## ThO2-UO2

Substitution of uranium for thorium in ThO2-UO2 system results in the decrease of lattice parameter, melting point, oxygen potential, thermal conductivity, while the density and the linear thermal expansion show an increase. In the uranium concentration range y > 0.2, less data are available but the same trend is expected. It was mentioned earlier that, at room temperature, the lattice parameter decreases linearly from 100 % ThO2 to 100 % UO2. Based on the results of calorimetric studies, thermal expansion studies, and other thermodynamic measurements on urania-thoria solid solutions [40], it is suggested that stoichiometric urania-thoria solid solutions are nearly ideal at least up to 2,000 K. Therefore, one can assume that this linear decrease in the lattice parameter also exists at high temperature. This linear decrease can only exist when the linear thermal expansion of (Th1-yUy)O2 (0 < y < 1) equals the linearly interpolated value of that of ThO2 and that of UO2.

The thermal linear expansion of the (Th1-yUy)O2 solid solution has been studied in much less detail than that of the pure compounds. Konings et al. [39] reviewed the thermophysical properties of ThO2-based fuels. They concluded from the results of eight studies on (Th1-yUy)O2, and suggested the equation of the type:

AL/L0 = (8.1635 x 10-4 + 3.8325 x 10-4y + 5.2423 x 10-4y2) (T/K — 298.15)

+ (1.2144 x 10-7 + 1.4936 x 10-8y + 1.5633 x 10-7y2)(T/K — 298.15)2,

(29)

where, the thermal linear expansion AL/L0 in %, and y is the molar fraction of UO2.

Bakker et al. [38] have recommended the percentage linear thermal expansion data of (Th1-yUy)O2 (0 < y < 1) by obtaining the linear interpolation of the values of Touloukian [61] and Martin [47] and obtained the following relations in two different set of temperature ranges:

(dL/L0) x 100 = -0.179 — y 0.087 + (5.097 x 10-4 + y4.705 x 10-4)

+ (3.732 x 10-7 — y 4.002 x 10-7) T2 — (7.594 x 10-11 — y 11.98 x 10-11) • Г3

(for 273 K < T < 923 K),

(AL/L0) x 100 = -0.179 —y 0.149 + (5.097 x 10-4 + y 6.693 x 10-4) • T

+ (3.732 x 10-7 — y 6.161 x 10-7) Г2 (31)

— (7.594 x 10-11 — y 19.784 x 10-11) • T3

(for 923 K < T < 2,000 K).

Momin et al. [60] measured lattice thermal expansion of (Th, U)O2 system by X-ray diffraction method. They obtained coefficient of expansion data for pure ThO2 and (Th0.8U0.2)O2 to be 9.5 x 10-6 K-1 and 7.1 x 10-6 K-1, respectively, in the temperature range 298-1,600 K. It was observed that the coefficient of thermal expansion of (Th08U02)O2 is lower than either of ThO2 and UO2, which is quite unreasonable.

Tyagi et al. [77] found CTE values for ThO2 and ThO2-2 wt% UO2 to be 9.58 x 10-6 and 9.74 x 10-6 K-1, respectively, in the temperature range of 298-1,473 K. The CTE value reported in IAEA-TECDOC [40] for ThO2 in the temperature range 300-1,473 Kis9.732 x 10-6 K-1 andforThO2-4 wt%UO2itis 9.85 x 10-6 K-1; both these values were found to be in close agreement with those reported by Tyagi et al. The CTE value 10.33 x 10-6 K-1 for composition (Th087U0.13)O2 in the temperature range 298-1,973 K as reported by Anthonysamy et al. [78] matches well with the value obtained in the IAEA study for ThO2-10 % UO2 which was found to be 10.21 x 10-6 K-1 in the temperature range 300-1,773 K. The average linear thermal expansion coefficients for (Th045U055)O2 and (Th0 09U0 9i)O2 were measured to be 10.83 x 10-6 K-1 and 11.45 x 10-6′ K-1, respectively, in the temperature range between 298 and 1,973 K. These data clearly show that thermal expansion coefficients increases with increase in UO2 content in ThO2-UO2 system. Figure 9 shows % thermal expansion plot of some typical ThO2-UO2 solid solutions.

The coefficient of expansion data of Momin et al. [60], Springer et al. [79], Turner and Smith [80], Kempter and Elliot [56] and Lynch and Beals [81] show a wide scatter of data points when plotted against composition. Rodriguez and Sundaram [82] in their review article reported an average linear thermal expansion coefficient of 9.67 x 10-6 K-1 for ThO2 (293-2,273 K) and 12.5 x 10-6 K-1 for (Th08U0.2)O2 (1,100-2,400 K). Powers and Shapiro [83] reported the same average linear thermal expansion coefficient value of 9 x

UO2 and (U0.064Th0936)O2. They obtained lower coefficient value (8 x 10-6 K-1 up to 1,073 K) for (Th0 8U0.2)O2.

Kutty et al. [84] measured thermal expansion of ThO2, ThO2-4 % UO2, and ThO2-20 % UO2 pellets fabricated by (Coated Agglomerate Pelletization) CAP route using ThO2 and U3O8 powders as the starting materials. They reported that the thermal expansion of ThO2-20 % UO2 pellet was different from that of ThO2 and ThO2-4 % UO2, e. g., it increased more rapidly with increasing temperature in the temperature range of 1,000-1,500 C which they attributed to the loss of oxygen of (Th, U)O2+x above 1,000 C. The thermal expansion behavior of polycrystalline samples of ThO2-3.45 % UO2 and SIMFUEL corresponding to the burnup of 43,000 MWd/Te has been investigated from room temperature to

I, 473 K, and for SIMFUEL corresponding to burnup of 28,000 MWd/Te has been investigated from room temperature to 1,173 K, using a high-temperature X-ray diffraction (HTXRD) by Bhagat et al. [85]. They reported that SIMFUEL has higher thermal expansion than ThO2-3.45 % UO2 and this is related to the higher thermal expansion coefficient of dissolved rare earth oxides and also to the lower melting point of SIMFUEL matrix.

The mean linear thermal expansivity for ThO2-SmOi.5 solid solutions containing 17.9, 41.7 and 52.01 % of SmOi.5 were determined by Subramanian et al. [86] in the temperature range 298-2,000 K. The mean linear thermal expansion coefficients for ThO2-SmO15 solid solution were found to be 10.47, 11.16, and

II. 45 x 10-6 K-1, respectively. The synthesis, characterization, and lattice thermal expansion studies of the ThO2-Nd2O3 phase with general compositions Thi-xNdxO2 — x/2 are reported by Mathews et al. [87]. The lattice thermal expansion (293-1,473 K) behavior of the solid solutions has been investigated by high temperature XRD and found to show a gradual increase with increasing content of NdOi.5 in Thi-xNdxO2-x/2 series. The lattice thermal expansion behavior of a number of single-phase compositions of CeO2-ThO2-ZrO2 in the temperature range from 293 to 1,473 K, as investigated by high-temperature XRD are reported by Grover et al. [88]. The average lattice thermal expansion coefficient of pure thoria was found to be 9.58 x 10-6 K-1, which increased to 11.91 x 10-6 K-1 in the compositionTh0.05Ce0.90Zr0.05O2.

Momin et al. [60] studied thermal expansion behavior of ThO2 and (Th08U02)O2 with 20 wt% Ln2O3. Ln2O3 contained oxides of La, Nd, Ce, Y, Sm, Gd, and Eu in equal proportions. Authors found that average thermal expansion coefficient of the solid solutions of ThO2 and (Th0.8U0.2)O2 with 20 wt% Ln2O3 show an increase as compared to those of the parent compounds. Authors related the higher values of coefficient of expansion to the partial substitution of U4+ or Th4+ with Ln3+ resulting in weakening the interatomic bonding in the solid solution matrix. Grover et al. [88] found that coefficient of linear thermal expansion of (Th, Ce, Zr)O2 is higher than ThO2 and increases with increase of cerium and zirconium content in (Th, Ce, Zr)O2. Dilatometric measurement on ThO2-10.09 % UO2 and ThO2-20.02 mol% UO2 by Springer et al. [79] and XRD determination on ThO2-50.05 % UO2 by Kempter and Elliot [56] have been reported. Variation in expansion with composition in any case is reported to be quite small.