surveys of the
object indicated contact radiation exposure rates of about 1000 roentgens
per hour (R/hour). The licensee believes that the source of the radiation
was a small fragment of radioactive material imbedded in a piece of floating
material. The floating material probably was a piece of a 5-gallon
polyethylene container in which irradiated components had been stored
underwater for more than a year. Apparently, the polyethylene container
began to disintegrate as a result of radiation from the irradiated
components stored inside it.
The problem of contaminated objects floating to the surface of SFS pools is
not new. In December 1984, complete canister filters from a portable
underwater vacuuming system floated at least 15 feet toward the top of the
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fuel racks at the Peach Bottom Atomic Power Station. Licensee personnel
speculated that the filters may have surfaced and then sank to the spent
fuel storage racks. Although no personnel received unexpected exposures,
this event could have caused significant exposure to personnel.
In May 1987, floating vacuum cleaner filters were also involved in a similar
event at the Millstone Nuclear Power Station, Unit 1. During that event,
the radiation level on the refueling floor increased momentarily to more
than 100 mR/hour, then quickly returned to a normal level of about 10
mR/hour. The high radiation level occurred at the same time that the spent
fuel pool cooling system was returned to use following valve maintenance.
Refilling of the system forced air into the SFS pool through the sparger.
The bubbles rose in the pool, causing agitation. The licensee postulated
that the agitation forced highly contaminated vacuum cleaner filter
cartridges stored in the pool to the surface. These filters contained
debris from past cleaning evolutions of the reactor vessel and were
suspended by nylon line in the SFS pool. After this event, the licensee
decided to remove disposable items from the pool and to replace nylon lines,
used for suspending items in the pool, with wire cables.
Events Involving Highly Radioactive Particles or Fragments of Radioactive
Material Removed from SFS Pools
During reracking operations in the SFS pool at the Diablo Canyon Nuclear
Power Plant, Unit 1, in December 1987, a highly radioactive particle (45.5
mCi of Co-60) was inadvertently removed from the SFS pool. The particle was
attached to an air grinder hose that was partially pulled from the pool by
the diver tender when the grinder hose became entangled with the diver's
hose. When the Co-60 particle was removed from the pool, the radiation from
the particle caused the fuel handling building (FHB) area radiation monitor
to alarm, resulting in a shift of the FHB ventilation system to the iodine
removal mode. The air grinder hose was not monitored for radiation while it
was being removed from the pool. Conservative estimates by the licensee
indicated that had the diver tender come in contact (0.5 second) with the
Co-60 particle with his hands, he could have received an extremity dose of
In October 1989, personnel at the Byron Station found unexpectedly high
activity Co-60 particles (8 mCi and 77 mCi) during efforts to decontaminate
and remove a portable filter assembly that had been used in an underwater
vacuum cleaner to clean the bottom of the SFS pool. Three individuals
received unexpected doses to their hands and forearms from these highly
radioactive particles. These doses were calculated to be 1.25, 1.95, and
Highly radioactive materials also have been lifted, unexpectedly, to the
surface of SFS pools. At the FitzPatrick plant in February 1987, a worker
received an overexposure to the hand during the cutting and removal of
in-core instrumentation dry tubes from the reactor vessel. During this
operation, the cutting tool was removed from the water for inspection. A
piece of highly radioactive dry tube that had been stuck in the tool fell
out on the refueling floor. A worker immediately picked up the piece of dry
tube and threw it back into the water. As a result of this brief contact
with the dry tube, the worker received a radiation dose of about 30 rem to
his hand, which exceeds the NRC limit of 18.75 rem per calendar quarter.
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At the Callaway Plant, personnel discovered that an inadequate survey had
been made on an underwater fuel elevator following its removal from the SFS
pool on April 23, 1989. On April 29, 1989, a metal object was found to read
1000 R/hour at contact, but the object was shielded by the elevator
structure so that only a localized area exceeded 100 mR/hour. The highly
radioactive metal object was found to be a piece of a torn grid strap from a
fuel assembly that had been repaired in the fuel elevator on April 21, 1989.
Personnel had attempted to flush the piece out of the bottom of the fuel
basket. However, because the piece was never seen in the fuel elevator
basket and because no abnormal radiation readings were reported, personnel
incorrectly assumed that the piece had been flushed from the elevator basket
and was at the bottom of the fuel pool.
Event Involving Inadvertent Lifting of Highly Radioactive Material by Hoist
In April 1982 at the Peach Bottom Atomic Power Station, Unit 2, personnel
inadvertently lifted a highly radioactive (in the million R/hour range)
control rod blade (CRB) near the surface of the water for 5 to 10 seconds.
This event occurred when a hafnium test blade (HFB) was to be moved from the
fuel preparation machine to a CRB rack location. This evolution was being
supervised by a senior licensed operator. When an operator attempted to
move the HFB using the fuel grapple, the operator could not engage the
temporary C clamp on the HFB. An auxiliary hoist was subsequently used to
successfully move the HFB to the CRB rack location. After completing the
HFB movement, the operator proceeded to pull the hoist grapple to its normal
storage location out of the water. While the hoist was rising, its grapple
engaged and picked up an adjacent CRB. The hoist limit switch stopped the
upward movement when the grapple was still under 8 to 10 feet of water. At
that time the operator, without looking, used the "override" button to
bypass the limit switch and raised the grapple with the attached CRB. The
area radiation monitor started alarming as the grapple with the CRB
approached the surface of the water. Upon hearing the alarm, the refueling
floor supervisor looked and saw a brown object approaching the water surface
and yelled, "Put it down, put it down." The grapple was stopped before the
CRB reached the surface. No radiological surveys were performed during this
evolution. However, individuals at the pool area were unnecessarily exposed
to radiation. Although these doses were about 300 mrem or less, if the CRB
had reached the surface, the doses could have been much greater.
Event Involving Radiation Streaming
Underwater tools are designed with flood holes to allow water to fill the
hollow sections of tubes. These flood holes provide shielding against
radiation streaming from highly radioactive materials that are stored under
water. At Indian Point Station, Unit 3, in July 1989, a worker using a tool
to perform reconstitution of a spent fuel assembly noticed that a
12-foot-long 3/4-inch-diameter hollow section of the tool did not have a
flood hole. As a result of this equipment design deficiency, the worker
received an unplanned exposure. Although the licensee estimated that the
worker received a radiation dose of only 30 mrem as a result of this
incident, a hollow tool filled with air rather than water can result in much
higher doses when the lower end of the tool is under water and near a highly
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Additional information on these events is provided in the documents listed
in Attachment 1. The NRC and licensee documents are available in the NRC
Public Document Room.
Although the events described above were associated with SFS pools, similar
problems can occur during the removal and handling of material from flooded
A review of licensee evaluations of these events, the licensee corrective
actions, and feedback from NRC inspectors indicate that the following
radiological control considerations can help minimize the possibility of
unexpected exposures from radiation sources in SFS pools:
x Thorough evaluations of the contents of, and activities involving, SFS
pools from a radiological perspective to identify potential unexpected
x Measures such as task analysis and training to ensure awareness of the
potential for uncontrolled, unplanned transfer of highly radioactive
materials, including highly radioactive particles, to the surface of
SFS pools with the attendant potential for high radiation exposure of
x Measures to ensure awareness of the need for thorough radiation surveys
of all materials being removed from SFS pools.
x Measures to eliminate or secure floatable material in SFS pools.
x Avoidance of the use of containers made of materials (particularly
plastics) that are subject to radiation damage and disintegration for
the storage of highly radioactive materials in SFS pools. If such
containers are used, limiting the radiation dose to the container can
be used to prevent disintegration of the container as a result of
x Measures to ensure that highly radioactive objects stored under water
at one end of a line whose other end is secured above the surface of
the pool are not unexpectedly pulled to the surface.
x Measures to ensure awareness of the need to prevent radiation streaming
through hollow sections of handling tools.
x Enhanced use of alarming personal dosimeters and of alarming area
radiation monitors around SFS pools.
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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 NRR project
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: John D. Buchanan, NRR
Ronald L. Nimitz, RI
1. Documents Providing Information on Events
Involving Spent Fuel Storage Pools
2. List of Recently Issued NRC Information Notices
May 9, 1990
Page 1 of 1
Documents Providing Information on Events Involving
Spent Fuel Storage Pools
James A. FitzPatrick, June 1989 Event
x Inspection Report Nos. 50-333/89-08, August 10, 1989; 50-333/89-13,
August 2, 1989; and 50-333/89-21, December 29, 1989.
Peach Bottom, December 1984 Event
x Memorandum from R. R. Bellamy, Region I, to L. J. Cunningham, NRR,
August 16, 1989.
Millstone Unit 1, May 1987 Event
x Inspection Report No. 50-245/87-11, July 1, 1987.
Diablo Canyon, December 1987 Event
x Inspection Report No. 50-275/88-01, February 12, 1988.
x LER 87-27-00 (Docket No. 50-275).
Byron, October 1989 Event
x Inspection Report No. 50-454/89-21, December 8, 1989.
James A. Fitzpatrick, February 1987 Event
x Inspection Report No. 50-333/87-07, March 11, 1987.
x Letter from John C. Brons, New York Power Authority, to the Director,
Office of Inspection and Enforcement, NRC, May 21, 1987.
Callaway, April 1989 Event
x Inspection Report No. 50-483/89-16, September 8, 1989.
Peach Bottom, April 1982 Event
x Inspection Report No. 50-277/82-11, July 13, 1982.
Indian Point Unit 3, July 1989 Event
x Inspection Report No. 50-286/89-18, September 12, 1989.
Page Last Reviewed/Updated Wednesday, March 24, 2021