United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-56: Shielding Deficiency in Spent Fuel Transfer Canal at a Boiling-Water Reactor

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555-0001

December 11, 1995


NRC INFORMATION NOTICE 95-56:  SHIELDING DEFICIENCY IN SPENT FUEL            
                               TRANSFER CANAL AT A BOILING-WATER REACTOR


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 an event at a boiling-water reactor where the
mishandling of highly activated materials in the flooded spent fuel transfer
canal caused unexpectedly high radiation fields in a hallway under the canal. 
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

On November 11, 1994, contractors were conducting underwater operations in the
Unit 1 spent fuel pool in the cutting tool working area at the Edwin I. Hatch
Nuclear Plant (Inspection Report 50-321/95-01 and 50-366/95-01 [Accession
Number 9502140081])--cutting coupons out of spent control rod blades
containing the upper guide roller bearings (see Attachment 1).  The highly
activated Stellite bearings (some measured as high as 160 sievert [16,000 rem]
per hour at 30 centimeters [12 inches] under water) were being collected
adjacent to the Unit 1 work area.  Periodically, the coupons containing the
upper guide roller bearings were transferred from the collection bucket to a
cask liner in the shipping cask storage area in the Unit 2 spent fuel pool. 
When the workers could not find the tool to open the liner, they decided to
transfer the coupons in the collection bucket (about half full, with 160
coupons) into another bucket for temporary storage, so that the cutting
process could continue.  To facilitate the task, the offload was performed in
the transfer canal since the canal was much shallower than the fuel pools. 
During the transfer, some of the coupons fell to the bottom of the transfer
canal.  Since the transfer canal was designed and routinely used as a transit
area for highly activated material, including spent fuel, the workers were not
concerned about dropping the coupons and saw no need to notify the unit shift
foreman or the control room of the incident.  They recovered the coupons from
the bottom of the canal and placed them in the storage bucket resting on the
canal floor.


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About 30 minutes after the coupons were dropped, a plant equipment operator
was walking through the Unit 1 (56.4-meter [185-foot] elevation) hallway
directly under the transfer canal (see Attachment 2), when his digital alarm
dosimeter alarmed on high dose rate (set at 0.5 msievert [50 mrem] per hour). 
The plant equipment operator left the area promptly and notified a health
physics supervisor, who sent a health physics technician to the area.  When
the health physics technician also found the elevated radiation levels
described above, he secured the area, investigated for the source of the high
radiation levels, and informed shift management.  The licensee measured
radiation levels of up to 1 sievert [100 rem] per hour on contact with the
hallway ceiling directly below the bottom of the canal, and 0.05 to 0.1
sievert [5 to 10 rem] per hour in the general area of the hallway.  The plant
equipment operator received a dose equivalent of about 0.1 msievert [10 mrem]
from the event.  The licensee has always required that all personnel entering
the radiological controlled area be issued dosimeters.  Before alarm
dosimeters were required, all workers entering the radiologically controlled
area were issued personal dosimeters (non-alarm), so any doses to workers from
this shielding deficiency would have been detected and accounted for as part
of the routine dosimetry program.  

In response to the event, Hatch Nuclear Plant management instituted procedural
controls to prohibit the use of the transfer canal until doors had been
installed, in order to exclude worker access to the affected hallway.  Before
the transfer gates are allowed to be lifted (allowing use of the transfer
canal), the doors at each end of hallway under the canal are locked, and
access to the hallway is controlled as a very high radiation area, as defined
in 10 CFR 20.1602, "Control of Access to Very High Radiation Areas."

At the Limerick Generating Station recently, the licensee took the initiative
to reevaluate the adequacy of its spent fuel pool shielding before commencing
the reracking of its Unit 2 fuel pool.  A thorough engineering review examined
the radiation levels in the accessible areas under the spent fuel pool and
associated transfer canals and cask pit area.  This review revealed (using
conservative shielding assumptions) that a dropped spent fuel bundle could
create a high radiation area in areas below the cask wash pit that were
accessible to personnel.  Before commencing fuel movement, the Limerick staff
established precautionary radiological controls (posted as high radiation
area/radiation work permit required for entry) and radiation monitoring
(portable area radiation monitor) for the affected area.        

Discussion

A portion of the shielding directly under the transfer canal at Hatch Nuclear
Plant is inadequate to prevent unacceptable external dose rates in the hallway
below, should a highly activated component be placed near, or in contact with,
the canal floor (with attendant loss of water shielding).  Therefore, even
with a successful bucket-to-bucket transfer (no dropped coupons), with the
loaded bucket stored and resting on the canal floor, the hallway radiation
levels would have been essentially the same as for the actual event.

Many licensees perform spent fuel pool modifications and major cleanup
activities involving handling and moving large quantities of highly activated.                                                            IN 95-56
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materials, including spent fuel.  In general, the industry has significantly 
improved its awareness of and controls for potentially high and very high
radiation areas caused by operational mishaps (e.g., dropping a spent fuel
bundle in the transfer canal directly over the upper drywell in a boiling-
water reactor, and the hazards of withdrawn incore thimbles under the reactor
vessel at pressurized-water reactors).  Initiatives such as those taken by the
Limerick Station should help prevent unexpected, uncontrolled worker exposures
with the potential for exceeding the regulatory limits.  A thorough prejob
evaluation of activities involving highly activated (or potentially highly
activated) components can help identify challenges to existing plant
shielding.

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:  Jim Wigginton, NRR
                     (301) 415-1059

                     Roger Pedersen, NRR
                     (301) 415-3162

                     Wade Loo, RII
                     (404) 331-2831

Attachments:
1.  Under Water Operations Event in the 
      Spent Fuel Pool At Hatch
2.  Unexpected High Radiation Area at Hatch

(See File IN95056.WP1 FOR Atts. 1 and 2)
Page Last Reviewed/Updated Monday, November 18, 2013