As a result of the end of the Cold War and related reductions in the nuclear arsenals in the United States and Russia, many tons of plutonium are now surplus to national security requirements. In both countries, the principal planned method for the disposition of surplus plutonium is to use the plutonium in the form of mixed-oxide (MOX) fuel to generate electricity in existing commercial reactors. The responsibility for this program resides in the Office of Fissile Materials Disposition of the National Nuclear Security Administration (NA-26).
Los Alamos is involved in two key technical aspects of the U.S. plutonium disposition program: the demonstration of key technologies for the disassembly and conversion of plutonium weapon components (pits), and the polishing-or final purification-of plutonium oxide to provide material for fabrication of MOX
The demonstration of key technologies for the disassembly and conversion of plutonium weapon components has been accomplished with the Advanced Recovery and Integrated Extraction System (ARIES) installed in the TA-55 Plutonium Facility. The ARIES equipment is configured in several connected modules to perform the sequential steps involved in disassembly and conversion of parts to unclassified forms.
Two demonstration phases have been completed, establishing the requirements for the disassembly of each of the 32 pit types in the surplus stockpile. Currently, several modules are being upgraded to reduce operator exposure and make the equipment more suitable for a future production environment.
The culmination of this effort is the development of build-to-print drawings of the key process equipment to be installed in the Pit Disassembly and Conversion Facility, to be constructed at the Savannah River Site. Currently, this design activity is in the latter stages of Detailed (Title II) Design.
The plutonium from the ARIES processing is an unclassified form of plutonium oxide. The Office of Fissile Materials Disposition initiated a project in Fiscal Year 2002 to purify this type of material to specifications that would allow direct use in the fabrication of MOX-fuel lead assemblies. The initial project plan included an initial 5-kilogram demonstration of the polishing process to confirm that the required product purity could be achieved, followed by a production phase to generate a total of about 140 kilograms of high-purity plutonium oxide. To meet external schedule requirements, the material needs to be ready to ship by August 2004.
Process-flow diagram for aqueous polishing.
During the course of the 5-kilogram demonstration, several issues were encountered that resulted in additional demonstration activities. First, there were some chemical impurities in the product material that either exceeded specification levels or were not able to be measured to the required sensitivity. Second, some of the original equipment used for the demonstration was shown to be unreliable. Third, there were deficiencies in the quality- assurance practices that were identified.
There has been a concerted effort to resolve all of these issues over the past year. The final confirmation that Los Alamos can meet required product quality and production capacity is targeted for this July, which is an Appendix F milestone for the Laboratory.
The aqueous polishing process uses a conventional process-flow diagram of dissolution, ion exchange, oxalate precipitation, and calcination. Several batches of calcined plutonium oxide are blended to reduce the overall number of samples needing to be analyzed to confirm that the final product meets specification. Finally, the plutonium oxide is packaged in crimp-sealed convenience cans, placed inside welded containers, and stored until future shipment to a MOX fuel fabrication facility.
The primary equipment used for the polishing function is the Advanced Testing Line for Actinide Separations (ATLAS). This glovebox line was installed in the early 1990s to provide a flexible capability for full-scale separations research. The configuration used for this polishing project includes:
Aqueous polishing of plutonium is performed in the Advanced Testing Line for Actinide Separations (ATLAS), a glovebox line installed in the early 1990s to provide flexibility in full-scale separations research. Some of the major equipment used in the aqueous and dry-powder processes includes (from top to bottom): a pair of 3-foot Reillex-HPQª anion exchange columns, a Turbula blender for product plutonium oxide homogenization, a Carbolite muffle furnace for rigorous calcination operations, and a Teflon dissolution apparatus.
The aqueous and dry-powder operations are performed in Actinide Process Chemistry (NMT-2), the trace and isotopic analyses by Actinide Analytical Chemistry (C-AAC), the physical analyses and canning by Pit Disassembly and Nuclear Fuels Technol
Several analytical techniques are being used to evaluate the product plutonium oxide. Most of these techniques are well established and have been used to support a variety of programs at Los Alamos, the exception being the Coulter Counter for particle-size analysis. This apparatus was installed after the early demonstration activities showed that the laser-scattering instrument was providing insufficiently accurate results.
The 5-kilogram demonstration was intended to establish the baseline unit operations in the process-flow diagram and should have been confirmatory in nature. However, several issuesÑimpurity, equipment problems, and quality controlÑwere discovered during the course of the initial operations that required corrective actions.
At the onset of the demonstration, it was recognized that removing gallium from the plutonium feed materials would be challenging. To avoid any issues with the process to license the MOX fuel, an extremely low specification for gallium was establishedÑcomparable to levels that would be seen in conventional fuels over time as a result of fission- product buildup.
The early results showed the final product gallium concentrations declining from run to run, perhaps as the process equipment was decontaminated. However, detection limits were also above the specification. Negotiation with DOE (NA-26) resulted in a slight relaxation of the specification. In addition, feed materials have been limited to those that are already relatively low in gallium. Also, the measurement sensitivities have been improved so that the detection limit is now below the fuel specification.
The net result is that the gallium levels in product materials are now routinely below the revised specification.
Over the course of the demonstration, it was also determined that there were similar issues with boron in the product material. The detection limits for boron were often at the specification limit, and routinely there would be an indication of impurity levels at or slightly above the detection limit. A potentially large source of boron impurity is the borosilicate glass dissolvers used in the dissolution process. The walls on these dissolvers are etched rapidly, requiring their replacement every five to six runs.
Teflon dissolvers have been installed to eliminate this source of boron. Several batches have been run through the Teflon dissolvers. However, analytical results for boron have not yet been completed. Even with the Teflon dissolvers, there is still an issue that detection limits are above the specification. Discussions are currently going on with NA-26 about relaxing the specification for boron as well, or using other DOE analytical laboratories that have lower detection limits.
During the initial demonstration, several of the major pieces of equipment were shown to have problems. The original calcination furnaces exhibited control problems, so new furnaces and controllers with computer-data logging were installed. The original V-blender was replaced with a Turbula blender to provide a more homogeneous product in a shorter time. The Turbula blender also allows for larger batch sizes to reduce the number of samples needed to characterize the final product material and to conform to standard industrial blending approaches.
As mentioned earlier, the laser-scattering particle-size analyzer was shown to provide inconsistent results. A Coulter Counter was installed to address this issue and to provide a capability that was directly comparable to previous industrial results.
The initial demonstration uncovered some significant quality-control issues. The primary methods for demonstrating product quality are the analytical measurements for product characteristics against the specification. These capabilities at Los Alamos have robust quality-management programs because of the recent efforts to re-establish pit manufacturing at Los Alamos.
There are other areas, however, that required an effort to establish an adequate quality program. For example, data traces for the calcination-furnace temperature profile were not archived. Although operators confirmed that requirements were met in the data records, it is appropriate to archive available data to allow verification that the requirements were met. Also, there were incomplete and inconsistent entries in the data records.
Even though NA-26 had reviewed and approved the NMT Division quality program, it was clear that a specific project-level Quality Assurance Plan was needed to clearly identify the appropriate process control and documentation requirements for the day-to-day operations by personnel. Implementation of this quality plan is nearly complete, in anticipation of several internal and external QA audits over the next few months.
In the top photo, Joe R. Martinez (left) and David Martinez, both with Actinide Process Chemistry (NMT-2), sample a stabilized plutonium oxalate batch (now oxide) for chemical trace analysis, particle morphology, and moisture content. The photo immediately above was taken through a glovebox window and shows plutonium oxalate cake in fused-silica roasting boats. The material is dried before it is placed in a stabilization furnace, where it is converted to plutonium dioxide. The actual color (which is skewed by the glovebox window reflection) is rusty orange; when converted to oxide it turns an "army" green.
Over the coming weeks, there will be a great deal of activity to demonstrate that all aspects of the product specification can be met, to demonstrate a complete implementation of the quality-assurance program, and to demonstrate that there is sufficient throughput to generate the required quantities by August 2004 even if there are unforeseen interruptions to the operations.
This article provides an overview of the significant work being performed by NMT-2, NMT-15, and C-AAC personnel to address the challenging endeavor of disposing of excess plutonium through aqueous polishing. Many aspects addressed briefly in this article will result in future articles that will describe in more detail the efforts that have been and will be accomplished to install and qualify equipment, improve measurement approaches, and establish quality operations. Readers are encouraged to look for articles in future issues of ARQ from the following people: Fawn Coriz, Liz Bluhm, and Simon Balkey of NMT-2; Lawrence Drake of C-AAC; and Brian Bluhm and Jane Lloyd of NMT-15.
This article was contributed by Randy Erickson of Nuclear Materials Technology Division (NMT-DO), Kent Abney of Actinide Process Chemistry (NMT-2), Technologies (NMT-15), and Deborah Dale of Actinide Analytical Chemistry (C-AAC).
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