CM Furnaces Helping Solve the Nuclear Fuel Disposal Problem
The U.S. Department of Energy is working to immobilize excess plutonium at its Savannah River Site (SRS) as part of a two track approach for the disposition of weapons usable plutonium. The Plutonium Immobilization process involves the disposition of excess plutonium by incorporation into ceramic pucks. The ceramic pucks, after pressing, are roughly 3-1/2 inches in diameter by 1-3/8 inches thick. After sintering, puck size is reduced to roughly 2-5/8 inches in diameter by 1 inch thick. After pressing, the pucks will be placed on trays. The loaded trays will be transferred into position beneath the furnace, where an integral lift mechanism will lift the loaded trays into the furnace. Due to the radiation levels and contamination associated with the plutonium material, the sintering process will be fully automated and contained within nuclear material gloveboxes. As such, the furnace used must incorporate water and air cooling to minimize heat load to the glovebox.
It was known that retort type furnaces had been used for glovebox applications in the past. This type of furnace utilizes heating elements located outside of a metallic furnace chamber in order to heat product contained within the chamber. This type of furnace technology was seen as advantageous from a maintenance standpoint since the heater elements are located outside of the furnace. However, since the heater elements are located outside of a metallic furnace chamber, retort furnaces are limited to temperatures below. Since process development work has determined that puck sintering temperatures in excess of 1300 degrees C are required for proper form development, this type of furnace was also abandoned as a candidate for sintering the ceramic pucks in the Plutonium Immobilization.
A search of all other commercially available furnace technologies identified suppliers of traditional, bottom loaded furnaces containing a ceramic insulating package and resistance heating elements located within the furnace chamber. These furnaces can also be supplied with an integral, automated lifting mechanism for loading the trays and pucks inside the furnace. Since research determined that bottom loaded furnace technology was the best available to meet the glovebox application and processing needs, a process for removing the pucks from the press, placing the pucks on trays, stacking the trays, and delivering the stack of trays loaded with pucks to the furnace was developed to accommodate the furnace technology selected. Several suppliers were identified that could supply bottom loaded furnaces to meet the application. After bidding, CM Furnace, who had supplied furnaces for glovebox use in the past, was contracted to supply a custom built, automated, bottom loaded furnace to meet the glovebox application need.
The furnace was specified to meet the following criteria
- Furnace chamber size to accommodate a furnace tray stack containing six trays, each loaded with nine pucks, making for a total furnace load of 54 pucks.
- Design to meet the furnace temperature schedule requirements, including a ramp up rate of 4 deg. C/min. to 300 deg. C, a two hour hold at 300 degrees C, a ramp up rate of 5 deg. C/min. to 1350 degrees C, a four hour hold at 1350 degrees C, and a rapid cool down (see Figure 3 for the required furnace temperature schedule).
- Design to accommodate incorporation of a linear transport system that transfers the tray stack and furnace door into position underneath the furnace for loading into the furnace.
- Design to lift the furnace door and tray stack from the linear transport system for final positioning inside the furnace and to deliver the furnace door and tray stack back to the linear transport system after puck sintering is complete.
- Features to facilitate ease of maintenance in a glovebox, such as glovebox replaceable thermocouples and heater elements.
- Trace cooling water and annular space air cooling systems designed to maintain furnace exterior shell temperatures below 50 degrees C in order to minimize heat load to the glovebox and to minimize cool down time for the tray stack.
Figure 3 – Furnace Temperature Schedule
Figure 4 shows an elevation view of the prototype furnace designed and supplied by CM Furnace, with tray stack included. Also shown in Figure 4 is a representation of the Linear Transport System (LTS), including Linear Synchronous Motor (LSM) and transport cart, needed to deliver the tray stack into position beneath the furnace. Design of the furnace to accommodate the LTS and tray stack resulted in elevating the furnace approximately 5 feet from floor level. Also shown in Figure 4 is the furnace door on which the tray stack resides during furnace lift operation, furnace sintering, as well as during transport to the puck loading and unloading stations. In other words, the stack of trays, both empty and full of pucks, always travels on top of the furnace door during loading, unloading, and puck sintering operations. Design of the furnace support structure and lift mechanism leaves roughly a 2’ wide by 3’ high space beneath the furnace to allow for transport of the tray stack and furnace door.
Figure 4 – Prototype Furnace
In order to accommodate the tray stack and furnace door, the interior furnace chamber is roughly 26 inches square. Based on a three layer insulation package that is 5.5″ thick in order to ensure temperature uniformity in the furnace and minimize power usage, along with a furnace annular space that is roughly 6″ wide on all four sides and on top of the furnace, furnace exterior dimensions are roughly 5’ square. Also shown in Figure 4 is the 3/8″ diameter trace cooling water tubing on the furnace chamber shell and the 2 inch outlet ports for the annular space air cooling system. These features are incorporated in order to minimize heat load to the glovebox and to aid in reducing furnace cool down time. Additional detail concerning furnace heat load and balance can be found in section 5.0, Furnace Support and Cooling Systems.
Automated Furnace Lift
The LTS delivers the tray stack and furnace door into position beneath the furnace. Once in position, the furnace lift engages the bottom of the furnace door and elevates the furnace door and tray stack roughly 3 feet into position inside the furnace for sintering. Once sintering is complete, the furnace lift mechanism lowers the furnace door and tray stack (with sintered pucks) onto the linear transport cart for transport to the puck unloading station. After delivering the furnace door and tray stack to the LTS, the furnace lift mechanism remains in the fully lowered position and is ready to receive another furnace door and tray stack for loading into the furnace and sintering. A picture of the furnace lift mechanism, with furnace door and tray stack, is provided as Figure 5.
Furnace Lift Assembly
The lift mechanism consists of two self lubricated ACME screws, four precision guide rails, and four bearings to support each end of the ACME lift screws. The ACME lift screws are chain driven. The chain drive consists of a 1/3 hp AC motor and gearhead, three drive sprockets (one driven by the AC motor and two that drive the lift screws), and three idler sprockets. Two of the idler sprockets have spring loaded tensioners and one has a position adjustment screw in order to maintain proper chain tension. Selection of ACME screws over ball screws was made in order to prevent back driving of the lift screws and furnace door with tray stack in the event of drive chain breakage. All drive components are located compactly in a metal enclosure beneath the furnace and in a manner to allow for uninhibited tray stack/furnace door motion beneath the furnace. The furnace lift hardware consists of four Frelon lined linear bearings, two ACME nuts to engage the lift screws, structural support members, and two locator pins to engage locating holes in the bottom of the furnace door to ensure proper positioning of the furnace door and tray stack inside the furnace.
The lift system is powered by an Allen Bradley variable frequency drive (VFD). Use of the VFD allows for lift speed adjustment and optimization. It also allows for communication with the furnace and supervisory control system in order to initiate door lifting or lowering upon command with the furnace stack in position for lifting or lowering after sintering. Also provided with the furnace lift system are four limit switches that are engaged at various times during lift operation. Two of these switches are used to detect when the furnace door is in the fully up or fully down position in order to stop lift function. The other two limit switches are used to detect when the furnace lift mechanism is near the fully up or fully down position so that deceleration of the lift mechanism can be initiated by the VFD. This is needed in order to decrease momentum and the force needed to stop a moving furnace door and tray stack, which weighs roughly 500 lbs. Currently, operation of the furnace lift mechanism is initiated by manual push buttons provided with the furnace control system. Future integration efforts, both at the CPTF and at the Plutonium Immobilization Plant, will incorporate automatic lift operation once the tray stack has been presented into position by the LTS and when puck sintering is complete.
Furnace Support and Cooling Systems
The process development work involves pressing and sintering ceramic pucks using surrogate materials as substitutes for the nuclear materials identified for immobilization. Also added to the materials prior to puck formation is a binder that provides strength to the pucks after pressing and prior to sintering. This binder, which consists mostly of organic hydrocarbons and water, is emitted from the pucks during heat up to 700 degrees C (known as binder burnout, most of the binder is emitted from the pucks at 300 degrees C). As such, support systems are provided with the furnace to treat the off gases emitted from the pucks during binder burnout as well as to supply purge air to sweep the gases emitted from the pucks out of the furnace during binder burnout. In addition, the purge air supply system also supplies cooling air directly to the furnace chamber during cool down. Annular space air and trace cooling water systems are also provided to minimize heat emitted from the furnace and to further reduce furnace cycle time. In order to account for and store the operating and process data generated during the furnace development runs, a computer data acquisition system (DAS) is employed so that real time processing parameters can be recorded for analysis. The DAS is also configured to calculate heat balance data so that heat load and balance issues can be observed in real time and stored during the furnace development runs.
The exhaust gas system consists of an exhaust gas blower used to pull air and gases from the furnace chamber during operation. The air and gases, during heat up, are pulled through an afterburner designed to convert the hydrocarbons emitted from the pucks during sintering into carbon dioxide and water. The afterburner uses a catalytic bed (platinum coated alumina beads) maintained at a specific temperature (300-500 C) in order to facilitate the required reaction. In addition, purge air is supplied to the furnace during heat up in order to dilute and sweep the gases and organics emitted from the pucks during sintering. The purge air supply system also provides cooling air directly to the furnace cavity during furnace cool down. Furnace cooling, as well as purging during heat up, uses both the exhaust gas blower to pull and purge air supply system to push the air and gases through the furnace in order to maintain a near atmospheric pressure in the furnace.
The trace cooling water system consists of 3/8″ diameter tubing formed in a coiled configuration and attached to all four sides of the furnace chamber shell wall. Cooling water is supplied to the coiling coils at a rate of 0.5-2 gallons per minute by piping/tubing and a closed loop, chilled water pumping system manufactured by Cornelius/Remcor, located in Glendale Heights, ILL. Installed as part of the supply system is a magnetic flow meter used to measure and transmit cooling water flow rate to the DAS. Resistant temperature detectors (RTDs) are also installed in the inlet and outlet piping in order to measure temperature gain and determine the amount of heat (BTUs/hr) removed by the cooling water system during operation. A picture of the trace cooling water tubing installed on the furnace inner shell is provided as Figure 6.
Furnace Trace Cooling Water Tubing
The annular space, cooling air system consists of a blower used to pull 150-200 standard cubic feet per minute of air through the furnace annular space to facilitate furnace cooling during cool down and to minimize furnace outer shell temperature and heat load to the glovebox. Eight, 1″ diameter openings are provided on the lower exterior of the furnace outer shell for supplying air to the furnace annular space. Also included are additional openings on the furnace lift drive train enclosure in order to draw air flow through the furnace lift drive train for cooling during furnace operation. Since the furnace interior chamber is not enclosed by a metal plate on the roof in order to accommodate heater element replacement and connection, the annular space cooling air pulled through the furnace also serves to cool the heating element terminal straps and connections on top of the furnace in order to extend heater element and connector strap life. In fact, and as preliminary experimentation has indicated, since the furnace chamber roof is not sealed by a metal plate with trace cooling water, a majority of the heat removed by the annular space cooling system is from the roof of the furnace. A picture of the top of the furnace showing the heating elements and connecting straps is provided as Figure 7.
Figure 7 – Furnace Heating Element Connections
Glovebox Considerations
Since maintenance aspects are always important considerations for equipment located in gloveboxes, the furnaces specified, designed, and fabricated for use in the Plutonium Immobilization Plant must possess aspects to allow for ease of maintenance. Especially in regard to those maintenance activities that will be routine for the specified equipment. In fact, equipment design that does not allow for effective performance of routine maintenance activities in gloveboxes can jeopardize plant throughput and significantly increase operating costs. The magnitude of the affect can result in a plant that is no longer functional or viable. This being said, the Pu Immobilization prototype furnace incorporates design aspects to facilitate glovebox maintenance. In particular, the prototype furnace includes design features for maintenance of the thermocouples and heater elements which are expected to require routine maintenance during furnace life.
Provided with the furnace are side access panels (three per side) that are attached to the furnace by toggle clamps. These access panels were designed to be removable using gloves in a glovebox to allow for access to the control and overtemperature protection thermocouples. The thermocouples are sheathed by alumina tubes and are fixtured in position by compression fittings using TeflonR ferrules. Thermcouples were provided six per side on the two sides of the furnace that would be closest to the glovebox walls. Ten of the twelve thermocouples are located in a retracted position and the other two are fully inserted in order to transmit the control and overtemperature thermocouple signals to the furnace control system. In order to change the control and overtemperature thermocouples, all that is needed is to insert one of the spare, retracted thermocouples into position inside the furnace and to reconnect the thermocouple extension wire (using standard thermocouple connectors) to the newly inserted thermocouple. In addition, if complete thermocouple replacement is required, the entire thermocouple can be removed from its position and replaced, as long as proper distance from the inside of the glovebox wall to the thermocouple compression fitting is achieved. Special hand tools can also be used to aid in replacement, as well as when changing the control and overtemperature thermocouple positions. Although complete thermocouple replacement is possible, it does require increased dexterity and effort as compared to inserting one of the retracted, spare thermocouples into position and reconnecting the thermocouple extension wire.
Heater element replacement must be accomplished through the top of the furnace as the heater elements in the furnace must hang from the roof during heating. This is needed since orientation of the heating elements (which are fabricated from molybdenum disilicide) must be vertical (and hang from the furnace roof) as the heating elements lose structural strength and form when heating at the required sintering temperature (1350 degrees C). This being the case, and since heater elements are required around the perimeter of all four sides of the furnace for even heating, reaching through the side of the glovebox in order to disconnect and remove heater elements is considered not possible by human means. Therefore, replacement of the heater elements must be accomplished through gloveports in the roof of the glovebox. As such, access panels held in place by toggle clamps are provided on top of the furnace to allow for access to the heater elements and connections from above the furnace. In addition, spatial considerations are needed to ensure that the heater elements can be completely removed from the furnace. Based on current configuration of the prototype Plutonium Immobilization furnace, is it believed that this will be possible by humans means with the aid of specially designed tools. In particular, disconnection and connection of the heating element straps during heating element replacement will be a task requiring dexterity and specially designed tools in order to perform the activity successfully.
Other than chain tension adjustment via an opening in the furnace lift drive train enclosure and lubrication of some of the mechanical components, furnace lift mechanism maintenance will require breaking glovebox containment and constructing plastic contamination containment huts around the breached glovebox. In addition, significant maintenance on the furnace, such as the replacement of failed insulation inside the furnace, will require complete replacement of the furnace. The Plutonium Immobilization Plant will be designed to allow for complete furnace replacement so that facility throughput requirements can be maintained for the life of the plant.
This article reprinted from http://sti.srs.gov
