Few binary, ternary, and quaternary mixed acti­nide dioxides have been investigated experimentally. The cell parameters at room temperature along the mixed oxides solid solutions usually follow the Vegard’s law quite well — that is, a linear evolution between the end members of the solid solution.

This has been evidenced for Th _ xUxO2 by Bakker et a/.59 on the basis of collected experi­mental data (see Figure 12) and observed later by Yang et a/.78

According to experimental work by Tsuji et a/.,79 Lyon and Bailey,48 and Markin and Street,53 Vegard’s law applies for Ui _ xPuxO2 too (see Figure 12), and this trend is nicely reproduced by MD calculations by Terentyev80 and Arima et a/.81 (see Figure 13). MD calculations are consequently currently used for more complex mixed dioxides, for example, by Kurosaki et a/.82 on the ternary mixed dioxides

U0.7 _ xPu0.3AmxO2.

Recently, experimental measurements done by Kato et a/.56 showed that the Vegard’s law is valid for ternary and quaternary mixed dioxides. The evolution of the lattice parameter a in U1 _ z _y_y’PuzAmyNpy’O200 for low contents of Am, Pu, and Np obeys quite well the following linear relation with the ionic radii rU, rPu, rAm, rNp, and rO and the composition:

l!! ,

z _ y _ У ) + rpuZ

+ ГАшУ + tNpy" + To]

Kato eta/.56 tried to extract the valence of americium in U1 _ z _JPuzAmyO2 00, from the evolution of the cell parameter as a function of the americium
content. They deduced that americium is +4 rather than +3 for the U1 _ z _y _y’PuzAmyNpy/O2 00 solid solution, owing to the fact that the ionic radii depend on both the nature and the valence of the element.

The thermal expansion of mixed actinide dioxides NpxPu1_xO2 has been measured by Yamashita eta/.71 The thermal expansion coefficients are so similar to each other along the mixed oxide solid solution (see Table 3) that Carbajo et a/.84 recommended in

their review a single equation for the whole solid solution. The thermal expansion coefficients as a function of neptunium and thorium composition in UO2 have been measured by Yamashita et a/.,83 and by Anthonysamy et a/.85 The data are reported in Tables 4 and 5. In the UxTh1 _ xO2 solid solution, the evolution of those coefficients b; (0 < i< 3, eqn [3]) follows a quadratic relation with the compo­sition, as shown by Anthonysamy et a/.85 or Bakker et a/.59 But in many cases, the simple Vegard’s law is applied to the evolution of lattice parameters as a function of composition and temperature. Results obtained by MD calculations show that such a simpli­fication works well in the MOX (see Arima et a/.81 in Figure 13 or Kurosaki et a/82 for ternary mixed (U, Pu, Am)O2).