WSRC-MS-2002-00442

Integrating Safeguards into the Pit Disassembly and Conversion Facility

T. G. Clark
Westinghouse Savannah River Company
Aiken, SC 29808

J. E. Gilmer and H. F. Kerschner
Battelle Pacific Northwest Division

 

This document was prepared in conjunction with work accomplished under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy.

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Background

In September 2000, the United States and the Russian Federation entered into an agreement which stipulates each country will irreversibly transform 34 metric tons of weapons-grade plutonium into material which could not be used for weapon purposes. Supporting the Department of Energy’s (DOE) program to dispose of excess nuclear materials, the Pit Disassembly and Conversion Facility (PDCF) is being designed and constructed to disassemble the weapon "pits" and convert the nuclear material to an oxide form for fabrication into reactor fuel at the separate Mixed Oxide Fuel Fabrication Facility. The PDCF design incorporates automation to the maximum extent possible to facilitate material safeguards, reduce worker dose, and improve processing efficiency. This includes provisions for automated guided vehicle movements for shipping containers, material transport via automated conveyor between processes, remote process control monitoring, and automated Nondestructive Assay product systems.

Such features afford an opportunity to plan and execute new strategies to meet Material Control and Accountability (MC&A) requirements, while offering new challenges to be considered. In addition, the mass balance engineered criticality control approach adopted affords the possibility of monitoring material in process perhaps more frequently than has generally been possible in the past. This paper presents the strategic approach developed by the design agency and accepted by the design authority to plan and engineer for implementing a design strategy that achieves enhanced facility safeguards by taking advantage of the automation and criticality measures incorporated in the design.

Discussion

The PDCF is being designed to accept surplus fissile material in pit form or as plutonium metal and generate an end product of plutonium oxide packaged in DOE Standard 3013 containers suitable for storage and disposition. Current facility design is based on a process developed at Los Alamos National Laboratory. This innovative approach is a low waste, modular pyroprocessing system chosen to convert plutonium and uranium metal into oxide. To remove plutonium from the pits, the pits are separated into hemishells with a cutting device, and the components separated and sized-reduced to batches for processing. The oxide products are to be sealed in 3013s, leak checked, and electrochemically decontaminated. The metal cans are placed in storage for ultimate transfer to other facilities to complete the disposition process. Non-plutonium pit components are separated and recovered for reuse or declassified and dispositioned.

Figure 1- 3013 container

Figure 1.  3013 container

 

Figure 2 - PDCF Process Overview

Figure 2.  PDCF Process Overview

A significant element of the national materials disposition process involves the transfer of the unclassified plutonium product to Nuclear Regulatory Commission (NRC) control at the MOX Fuel Fabrication Facility (MFFF) where commercial Mixed Oxide Fuel (MOX) will be fabricated. Both facilities are to be constructed at the Savannah River Site in South Carolina.

The PDCF, as a denial facility, will be a hardened building within an alarmed area housing the plutonium and uranium processing operations, an analytical support laboratory, and systems for packaging, storing and shipment of waste, and various support operations. Material will be transferred between process steps using an automated conveyor system; and analytical samples will be transferred utilizing a pneumatic transfer system. Conventional buildings and structures will house administrative areas, a control room and personnel, personnel change rooms and facilities, and ancillary equipment such as chillers, filters, exhaust fans, etc.

MC&A Considerations

The design architect (Washington Group International, Inc. / Battelle Pacific Northwest Laboratory) has developed a design strategy to include MC&A features in the design that effectively integrate radiological (ALARA) and engineered criticality controls. This approach ahs been accepted by the design authority (Westinghouse Savannah River Company). This strategy entails the initial subdivision of a single facility Material Balance Area (MBA) into sub-MBAs for the vaults, process modules, waste management and analytical laboratory areas to aid in isolating potential inventory differences during startup and operations. Also, the maximal automation of data collection and transfer from various Nondestructive Assay (NDA) devices to the accountability system for accounting will increase data accuracy and integrity of records. Further, utilization of the engineered criticality mass balance equipment design philosophy and automated transfer components are employed to the best advantage for MC&A purposes. These are discussed in the following sections.

Automated processing equipment will be utilized throughout the facility to achieve personnel exposure to As Low As Reasonably Achievable (ALARA). A process control system will control material movements through a series of automated conveyors and stations called Criticality Pass Throughs (CPTs) designed in concert with the engineered criticality control approach. Material movement from origin to destination will require two-person rule enforcement for access to a process module and to initiate material movements. Within a module the CPTs will be interlocked to prevent movement into and within a process glovebox train unless the criticality mass balance imposed on the particular process step has sufficient margin to allow the movement, or until sufficient margin is created by the transfer of fissile material to a different process. The accountability system will be updated at each introduction and removal from a glovebox train at each CPT location as the mass balance allows material movement, as well as, introduction to and removal from locations such as a Direct Metal Oxidation (DMO) furnace. While material is in a furnace, for example, the status of the device control parameters may be monitored in the module and the control room, providing data for material control requirements such as Daily Administrative Checks and inventory location and quantity at any time. In this controlled, automated manner, the accountability system will be updated more frequently than has perhaps been typical of the historical paper traveler methods commonly used to capture item location and status data.

Scales with integral bar code readers to detect unique device identification will be placed at many locations throughout various process modules to record weight changes and track item or container locations. This data will be uploaded to the accountability system automatically. Data transferred will include the scale and container identification, weight, the involved personnel identifications, and a date/time stamp at a minimum. Containers of various sizes and types will require that a unique etched or machined bar code be placed on each container.

Figure 3 - PDCF MC&A Data Flow

Figure 3.  PDCF MC&A Data Flow

The automated facility conveyor system typically includes drop boxes at the head and rear ends of each process module or glovebox train. To initiate movement, two personnel must be present at both the origin and destination conveyor locations prior to movement, and material accountability data such as container or item identification and weight must be provided to the accountability system. The typical transit time between two locations will be monitored by the process control system to provide material control oversight. Failure to arrive within a stipulated time will be treated as a material control alarm and will require MC&A personnel to investigate. Additionally, the mass balance of the destination glovebox will be checked prior to permitting material introduction through the interlocked airlock/drop box interface. If the mass balance is not acceptable, the airlock will not open until the mass meets the criticality requirements imposed at that location. This will also require human intervention to resolve, adding to material control oversight.

Automated Guided Vehicles (AGVs) will be used to place and retrieve items in vaults. The vault locations will be barcode labeled with unique location identification, read by a bar code reader or digital camera. Items stored in the vaults will be configured on a transport / storage pallet that allows the AGV to handle as many as four items at a time. Each of these pallets will also be marked with a machine-readable identification code. The AGV will read both barcodes prior to retrieving and after storing an item. Movement of the AGV into the secure vault space will not be automatic, but will involve standard vault access procedures with the appropriate operations, radiological controls, and security personnel. Once the secure vault space is accessed, movement and operation of the AGV will be automatic except in any rare case requiring special operations. The AGV control data download will require two-person concurrence for the download and prior to initiating an AGV movement. Data for a movement will be uploaded to the accountability system to record item removal and its arrival at destination so the system captures a location change in near real time. This method should provide a significant improvement to the lengthy lag times typically expected of a paper traveler system.

All product plutonium oxide will be milled to achieve the particle size range required, blended, placed in a convenience can, welded into a primary containment vessel, and the container electrolytically decontaminated. Weight and convenience can ID will have been recorded in the accountability system previously; the primary container ID and weight will require recording. The inner container will be placed in a machine-readable outer container and welded. Both the inner and outer containers will be leak tested following the welding operation. In addition, an X-ray of the product container will be made to meet the DOE Standard 3013 container baseline requirement.

Figure 4 - Accountable Material Movements

Figure 4.  Accountable Material Movements

The design includes provision for calorimeters, gamma ray isotopic and neutron multiplicity systems for product container NDA measurements. A gantry crane controlled by a host processor will schedule all container movements through the NDA suite devices, initiate assays, receive transmitted assay data, calculate results, put results in the proper format, and route them to the accountability system for uploading to the data base. This is being done to prevent human errors during entry of the accountability data; errors require human intervention to resolve, a sometimes time consuming process. The rapidity with which this can be accomplished far surpasses keyboard entry and should eliminate errors. Similar measures are planned for waste assay measurement devices and confirmatory and verification measurements of receipts and shipments. In the latter instance, both a 252Cf shuffler and gamma ray systems will be available for use.

The analytical laboratory is intended to qualify the PDCF product to the strict specification requirements imposed by the facilities performing the next phase of disposition for the product material. A commercial laboratory data management system is being contemplated for the same purpose as the NDA system hand entry error concerns. Inclusion would lead to the same rapid update and error minimization discussed previously.

Thus far, major components and features of a somewhat typical MC&A system have been described. The component that ties all these components, devices, and the data they generate is a classified, distributed process control system (PCS) and data management system with a high speed communication backbone installed throughout the facility that will feed the facility Local Area Nuclear Material Accountability System (LANMAS), and the site Comprehensive Nuclear Materials Management System (CNMMS). Communication between these systems will follow established site practices and encryption methodologies.

The challenges inherent in this approach or any complex, integrated data communication system are many. The selection of host processors will entail conscious consideration of file structures and the destination data base field and record lengths, the communication protocols used, and the classification levels involved. Much of the data transferred to the accountability system will be classified; however, large portions of the data collected from other devices will not be classified. Maintaining proper separation will be a large challenge, facilitated somewhat by having both a classified and unclassified communication backbone. Finally, imposing need to know criteria for portions of the data generated will be an additional challenge to be addressed.

Several devices and controls described earlier mesh well with the PCS and data management system as envisioned. Common features required of the overall system are unique identifiers for individual devices, denoted by their process location, device type, and whether or not they are key measurement points; a data capture capability, embodied in the data management system which will maintain data for archival and process control purposes; system capability to require two-person concurrence where appropriate; and an area access control capability to limit access to personnel with a need to know, proper authorizations, and all necessary training for admittance to a particular location, including enrollment in the site Personnel Assurance Program. These features in concert with the robust nature of the facility physical security design represent a joint effort to meet intent of the graded approach mandated in the orders for the protection of special nuclear material (SNM) and classified materials.