Issue Date: 06/14/93 - 1 - PART 9900: TECHNICAL GUIDANCE

NRC INSPECTION MANUAL ILPB

PART 9900: TECHNICAL GUIDANCE

FPLUGS.TG

MECHANICAL - FREEZE PLUGS

A. PURPOSE

To provide guidance on the use of freeze plugs during system

maintenance or modification on reactor fluid systems, which, for

various reasons, cannot be isolated through the use of existing

valves.

B. DEFINITIONS

Freeze Plug - Freeze Seal. A method of preventing fluid flow in a

piping system by freezing the fluid. The area of fluid frozen

performs the same function as a valve. The terms freeze plug and

freeze seal are synonymous.

C. BACKGROUND

On April 19, 1989, at the River Bend Station, a freeze plug failed

on a 6-inch service water line. The freeze plug had been established to allow inspection and repair work on manual isolation

valves to a safety-related auxiliary building cooler. The failure

of the freeze plug resulted in flooding portions of the auxiliary

building. Approximately 15,000 gallons of service water covering

portions of the 141-foot level discharged through a disassembled

isolation valve. A portion on the water flowed through holes in the

floor under safety-related 480 VAC motor control centers on to

non-safety-related cabinets on the 114-foot level containing

disconnect links and a 13.8/480 VAC transformer. Since the cabinets

were not designed to shed the water, an electrical fireball resulted

that damaged the cabinet and components.

During an Oconee 1 shut down for a refueling outage, a freeze plug

was used to facilitate replacement of a 3-inch diameter section of

low pressure injection piping, because no valves were available to

isolate the affected piping. The freeze plug was in a line

connected to the borated water storage tank, which has a 388,000

gallon capacity, and supplies borated water for the low pressure

injection system. The freeze plug failed, and approximately 30,000

gallons of water leaked into various areas of the auxiliary building

and a portion drained through the station yard drainage system and
unintentionally flowed past the site boundary before the leak was

brought under control 8 hours after the freeze plug failed.

Information Notice 91-41, "Potential Problems With the Use of Freeze

Seals," identified potential problems related to the freeze seal in

PWRs and BWRs, specifically including both the River Bend and Oconee

1 incidents. The information notice indicated that freeze seal

failure in a PWR reactor boundary system could result in immediate

loss of primary coolant. In BWRs, failure of a freeze seal in a

system connected to the vessel's lower plenum region, such as the

reactor water cleanup (RWCU) system, could result in the water level

in the reactor vessel falling below the top of the active fuel. The

estimated time for this to occur is less than 1 hour if the seal

failed completely and makeup water was not added to the reactor.

The information notice indicated concerns that freeze seal failures

in secondary systems can also be significant because of the

potential for consequential failures, such as the loss of RHR in the

River Bend event. The information notice identified procedural

inadequacies that resulted in a failure to install and monitor a

temperature detection device, and a lack of personnel training in

the use of freeze seals. Other important considerations identified

in the notice included: "examining training, procedures, and

contingency plans associated with the use of freeze seals, and

evaluating the need for and availability of additional water makeup

systems and their associated support systems."

D. DISCUSSION

Freeze plugs are routinely used in nuclear reactor fluid systems to

drain or isolate components which for various reasons cannot be

conveniently isolated by valving. Basically, a freeze plug is

produced by chilling the outside of a pipe, usually by immersing a

portion of the pipe in liquid nitrogen. Eventually, the water at

the inner surface of the chilled pipe freezes and the ice/water

interface grows towards the center of the pipe and also along its

axis. Although this description is rather simplified, there exists

certain problem areas about which care must be taken, both for the

establishment of an effective freeze seal and for the assurance that

the freeze seal will be adequately maintained for the period of time

it is required. It is essential, therefore, that written procedures

be established and followed, and that adequate training be conducted

for personnel to correctly implement these procedures.

In the course of the NRC staff's evaluation of shutdown and lowpower operations (NUREG-1449), the staff identified and examined

plant configurations used during shutdown operations involving

temporary seals in the reactor coolant system. This includes freeze

seals that are used in a variety of ways to isolate fluid systems

temporarily, temporary plugs for nuclear instrument housings, and

nozzle dams in PWRs. The staff has identified instances in which

failure of these seals, either because of poor installation or an

overpressure condition, can lead to a rapid non-isolable loss of

reactor coolant. This concern is of special importance in PWRs

because the emergency core cooling system (ECCS) is not designed to

automatically mitigate accidents initiated at pressures below a few

hundred psig and is not normally fully available for manual use

during these conditions. In BWRs, the ECCS is required to be

operable during cold shutdown, and during refueling when there is

fuel in the reactor vessel and the vessel water level is less than
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23 feet above the reactor pressure vessel flange. In addition, the

ECCS is actuated automatically when water level is low in the

reactor vessel.

At the River Bend Station (RBS), freeze seals were being produced

by both outside contractor personnel and plant maintenance

personnel. Each organization had its own freeze seal procedure.

The RBS procedure permitted a freeze seal contractor to use his own

procedure. Much of the licensee's knowledge on freeze seal

production was gleaned from observation of the freeze seal

contractor during the first refueling outage. There had been no

formal training or a list of personnel qualified on freeze seal

techniques established for RBS maintenance personnel.

The RBS and contractor procedures had some notable differences. The

freeze seal contractor's procedure required installation of a

temperature measuring device into a sleeve in the freeze seal

chamber. The RBS procedure did not require installation of a

temperature measuring device. While the RBS procedure discussed use

of such a device, it was ambiguous in that it required a resistor

temperature sensor probe be taped to the pipe surface, but showed

a sketch with a thermocouple protruding from the chamber. The

freeze seal contractor required recording temperature every 5

minutes during establishment of the freeze plug and every 15 minutes

while the plug was being held. There were no temperature monitoring

or recording requirements in the RBS procedure. The freeze seal

contractor utilized a manifold boot and controlled flow by dripping

liquid nitrogen from a vent, whereas the RBS procedure vented

gaseous nitrogen (which is not considered to be a very reliable

method). In addition to temperature measurement, there are other

indications of freeze conditions. These indications are frosting

of the pipe at each end of the boot and observation of water flow

downstream from the freeze seal. Neither of these methods are very

reliable, but both were used in the RBS procedure. In addition, the

freeze seal contractor prohibited multiple seals from a single

nitrogen bottle, but no such prohibition was stated in the RBS

procedure.

In the RBS incident two freeze seals were produced from the same

nitrogen bottle. Nitrogen flow was controlled by observation of the

nitrogen plume at the vent, and some valve manipulation was required

to produce equal plumes from both vents and to maintain roughly

uniform plume size for the duration of the freeze. Temperature

indication was estimated by the axial length of frosting on the pipe

on either end of the boot. The freeze plug failed even though vapor

flow out of the jacket vent was observed and frost lines indicated

that a plug had formed. 

At Oconee the freeze plug procedure did not provide adequate

guidance for determining the amount of liquid nitrogen required.

Two bottles of liquid nitrogen were considered by the supervisor to

be a sufficient supply for the task and were available at the freeze

site. When the first bottle was depleted, personnel took it away

to be refilled. After refilling the first bottle, it was

subsequently returned to the freeze site. When the first bottle was

next used, it was determined that it had been only partially

refilled due to operator and maintenance personnel unfamiliarity

with the refilling process. The refilled first bottle lasted for
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about 15 minutes, which did not allow enough time to refill the

second bottle. While the second bottle was being refilled, the

freeze seal failed.

As a result of these events, the inspector should ensure that the

licensee has considered the following items in establishing

procedures and controls:

1. Maintenance procedures ensure that an adequate supply of

liquid nitrogen is available prior to commencing the freeze

seal operation.

2. Personnel are trained in the filling of nitrogen bottles.

3. Each freeze plug will be supplied by a single source of liquid

nitrogen.

4. Freeze plug installation and maintenance will be performed by

plant personnel who have been trained on freeze plug

installation and maintenance techniques and procedures.

5. The licensee has a documented contingency program with

implementing procedures to use in the event that a freeze plug

fails.

6. Freeze plug procedures require communications between the

control room and the maintenance personnel performing the work

to keep the control room apprised of the status of ongoing

work.

7. Prior to establishment of a freeze seal, the area of pipe

scheduled to contain the seal should be subjected to surface

inspection (MT or PT) to assure that no surface defects are

present.

8. The equipment used to establish and maintain the seal should

be designed to provide uniform distribution of coolant.

Provision should be provided for temperature monitoring

instrumentation and dependable flow monitoring of some sort

(Observation of gaseous plume is not considered an adequately

dependable flow monitoring method).

9. Freeze seals should not be performed within some minimum

distance (e.g., a minimum of 3 pipe diameters) from piping

discontinuities.

10. Freeze plug jacket temperature will be monitored to ensure

that liquid nitrogen flow is maintained.

11. Provide adequate ventilation to assure adequate circulation

of volatile coolants. In confined areas, an air analyzer

should be in operation and frequent observations performed.

12. During freeze plug thaw, a minimum  p across the seal is

necessary to prevent downstream damage from movement of the

plug.
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13. Recommended modifications to basic freeze seal procedure for

variations in seal orientation (i.e. pipe axis horizontal or

vertical) should be provided where needed (i.e. temperature

and flow monitor locations).

14. Adequate measures have been taken to protect electrical

equipment and other systems whose operation could be impacted

by a failure of the freeze plug.

15. Planning and supervision of freeze seal operations should be

conducted by qualified and experienced personnel. In many

cases, contractors with special expertise and experience in

conducting freeze seal operations will be used. A more indepth inspection may be warranted if a recognized specialist

in the area of freeze sealing is not being used.

16. Planning for freeze seal operations should include

consideration for the potential for causing structural damage

to the piping based on the evaluation of a qualified engineer.

END