5.1.1 UO2 and MOX

As a ceramic fuel, Uranium Dioxide (UO2) is a hard and brittle material due to its ionic or covalent interatomic bonding. In spite of that, the uranium dioxide fuel is currently used in PWRs, BWRs, and CANDU reactors because of its properties. Firstly, oxygen has a very low thermal-neutron absorption cross-section, which does not result in a serious loss of neutrons. Secondly, UO2 is chemically stable and does not react with water within the operating temperatures of these reactors. Thirdly, UO2 is structurally very stable. Additionally, the crystal structure of the UO2 fuel retains most of fission products even at high burn-up (Cochran and Tsoulfanidis, 1999). Moreover, UO2 has a high melting point; however, its thermal conductivity is very low, minimizing the possibility of using UO2 as a fuel of choice for SCWRs. The thermal conductivity of 95% Theoretical Density (TD) UO2 can be calculated using the Frank correlation, shown as Eq. (1) (Carbajo et al., 2001). This correlation is valid for temperatures in the range of 25 to 2847°C.

kuo (T) =————————————- 10°————————— + ^°°5/2exp-16-35/(10-3T) (1)

2 7.5408 + 17.692 (10-3 T) + 3.6142 (10-3 T )2 (10-3 T )5/2

Mixed Oxide (MOX) fuel refers to nuclear fuels consisting of UO2 and plutonium dioxide (PuO2). MOX fuel was initially designed for use in Liquid-Metal Fast Breeder Reactors (LMFBRs) and in LWRs when reprocessing and recycling of the used fuel is adopted (Cochran and Tsoulfanidis, 1999). The uranium dioxide content of MOX may be natural, enriched, or depleted uranium, depending on the application of MOX fuel. In general, MOX fuel contains between 3 and 5% PuO2 blended with 95 — 97 % natural or depleted uranium dioxide (Carbajo et al., 2001). The small fraction of PuO2 slightly changes the thermophysical
properties of MOX fuel compared with those of UO2 fuel. Nonetheless, the thermophysical properties of MOX fuel should be selected when a study of the fuel is undertaken.

Most thermophysical properties of UO2 and MOX (3 — 5 % PuO2) have similar trends. For instance, thermal conductivities of UO2 and MOX fuels decrease as the temperature increases up to 1700°C (see Fig. 9). The most significant differences between these two fuels have been summarized in Table 2. Firstly, MOX fuel has a lower melting temperature, lower heat of fusion, and lower thermal conductivity than UO2 fuel. For the same power, MOX fuel has a higher stored energy which results in a higher fuel centerline temperature compared with UO2 fuel. Secondly, the density of MOX fuel is slightly higher than that of UO2 fuel.

The thermal conductivity of the fuel is of importance in the calculation of the fuel centerline temperature. The thermal conductivities of MOX and UO2 decrease as functions of temperature up to temperatures around 1527 — 1727°C, and then it increases as the temperature increases (see Fig. 9). In general, the thermal conductivity of MOX fuel is slightly lower than that of UO2. In other words, addition of small amounts of PuO2 decreases the thermal conductivity of the mixed oxide fuel. However, the thermal conductivity of MOX does not decrease significantly when the PuO2 content of the fuel is between 3 and 15%. But, the thermal conductivity of MOX fuel decreases as the concentration of PuO2 increases beyond 15%. As a result, the concentration of PuO2 in commercial MOX fuels is kept below 5% (Carbajo et al., 2001). Carbajo et al. (2001) recommended the following correlation shown as Eq. (2) for the calculation of the thermal conductivity of 95% TD MOX fuel. This correlation is valid for temperatures between 427 and 2827°C, x less than 0.05, and PuO2 concentrations between 3 and 15%. In Eq. (2), T indicates temperature in Kelvin.

Where x is a function of oxygen to heavy metal ration and

A(x) = 2.58x + 0.035 (mK/W), C (x) = -0.715 x + 0.286 (m/K)