INPO Document Freeze Sealing
Events
ACADs (08-006) Covered
Keywords
Description
Supporting Material
Just-In-Time Operating Experience
Freeze Sealing
Rev. 1, July 2000
Deficiencies in freeze sealing have resulted in industrial safety hazards,
personnel and area contaminations, internal flooding, and emergency system
actuations.
Events
Plant: Limerick Unit 1
Freeze Seal Activities Result in an Atmosphere Dangerous to Life and Health, and Air Monitor Operation
Problems -- Reference: OE4217
On September 29, 1990, an oxygen deficient atmosphere was created when cold nitrogen gas, a byproduct of the
liquid nitrogen used in the freeze seal process, displaced the oxygen at floor level below a freeze seal. The workers
maintaining the freeze seal did not know that a hazardous condition existed until operators performing an unrelated
task became dizzy and light-headed. It was determined that the portable air monitor in use was not providing
accurate readings. Subsequent samples showed that the oxygen content in the area below the freeze seal was less
than 13 percent rather that an expected 21 percent.
Important Point:
 Pure, extremely cold nitrogen gas from the freeze seal process is heavier than air
and will displace ambient temperature breathable air.
Contributors:
 Air flow from temporary ventilation was insufficient to remove the volume of
nitrogen from the freeze seal work area.
 The workers were unaware that the air quality monitor in use must be in fresh air
when turned on, prior to placement in a confined space. If the monitor is turned
off or loses power during use, it must have a fresh air supply for calibration, prior
to continuing use, or the reading will be erroneous.
Plant: Callaway
Contamination of Mechanics and Work Area Caused by Freeze Seal Failure -- Reference: OE11222
On October 11, 1999, mechanics were contaminated while replacing valve bonnets on two residual heat removal
(RHR) instrumentation root valves when the freeze seal for one of the valves failed causing water to exit through the
valve bonnet. The freeze seal failed when orifices in the heads that clamp onto the pipe became clogged because of
degradation of the refrigeration unit hoses. The mechanics noticed the frost melting from the hoses, but it took less
than one minute from that point until the freeze seal failed. The valve bonnet bolts were tightened and the water flow
stopped.
Important Point:

The station did not have a method to test the refrigeration unit performance prior
to placing it in service.
Plant: Oconee Unit 1 and River Bend
Internal Flooding Resulting From Freeze Plug Failures -- Reference: SER 15-89
On October 7, 1987, at Oconee Unit 1, 30,000 gallons of slightly contaminated water from the refueling water
storage tank flooded the auxiliary buildings when a freeze seal failed. On April 1, 1989, at River Bend, one of two
freeze seals failed causing 15,000 gallons of service water to flood the auxiliary building. A similar nonflooding
event at Ginna is discussed in OE8380.
Important Points:
Contributors:
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Temperature monitoring to verify freeze seal integrity was not performed.
The liquid nitrogen supply available at the work area was insufficient to maintain
the freeze seal.
The piping was not water solid in the area of the freeze seal at River Bend.
Positive methods, such as ultra sonic examination, were not used to detect air
voids in the piping at the proposed seal area.
The personnel involved in the freeze seal activities were not trained on seal
integrity problem identification.
At Oconee, a portable bottle used to transport liquid nitrogen to the freeze seal
location was not completely refilled and ran out before another bottle was
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
brought to the work location.
At River Bend, the use of a single liquid nitrogen entry-point created difficulty
supplying two freeze seal plug jackets.
Plant: LaSalle Unit 2 and Limerick
Safety-System Actuation and Water Hammer Events Caused By Freeze Seal Thawing -- Reference: LER 37492002 and OE5283
On February 26, 1992, at LaSalle, the Division I emergency core cooling system actuated when air on the
maintenance side of a freeze seal isolation entered an instrumentation reference leg as the seal was allowed to thaw.
On June 6, 1991, at Limerick, the fire protection system depressurized initiating an automatic fire pump start and
system water hammer. Although station procedures directed system voids be backfilled before freeze seal thaw,
backfilling did not occur.
Important Point:

Contributor:
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Workers did not understand the importance of backfilling voids before thawing
freeze seals.
Station personnel were not trained on freeze seal and seal thaw evolutions.
Important Considerations Prior to the Freeze Sealing (Lessons Learned)
 How have we verified that we will not create an oxygen deficient atmosphere in the area of our freeze
seal activities or in areas adjacent to our work area? How have we assured that permanent and
temporary ventilation will remove the nitrogen gases from our freeze seal activities?
 What actions have we taken to verify that our freeze seal activities will not form hydraulic pockets and
overpressurize system piping? What is the “rule of thumb” for the distance between freeze seals or
freeze seal jackets? What will happen if our freeze seal traps water pockets in the piping or
components?
 What action have we taken to verify that each freeze seal plug jacket receives its own independent
supply of liquid nitrogen? How have we verified that the supply of liquid nitrogen is adequate to
establish and maintain our freeze seal and that we are capable of delivering the liquid nitrogen to the
area in sufficient quantity to adequately support our freeze seal activity?
 How have we verified that our liquid nitrogen bottles are full?
 What contingency actions have we established if our liquid nitrogen inventory is consumed faster than
our estimates or the repair time exceeds the time estimated in the work plan?
 What indications will we use to verify that our freeze seal is effective? How will we identify a void in
our freeze seal? What indications, processes, and procedures have we established to identify problems
with establishing or maintaining our freeze seal?
 What contingency plans have we established to prevent or minimize flooding in the event our freeze
seal fails? How have we verified that contingency isolation valves are unlocked and available for use,
that workers are familiar with the location and operation of these isolation devices, and the control
room staff is aware of our activities and ready to act quickly if our contingency isolation plan is
implemented?
 Before we allow our freeze seal to thaw, what indications will we monitor to verify that our back-filling
activities have been successful? What could be the result if there is a void in the system when our
freeze seal thaws?
 If we are using a refrigeration unit to create a freeze seal, do we have a method to test the performance
of the unit before it is placed in service? How do we perform testing at routine intervals to ensure
continued satisfactory performance? Does the refrigeration unit have an indication device allowing
personnel to know the device is cooling satisfactorily?
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