Thorium

Thorium is a radioactive element similar to uranium. Formerly used to make gas mantles, it is about three times as abundant in the earth's crust as uranium. Soil commonly contains around 6 parts per million of thorium, and some minerals contain 12% thorium oxide. Seawater contains little thorium, because thorium oxide is insoluble. Thorium can be completely burned up in simple reactors (in contrast to standard uranium reactors which use only about 1% of natural uranium). Thorium is used in nuclear reactors in India. If uranium ore runs low, thorium will probably become the dominant nuclear fuel.

Thorium reactors deliver 3.6 billion kWh of heat per ton of thorium, which implies that a 1 GW reactor requires about 6 tons of thorium per year, assuming its generators are 40% efficient. Worldwide thorium resources are estimated to total about 6 million tons, four times more than the known reserves shown in table 24.7. As with the uranium resources, it seems plausible that these thorium resources are an underestimate, since thorium prospecting is not highly valued today. If we assume, as with uranium, that these resources are used up over 1000 years and shared equally among 6 billion people, we find that the "sustainable" power thus generated is 4 kWh/d per person.

An alternative nuclear reactor for thorium, the "energy amplifier" or "accelerator-driven system" proposed by Nobel laureate Carlo Rubbia and his colleagues would, they estimated, convert 6 million tons of thorium to 15000TWy of energy, or 60kWh/d per person over 1000 years. Assuming conversion to electricity at 40% efficiency, this would deliver 24kWh/d per person for 1000 years. And the waste from the energy amplifier would be much less radioactive too. They argue that, in due course, many times more thorium would be economically extractable than the current 6 million tons. If their suggestion - 300 times more - is correct, then thorium and the energy amplifier could offer 120 kWh/d per person for 60 000 years.

Country

Reserves (1000 tons)

Country

Reserves (1000 tons)

Turkey

380

Australia

300

India

290

Norway

170

USA

160

Canada

100

South Africa

35

Brazil

16

Other countries

95

World total

1580

Table 24.7. Known world thorium resources in monazite (economically extractable).

Table 24.7. Known world thorium resources in monazite (economically extractable).

Mined Thorium

Conventional reactor

"Energy amplifier"

4 kWh/d

Mined Thorium

4 kWh/d

"Energy amplifier"

Figure 24.8. Thorium options.

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