Currently, there is an interest in using thorium based fuels in nuclear reactors. Thorium is widely distributed in nature and is approximately three times as abundant as uranium. However, ThO2 does not have any fissile elements to fission with thermal neutrons. Consequently, ThO2 must be used in combination with a "driver" fuel (e. g., UO2 or UC), which has 235U as its initial fissile elements. The presence of a "driver" fuel such as UO2 in a nuclear-reactor core results in the production of enough neutrons, which in turn start the thorium cycle. In this cycle, 232Th is converted into 233Th, which decays to 233Pa. The latter element eventually results in the formation of 233U, which is a fissile element (Cochran and Tsoulfanidis, 1999).

In regards to PT reactors, there are two possibilities when ThO2 is used. One option is to place ThO2 and a "driver" fuel in different fuel channels. The separation between ThO2 fuel and the "driver" fuel allows ThO2 fuel to stay longer inside the core. The second option is to
enclose ThC>2 and the "driver" in same fuel bundles, which are placed inside the fuel channels throughout the reactor core. This option requires the enrichment of the "driver" fuel since it has to be irradiated as long as ThO2 fuel stays inside the core (IAEA, 2005). Nevertheless, the current study considers the thermal aspects of one single fuel channel, which consists of ThO2 fuel bundles (i. e., first Option). However, this assumption does not suggest that the whole core is composed of fuel channels containing ThO2.

The use of thorium based fuels in nuclear reactors requires information on the thermophysical properties of these fuels, especially thermal conductivity. Jain et al. (2006) conducted experiments on thorium dioxide (ThO2). In their analysis, the thermal conductivity values were calculated based on Eq. (3), which requires the measured values of the density, thermal diffusivity, and specific heat of ThO2. These properties were measured for temperatures between 100 and 1500°C (Jain et al., 2006). In the current study, the correlation developed by Jain et al. (2006), which is shown as Eq. (4), has been used.

k = apcp (3)

1

kThC2 0.0327+1.603X10-4 T ^