1 Copyright © 2014 by ASME

REMOTE SEALING OF CANISTERS FOR HOT ISOSTATIC PRESSING

Dennis Wahlquist

Idaho National Laboratory Idaho Falls, Idaho, USA

Ken Bateman Idaho National Laboratory Idaho Falls, Idaho, USA

Tim Malewitz Portage Inc.

Idaho Falls, Idaho, USA

ABSTRACT Battelle Energy Alliance, LLC, has successfully tested a

remote welding process to seal radioactive waste containers

prior to hot isostatic pressing (HIP). Since the 1990s, a variety

of radioactive and hazardous waste forms have been remotely

treated using HIP during trials within Idaho National

Laboratory (INL) hot cells. For HIP treatment at INL, waste

was loaded into a stainless-steel or aluminum canister, which

was evacuated, seal welded, and placed in a HIP furnace. HIP

simultaneously heats and pressurizes the waste, reducing its

volume and increasing its stability, thus lowering the cost and

risk associated with disposal. Weld integrity must be ensured in

order to prevent the spread of contamination during HIP. This

paper presents a process for sealing HIP canisters remotely

using modified, commercially available equipment. This

process includes evacuation, heating, welding, and weld

inspection. The process and equipment have proven to reliably

seal canisters in continued HIP trials.

INTRODUCTION One of the challenges facing Idaho National Laboratory

(INL) is how to safely and economically store and dispose of

4,400 m3 of granular radioactive calcine that was produced

there from 1953 to 1992. Hot isostatic pressing (HIP) is a

promising method of significantly reducing risks associated

with this calcine, its volume, and, in turn, the cost of storing it.

A study conducted by the Australian Nuclear Science and

Technology Organisation (ANSTO) reported that using the HIP

process for volume reduction of INL granular calcine and

solidified sodium-bearing waste would save an estimated

$2 billion in disposal costs (a 50% reduction) over the U.S.

Department of Energy baseline treatment of immobilization in

a borosilicate glass matrix (Begg et al. 2005). HIP uses pressure

and heat to decrease the volume of materials by reducing their

internal void spaces. However, while it may lower INL’s

calcine risks and storage costs, HIP also presents new

challenges—for example, how to safely load radioactive

calcine into manageable-sized containers (i.e., HIP canisters)

that can be sealed and maintain their integrity after being

subjected to HIP processing. INL researchers determined that

the safest way to accomplish this is to remotely fill HIP

canisters with the calcine and seal them via welding in a

shielded hot cell before they undergo HIP.

INL HIP PROCESS In the 1990s, a Model MIH-9 “mini” hot isostatic press

manufactured by ABB Autoclave Systems, Inc., (Fig. 1) was

installed in the Hot Fuel Examination Facility (HFEF)

decontamination hot cell at INL (Bateman et al. 2002). Due to

the high radioactivity of the waste forms that were to undergo

HIP treatment, trials were conducted in the HFEF hot cell using

remotely operated equipment to prevent the spread of

contamination to workers and the public. Extensive research

and pilot-scale testing has been conducted into the HIP process

as a method for treatment of a variety of waste forms, including

ceramic wastes, iodine, corroded fuel pins, irradiated materials,

and sludge. Trials have also been conducted with an INL

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-36919

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calcine surrogate consisting of simulated alumina provided by

ANSTO and spiked with radionuclides by Battelle Energy

Alliance (Fig. 2; Bateman 2011). In all cases, HIP produces a

volume-reduced, monolithic waste form with increased stability

for disposal in a repository.

Materials to be HIP processed during the testing at HFEF

were placed inside of stainless-steel or aluminum canisters,

which were evacuated, heated to remove residual moisture, and

then sealed. The sealed HIP canister was placed inside of a

monolithic pressure vessel with a working diameter of 5.9 in.

(15 cm), a height of 11.81 in. (30 cm), and a maximum pressure

rating of 200 MPa. A molybdenum heating element provided a

maximum temperature of 1,450 C to the pressure vessel

(Bateman et al. 2002).

FIG. 1. INL HOT ISOSTATIC PRESS INSTALLED IN A DECONTAMINATION HOT CELL AT INL (BATEMAN 2002).

FIG. 2. SURROGATE INL CALCINE BEFORE (LEFT) AND AFTER (RIGHT) HIP.

HIP CANISTER SEALING CONSIDERATIONS Prior to HIP, the vessel holding the material that was to

undergo the HIP process had to be filled, evacuated of excess

air, sealed (welded), and weld inspected.

INL HIP canister loading and sealing equipment was

designed to (Wahlquist and Bateman 2002):

Provide remote operation from a window of the HFEF

main hot cell in an argon atmosphere

Contain material during operation and minimize the spread

of contaminants during transfer operations.

Allow for visual inspection of completed welds.

HIP CANISTER DESIGN HIP canisters may vary in size and material of

composition. Common HIP canister materials are stainless steel

and aluminum. HIP demonstrations for material batches up to

375 lb (170 kg) have been reported (Whittaker 2012).

For INL HIP trials, two different types of canisters were

used. The first was a cylindrical bellows canister fabricated of

304 stainless steel (Fig. 3) developed at INL. The canister had a

height of 9 in. (22.86 cm) and a diameter of 4.5 in. (11.4 cm)

prior to HIP (Goff et al. 1996). The wall of the canister was a

Hyspan No. 7541P-7.0-1.0 bellows. Seam-welded plates

formed the top and bottom of the canister. The second canister

was an ANSTO 4-in. (10.16-cm)-diameter × 6-in. (15.24-cm)-

high container with two large bellows (Fig. 2). The HIP process

was designed to accommodate both types of canisters.

Integral to the design of HIP canisters was a 7.5-in. (19.05-

cm)-long, 1-in. (2.54-cm)-diameter, 0.035-in. (0.088-cm)-thick

fill tube that was welded into an opening in the top plate. At a

loading station within the hot cell, material was proportioned

from an intermediate container (Fig. 4) through the fill tube into

the HIP canister. The intermediate container was sized to match

the volume of the HIP canister assembly. Butterfly valves were

used to proportion the correct amount of material to fill a given

type of canister. During loading, the HIP canister was vibrated

to facilitate material transfer and reduce the void volume within

the canister (Wahlquist and Bateman 2002).

HIP CANISTER SEALING PROCESS A critical aspect of using HIP for volume reduction of

radioactive calcine at INL is sealing the HIP canister to contain

contamination. Although other techniques are available,

welding is considered to be the best option for sealing the HIP

canisters. For various reasons, techniques other than welding

are unsuitable for canisters that are subjected to the HIP

process. For example, elastomeric seals, solders, and brazing

would not withstand the temperatures associated with the HIP

process. Mechanical joining techniques, such as threading,

swaging, and crimping alone, could result in the joint opening

as the canister wall is deformed during the HIP process. Unlike

the joint methods mentioned above, welding results in physical

joining of the lid or plug to the canister itself. Thus, the wall of

the canister would have to be ruptured before leakage would

occur (Berry et al. 1993).

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FIG. 3. PHOTOGRAPH OF A BELLOWS CANISTER WITH THE FILL/VACUUM TUBE PROTRUDING FROM THE LID BEFORE UNDERGOING THE HIP PROCESS (LEFT). THE

BELLOWS CANISTER AFTER UNDERGOING HIP AND SEALING WITH A WELD SEAL IS SHOWN ON THE RIGHT.

FIG. 4. BALL VALVES AT EACH OPENING OF THE INTERMEDIATE CONTAINER ARE USED TO LOAD

AND UNLOAD THE CONTAINER.

CANISTER EVACUATION AND FILL TUBE CRIMPING Prior to undergoing HIP, the canister had to be evacuated

and excess moisture removed. To prepare for this process, a

piece of quartz wool was fed into the top of the HIP canister via

the fill/vacuum tube to prevent calcine from being removed

during movement and evacuation. A guide wire was used to put

into place a plug that was press fitted into the container to

protect the weld between the fill tube and the canister. Prior to

evacuation, an initial crimp was made in the fill/vacuum tube.

Tube Crimping: The fill/vacuum tube was crimped using a

modified emergency gas pipe shut-off tool that compressed the

tubing. A Mustang Tool A 1-in. (2.54-cm) crimping blade

crushed the tubing together using a 10,000-psi hydraulic pump

(Power Team P A9H) that was modified for in-cell use. An

automatic control valve shut the pump down when a preset

hydraulic pressure was reached (Fig. 5).

Tube Evacuation: Following initial crimping, the HIP

canister was evacuated using a combination of heating and

pressure reduction. Via an NW25 KF vacuum fitting welded to

the end of the fill tube, a Varian TriScroll pump was connected

to the tube and used to lower the canister pressure. This pump

is oil-less, requires very little maintenance, and is capable of

achieving pressures as low as 10 millitorr. A condensate trap

and a dust filter installed upstream of the pump prevented the

introduction of particles and moisture to the HIP canister.

Canister Heating: Concurrent with vacuum system

operation, the HIP canister was heated in a modified

Thermcraft ceramic heater capable of temperatures up to

1,100 C. The typical bake-out temperature was 500 C.

Heating and vacuum treatment was continued until pressure

was less than or equal to 100 millitorr. After this was achieved,

the canister was allowed to cool and a second crimp was added

above the first crimp to flatten the tube for cutting and welding.

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FIG. 5. CRIMPING OF THE FILL/VACUUM TUBE FOR THE CALCINE SIMULANT HIP PROCESS (BATEMAN 2011).

Tube Cutting: Following crimping, an oil-less pneumatic

cutter was used to cut the crimped fill tube just above the tube

crimper jaws in order to prepare the container for welding and

HIP. The cutter was a Turbine Tools J30XHD pneumatic cutting

tool and used a 3-in. (7.62-cm) cutting wheel turning at

25,000 rpm. Modifications to the hot cell pneumatic flow

system were required to allow for the use of the cutting tool.

The flow required for this cutter was about 40 to 50 cfm. The

carriage was used to move the cutter forward through the

tubing. A splatter guard held on by magnets kept debris from

scattering.

HIP CANISTER WELDING The HIP canister weld station that was designed for hot cell

use was built around a Jetline Engineering tungsten inert gas

(TIG) welder. The TIG welding process uses a tungsten

electrode connected to a power supply. An arc is drawn through

a highly ionized column of inert gas, such as argon. The high

temperature of the arc melts the metal to form a seal. An arc

length controller (ALC), in combination with the drive

assembly, is used to control the length of the arc. Major

components of the welder are described below (Wahlquist and

Bateman 2002).

Carriage Assembly: A Jetline Engineering carriage and

slide were modified with new limit switches and mounting

hardware. A new bracket that held the arc length drive assembly

and cutter assembly was attached to the carriage. The forward

limit switch was used to position the tungsten inert gas torch for

the welding operation.

Arc Length Drive Assembly: The purpose of the arc length

drive is to keep the welding arc at a constant distance from the

work piece. A Jetline ALC was used to control the weld. The

torch was started in front of the flattened vacuum/fill tube and

then automatically rose when it reached the tube. The ALC

maintained a proper arc distance to produce the desired weld. A

touch circuit was used to position the welder horizontally.

When the ALC was in manual mode and the tungsten touched

the flattened tube on the side, an audible signal and light turned

on. An Intellitig 40 welding controller was also used to set the

parameters for the seal weld. Once the correct welding

parameters were found, they were programmed into the system

to allow repeated performance.

Welder: A Miller XMT 200 constant current/constant

voltage welder was used to perform the TIG welding. The unit

was not modified for use in-cell. A Miller high-frequency arc

starter was also integrated into the system. The Intelletig 40

controller was used to adjust the welding parameters. The

Jetline carriage assembly and ALC were used to control the

physical attributes of the weld process.

Control Console: The sealing equipment was controlled

from a single console. The control console contained all of the

major components: welder, ALC, Intellitig 40 controller, high-

frequency arc booster, heater controller, and various other

items. An “A” scale at the worktable was used for weighing and

accountability purposes.

The weld process was repeated up to three times at

increasing amperage levels to ensure a satisfactory seal. Weld

inspection equipment was then used to visually examine the

weld (Fig. 6). Use of binoculars and a magnifying glass proved

to be the best inspection method. The integrity of the weld seal

was tested using helium. Helium leak checks were performed in

the air cell section of the HFEF hot cells . A helium leak check

verified that the seal weld integrity was acceptable for HIP. In

addition, physical measurements of the HIP container were

taken to verify satisfactory HIP and seal welding.

Many parameters affect the quality of a weld, including arc

length, temperature, and atmosphere surrounding the weld.

Initially, it was thought that the inert atmosphere of the hot cell

was sufficient to use as the cover gas for the TIG weld process.

Test welds produced an unsatisfactory result, as shown in

Fig. 6. A sample of the atmosphere showed that the argon

atmosphere contained 1,000 ppm oxygen, so purified argon gas

was used as the cover gas, producing a satisfactory weld, as

shown in Fig. 7.

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FIG. 6. UNSATISFACTORY WELD.

FIG. 7. SATISFACTORY WELDS.

CONCLUSION The remote HIP container sealing process and equipment

have been developed and refined to a proven reliable method of

sealing containers prior to HIP. A variety of waste forms,

including simulated INL calcine, have been sealed and

undergone HIP in the HFEF hot cell without container breach

or incident. Using the methods established for canister sealing,

HIP treatment is feasible for use in treatability studies using

actual INL calcine.

ACKNOWLEDGEMENT This work was supported by the U.S. Department of

Energy, Office of Environmental Management, under DOE

Idaho Operations Office Contract DE-AC07-05ID14517.

NOMENCLATURE ALC arc length controller

ANSTO Australian Nuclear Science and

Technology Organisation

cfm cubic feet per minute

HFEF Hot Fuel Examination Facility

HIP hot isostatic pressing

INL Idaho National Laboratory

MPa megapascal

ppm parts per million

psi per square inch

rpm revolutions per minute

TIG tungsten inert gas

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