PROPOSAL FOR DEVELOPMENT OF ELECTROMETALLURGICAL TECHNOLOGY FOR TREATMENT OF DOE SPENT NUCLEAR FUEL
Argonne National Laboratory
A total of approximately 2,700 metric tons of spent nuclear fuel has accumulated within the U.S. Department of Energy (DOE) complex. This government-owned fuel was used in a variety of nuclear reactors, including reactors for the production of national defense materials, experimental and research reactors, and commercial reactors. Although the quantity of spent fuel in the DOE inventory is much less than that in storage at U.S. commercial reactors (estimated to reach 40,000 tons by the end of the century), the DOE spent fuel presents special problems that demand prompt attention. The DOE spent nuclear fuel inventory differs in several ways from the commercial spent fuel inventory, in that the DOE spent fuel represents the arisings from over 40 years of the evolution of nuclear power and reflects a broad spectrum of fuel types, cladding materials, levels of enrichment in the fissile isotopes of the actinide elements, and degrees of chemical reactivity.
Over 150 different fuel designs are represented in the DOE inventory; these can be grouped into 53 different categories according to common design features such as composition, cladding material and enrichment level. There are six major categories of fuel composition (metal, oxide, graphite, cermet, hydride, and aluminum alloy) alone. Included in the DOE spent nuclear fuel inventory are fuels that (1) have seriously degraded during storage, (2) are highly enriched in fissile isotopes, (3) are chemically reactive or contain reactive materials, and (4) cannot be expected to retain their integrity or remain stable over an extended period of wet or dry storage.
The electrometallurgical treatment technique, developed by the Argonne National Laboratory, appears to have great potential for application in the treatment of the collection of DOE spent nuclear fuel types for ultimate disposal. This process is applicable in its current stage of development to over 90% of the DOE spent fuel inventory and offers the advantages of a simple, compact system that is both economical and technically sound.
Electrometallurgical treatment refers to the complete set of unit operations required to separate the actinide elements from fission products present in spent fuel and to place the waste products in stable forms suitable for disposal. In the case of most spent fuel types, these operations begin with the dismantling of the irradiated fuel assembly and the removal of individual fuel elements. These fuel elements are sent to a chopper, where they are chopped into pieces suitable for the next operation. If the fuel is metallic, regardless of the cladding type or any internal components such as hardware or heat transfer media, it is sent directly to the electrorefiner for electrotransport of the actinides to appropriate electrodes where they are collected for further processing. If the fuel is oxide, it is first sent to a reduction step where the oxide compounds of actinides and fission products are reduced to the metallic state by reduction with metallic lithium. The reduced metal product is then transferred to the electrorefiner for separation into the various product streams. If the fuel is of a type other than metal or oxide, it must first