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i35

Thorium-based nuclear power

Thorium cycle

Thorium is a radioactive metal, slightly more abundant in the Earth's crust than uranium and with similar chemical properties as uranium. The concept of the thorium reactor is based on the conversion by neutron capture of non-fissile thorium-232 atoms into uranium-233 atoms, which are fissile. The use of thorium as nuclear fuel would imply a breeder cycle, similar to that of the uranium-plutonium breeding cycle.

A thorium reactor would consist of an active reactor core in which fissile nuclides are fissioned, surrounded by a blanket with thorium-232. The active core would generate heat for power generation and the neutrons for the transmutation of thorium-232 into uranium-233. After a given amount of absorbed neutron radiation the material would be removed from the blanket and transported to a reprocessing plant. There the material would be separated into several fractions: remaining thorium-232, newly formed uranium-233 and unwanted nuclides. The thorium would be replaced into the blanket of the reactor and the uranium-233 would be used to fabricate fuel elements for the core of the reactor.

Research and development on the thorium cycle has been less intensive than on the U-Pu cycle and never reached the prototype phase. Among a number of other countries, the USA conducted Th-232/U-233 research in the 1960s and 1970s. For unclear reasons the research has not been continued.

Uranium-233

Uranium-233 does not occur in nature, due to its relatively short half-life of 158000 years. In the USA, during the 1950s and 1960s, U-233 has been envisioned as fuel in very compact military reactors for special applications, e.g. for electricity generation on remote locations and in large (military) spacecraft, in nuclear-powered rockets and even in a nuclear-powered bomber. Although U-233 reportedly would have about the same properties as plutonium for use in nuclear weapons, no such weapons seem to have been developed. Undoubtedly the former Sovietunion also has done research on the use of thorium, but little is known about that.

Apparently there exist good reasons not to use U-233 in military reactors or in weapons and to discontinue the research towards the thorium power reactor. The high radioactivity of uranium-233 and unavoidable contamination with unwanted radionuclides impeding the controllability of the fission process in uranium-233 may be a factor.

India seems to be the only country at this moment still conducting some research on the Th-232/U-233 fuel cycle.

Only small quantities of U-233 exist in the world at this moment, the USA has 1710 kg of it in storage, 905 kg of which still contained in spent fuel. The U-233 stocks in other countries are unknown. The largest DOE reactor currently operating could produce only about 0.3 kg/year.

Time frame of any thorium cycle

To generate sufficiently pure U-233, special reactors are required, likely not appropiate for use as power reactors. It would take decades to construct these reactors and to generate sufficient U-233 to start up the first operating Th-U breeder system. Then it would take centuries to attain a thorium breeder capacity equalling the current nuclear capacity (370 GWe), assumed the cycle would work. Hybrid reactor A major drawback of the thorium cycle is that a genuine thorium breeder reactor cannot sustain a fission process in itself, but always would need an external accelerator-driven neutron source, or the addition of extra fissile material, such as plutonium or uranium-235 from conventional reactors.

Feasibility

The feasibility of the thorium breeding cycle is even more remote than that of the U-Pu breeder. This is caused by specific features of the thorium cycle on top of fundamental limitations. The realisation of the thorium-uranium cycle would require the availability of 100% perfect materials and 100% complete separation processes. None of these two prerequisites are possible, as follows from the Second Law of thermodynamics [more i42, i43]. It can be argued beforehand that the Th-U breeder cycle will not work as envisioned. In addition it would be questionable if the energy balance of any thorium fuelled nuclear power system could be positive.

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