William F. Hamel, Jr.
U. S. Department of Energy
West Valley Demonstration Project

Paul J. Valenti and Daniel I. Elliott
West Valley Nuclear Services Company, Inc.

The Vitrification Facility (VF) at the West Valley Demonstration Project (WVDP) was designed and constructed to convert high-level radioactive waste (HLW) stored in underground tanks into a stable waste form (borosilicate glass) suitable for disposal in a federal repository. After completion of nonradioactive testing with simulated waste, radioactive waste processing commenced in July 1996. The HLW, consisting of a blend of washed plutonium-uranium extraction (PUREX) sludge, cesium-loaded zeolite, and neutralized thorium extraction (THOREX) waste, is batch-transferred from the Waste Tank Farm (WTF) to the VF. In the VF the waste is mixed with glass-forming chemicals and fed to a joule-heated melter at 1,150^o C. The glass is then poured into stainless steel canisters. The canisters are sealed, decontaminated, and stored.

During the first year of radioactive processing, glass production rates in excess of 35 kilograms per hour have typically been achieved. As of June 1997, more than 320,000 liters of HLW have been processed. This has resulted in the vitrification of over 220,000 kilograms of glass that contains more than five million curies of cesium-137/strontium-90. Glass production system availability has surpassed 75%, due in part to design modifications and changes in operational and maintenance strategies developed from operational experience.

This paper will discuss the vitrification operational experiences gained during the first full year of radioactive waste processing and includes lessons learned in maintaining continuous operation of a remotely operated HLW vitrification melter.

At the inception of the West Valley Demonstration Project (WVDP), two types of high-level radioactive waste (HLW) were stored in underground tanks. As a result of commercial reprocessing of spent reactor fuel, roughly 2,200 cubic meters of neutralized liquid plutonium-uranium extraction (PUREX) waste was produced. Another 40 cubic meters of acidic thorium extraction (THOREX) waste was stored separately. The PUREX waste was washed to minimize the sodium and sulfate inventories in the waste. The supernatant and wash solutions were pretreated using zeolite. This process removed more than 99.9% of the cesium-137, allowing the remaining decontaminated salt phase to be processed as low-level waste. Prior to the start of radioactive operations, the THOREX and zeolite wastes were combined with the washed PUREX sludge, resulting in approximately 1,000 cubic meters of HLW to be vitrified.

In the vitrification process, the pretreated HLW is combined with glass-forming chemicals such as silica, sodium tetraborate, potassium hydroxide, lithium hydroxide, aluminum hydroxide, and titanium dioxide. Sugar (sucrose) is added to control the oxidizing conditions in the melter. The resulting slurry is pumped into a ceramic melter where electric current is passed through the molten glass pool to maintain the temperature needed (1,150^o C) to vitrify the waste and glass-formers into a borosilicate glass product. The molten glass is poured into canisters which are then cooled, sealed, decontaminated, and stored on site in preparation for shipment to a federal repository or other interim storage location (see Figure 1).

Prior to transferring the HLW from its underground storage tank to the Vitrification Facility (VF), the waste is mobilized within the tank using up to five 150-hp pumps. By suspending and mixing the waste solids, a more consistent waste composition is achieved. The mobilized HLW slurry is then batch-transferred to the concentrator feed makeup tank (CFMT) where the slurry is added to the existing CFMT tank heel and recycled off-gas scrubber solution. The amount of waste transferred for each batch is dependent on the available volume in the CFMT (23,000 liter capacity) and the expected volume of the subsequent glass-former addition. Typical waste transfer volumes are on the order of 12,000 liters.

Following the waste transfer, slurry samples are drawn using remotely operated inline sampling equipment. Based on the chemical analysis of the samples, the total amount of glass-forming chemicals required to produce the WVDP target glass composition is calculated. Typically, more than 20 elements are analyzed, although only 15 are required for the composition control of the resultant glass. These 15 elements (Al, B, Ca, Fe, K, Li, Mg, Mn, Na, P, Si, Th, Ti, U, and Zr) normally account for more than 98% of the glass composition.

The Cold Chemical System, a nonradioactive facility, consists of 10 tanks and material handling equipment that are used to prepare each batch of glass-forming chemicals. The glass-former slurry is made up of up to 14 separate chemical constituents, added in a specific sequence to minimize preparation time.

Once the waste sampling is complete, the contents of the CFMT are heated to concentrate the waste. The dilute waste is concentrated from approximately 12 weight percent solids to greater than 50 weight percent solids through the use of an external steam jacket on the CFMT to evaporate water from the waste. The glass-former slurry from the Cold Chemical System is then added to the waste. The waste-feed slurry is again sampled to verify that the feed batch composition is acceptable for glass-making. If the sample analysis shows an unacceptable composition, a chemical shim is formulated, prepared, and added to the CFMT.

Acceptable feed slurry is transferred to the melter feed hold tank (MFHT) using a steam jet system. The 22,000 liter MFHT allows for continuous feeding of the melter while simultaneously preparing the subsequent feed batch. Feed to the melter is provided using a submerged air displacement slurry (ADS) feed pump within the tank. The pump chamber (roughly 1.4 liters) is alternately filled and displaced with air to transfer the feed to the melter in pulses. Average slurry flowrates as high as 150 liters per hour can be attained, while the typical feed flowrate is 70 to 80 liters per hour. The air pressure used to displace the feed in the pump chamber is adjustable and is set to provide slurry velocity that is high enough to prevent settling and plugging, while still being low enough to avoid atomizing the slurry as it enters the melter plenum. During pump operation, this air pressure is graphically displayed on the distributed control system. The resulting graphic pressure pulses have proven to be an effective diagnostic tool for evaluating pump performance.

The slurry-fed ceramic melter (SFCM) is the key component in the vitrification process (see Figure 2). The function of the SFCM is to dry and melt the slurry feed, converting it to glass. The ceramic-lined melter has three Inconel 690 electrodes to pass electric current. The electrodes are in continual contact with the glass pool. The resistivity of the glass pool generates the heat necessary to maintain an operating temperature range of 1,100^o to 1,200^o C -- an effect referred to as joule-heating. The nominal volume of the glass pool in the melter is 680 liters.

As melter feed is added to the melter, water in the feed is evaporated and carried off with the melter off-gases. The remaining solids accumulate on the surface of the glass pool to form a crust of dried and calcined wastes, referred to as the cold cap. The size of the cold cap, ideally 70% to 90% coverage of the glass pool, is reflected in the temperature of the air space above the glass pool (plenum) and is maintained by controlling the rate of melter feed. Feed rate is periodically manually adjusted to maintain melter plenum temperature between 400^o and 600^o C.

The molten glass exits the melt cavity via a discharge passage through the refractory, originating near the floor of the melter. The molten glass must rise up through the passage to a trough located in a discharge chamber. The glass flows off the trough and falls through a heated discharge chamber into a stainless steel canister below the melter. Molten glass is batch-poured every four to six hours to maintain the melter glass level within a two-centimeter band. Air is bubbled into the overflow passage via a platinum tube in order to lift the glass up to the trough. During an air lift, the pour stream is visually monitored by closed circuit TV (CCTV). Glass level within the canister is visually monitored with the infrared level detection system (ILDS). A shielded infrared camera scans the canister and detects an increased surface temperature as the thermally hot glass accumulates within the canister. Glass production rate typically exceeds 35 kilograms per hour, resulting in a normal canister fill time of roughly 58 hours.

The molten glass is poured from the SFCM into HLW canisters made of 304L stainless steel. The canisters have a minimum wall thickness of 0.34 centimeters, an outside diameter of 0.61 meters, an overall height of 3.0 meters, and an opening diameter of 0.42 meters. They are designed to contain approximately 1,900 kilograms of glass at an 85% fill level.

A four-position rotary turntable positions the canisters below the melter discharge chamber and provides for remote canister changeout. Once filled and cooled, the canisters are removed and transferred to a welding station using an overhead crane. Prior to welding a lid on the canister, the level of solid glass within the canister is measured and recorded and glass shards are collected from the canister for analysis and archiving. Stainless steel lids are remotely welded onto the canisters using a pulsed gas tungsten arc welding process. The quality of the canister lid weld is assured by process control of the weld parameters. A visual inspection of the completed weld is performed and a computer printout of the weld parameters is reviewed to ensure that critical process parameters are maintained within acceptable ranges.

Once welded, the canister is decontaminated through electrochemical dissolution of a thin layer of the surface metal. The canister is placed in a titanium tank and soaked in a 65^o C bath of cerium nitrate and nitric acid, then rinsed with nitric acid and water. The decontamination solutions are recycled back into the vitrification process. Finally, the canisters are moved from the vitrification cell into another shielded cell referred to as the High-Level Waste Interim Storage (HLWIS) Facility.

The off-gas from the SFCM is composed primarily of steam, air, decomposition gases, and volatile species. Treatment consists of quenching/scrubbing, filtering, and destruction of nitrogen oxide (NO_x ) gases prior to atmospheric discharge (see Figure 3).

Melter off-gases exiting the melter first pass through a film cooler located at the inlet to the off-gas piping. The film cooler, a louvered insert extending into the melter, supplies air at roughly 4 cubic meters per minute along the inside surface of the off-gas piping. This creates a relatively cool boundary layer between the 500^o C (nominal) off-gas stream and the inside surface of the pipe, minimizing sticky deposits especially around the leading edge of the insert.

The off-gases are directed through a submerged packed bed called the submerged bed scrubber (SBS) where they are quenched to less than 45^o C. The particulates that are removed from the off-gas stream as it percolates up through a bed of ceramic spheres are eventually recycled back to the CFMT and included in a subsequent feed batch. The off-gas next encounters one of two redundant high-efficiency mist eliminators that remove entrained moisture prior to preheating and high-efficiency particulate air (HEPA) filtration within the vitrification cell. After exiting the vitrification cell, the off-gas stream passes through underground piping to another building for final filtration and NO_x treatment.

The ex-cell, off-gas treatment processes include moisture removal, reheating, HEPA filtration, and catalytic NO_x reduction. After passing through an entrainment separator, the off-gas flows through one of two redundant electrical heaters, one of two parallel HEPA filter trains, then on to one of three process off-gas blowers. From the blower, the off-gases are heated again to approximately 320^o C prior to passing through a catalytic converter where, in the presence of ammonia, the NO_x gases are reduced to nitrogen and water vapor which are finally exhausted to the atmosphere.

Initial waste transfers from the Waste Tank Farm (WTF) to the VF had a lower curie content than anticipated (i.e., less than 100,000 curies cesium-137/strontium-90 per batch). An additional sludge mobilization pump was installed into an existing HLW tank riser in a low-flow area within the tank where solids were apparently mounding. Careful management of the HLW tank level also increased batch curie inventory. By periodically decanting the accumulated liquid, the solids concentration in the transfers to the VF was optimized while minimizing the transfer pump plugging problems associated with pumping slurries. Recent waste transfer batches have typically exceeded 200,000 curies.

Batch makeup cycle time has been reduced since the start of radioactive operations. Due to the consistency of the waste, slurry acceptance engineers now generate chemical premix recipes well in advance of the final (post waste analysis) recipe. Independent verification of recipe calculations have virtually eliminated the need for corrective (and time consuming) chemical shims. The turnaround times for chemical analyses in support of feed batch preparation have experienced dramatic improvement relative to initial estimates. Early analytical cell mockup training and the experience gained by the lab technicians has reduced the time required for the various feed batch analyses by more than 50% thereby saving 66 hours on the total batch cycle time. This is significant in that if this were not the case, batch preparation would be the limiting factor in the vitrification process. As a result, the melter can be fed continuously from the MFHT.

The vitrification cell in-line slurry sampling units were modified based upon input from plant operators to improve the ease of remote operation. Typical sampling time was reduced from eight hours to one hour.

Processing experience gained during the five years of initial nonradioactive melter operational testing, combined with recipe experimentation in a scale vitrification (mini-melter) system, has been successfully applied to radioactive operation. During these tests, glass chemistry (largely oxidation-reduction) studies, and methods of chemistry control were developed and have yielded excellent correlation with radioactive operations.

Shortly following the start of radioactive operation, a significant restriction in the melter off-gas piping (prior to the SBS) developed. The restriction was suspected to be caused by the collection of dried melter feed adhering to the inside surface of the centimeter diameter piping. Remote radiation surveys showed the buildup to be occurring at an acute (45 degree) elbow just a few feet from the melter. Design modifications incorporated to mitigate the blockage included the following:

• A water flush line was installed into the melter air injection line. Water is periodically introduced into the off-gas stream to steam clean the piping.
• A flow-restricting orifice was installed in the air line supplying motive force air for the melter ADS pump. Reduction in the airflow rate reduced the atomization of the melter feed as it was pulsed into the melter plenum. This made it less likely for the feed material to be swept into the off-gas piping.

The first set of silicon-carbide discharge heater elements exceeded their expected life by approximately three months (12 months vs 9 months). Changes in operational strategies for the heaters and providing automatic backup power minimizes heater cycling and prolongs heater life. This modification is significant in that remote heater replacement requires idling the melter for two to three weeks.

Melter pressure control has been a challenge from the start. Irregular off-gas flow characteristics attributed to the SBS, combined with the pulsing action of the ADS feed pump, create a dynamic environment for pressure control. The control system was modified by installing a high-speed data acquisition system with a quick-acting control valve. An adaptive gain feature was also incorporated into the control scheme. These modifications made melter pressure control more manageable, however melter pressure still fluctuates. Melter pressure, controlled to maintain -12.7 centimeters H₂O, varies as much as 7.6 centimeters H₂O from the nominal setpoint. Consequently, the melter glass level is maintained at least 2.5 centimeters below the overflow level to preclude excessive dripping of glass into a canister between glass pours.

Maintaining a steady pour stream and clear discharge port is essential to continued glass production. During startup testing with nonradioactive glass, the WVDP melter experienced chronic blockages in the glass pour stream. The blockages were the result of a failed barrier between the melt chamber and the discharge chamber, allowing glass migration and subsequent buildup at the bottom of the discharge chamber. The melter was repaired and placed back into service. Just prior to radioactive operations, excessive production and an accumulation of very thin glass fibers (angel hair) led to blockages in the glass pour stream. Prior to radioactive operations, the glass buildup was removed and a flow-reducing orifice was installed to reduce the airflow from the discharge chamber to the main melt chamber. Limiting the airflow through the discharge chamber seemed to successfully reduce angel hair production to an acceptable level. Angel hair formation, however, has not been nor will it ever be completely eliminated. Periodic accumulation of angel hair in the melter discharge port has been successfully overcome by rebalancing the discharge heater loads to maximize the temperature around the discharge port and melt the accumulated glass from the chamber.

The WVDP HLW canisters are required to be filled at least 80% full. Originally, canisters were typically filled to the 85% level. Through the field experience gained with the video imaging capabilities of the ILDS system, canisters are now normally filled approximately 90% full. While the ILDS- based levels are backed up by mass balance calculations, the accuracy provided by the ILDS system for determining the canister fill level has been outstanding. The final (reported) canister fill verification is performed by direct measurement of the glass level (using a measuring stick device) at the vitrification weld station. The variation between the canister fill level provided by direct measurement and that provided by the ILDS is usually within 1%.

A significant lesson learned is that a large canister opening is vital to the success of the WVDP vitrification process. The glass pourstream must fall a distance of over 1.5 meters from the end of the trough before entering the canister. Lateral movement in the pourstream can (and does) occur due to melter pressure fluctuations, variations in the airlift flowrate, or effects of air in-leakage to the discharge chamber. The large canister opening (0.42 meters in diameter) has proven to be an effective design feature to accommodate pourstream deflections.

Slurry transport within the vitrification cell is accomplished by either steam jet transfer or ADS pumps. Steam jet operation has been as expected with no significant problems associated with these routine transfers. The throughput capability of the WVDP melter requires a low average slurry feed flowrate. The ADS pump has provided a low average feed rate while generating high slurry velocity and high back pressure. These characteristics have resulted in essentially no plugging problems normally encountered with slurry transport.

Vessel level, density, and pressure indications within the vitrification cell are provided by bubbler assemblies. A control scheme that includes automatic periodic air blowdowns has maintained the bubblers in the CFMT and MFHT relatively free from plugging. Recent modifications to the automatic blowdown cycles have included a small (~100 milliliters) injection of demineralized water into the bubbler probes just prior to the air blowdown and are proving to be effective in maintaining clear bubbler lines.

Canister closure (lid welding) using a pulsed gas tungsten arc process has become routine. There have been only a few canister lid welds that were just outside the normal range for weld parameters. The welds were corrected by simply reperforming the automatic weld. The ease of reworking a canister lid by rewelding has proven to be one of the major advantages of this system.

Canister decontamination operations have also become routine. Early difficulties with tank temperature control and cooling coil pressurization were corrected with minor piping modifications and control loop tuning adjustments. The process generates significant volumes of neutralized decontamination and rinse solutions. Through careful scheduling of vitrification evolutions, decontamination and rinse solutions have been effectively managed by reintroducing them into the melter feed batch makeup process.

Most of the down-time associated with the WVDP vitrification campaign can be attributed to failure and repair of remotely operated equipment. Repair and replacement of the melter feed jumper resulted in nearly a two month outage. However, considerable preplanned vitrification maintenance was rescheduled and accomplished during the unscheduled downtime. In addition, the WVDP was successful in their first attempt to perform hands-on maintenance to a vitrification cell remote component, following remote decontamination in the vitrification cell. The melter feed jumper was decontaminated to allow hands-on repair of its remotely operated piping connectors.

One method used to help manage the vitrification cell water inventory is to raise the SBS operating temperature to maximize the amount of water vapor leaving in the off-gas stream. This moisture, combined with demineralized water which is injected into the off-gas stream to cool the in-service blower, led to some early difficulties in accurately monitoring NO_x and NH₃ concentrations in the off-gas. Air dryer and chiller units were installed in the sampling lines to condense and extract the moisture prior to reaching the analyzers, increasing the system's NO_x abatement efficiency.

Abatement efficiency was also enhanced by initially saturating the selective catalytic reduction bed with ammonia reactant during melter feed initiation. The ammonia addition rate to the bed is then gradually reduced as the melter reaches steady-state feeding.

On 19 June 1996 the Secretary of the Department of Energy approved the initiation of radioactive operations at the WVDP. On 5 July 1996 the first HLW canister was filled. As of 15 August 1997, the WVDP Vitrification Facility has operated at an overall availability (actual time feeding slurry to the SFCM) of just over 77% and has produced 130 HLW canisters representing the processing of over 5.8 million curies of cesium-137 and strontium-90. This accomplishment represents just over 60% of the total activity to be processed. The average glass production rate is in excess of 35 kilograms per hour. The average radiation measurement on contact with a filled HLW canister is 2,700 Rads.

The vitrification process systems have been performing as designed since initiating radioactive operations. Exhaustive functional and integrated testing using waste simulant combined with adherence to design requirements has limited the unscheduled downtime within the WVDP vitrification process. Strong problem solving skills and efficient use of unscheduled process downtime for scheduled maintenance has contributed greatly to the high system availability at the WVDP.

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