As (U, Pu)N were some of the most promising candidates for the first breeder reactors, they are the best studied nitride solid solution fuels. UN and PuN form a continuous solid solution, and the lattice param­eter increases with an increase in the plutonium con­tent, and is accompanied by a large deviation from Vegard’s law, as shown in Figure 7,26 suggesting the nonideality of the solution. A diagram of the calculated U-Pu-N ternary phase at 1000 °C, shown in Figure 8,1 suggests that there is a relatively narrow range of pos­sible (U, Pu)N compositions, as is the case with U-N and Pu-N binary systems. It is suggested that the sesquinitride solid solution (U, Pu)N15 exists in a sys­tem in which PuN may constitute up to 15mol%27, although this is not depicted in Figure 8.

As uranium monocarbide and plutonium mono­carbide, as well as other actinide carbides, have an NaCl-type fcc structure, actinide nitrides and acti­nide carbides form solid solutions. Some research performed on actinide nitride carbides, for example, U-N-C, Pu-N-C,28-30 have investigated the suit­ability of these carbonitride fuels and the impurities in nitride fuels after carbothermic reduction. Phase

UN NpN

NpN PuN

Composition

stability graphs of U and/or Pu-N-C, both with and without oxygen, also have been constructed in order to make pure nitride fuels.31, 2 The irradiation behav­ior of (U and/or Pu)-N-C fuels also has been reported,33 but the details of this data are out of the scope of this chapter.

As MAs are usually burnt with uranium and plu­tonium for transmutation, and as Am originally exists in Pu, (MA, U)N or (MA, Pu)N have also been well studied. As mentioned above, the vaporization behav­ior of (Pu, Am)N has been studied34, and abnormal vaporization of Pu and Am was observed. The lattice parameters of (U, Np)N and (Np, Pu)N increase with increase in Np and Pu content, and with a small deviation from ideality, as shown in Figure 7.24 Although scarcely any data for pure CmN has been obtained, X-ray diffraction data for (Cm04Pu06)N has been reported, as shown in Figure 9.24

Inert matrix fuels, where MA as well as uranium and plutonium are embedded in a matrix, are also being considered for use in ADS for transmutation. Recent research in MAs has focused on using various nitride solid solutions and nitride mixtures as inert matrix fuels. For ADS targets, matrices have been designed and selected so as to avoid the formation of hot spots and to increase the thermal stability, especially in the case of Americium nitride. Con­sidering their chemical stability and thermal conduc­tivity, ZrN, YN, TiN, and AlN were chosen as candidates for the matrix.16,35 ZrN has an NaCl-type

0.48

0 0.2 0.4 0.6 0.8 1

PuN CmN

PUO2 Composition cmO2

Figure 9 Lattice parameter of (Pu, Cm)N and (Pu, Cm)O2. Reproduced from Minato, K.; etal. J. Nucl. Mater. 2003, 320, 18-24, with permission from Elsevier.

fcc structure with а = 4.580 A and has nearly the same thermal conductivity as UN, has a high melting point, good chemical stability in air, and a tolerable dissolu­tion rate in nitric acid. Recently, abundant data have been made available for ZrN-based inert matrix fuels. It is planned that (Pu, Zr)N, with about 20-25% Pu, will be used to burn Pu in a closed fuel cycle.36 The lattice parameter of (Pu, Zr)N decreases with an increase in the Zr content, and is between that of PuN and ZrN, in accordance with Vegard’s law.24 It has also been estimated, using a model, that (Pu, Zr)N with 20-40 mol% PuN, does not melt till up to 2773 K; this is based on experimental thermody­namic data which show that U0.9Zr0 8N does not melt till up to 3073 K.37 In the case of (Am, Zr)N, it is reported that two solid solutions are obtained when Am content is over 30%24, as shown in Figure 10. The Am content of the two phases have been estimated, from the lattice parameter, to be 14.5 and 43.1 mol%. A thermodynamic modeling of a uranium-free inert
matrix fuel, for example, (Am0.20Np0.04Pu0.26Zr0.60), has also been accomplished.38

In contrast to ZrN, TiN does not dissolve MA nitrides even though TiN also has an NaCl-type fcc structure. This is explained by the differences in lattice parameter, which was estimated by Benedict.39 A mixture of PuN and TiN was obtained by several heat treatments above 1673 K, and the product, in which one phase was formed, did not contain the other phase.40 TiN, as well as ZrN, have nonstoichio­metry. It is also reported that a TiN + PuN mixture may be hypostoichiometric although (Pu, Zr)N is hyperstoichiometric.