NMT-2 Responds as National Prioities Change

Overview of Plutonium Processing at Los Alamos
The mission of the Actinide Process Chemistry Group (NMT-2) is to effectively manage nuclear materials and supply plutonium for defense and materials disposition programs for the Department of Energy (DOE). (See Figure 1.)

Figure 1. NMT-2 manages the division's nuclear material holdings, allowing for an efficient flow of materials into and out of the production lines.

NMT-2 has 70 technical personnel including 45 technicians and 25 staff members working at the Los Alamos Plutonium Facility at TA-55. They include chemists, chemical engineers, chemical process technicians, mechanical technicians, and drafting support. The technical staff are supported by a small core of project management specialists who formally track budgets, schedules, and quality.

NMT-2 currently encompasses four major technical areas: 1) providing high-quality-specification plutonium metal to the pit manufacturing program and high-purity oxide for standards, long-term storage, and disposition programs; 2) managing the vault to allow safe, secure storage of special nuclear materials (SNM); 3) providing special isotopes for dynamic experimentation testing; and 4) the proper management and disposition of secondary residues and wastes. To accomplish these objectives, NMT-2 is organized around major technology areas: pyrochemical processing; aqueous processing, which includes chloride and nitrate based aqueous recovery as well as dry powder handling; vault operations; and advanced projects.

The pyrochemical processes are performed in molten chloride salt at high temperatures. First, plutonium oxide is reduced to metal by reacting the oxide with calcium metal. After it is converted to metal, americium is removed by sparging chlorine gas through the molten plutonium metal. The americium is oxidized into AmCl₃, which is then absorbed and removed in a small salt blanket. The impure plutonium metal is cast into cylindrical anodes and electrorefined. The highly-pure (99.9% wt. Pu) plutonium metal is available for pit manufacturing while the secondary salt and crucible residues along with the anode heels are sent on to the aqueous operations for further recovery.

Residues are dissolved in either hydrochloric acid for pyrochemical items or nitric acid for impure oxides, ashes, and other miscellaneous compounds. After the plutonium is in solution, impurities are separated using solvent extraction or ion exchange. The solvent extraction process is a modified PUREX process in centrifugal contactors. The ion exchange process uses a polyvinylpyridine resin developed 10 years ago in collaboration with Reillex industries. The purified plutonium is removed from solution via a Pu(III) oxalate precipitation, and the remaining liquid effluents are either neutralized and discarded or reduced in volume in an evaporator. The lean evaporator overheads are currently sent to the Radioactive Liquid Waste Treatment Facility while the concentrated bottoms are fixed in Portland cement to be discarded as transuranic waste.

The PF-4 vault safely stores the majority of SNM. The vault operations are also closely tied to those of the nearby nondestructive assay laboratory, which performs measurements of the SNM before storage. The PF-4 vault maintains adequate space to receive materials from off-site and store them separately from the Plutonium Facility processing operations.

Advanced projects include the research, development, and deployment of new technologies and process unit operations to decrease waste and improve product quality. Projects include an enhanced pyrolysis unit with off-gas treatment, an acid fractionator for nitrate removal and nitric acid recovery, a glass vitrification unit to increase transuranic waste loading and minimize the final waste volume, and a low-pressure salt still for the efficient separation and recovery of monovalent chloride salts from metal oxides. In addition, an aqueous chloride processing line, CLEAR, will use enhanced organic extractants to reduce the amount of radioactivity discharged in the liquid caustic effluents by several orders of magnitude.

Tying Plutonium "Value" to National Priorities
Historically, nuclear reactors at Hanford and Savannah River produced "weapons grade" plutonium. The plutonium was extracted from the spent fuel and associated fission products using the PUREX solvent extraction process. Because the intense radiation required complex, remotely operated facilities, this process was expensive and generated large quantities of radioactive waste. Often the value of the plutonium in the residues from weapons production exceeded that of newly generated plutonium, and it was thus cost effective to recover it.

Economic discard limits (EDLs) were established to segregate waste materials from recoverable residues. Plutonium in residues was aggressively recovered for reuse until the concentration fell below the EDL, at which point the residue was declared a waste and packaged appropriately for disposal.

Before the end of the Cold War, Los Alamos provided pure plutonium metal to support the Rocky Flats Plant operations. The capacity for metal production exceeded that of the related scrap recovery operations so only the richest residues were recovered. Lean residues that accumulated throughout this period were stored in the PF-4 vault. Supporting research and development (R&D) focused mainly on methods to improve the metal and oxide production steps. In addition, we made reactor-grade plutonium oxide feed for mixed oxide (MOX) fuel to be burned in the Fast Flux Test Facility breeder reactor at Hanford.

Transitions in Plutonium Processing Programs
By the early 1990s, no defense or civilian program needed plutonium. There was no longer a need to produce additional pure plutonium metal or oxide and no programmatic driver to recover plutonium from scrap materials. Consequently, with no processing requirements, there were no new R&D initiatives.

During this period, the DOE reduced the size of the national weapons complex, shutting down, decontaminating, and decommissioning aged facilities. Los Alamos was selected to carry on plutonium pit programs. The Defense Nuclear Facility Safety Board Recommendation 94-1 mandated complex-wide programs to stabilize the legacy scrap holdings. International treaty agreements mandated reduced amounts of available weapons grade plutonium. In response, the DOE initiated a materials disposition program with the dual approaches of immobilizing and isolating plutonium or blending it into a MOX fuel for burning in commercial or government-owned reactors. As a result of these national changes, the focus at the Plutonium Facility and in NMT-2 changed from plutonium recovery to waste stream treatment and polishing, waste minimization, and pollution prevention.

We stopped using metal production methods that generated large volumes of waste. We optimized the balance of the pyrochemical processes to minimize the amounts of secondary salt wastes by recycling reagents or switching to new ones.

For the aqueous chloride processes, we designed and produced better extractants to remove residual activity from the secondary liquid waste stream. For the aqueous nitrate processes, we developed nitric acid recycling to remove nitrates to meet ground-water discharge standards at the Laboratory outfall and to reduce residual activity in the liquid waste stream leaving the facility. We developed pyrolysis to reduce the volume of combustible rags and plastics without incineration.

Plutonium Processing Programs Today
As we approach the new millennium, the objectives for NMT-2 are again changing. The original goal of the 94-1 program was to stabilize high-risk or "high-priority" legacy residues for worker safety reasons; after three years all the high-risk items have been processed and disposed of. The focus is now on reclaiming vault storage space to allow the facility greater programmatic flexibility. We have identified approximately 700 items as potentially unnecessary for national strategic reserves. These may be consolidated and potentially disposed of as waste (transuranic or other) freeing valuable vault space. Resources are being allocated to develop new EDLs and apply them to a broader range of residue types and thus more effectively dispose of both legacy and newly generated residues.

The crucial first step in several defense programs is to supply pure plutonium metal that meets stringent quality specifications. Recent electrorefining runs have produced metal that exceeds purity and production requirements. The highly pure (99.9 wt %) plutonium metal is available for pit manufacturing. Using sealed interim storage containers has allowed the product metal to be properly stored and safely stockpiled. Unfortunately, the program funding does not concurrently support processing of the associated residues; low-grade residues are once again accumulating in the PF-4 vault.

A final objective is to supply pure plutonium oxide as feed to the Materials Disposition Program for processing into MOX fuel. NMT-2 is responsible for dissolving and purifying plutonium oxide for the demonstration lead test assembly. Process performance requirements and waste acceptance criteria are driving the current R&D in actinide science. Electrorefining is the only process that can consistently meet the exacting specification (99.9 wt %) for plutonium metal. Yet yields are averaging only 70% so residue recovery is necessary to maintain the plutonium stockpile. We will accomplish this through salt distillation and improved americium extraction techniques. Plutonium oxide purity must exceed 87.5% plutonium of a theoretical maximum of 88.2% in PuO2. In addition, the oxide must have less than 100 ppm of any element impurity and must be a reactive (nonsintered) oxide to allow for either complete dissolution or reduction to metal. Presently, only aqueous recovery can do this. Finally, there can be no hydrogenous transuranic waste, according to the Waste Isolation Pilot Plantıs very restrictive waste acceptance criteria. Waste vitrification with the associated off-gas scrubbing is being developed to address these criteria.

The Future of Plutonium Processing
Research is just beginning on the basic structure and chemistry data for the use of ionic liquids for both pyrochemical and aqueous operations. Advanced materials manufacturing might be achieved through the use of emulsion membranes for fabricating oxides with special physical properties. Similarly, metallic powders and foams might be generated for advanced metal-shaping techniques. We are investigating improved equipment and techniques for existing plutonium processing operations. Finally, we are currently looking for improvements in waste forms, waste treatment methods, and engineering models of the individual unit operations as well as their interactions in the overall process.

The bottom line is that through all the programmatic transitions, by using better technology and more reliable process controls that have resulted in numerous process improvements, the amount of waste per unit of product has steadily declined and will continue to do so. The Actinide Process Chemistry group is thus moving into the twenty-first century with the integrated goals of producing pure plutonium products while reducing secondary waste and residues and by fully complying with all regulatory and environmental constraints.

This article was contributed by Stephen B. Schreiber and Stephen L. Yarbro (NMT-2).

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