The initial driver for thorium fuel development was to provide an alternative fuel cycle in anticipation of a projected rapid growth in nuclear power and possible shortage of natural uranium. An added stimulus was thorium’s supposed abundance in nature, based on the fact that the average concentration in the earth crust is approximately three times that of uranium, as mentioned above. Further, by the mid-1970s, the uranium price reached $40.00/pound U3O8 and this resulted in a perceived shortfall of low-price uranium based, in part, on one large nuclear power plant vendor being unable to meet uranium supply commitments to its customers. Along with the abundance of thorium in nature and breeding U-233, there were a number of other reasons at that time for rising interest in the thorium fuel cycle. Some of them included:

• the absence of uranium resources but large amounts of identified thorium resources in some countries having an ambitious civil nuclear programme, such as India

• the good in-core neutronic and physical behaviour of thorium fuel under irradiation

• a lower initial excess reactivity requirement (higher thermal conversion factor) of thorium-based cores using particular configurations

Thus, as illustrated in Table 8.1, the feasibility of different types of reactors based on Th fuels has been successfully demonstrated and significant experience has been accumulated so far, theoretically as well as practical and engineering-wise.

By the early 1980s, a number of factors had essentially killed enthusiasm for alternative fuel cycles. First, interest in the nuclear option waned significantly, especially in the US where public support for nuclear power dramatically declined following the Three Mile Island event of 1979. This anti-nuclear trend intensified and was further exacerbated in Europe by Chernobyl, seven years later. Second, starting in the early 1980s, the price of uranium remained low for over two decades so that, again, there was less interest in developing alternative fuel cycles. A contributing factor was the introduction into the market of down — blended uranium obtained from nuclear weapon disarmament programs (e. g., the US’s collaboration with Russian in the Megatons to Megawatts Program). Third, by the end of the 1970s, the Ford and Carter administrations had put an end to commercial reprocessing in the US so that it no longer had the capability to recover the fissile material from any non-military used fuel, let alone thorium-based fuel. Finally, there were proliferation concerns because, at that time, the reference option for implementing the thorium cycle was to deploy it with HEU. Not only is HEU chemically separable from thorium (assuming seed and fertile material are combined), but some fuel designs completely separated the HEU driver fuel from the fertile thorium. Consequently, the infrastructure needed for large-scale commercialization of thorium fuels never came about.

In the last decade, however, there has been a revival of interest in thorium — based fuels. This seems to have been initially motivated by the development of a LWR proliferation-resistant fuel cycle (i. e. the Radkowsky Concept),[15] and also by the so-called nuclear renaissance and resource scarcity that it might entail. It was also stimulated by some of the same factors that were the drivers for thorium cycles development in the 1950s and 1960s. These new factors vary from country to country, of course, but they include:

• The potential for a low production of plutonium and minor actinides in thorium based-fuel cycles. This is explained by the lower position of thorium in the Mendeleev’s table.

• The capability of destroying plutonium by fissioning it in a plutonium/thorium cycle in thermal reactors. These investigations include advanced reactor concepts based on thorium fuel cycles for future nuclear applications such as LWRs, HTRs, molten salt reactors (MSRs), accelerator-driven systems (ADS) and even fusion blanket systems.

• Transmutation of minor actinides.

• The possibility of breeding fissile isotopes (i. e. a conversion factor greater than one) with a thorium cycle in some thermal reactors such as MSRs, which is one of the concepts included for Generation IV systems.

• More recently, the dramatic increase in the price of uranium, which is closely tied to the perceived shortage of this material in light of a rapid growth of nuclear energy especially in Asian countries.

In this chapter we will look at some of these points in more detail.

Because of its long-term prospects, thorium continues to be studied. There is even currently a new upswing of interest in thorium both within academic institutions and R&D organizations but more importantly by industry. Indeed, some utilities as well as fuel vendors are revisiting thorium to investigate the industrially viable paths for the use of thorium as a complement to uranium/plutonium in LWRs, the main goal being savings in the usage of natural uranium when market conditions might render this thorium option viable. In Japan the HTTR could well be used in the future with thorium (as well as HTR-10 in China). Furthermore, India is still considering thorium as an industrial fuel for use in the not too distant future.