It is well known that the thermal conductivity of ThO2 is higher than that of UO2 by *50 % over a significant range of temperature. Berman et al. [123] made a systematic attempt to correlate thermal conductivity, temperature, and composi­tion for ThO2-UO2 system in the early 1970s. Belle and Berman [12] updated the thermal conductivity correlation to 3,400 K by making use of the enthalpy data. Some information is available in literature for thoria—urania mixtures are from the work of Murti and Mathews [124], Lucuta et al. [125], Pillai et al. [93], Belle et al. [104], Kingery et al. [109], Berman et al. [123], IAEA-TECDOCs etc. (Table 13) but more data are still needed to completely characterize the thermal conductivity of (Th, U)O2 fuel pellets. As a rule, in a homogeneous unirradiated mixture of ThO2-UO2, the thermal conductivity is somewhat higher than the thermal conductivity of unirradiated UO2, depending on the temperature and the relative content of the ThO2. However, it is worth mentioning that thermal con­ductivities of (Th0 655U0.345)O2 and (Th0 355U0.645)O2 pellets were found to be lower than that of both pure ThO2 and UO2 and degradation is large at low temperatures, but smaller as the temperature increases [67].

Mcelroy et al. [110] have measured the thermal conductivity of sol-gel-derived ThO2 fuels from 80 to 1,400 K and compared with similar measurements on UO2. Murabayakshi et al. [114] reported the thermal conductivity of ThO2 pellets having densities ranging from 90 to 95 %. In respect of the porosity dependence of the thermal conductivity, the experimental results deviated significantly from the relationship derived by Loeb, and a modified Maxwell model was introduced to explain the data. Jain et al. [126] reported thermal diffusivity of a range of thoria — lanthana solid solutions in the compositional range from pure thoria to 10 mol% LaOi.5 by the laser-flash method covering a temperature range from 373 to 1,773 K, and reported that thermal conductivity of thorium oxide-lanthanum oxide solid solutions decreases with increasing lanthanum content and tempera­ture. Ronchi et al. [118] measured thermal conductivity of (Th088U0.12)O2 in the temperature range of 573-1,573 K. Ferro et al. [127] evaluated diffusivity of (Th0.94U0.06)O2 and (Th0.90U0.i0)O2 from 650-2,700 K. Lemehov et al. [4] pre­sented a model for the lattice thermal conductivity of pure and mixed oxides based on the Klemens-Callaways approach for the dielectric heat conductance modeling

and on some correlations between thermoelastic properties of solids. The thermal conductivity of ThO2 and Th0 .98U0. 02O2 was measured from 300 to 1,200 K by Pillai and Raj [93] and they showed that the decrease in thermal conductivity of Th0.98U0.02O2 over that of ThO2 is due to the enhanced phonon-lattice strain interaction in the oxide. Murti and Mathews [124] measured thermal conductivity on thorium-lanthanum mixed oxide solid solutions covering a temperature range from 573 to 1,573 K and a compositional range from 0 to 30 mol% LaO1.5 and reported that thermal conductivity of the solid solutions were found to decrease with increase in lanthanum oxide content or temperature. Kutty et al. [84] mea­sured thermal conductivity of ThO2, ThO2-4 % UO2, ThO2-10 % UO2 and ThO2- 20 % UO2 made by coated agglomerate pelletization (CAP) process and reported that thermal conductivity decreased with UO2 content. A study carried out by

INEEL [67] shows that ThO2 has a higher thermal conductivity than UO2, but (Th, U)O2 containing 65 or 35 wt% ThO2 has similar in thermal conductivity of UO2.

An assessment of thermal conductivity data of both irradiated and unirradiated ThO2 and Th1-yUyO2 solid solutions has been made by Berman et al. [123]. They analyzed data of Springer et al. [57], Jacobs [128], Matolich and Storhok [129] and Belle et al. [12]. Berman et al. [123] suggested complex behavior of the parameters A and B of Eq. (39) on variation of the uranium content which is inconsistent with theory and data on other ThO2 or UO2 compounds containing substitutions. The assessment by Bakker et al. [38] used only those data sets that contain pure ThO2, which show a systematic decrease of the thermal conductivity on increasing UO2 content (for UO2 concentrations up to 20 %). Since good agreement exists between the variation of the A and B parameter on substitution as determined by Murabayashi [114] and the variation of A and B of comparable compounds as well as that predicted by theory, these parameters are used to obtain a recommended thermal conductivity for Thi_yUyO2. The uranium concentration dependence of the thus obtained A and B parameters were fitted to obtain an equation that is valid for uranium concentration up to 10 % and a theoretical density of 95 %:

A = 4.195 x 10-4 + 1.112 • y — 4.499 • y2, (57)

B = 2.248 x 10-4 — 9.170 x 10-4 • y + 4.164 x 10-3 • y2 (58)

The recommended equation for (Thi_yUy) O2 containing up to 10 % UO2 is:

k(Th-u)02 = [4.195 • 10-4 + 1.112 • y — 4.499 • y2

+ (2.248 • 10-4 — 9.170 • 10-4 • y + 4.164 • 10-3 • y2) • t]

The above equation is valid in the temperature range 300-1,173 K. Figure 17 shows thermal conductivity of ThO2-UO2 for various UO2 contents.

An elaborative study has been reported in IAEA-TECDOC [40] on ThO2 containing 4, 6, 10, and 20 % of UO2. The following are the recommended equations for the thermal conductivity (k) as a function of temperature (T/K) which is valid from 873 to 1,873 K:

k[Th0.96U0.04]O = 1/(-0.04505 + 2.6241 • 10-4 • T) (60)

k[Th0.80U0.20]O2 = 1/(0.02771 + 2.4695 • 10-4 • T) (61)

Subsequently, best-fit equation for thermal conductivity of (Th1-yUy)O2 of 95 % theoretical density as a function of composition (y in wt%) and temperature (T/K) has been derived, which is valid through 873-1,873 K.

k(y, T) = 1/[-0.0464 + 0.0034 • y +(2.5185 • 10-4 + 1.0733 • 10-7 • y)- T]

(62)

600 800 1000 1200 1400 1600 1800 2000

Temperature, K