S. A. Erickson
Westinghouse Savannah River Company
Aiken, SC 29808

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The Plutonium Immobilization Facility will encapsulate plutonium in ceramic pucks and seal the pucks inside welded cans. Remote equipment will place these cans in magazines and the magazines in a Defense Waste Processing Facility (DWPF) canister. The DWPF will fill the canister with high level waste glass for permanent storage. Ceramic feed batching (CFB) is one of the first process steps involved with first stage plutonium immobilization. The CFB step will blend plutonium oxide powder before it is combined with other materials to make pucks. This report discusses the Plutonium Immobilization CFB process preliminary concept (including a process block diagram), batch splitting component test results, CFB development areas, and FY 1999 and 2000 CFB program milestones.

The ceramic pucks produced in the Plutonium Immobilization Facility are made of three primary constituents: plutonium oxide powder with impurities, uranium oxide powder, and ceramic precursors. Criticality safety requirements limit the amount and configuration of plutonium oxide powder in feed batching. Ceramic feed batching is needed to thoroughly blend the plutonium oxide powder to achieve isotope and impurity "leveling" to ensure product consistency and the integrity of the ceramic form. The Plutonium Immobilization Facility design will minimize operator exposure and prevent the spread of contamination. To achieve these goals in feed batching the process must be "dustless" and remotely operated.

The following are the CFB milestones for FY 1999.

• 1/31/99 – Start commercial splitter testing

• 3/28/99 – Complete blending system concept

• 5/31/99^* – Complete commercial splitter evaluation

• 8/31/99 – Complete component testing

• 9/30/99^* – Complete component test report

^* D & T Milestone

Feed material for the Plutonium Immobilization process will consist of oxide from the conversion process, as well as oxide produced from plutonium pits. The plutonium content and composition of individual feed cans will vary significantly. A preliminary analysis by Lawrence Livermore National Laboratory (LLNL) has determined that up to 20% of the oxide blends will require recycling to produce an acceptable feed for the first stage immobilization process.

The Plutonium Immobilization Facility must produce approximately 530 pucks per day (including allowance for two rejects) to meet production requirements. The feed batching production requirements are dependent upon the puck production requirements and blending feed specifications. According to a Plutonium Immobilization materials report³, the nominal plutonium concentration goal is 10.39 % weight of the ceramic puck. An estimate of the mass of one ceramic puck is 455.7 grams. Based on these initial numbers, recycling needs, and puck production requirements, a simple calculation can be performed to estimate the amount of plutonium powder to be blended per day:

Normal feed batching system operations will be performed remotely, but maintenance and repairs will be performed manually. The feed batching system shall be developed to minimize complexity and amount of equipment in containment, minimize equipment maintenance, ensure serviceability and replacement of equipment, and minimize radioactive material in proximity to personnel during maintenance and repair. Feed batching transfers shall be designed to minimize contamination of the glovebox interior to minimize radiation exposure and material holdup, and reduce waste volume. Standard oxide receipt cans shall be configured such that they are critically safe at all times to eliminate the risk of a nuclear incident from equipment failure or transfer error.

• One batch will consist of 80 pucks = 4 kg Pu
• Six to seven batches will be processed per day
• One blend cycle will consist of 40-50 kg Pu, 40-50 kg U, and 20-25 kg impurities
• Each standard oxide feed can will contain ~ 1 - 5 kg Pu
• One blend cycle will be processed approximately every two days
• Particle size distribution⁴: 20% £ 5 m m, 60% £ 40 m m (± 25 m m), 20% ³ 100 m m
• Acceptable error in splitting is 5 % by weight
• Pu powder will be handled in standard oxide cans (5 kg capacity, 2.5 l volume)
• Feed and receipt standard oxide cans will be weighed for accountability and material holdup measurement
• 1 standard oxide can per batch will be removed after splitting to be blended and sampled
• Slow blend cycle time will allow for adequate dust settling between process steps

Attachment 1 is the feed batching process block diagram, which shows the main feed batching process steps. The process block diagram is based on the feed batching steps contained in the Plutonium Immobilization Plant, First Stage Immobilization, Process Flow Drawing (P-4, Rev. A, 4/6/98).

The following section describes the current feed batching process concept.

Attachment 2 illustrates a side view of the feed batching process concept. Standard oxide cans filled with plutonium oxide powder are removed from a storage vault and enter the process glovebox. Once inside the glovebox, a transport cart or conveyance system carries the standard oxide can to a pick and place device. The pick and place device removes the lid from the standard oxide can. The standard oxide can is then coupled to an inverting device. A valve is installed on the inverting arm for dust and powder release control. The standard oxide can is raised to the necessary height above the batch splitter and inverted. Once inverted, the standard oxide can is coupled to the batch splitter via an additional valve. The valves are opened and the contents of the standard oxide can are released into the batch splitter. The batch splitter divides the powder and distributes it into multiple divisions. (Note: To maintain criticality safety, neutron shielding will be required between splitter divisions⁵).

After a sufficient amount of powder has accumulated in the divisions, due to multiple standard oxide cans being processed, splitter operation is stopped. An empty standard oxide can is positioned via cart or conveyance system and coupled to the batch splitter. A valve is opened on the dispensing portion of the batch splitter, and the contents of one division are released into the empty standard oxide can. The standard oxide can is decoupled from the batch splitter, weighed and moved to a lid installation station. A pick and place device installs a lid on the can. The filled can then leaves the feed batching process area and is transported to a storage vault. This process is repeated until all divisions have been emptied.

For accountability measurement, one of the standard oxide cans will require mixing, and subsequently isotope and impurity sampling. After splitting has been completed, one standard oxide can is coupled to a second inverting device. A valve is installed on the inverting arm for dust and powder release control. The standard oxide can is raised to the necessary height above a V-blender and inverted. Once inverted, the standard oxide can is coupled to the V-blender via an additional valve. The valves are opened and the contents of the standard oxide can are released into the V-blender. The V-blender is then decoupled from the standard oxide can. Once decoupled the V-blender rotates causing the powder to mix thoroughly and uniformly. An empty can is positioned via cart or conveyance system under the V-blender. The can is coupled to the V-blender and is filled with the mixed contents of the V-blender. The can is decoupled from the V-blender and moved to a lid installation station. A pick and place device installs a lid on the standard oxide can. The filled can then leaves the feed batching process area to be weighed and sampled.

After issuing reports concerned with the CFB system concept¹ and splitter equipment testing², feed batching equipment development efforts were focused on modifying the rotary tube divider shown in Figure 1. In review, the unit shown in Figure 1 divides one sample (1/11^th) of the powder and collects the remaining powder in bulk. The sampling capability of the rotary tube divider was acceptable², but gave little indication as to how well the rotating tube concept could split powder into multiple divisions. Consequently, modifications to the unit shown in Figure 1 were proposed to include a receiver with 12 collection ports. These modifications are discussed below.

The equipment shown in Figure 1 was rented and could not be altered. In order to minimize fabrication efforts and costs, the modified receiver was designed to mate to the upper dispensing portion of the rotary tube divider shown in Figure 1. Consequently, the geometry of the modified receiver, i.e. diameter and wall angle, matched the upper portion of the rotary tube divider. It was desired to place the divider ports in the new receiver directly under the path of the rotating tube, as is the laboratory sample port in Figure 1. However, in order to accommodate the correct wall angle, a 1" diameter port size, and a sharp knife-edge between each consecutive port, it was necessary to locate the ports near the bottom of the receiver. A conceptual depiction of the modified receiver mated to the rotary tube divider is shown in Figure 2. In theory, the rotating tube would distribute the powder centrifugally along the smooth aluminum surface of the cone-shaped receiver wall. The powder would then flow down the wall surface and its momentum would allow it to slide into the receiver ports.

A picture of the modified receiver fabricated by SRTC is shown in Figure 3. This receiver was mated to the dispensing half of the tube divider and tested using surrogate materials. Initial testing provided immediate evidence that powder flow problems existed in the modified receiver. Figure 4 shows the receiver core after a test has been completed. As can be seen in the figure, powder has accumulated and has begun to bridge at each port opening. This accumulation problem is most likely attributed to a small port diameter size and a rough surface finish on the receiver interior due to the lathe machining operation. It is documented that lathe machine operations produce surfaces with roughness values on the order of 6 microns.⁶ As the powder flows down the wall of the receiver gaining momentum, it impinges on the conical section in the base of the receiver. The powder then tends to adhere to the surface due to a poor machine finish. As more powder flows down and impinges against this surface, agglomeration occurs. Powder is also seen accumulating around the circumference of the receiver wall. This strip of powder represents the material that is slung from the tube and has adhered to the wall surface.

Figure 1. Rotary Tube Divider

An analysis of the flow problems associated with the modified receiver led to two conclusions. First and foremost, the placement of the ports at the bottom of the receiver required the powder to travel a large distance before being divided. This distance introduced potential for powder to adhere to surfaces, which is seen in Figure 4. Secondly, the receiver ports are not sufficiently sized to allow powder to flow freely. Based on these conclusions, a second receiver modification was proposed. This modification is discussed below.

Design Modification #2

The second receiver modification is a derivative of the first modification. It implements potential solutions to problems encountered in the first design. Figure 5 illustrates the proposed dimensions for the second receiver modification. Figure 6 shows a rendering of the modification design. As shown in Figure 6, the path of the rotating tube will flow directly over the receiver ports. Thus, as the powder falls out of the rotating tube it will be deposited directly in the port (as opposed to sliding down a lengthy surface in the first modification). The receiver port opening size will be 3" in diameter and will be reduced to a 1.5" diameter throat (note: it may also be advantageous to angle the receiver ports to match the geometry of the laboratory sample port shown in Figure 1). Adjacent ports are separated by sharp edges that include a cutout to accommodate the path of the rotating tube. The sharp edges are necessary to ensure that no powder is deposited on surfaces that separate the ports. The rotating tube travels around a center cone; the cone is intended to guide any misdirected powder back down into the receiver ports. The receiver will be mated to the upper portion of the rotary tube divider in a similar fashion as the equipment shown in Figure 2.

As a final measure to reduce powder flow difficulties, the receiver will be electropolished. Electropolishing is a process by which metal is removed from a work piece by passage of electric current while the work is submerged in a specially designed solution. The quantity of metal removed from the work piece is proportional to the amount of current applied and the time. In the course of electropolishing, the work piece is manipulated to control the amount of metal removal so that polishing is accomplished and, at the same time, dimensional tolerances are maintained. Electropolishing literally dissects the metal crystal atom by atom, with rapid attack on the high current density areas and lesser attack on the low current density areas. The result is an overall reduction of the surface profile with a simultaneous smoothing and brightening of the metal surface. The electropolishing process accomplishes all of the following: removal of surface occlusions, increased corrosion resistance, reduced buffing and grinding costs, removal of directional lines, radiusing of sharp edges, and reduced surface friction. The resulting finish often appears bright, shiny, and comparable to the mirror finishes of bright chrome. It is strongly felt that this operation will improve the surface finish enough to allow powder to flow smoothly along the receiver surfaces.

The second receiver modification will be fabricated in the first quarter of FY 2000. Testing will be initiated immediately following fabrication completion.

Efficient powder handling and transfer in the Plutonium Immobilization process will be a necessity. Efforts have been made by LLNL in the past fiscal year to research powder handling techniques and practices. As a result of this research, development was initiated on powder handling equipment, i.e. valves and hoppers. The points listed below summarize LLNL’s lessons learned:

• Throughout the first stage of the PIP process it will be necessary to handle powder in equipment and containers with varying geometries. Four physical parameters are critical in powder handling equipment design: diameter, height, side slope, and opening size.

• A 70° (from horizontal) wall slope in powder handling equipment is desirable to enhance free powder flow with minimum bridging.

• LLNL tested a hopper with 60° · walls. The batch size consisted of 10 kg of powder with a volume of 11-12 liters. Three discharge diameters were tried: 2", 3", and 4". The 2" and 3" were insufficient to handle free powder discharging. The 4" diameter provided better flow conditions. However, it was necessary to apply vibration to the hopper when the powder was either too moist or compact. The vibrator was also used as a discharge aid to ensure that bridging did not occur.

• Powder flowed from the hopper, through a valve, and into a 2"i.d. hose. No powder flow problems were noticeable in the hose.

• Four valve types were considered for use in the material transport system: gate, ball, butterfly, and pinch. The ball valve was dismissed due to potential powder seepage in the gap between the ball and seat. The pinch valve was rejected due to incompatible geometry with the hopper as well as difficulty in operation. Four inch commercial gate and butterfly valves were suitable from an operational perspective, but are unattractive due to their size and bulk. As a result of these shortcomings, custom valves were developed.
  

• Ideally, a material transport valve should be simple in operation, well sealed, and durable. These desirable valve characteristics led to the design and fabrication of the gate valve shown in Figure 7.

• Valve jamming can occur if powder builds up between moving parts. To prevent valve jamming, it is necessary to use a valve seal made of a soft material. Polyurethane (with a 60-durometer hardness rating) was chosen as the sealing material for this application. In addition, dry lubricating the moving parts improved valve performance by reducing friction.

The following are the ceramic feed batching milestones for FY 2000.

• 1/31/00 – Complete mock-up design

• 3/31/00 – Place mock-up orders

• 4/30/00^* – Provide preliminary TDR input

• 7/31/00 – Start mock-up installation

• 9/30/00^* – Complete mock-up installation

^* D & T Milestone

This report discusses the Plutonium Immobilization CFB component development progress. The process block diagram, see attachment 1, details the feed batching steps and is based on the Plutonium Immobilization Plant, First Stage Immobilization, Process Flow Drawing. A description is presented for the feed batching process concept that discusses the steps required to blend the plutonium oxide powder. Attachment 2 illustrates the proposed process concept. Batch splitting components related to the rotary tube divider are reviewed and future development work is discussed. Brief points concerning LLNL’s work on powder handling and valve development are also covered.

1. Erickson, S. A., Plutonium Immobilization Feed Batching System Concept Report (U). WSRC-TR-99-00073, Savannah River Site, Aiken, SC 29808, (1998).
2. Marshall, K. M., Evaluation and Testing of Splitters for the Plutonium Immobilization Program (U). WSRC-TR-99-00080, Savannah River Site, Aiken, SC 29808, (1999).
3. Gray, L. W., Materials Disposition Acceptance Specifications for the Plutonium Immobilization Project. PIP-98-047, Lawrence Livermore National Laboratory, Livermore, CA 94551, (1998).
4. Melton, D. R., Ceramic Feed Batching Plutonium Oxide Particle Size Distribution (U). NMP-PLS-990010, Savannah River Site, Aiken, SC 29808, (1999).
5. Revolinski, S. M., Splitter Design Estimate for Plutonium Immobilization Facility. WSMS-CRT-98-0042, Westinghouse Safety Management Solutions, Aiken, SC 29808, (1998).
6. Oberg, E., and Jones, F. D., and Horton, H. L., and Ryffel, H. H. Machinery’s Handbook. 25^th ed., p. 708, Industrial Press Inc., New York (1996).

Attachment 1. Feed Batching Process Block Diagram

 

Attachment 2