Thorium (atomic number 90) is a soft, silvery-white metal when present in pure form. It retains its luster for a long period of time. However, given a chance to oxidize to thorium oxide (ThO2), it quickly loses its luster turning into gray and

finally black in color. It is another nuclear fuel that has not been tapped to its full potential to date. As we have already discussed in Chapter 1, Th232 is a fertile iso­tope that could produce fissile U233 isotope upon capturing a neutron and then fis­sion to produce energy needed for electric power. The relevant reaction has been shown in Eq. (1.3). Thus, thorium is an important breeder material. Thorium exists in only one isotope form (Th232) in nature and decays very slowly (its half-life is

14.5 billion years, that is, thrice the age of the earth). Other isotopic forms of tho­rium (Th228, Th230, and Th234) occur as decay products of thorium and uranium at some point, but they are present in trace amounts only.

Thorium is far more abundant than uranium in nature. Most rocks and sands on earth surface contain minute amount of thorium. Monazite sand that is rare earth phosphate mineral is an important source of thorium, and almost two-third of this high-quality deposit (thorium content of 6-7 wt% on average) is found in southern and eastern coasts of India. Other thorium resource also includes thorite or tho­rium silicate (ThSiO4). A large vein deposit of thorium is present in the state of Idaho of the United States.