Nuclear fuels can be classified into two main categories; metallic fuels and ceramic fuels. The most common metallic fuels include uranium, plutonium, and thorium (Kirillov et al.,

2007) . The advantage of metallic fuels is their high thermal conductivity; however, they suffer from low melting points and also that the fuel undergoes phase change. The three phases in a metallic uranium fuel includes a-, p~, and Y-phase. A phase changes to another phase as a function of temperature, resulting in a volume change in the fuel. In addition, metallic fuels undergo oxidation when exposed to air or water. For use in high-temperature applications, a potential fuel must have a high melting point, high thermal conductivity, and good irradiation and mechanical stability (Ma, 1983). These requirements eliminate various nuclear fuels categorized under the metallic fuels mainly due to their low melting points and high irradiation creep and swelling rates (Ma, 1983). On the other hand, ceramic fuels have promising properties, which make these fuels suitable candidates for SCWR applications. Table 2 provides basic properties of selected fuels at 0.1 MPa and 25°C (Chirkin, 1968; IAEA, 2008; Frost, 1963; Cox and Cronenberg, 1977; Leitnaker and Godfrey, 1967; Lundberg and Hobbins, 1992).

In general, ceramic fuels have good dimensional and radiation stability and are chemically compatible with most coolants and sheath materials. Consequently, this section focuses only on ceramic fuels. The ceramic fuels examined in this chapter are U02, MOX, Th02, UC, UN, UC>2-SiC, U02-C, and U02-Be0. Further, these ceramic fuels can be classified into three categories: 1) low thermal-conductivity fuels, 2) enhanced thermal-conductivity fuels, and 3) high thermal-conductivity fuels. Low thermal-conductivity fuels are U02, MOX, and Th02. Enhanced thermal-conductivity fuels are U02-SiC, U02-C, and U02-Be0; and high thermal — conductivity fuels are UC and UN.

In addition to the melting point of a fuel, the thermal conductivity of the fuel is a critical property that affects the operating temperature of the fuel under specific conditions. U02 has been used as the fuel of choice in BWRs, PWRs, and CANDU reactors. The thermal conductivity of U02 is between 2 and 3 W/m K within the operating temperature range of SCWRs. 0n the other hand, fuels such as UC and UN have significantly higher thermal conductivities compared to that of U02 as shown in Fig. 9 (Cox and Cronenberg, 1977; Frost et al., 1963; IAEA, 2008; Ishimoto et al., 1995; Leitnaker and Godfrey, 1967; Khan et al., 2010, Kirillov et al., 2007; Lundberg and Hobbins, 1992; Solomon et al., 2005). Thus, under the same operating conditions, the fuel centerline temperature of high thermal conductivity fuels should be lower than that of U02 fuel.