Fig. 7-1 Thermal conductivity of neptunium nitride Thermal conductivity of NpN is almost halfway between those of UN and PuN and its temperature dependence is near to that of PuN. The nitrides possess higher thermal conductivities than those of the oxides by about an order of magnitude.

Fig. 7-2 Thermal conductivity of actinide nitrides at 1000 K Measured values are gradually decreased with the atomic number of the actinide series.

Actinide mononitride is proposed as a potential fuel of actinide burning reactors in the partitioning-transmutation scheme of the nuclear fuel cycle which has been enthusiastically promoted in the Japan Atomic Energy Research Institute (JAERI). In order to clarify the feasibility of this fuel in the transmutation ("actinide-burner") cycle, it is essential to construct a data base of thermal properties of these actinide mononitrides. Neptunium is a key element among these actinides, because it occupies about 60% of the larger generating amount and the longer half-life compared with the others. Therefore it is very important to determine the thermal conductivity of neptunium mononitride NpN. As the actinides are highly toxic and difficult to handle, thermal conductivity, vapor pressure and other thermal properties of NpN have to be measured in glove boxes. The temperature dependence of the thermal conductivity deduced from the thermal diffusivity of NpN measured by laser flash method, using highly purified materials which are synthesized in gloveboxes by the carbothermic reduction of neptunium dioxide NpO2, is shown with those of other nitrides and oxides in Fig. 7-1 and compared to other actinide nitrides at 1000 K in Fig. 7-2. Data of UN and PuN were measured in the JAERI but the one of ThN was obtained from the literature. The above-mentioned restrictions of handling result in the limited data base of thermal properties of the actinides, therefore this thermal conductivity should be a valuable datum. Further-more, the synthesis and the decision of thermal properties of (U, Pu, Np) mixed nitride are being progressed in order to evaluate the feasibility of the nitride as a practical nuclear fuel of a burning reactor and a fast breeder reactor.

Reference Y. Arai et al., Thermal Conductivity of Neptunium Mononitride from 740 to 1600K, J. Nucl. Mater., 211, 248 (1994).

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