ThO2 and PuO2 form a continuous series of solid solutions over the entire range of composition. At the Pu-rich end, mixed oxide may be heterogeneous if prepared under reducing conditions, as a result of the formation of Pu2O3. The lattice parameter of (Th1-yPuy)O2 decreases linearly from pure ThO2 to pure PuO2 [26]. Lattice parameters of (Th, Pu)O2 with various PuO2 contents are given in Table 11. Assuming ideal solid solution behavior at high temperatures for ThO2 and PuO2, it would be expected that this linear decrease in lattice parameter would also happen at elevated temperatures.

The available data on ThO2-PuO2 are scanty. One way to overcome this problem is to use CeO2 in place of PuO2 as they both have quite similar physico­chemical properties viz., ionic radii in octahedral and cubic coordination, melting points, standard enthalpy of formation and specific heat etc. Thus, the plutonium chemistry can be well simulated using CeO2 in place of highly active PuO2. Mathews et al. [89] have recently measured bulk thermal expansion of (Th, Ce)O2 system. Bulk and lattice thermal expansion studies on (Th1-yCey)O2 (y = 0.0, 0.04, 0.08 and 1.0) were carried out by dilatometry and high temperature XRD from room temperature to 1,123 and to 1,473 K, respectively. The average linear thermal expansion coefficients of ThO2, Th096Ce004O2, Th092Ce008O2, and CeO2 were found to be 9.04 x 10-6, 9.35 x 10-6, 9.49 x 10-6, and 11.58 x 10-6 K-1, respectively, between 293 and 1,123 K. Some data on (Th1-yPuy)O2 generated at BARC was reviewed in IAEA-TECDOC [40]. Thermal expansion curve for (Th1-yPuy)O2 for y = 0.02, 0.04, 0.06, 0.10 are shown in Fig. 10.

The thermal expansion of the solid solutions (Th1-yPuy)O2 could be reasonably approximated at various temperatures by taking linear interpolated expansion data of ThO2 and PuO2 as per their weight fraction. IAEA-TECDOC [40] used “interpolation method,’’ using the recommended equation by Touloukian [61] for ThO2 and the following equation for pure PuO2 as recommended by MATPRO. The equation for linear strain calculations is as given below:

s = K • T — K2 + K • exp(-Eo/ksT); (32)

where, e is the linear strain which is taken as zero at 300 K, T is the temperature (K), kB is Boltzman’s constant (1.38 x 10-23 J/K), and ED, K1, K2, and K3 are constants having values 7 x 10-20 (J), 9 x 10-6 (K-1), 2.7 x 10-3 (unit less), and 7 x 10-2 (unit less), respectively.

Percentage linear thermal expansion for (Th1-yPuy)O2 (0 < y< 1) obtained by linear interpolation of the data of ThO2 and data for PuO2 and can be expressed as (in the temperature range of 300-1,773 K) [40].

(Th1-yPuy)O2 where 0 < y <1:

(AL/L0) x 100 = — 0.179 — 0.049 • y + (5.079 x 10-4 + 2.251 x 10-4 • y) • T + (3.732 x 10-7 — 2.506 x 10-7 • y) • T2 + (-7.594 x 10-11 + 12.454 x 10-11 • y) • T3

(33)

As part of thorium-based fuel development program for fast breeder reactors, the thermophysical properties of mixed thorium-plutonium oxide pellets of both thorium — and plutonium-rich compositions were evaluated in India [58]. The plutonium-rich mixed oxide pellets contained 70-80 % PuO2 which could be considered as candidate fuel for small LMFBR core like the operating fast breeder test reactor (FBTR). The thorium-rich compositions contained 20-30 % PuO2 which could be considered as alternative fuel for large LMFBRs like the forth­coming prototype fast breeder reactor (PFBR-500). The mixed oxide pellets were prepared by ‘‘powder-pellet’’ route involving mechanical mixing of ThO2 and PuO2 powders followed by cold pelletization and high temperature sintering. Small amount of Nb2O5 (0.25 wt%) or CaO (0.5 wt%) powder were used as ‘‘sintering aid’’ and admixed with the powder during co-milling. The coefficient of thermal expansion of mixed (Th03Pu07)O2, (Th07Pu03)O2, and (Th08Pu02)O2

Fig. 11 Thermal expansion 14

curves for high Pu bearing (Th1-yPuy)O2 samples [58].

(permission from IAEA) 12

10

CD

О

5

Ш 8 —

О

6 4

were evaluated by a high-temperature dilatometer and is summarized in Fig. 11. XRD pattern of ThO2 and the pellets containing lower amounts of PuO2 (30 % PuO2) sintered in either Ar or Ar-8 % H2 showed only single-phase isostructural with fluorite phase. But ThO2-PuO2 pellets with higher plutonium content such as ThO2-50 % PuO2 and ThO2-75 % PuO2 pellets sintered in either Ar or Ar-8 % H2 showed the presence of two phases. In addition to the phase that is isostructural with PuO2 (fluorite), another phase which is isostructural with bcc a-Pu2O3 has been observed [90]. Hence, no conclusion could be drawn from the above results.