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Fig. 7-3 Ternary U-Np-O phase diagram at 1273 K ,single phase; two phases; limiting composition of MO2+X phases in two-phase mixtures; the broken line is tentative

Transuranium elements (TRU) such as neptunium (Np), plutonium (Pu), americium (Am) etc., are accumulated during the burn-up of nuclear fuels. The development of the advanced treatment technology to transmute TRU into shorter-lived nuclides is clearly required because TRU generated by the reprocessing of spent fuels consists of very long-lived nuclides with half lives of more than several ten thousand years. On the other hand, the possibility of the high burn-up of fuel is being studied with the aim of reducing the power-generating cost. In this case Np is a noticeable key element, as the accumulated amounts of TRU are increased with the burning of fuel. The Np concentration increases to about 60% but the Pu nuclide concentration is reduced. In the Japan Atomic Energy Research Institute (JAERI) the research and development of advanced mixed oxide fuels containing Pu and TRU (Np) has been conducted with the intention of exploring a durable high burn-up fuel and reusing TRU wastes as a resource. In this context, the basic properties of the oxides containing Np have been systematically studied. The new ternary U-Np-O phase diagram at 1273K as deduced from this research, is shown in Fig. 7-3. From the experimental results that the area surrounded by UO2 - NpO2 - U0.4Np0.6O2.27 - UO2.25 is the single phase of solid solution (MO2), Np accumulated during burn-up seems to be homogeneously distributed in the matrix of fuel in solid-solution. This fact shows the crystal structure of the oxide fuel, even though Np is dissolved, is not changed and the durability of the oxide fuel is maintained.

Reference T. Yamashita et al., Phase Relations of Uranium-Neptunium-Oxygen System, J. Alloys Comp. 213/214, 375 (1994).

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