Physical properties of thorium as an atomic nucleus

All known isotopes of thorium are unstable but one of them, thorium-232 (Th-232) has a very long half-life of approximately 1.41 x 10 1 0 years (it is an alpha emitter). The half-lives of all other isotopes being less than 100 000 years, Th-232 is the sole thorium isotope of naturally occurring thorium,[12] which has an atomic weight of 232.038 g/mol. It undergoes natural disintegration and is eventually converted through a 10-step chain of isotopes to lead-208, a stable isotope. Alpha and beta particles are emitted during this decay. One intermediate product is the gas radon-220 also called thoron.

Th-232 and U-238 are fertile materials. Just as the absorption of a neutron by U-238 generates Pu-239, so, too, U-233 is generated from Th-232. The reactions are very similar.

232Th П > 233

In a reactor core, Th-232 absorbs a neutron to first produce Th-233, which decays very rapidly (with a radioactive decay period of 22 min) into protactinium-233 (Pa-233), which itself decays (with a radioactive period of 27 days) to produce U-233.

It is also possible, however, for Pa-233 to capture a neutron so that the formation of U-233 is, in effect, in competition with the formation of U-234, the balance depending on the average flux level:

Th-232+n ^Th-233 (22 m) ^Pa-233+ n ^Pa-234 (6.7 h) ^ U-234

For thermal neutrons, U-233 has a higher neutron yield per neutron absorbed than either uranium-235 or plutonium-239. The average number of fission neutrons produced per absorption of a thermal neutron (called the ‘eta’ factor) is typically 2.27 for U-233 in a standard PWR compared to 2.06 for U-235 and 1.84 for Pu-239. This is one of the principal advantages of the thorium cycle: the high eta value of the generated fissile isotope, U-233, makes it the best fissile isotope in the thermal range among all existing fissile isotopes. It is therefore theoretically possible to achieve breeding in today’s reactors using Th/U-233 based fuel.

To generate U-233, fissile materials — such as U-235 or Pu-239 — are required to provide the neutrons that will transform the Th-232 into U-233. After being discharged from the reactor, used fuel can be reprocessed. The fissile materials (U-233, U-235 and Pu) as well as the remaining fertile Th-232 are then retrieved to be recycled into new fuel assemblies.

However, one of the principal drawbacks of the thorium cycle is U-232 production through various nuclear reactions on Th-232 and U-233.[13] U-232 is an alpha emitter with a 72 year half-life and is always associated with U-233 at concentrations ranging from tens to hundreds of parts per million. The U-232 decay chain is as follows:

U-232 (a, 72 yrs) ^ Th-228 (a, 1.9 yrs) ^ Ra-224 (a, 3.6 d / /0.24 MeV) ^ Rn-224 (a, 55 s / /, 0.54MeV) ^ Po-216 (a, 15 s) ^ Pb-212 в-,10.6 h/ /, 0.3 MeV) ^ Bi-212 (a, 60 m / /, 0.78 MeV) ^ Tl-208 (в-, 3 m / /, 2.6 MeV) ^ Pb-208 (stable)

It can be seen that this chain includes hard gamma emitters such as thallium-208 (up to 2.6 MeV). Therefore, the presence of U-232 requires that fabrication of U-233 based fuels be performed remotely in a gamma-shielded environment and this may entail significant additional cost.1

If uranium is chemically purified so that its decay products are removed, freshly separated U-233 (with significant concentrations of U-232) can be processed and converted into desired forms in lightly shielded enclosures without significant radiation exposure to workers. Depending on the U-232 concentration, it will take days or weeks for U-232 decay products that emit gamma rays to build up sufficiently to require heavy shielding to protect the workers.

The nuclear characteristics of U-233 are significantly different from those of weapons grade plutonium (WgPu) or highly enriched uranium (HEU). The minimum critical mass of U-233, in a uniform fluoride aqueous solution, is 0.54 kg (American National Standards Institute [ANSI] 1983). This is somewhat less than that of WgPu or HEU. Thus, facilities designed for WgPu or HEU might not be suitable for storage or processing of U-233 unless more restrictive criticality precautions are instituted.2 It is likely that fabrication of reprocessed U-233/ thorium based fuel would be performed in a dedicated facility whose criticality safety will be designed considering U-233.

Chemical characteristics of uranium-233

Uranium-233 is chemically identical to natural, depleted and enriched uranium. Consequently, the same chemical processes used for natural, depleted and enriched uranium are applicable to U-233. As a consequence of its shorter half-life, however, the U-233 isotope has a higher specific radioactivity than the naturally occurring isotopes of uranium (i. e., U-234, U-235 and U-238). Thus, certain radiation-induced chemical reactions are faster in uranium containing significant quantities of U-233. This is of some importance in situations such as long-term storage. The higher radiation levels of U-233 require that storage containers and U-233 storage forms should not contain either organics (plastics, etc.) or water that, through radiolysis, could degrade to form potentially explosive concentrations (unless they can somehow be vented) of hydrogen gas.