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Thorium Fuel Cycle

The thorium fuel cycle is a process chain consisting of various stages. The thorium fuel cycle uses thorium 232 as a fertile material. During the fuel burning, the thorium 232 transforms into a fissile uranium 233.

Thorium reactors are based on the thorium fuel cycle and use thorium 232 as a fertile material. During the fuel burning, thorium 232 transforms into a fissile uranium 233. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as thorium 231), which are insufficient to initiate and sustain a nuclear chain reaction. Therefore, additional fissile material is necessary to initiate the fuel cycle.

According to proponents, the thorium fuel cycle offers several potential advantages over a uranium fuel cycle, including thorium’s greater abundance, better physical and nuclear properties (e.g., the lower capture-to-fission ratio for thermal neutrons), reduced plutonium and actinide production, and better resistance to nuclear weapons proliferation when used in traditional light water reactors.

Thorium – Fuel Breeding

n+_{90}^{232}\textrm{Th} {\rightarrow} _{90}^{232}\textrm{Th}+\gamma \rightarrow _{91}^{233}\textrm{Pa} \rightarrow _{92}^{233}\textrm{U}

232Th is the predominant isotope of natural thorium. If this fertile material is loaded in the nuclear reactor, the nuclei of 232Th absorb a neutron and become nuclei of 233Th. The half-life of 233Th is approximately 21.8 minutes. 233Th decays (negative beta decay) to 233Pa (protactinium), whose half-life is 26.97 days. 233Pa decays (negative beta decay)  to 233U, a very good fissile material. On the other hand, proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

What is Thorium

Thorium is a naturally-occurring chemical element with atomic number 90, which means there are 90 protons and 90 electrons in the atomic structure. The chemical symbol for thorium is Th. Thorium was discovered in 1828 by Norwegian mineralogist Morten Thrane Esmark. Joens Jakob Berzelius, the Swedish chemist, named it after Thor, the Norse god of thunder.

Thorium is a naturally-occurring element estimated to be about three times more abundant than uranium. Thorium is commonly found in monazite sands (rare earth metals containing phosphate minerals).

Thorium has 6 naturally occurring isotopes. All of these isotopes are unstable (radioactive), but only 232Th is relatively stable with a half-life of 14 billion years, which is comparable to the age of the Earth (~4.5×109 years). Isotope 232Th belongs to primordial nuclides, and natural thorium consists primarily of isotope 232Th. Other isotopes (230Th, 229Th, 228Th, 234Th, and 227Th) occur in nature as trace radioisotopes, which originate from the decay of 232Th, 235U, and 238U.

Thorium 232

Thorium 232, which alone makes up nearly all natural thorium, is the most common isotope of thorium in nature. This isotope has the longest half-life (1.4 x 1010 years) of all isotopes, with more than 83 protons, and its half-life is considerably longer than the age of Earth. Therefore 232Th belongs to primordial nuclides.

232Th decays via alpha decay into 228Ra. 232Th  occasionally decays by spontaneous fission with a very low probability of 1.1 x 10-9 %.

232Th is a fertile isotope, and 232Th is incapable of undergoing a fission reaction after absorbing a thermal neutron. On the other hand, 232Th can be fissioned by a fast neutron with energy higher than >1MeV. 232Th does not also meet alternative requirements for fissile materials. 232Th is incapable of sustaining a nuclear fission chain reaction because too many neutrons produced by fission of 232Th have lower energies than the original neutron.

Isotope 232Th is the key material in the thorium fuel cycle. Radiative capture of a neutron leads to the formation of fissile 233U. This process is called nuclear fuel breeding.

The fertile-to-fissile conversion characteristics depend on the fuel cycle and the neutron energy spectrum. For a thermal neutron spectrum (E < 1 eV) and the uranium fuel cycle, fuel breeding (C>1) is not feasible, although η for both isotopes is greater than 2. This is because η is not large enough to compensate for the neutron leakage and its parasitic capture.

The situation is considerably better for a thermal neutron spectrum (E < 1 eV) and the thorium fuel cycle. Due to the very low capture-to-fission ratio, the reproduction factor for uranium 233 is about η = 2.25. From this point of view, the thorium fuel cycle is promising, and a thermal reactor of this type could successfully be bred.

See also: Conversion Factor

References:
Nuclear and Reactor Physics:
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      2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
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      8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

      1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
      2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
      3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
      4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Nuclear Fuel Cycle