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Fig. 7-5 Concept of an actinide transmutation (burning) cycle using the nitride/pyrochemical process Actinide salts (nitrate) are partitioned from high-level radioactive wastes which are fed into the first-stratum commercial fuel cycle. These salts are converted to solid microspheres by a sol-gel technique. The gel microsphere is formed as a mixture of actinide oxide and carbon, and is converted to mononitride by heating in a N2+H2 stream. The microstructure of uranium mononitride produced is shown in the photograph. In this carbothermic reduction process, 15N2 enriched nitrides can be synthesized, by using 15N2 gas instead of natural nitrogen. The nitride fuel is completed in the form of TiN-coated particles. After irradiation to high burnup in a dedicated actinide burner, the spent nitride fuels are reprocessed with the pyrochemical process in the second stratum actinide burner cycle. The nitrides are fed into the molten-salt electro-refiner, as shown in this figure. Since the solubility of nitrogen in molten salt is small (~100 ppm), 15N2 gas enriched in the nitrides is readily recovered. While the nitrides are anodically dissolved, purified actinide metals are recovered at the cathode.

Highly toxic and long-lived actinide elements such as neptunium, plutonium, americium and curium are present in high-level radioactive wastes which result from nuclear reprocessing. Treatment and disposal of these wastes are current critical issues. The partitioning-transmutation scheme being developed in the Japan Atomic Energy Research Institute (JAERI) is based on a double-strata fuel cycle concept, where the minor actinides from the commercial fuel cycle flow into the second stratum transmutation ("actinide-burner") cycle. The minor actinides are concentrated and confined in the second stratum, exciting only after being converted into fission products. It is preferable for this second stratum, that high atom densities of the actinides are maintained throughout the whole cycle in order for the system's volume and physical envelope to be minimized. JAERI is studying the feasibility of using nitride fuels and pyrochemical reprocessing for this purpose, with metal fuel regarded as an alternative. The favorable thermal properties of nitride fuels make full utilization of a cold-fuel concept possible because, 1) lower fuel temperatures result, and hence less fission gas is released, 2) a thinner cladding is needed which achieves a harder neutron spectrum, providing more effective actinide burning, and 3) there is a negative Doppler reactivity coefficient in case of accidents. It has been estimated that the size of the fuel cycle facility based on the pyrochemical process can be very small, and accordingly the capital cost can be significantly reduced from that of the equivalent PUREX aqueous plant. On the basis of experiences with uranium nitride electro-refining, a laboratory-scale electrochemical cell has been installed in a plutonium glove box at the JAERI Oarai Establishment. The electro-refining process of plutonium nitride and neptunium nitride was performed in 1997.

Reference T. Ogawa et al., Concepts of Dense Fuel Cycle Processing for Actinide Burning/Breeding, Pacific Basin Nuclear Conf., Oct. 20-25, 1996, Kobe, Japan, 1179 (1996).

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