Plutonium processing development

An overview is given of plutonium process chemistry used at the U. S. Department of Energy Hanford, Los Alamos National Laboratory, Rocky Flats, and Savannah River sites, with particular emphasis on solution chemistry involved in recovery, purification, and waste treatment operations. By extrapolating from the present system of processes, this paper also attempts to chart the future direction of plutonium process development and operation. Areas where a better understanding of basic plutonium chemistry will contribute to development of improved processing are indicated. 

Nuclear Waste Reprocessing. Liquid waste remaining from processing of spent reactor fuel for military plutonium production is typically acidic and contains substantial transuranic residues. The cleanup of such waste in 1996 is a higher priority than military plutonium processing. Cleanup requires removal of long-Hved actinides from nitric or hydrochloric acid solutions. The transuranium extraction (Tmex) process has been developed for... 

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

The Chemistry Research and Development group has a large variety of plutonium process chemistry projects underway. The work will certainly add to our understanding of plutonium chemistry and will result in plutonium process improvements. 

Recent process development efforts have been devoted to more expeditious and less costly pyrochemical reprocessing of residues created by the metal preparation and purification process. We intend to establish an internal recycle which yields either reusable or discardable residues and recovers all plutonium for feed to the electrorefining purification system. This internal recycle is to be performed in a more timely and less costly operation than in the present reprocessing mode. 

In order to achieve this goal of a fully integrated process sequence, a concerted research and process development effort must take place. Present R D efforts are devoted to the development of cost-effective pyrochemical processes for the recycle of plutonium in residues. Future efforts will be aimed at the recycle of reagents in each individual process. The objectives of the recycle are to produce plutonium metal which can be further purified, and to generate small volumes of residues which can be discarded or recycled. 

As U is the major component of a SNF see Table 1.2, its initial separation in reprocessing alleviates the mass burden of following steps and is considered preferable. The UREX process developed in the AFCI program of the United States is based on the PUREX process (30 vol % TBP in n-dodecane) and suppression of extractions of Pu and Np by reduction/complexation (175-182). Plutonium and Np are reduced by acetohydroxamic acid (AHA, CH3CONHOH) to Pu(III), Np(V), and Np(IV). U is kept in an extractable U(VI) state. Although Np(IV) is also extractable, AHA forms a complex with Np(IV) that is soluble in the aqueous phase. In the case where reoxidation of Pu(III) occurs, the Pu(IV) also transfers to the aqueous phase by forming a Pu(IV)-AHA complex. Thus, U is exclusively extracted. AHA decomposes to hydroxylamine and acetic acid (176). 

The flowsheet of the UREX process, developed in the United States, includes the following extraction cycles (1) separation of uranium and technetium, (2) separation of plutonium, (3) separation of cesium and strontium, (4) separation of MAs and Rare Earth Elements (REE), and (5) group separation of MA from REE metals.9,10 Flowsheet development in Europe11 includes a modified PUREX process and, after that, the DIAMEX process for separation of MAs and lanthanides, the SANEX process for separation of MAs from lanthanides, and a special cycle for Am/Cm separation. Cesium and strontium will be in the raffinate of the DIAMEX process, and this raffinate will be vitrified, or cesium can be preliminarily extracted.12... 

A more radical modification of the Purex process, the Aquafluor process, developed by General Electric for its Midwest Fuel Recovery Plant, retained only a single TBP co-decontamina-tion cycle followed by a continuous anion exchange contactor in which plutonium was to be removed from the U-Pu nitrate solution. The performance of this plant was never tested with plutonium, since General Electric decided to forego operation of the plant after technical difficulties developed during the "cold" checkout trials. 

The first microgram quantities of plutonium were produced [S6] in 1942 by irradiation of natural uranium with deuterons in the cyclotron of Washington University in St. Louis. This plutonium was separated at the Chicago Metallurgical Laboratory of the Manhattan Project by Seaborg and his collaborators, who employed the method of carrier precipitation frequently used by radiochemists to extract small amounts of radioactive material present at low concentration. As wartime urgency required that a plutonium separation plant be designed and built before macro quantities of plutonium could be available for process development, it was decided to use the same carrier precipitation process that had successfully produced the first small quantities of this element. 

The process developed by Belgonucleaire (Fig. 1) is called MIMAS for MIcronized MASter blend. The MIMAS MOX pellets are composed of a solid solution of UO2 and PUO2 homogeneously dispersed in a UO2 matrix. This result is achieved through two blending steps (1) the primary (or master) blend is obtained by ballmilling and (2) the secondary (or final) blend is used to produce the specified plutonium content in the pellets. 

The PAMELA vitrification process development as well as the operating experiences have been presented at several international conferences and symposia on radioactive waste management [1-5]. Presentations were also made at the NATO Advanced Research Workshop Disposal of Weapons Plutonium—Approaches and Prospects, held in St. Petersburg, Russia, May 14-17, 1995 [6-7]. 

Other treatment processes like acid digestion and washing processes, developed primarily for the recovery of plutonium at a time when plutonium was considered as a rare and valuable material, are presently of reduced interest. Now volume reduction and production of a stable waste form for disposal are the main objectives for conditioning. 

These variations permit the separation of other components, if desired. Additional data on uranium, plutonium, and nitric acid distribution coefficients as a function of TBP concentration, solvent saturation, and salting strength are available (24,25). Algorithms have also been developed for the prediction of fission product distributions in the PUREX process (23). 

When the Plutonium Project was established early in 1942, for the purpose of producing plutonium via the nuclear chain reaction in uranium in sufficient quantities for its use as a nuclear explosive, we were given the challenge of developing a chemical method for separating and isolating it from the uranium and fission products. We had already conceived the principle of the oxidation-reduction cycle, which became the basis for such a separations process. This principle applied to any process involving the use of a substance which carried plutonium in one of its oxidation states but not in another. By use of this... 

A limited overview of the process chemistry used at these sites is presented. This paper will also attempt to bridge, at least partly, the gap between ongoing fundamental plutonium research and development and applied technology needs. We believe it is important to bridge this gap, since a continuous flow of knowledge about plutonium chemistry from academic and government laboratories to the plant is necessary and beneficial... 

The complete chemistry of plutonium 1 iquid-to-solid conversion processes, especially peroxide and oxalate precipitation, should be further studied. Research and development of direct thermal denitration methods should also be pursued. 

The chemistry of waste treatment processes and the development of new processes are fertile areas of research work. The speciation of plutonium in basic and laundry wastes is needed. For example, if soluble plutonium complexes in basic wastes can be destroyed, perhaps ultrafiltration could replace the flocculent-carrier precipitation process. The chemistry of plutonium(VII) and of ferrites—a candidate waste treatment process—needs to be explored.(23)... 

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