Thorium has cubic crystal structure and is thus isotropic. It does not show radia­tion growth effect and thus has better dimensional stability than a-U under irradiation.

7.2.3.1 Pros and Cons ofThorium-Based Fuel Cycles

Thorium is more abundant in nature compared to uranium and naturally there is interest in having an economic fuel cycle based on thorium. Thorium-based fuel cycles offer attractive features that are low level of waste generation along with a less amount of transuranics in the waste and provide a robust diversification option for nuclear fuel supply. Also, the use of thorium in majority of reactors leads to significant additional safety margins. However, the full commercial exploitation of thorium fuels has some significant obstacles in terms ofbuilding an economic case to undertake the necessary developmental work. A great deal of R&D, testing, and qualification work is required before any thorium fuel can be considered for rou­tine commercial application. Other obstacles to the development of thorium fuel cycle are the greater fuel fabrication and reprocessing costs to include the fissile plutonium as a driver material. The high cost of fuel fabrication is partly because of the high level of radioactivity that is involved in the presence of U-233, chemi­cally separated from the irradiated thorium fuel. But the U-233 gets contaminated with traces of U-232 that decays (69-year half-life) to daughter nuclides such as thal­lium-208 that are high-energy gamma-emitters [14]. Even though this improves the proliferation resistance of the fuel cycle, it also makes U-233 hard to handle and easy to detect. Notwithstanding, thorium fuel cycle provides hope for long-term energy security benefits without the need for fast reactors.

Metallic fuels with their high neutron economy, good thermal conductivity, and thermal shock resistance should be the natural choice for fuels. However, they are not adequate for high-temperature reactors due to low strength at high temperatures, phase transformations, and so on. The other ceramics have superior strength at higher temperatures, low thermal expansion, good corro­sion resistance, and good radiation stability. A wide range of compounds are considered as ceramics, which include oxides, carbides, nitrides, borides, sili — cides, sulfides, selenides, and so forth. But ceramics also suffer from brittle­ness especially at lower temperatures. Here, we will discuss few ceramic nuclear fuels and discuss their salient features.

7.3.1