Uranium 238, which alone constitutes 99.28% of natural uranium is the most common isotope of uranium in the nature. This isotope has the longest half-life (4.47×10⁹ years) and therefore its abundance is so high. ²³⁸U belongs to primordial nuclides, because its half-life is comparable to the age of the Earth (~4.5×10⁹ years). For its very long half-life, it is still present in the Earth’s crust.

²³⁸U decays via alpha decay (by way of thorium 234 and protactinium 234) into ²³⁴U. ²³⁸U occasionally decays by spontaneous fission with probability of 0.000055%.

²³⁸U is a fissionable isotope, but is not fissile isotope. ²³⁸U is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand ²³⁸U can be fissioned by fast neutron with energy higher than >1MeV. ²³⁸U does not meet also alternative requirement to fissile materials. ²³⁸U is not capable of sustaining a nuclear fission chain reaction, because too many of neutrons produced by fission of ²³⁸U have lower energies than original neutron.

²³⁸U belongs also to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile ²³⁹Pu. This is the way how ²³⁸U contributes to the operation of nuclear reactors and production of electricity through this plutonium. For example, at a burnup of 40GWd/tU, about 40% of the total energy released comes from bred plutonium. This corresponds to a breeding ratio for this fuel burnup of about 0.4 to 0.5. This effect extends the cycle length for such fuels to sometimes nearly twice what it would be otherwise.

See also: Uranium 238

See also: Nuclear Breeding

See also: Neutron Cross-section

Uranium 235, which alone constitutes 0.72% of natural uranium is the second common isotope of uranium in the nature. This isotope has half-life of 7.04×10⁸ years (6.5 times shorter than the isotope 238) and therefore its abundance is lower than ²³⁸U (99.28%). ²³⁵U belongs to primordial nuclides, because its half-life is comparable to the age of the Earth (~4.5×10⁹ years). For its very long half-life, it is still present in the Earth’s crust.

 ²³⁵U decays via alpha decay (by way of thorium-231) into ²³¹Pa. ²³⁵U occasionally decays by spontaneous fission with very low probability of 0.0000000072%.
²³⁵U is a fissile isotope, which means ²³⁵U is capable of undergoing fission reaction after absorbing thermal neutron.

Moreover, ²³⁵U meets also alternative requirement that the amount (~2.43 per one fission by thermal neutron) of neutrons produced by fission of ²³⁵U is sufficient to sustain a nuclear fission chain reaction. ²³⁵U was the first isotope that was found to be fissile. In fact, ²³⁵U is the only existing fissile nucleus from naturally-occurring isotopes and therefore is a highly strategic material. At the time of the formation of the Earth, ²³⁵U was 85 times more abundant. The 0.72% observed today are only a residue caused by the difference in the half-lifes of ²³⁵U and ²³⁸U. If mankind had been present at the beginning of the Earth, they would not have needed to enrich uranium, because the content of fissile ²³⁵U was significantly higher.

See also: Uranium 235

See also: Neutron Cross-section

Uranium 234, which alone constitutes only 0.0054% (54 parts per million) of natural uranium is the last naturally-occurring isotope of uranium. This isotope has the half-life of only 2.46×10⁵ years and therefore it do not belong to primordial nuclides (unlike ²³⁵U and ²³⁸U). On the other hand, this isotope is still present in the Earth’s crust, but this is due to the fact ²³⁴U is an indirect decay product of ²³⁸U. ²³⁸U decays via alpha decay (by way of thorium-234 and protactinium-234) into ²³⁴U. ²³⁴U decays via alpha decay into ²³⁰Th, except very small fraction (on the order of ppm) of nuclei which decays by spontaneous fission.

In a natural sample of uranium, these nuclei are present in the unalterable proportions of the radioactive equilibrium of the ²³⁸U filiation at a ratio of one atom of ²³⁴U for about 18 500 nuclei of ²³⁸U. As a result of this equilibrium these two isotopes (²³⁸U and ²³⁴U) contribute equally to the radioactivity of natural uranium.

Since an uranium enrichment is a separation of light isotopes from heavy isotopes, the uranium enrichment results also in enrichment of ²³⁴U. Therefore enriched uranium contains also more isotope ²³⁴U than natural uranium. On the other hand enrichment tails or also depleted uranium contains much less ²³⁴U. In nuclear reactors, which use enriched uranium as a fuel, such increased content of 234U is acceptable. An undesirable concentrations may be reached when using reprocessed uranium, because spent nuclear fuel may contain a much higher concentration of about 0.01% of ²³⁴U.

²³⁴U is a non-fissile isotope and it is therefore not capable of undergoing fission reaction after absorbing thermal neutron. ²³⁴U belongs to the group of fertile isotopes.  Radiative capture of a neutron leads to the formation of fissile ²³⁵U similarly to ²³⁸U which radiative capture leads to the formation of fissile ²³⁹Pu. The radiative capture cross-section for ²³⁴U is about 100 barns for thermal neutrons and therefore ²³⁴U is converted to ²³⁵U more easily and therefore at a greater rate than ²³⁸U is to ²³⁹Pu (nuclei of ²³⁸U have a much smaller cross-section of 2 barns). On the other hand, the effect of ²³⁵U breeding is almost insignificant in comparison with the ²³⁹Pu breeding.

See also: Uranium 234

See also: Neutron Cross-section

Uranium 233 is not naturally-occurring isotope of uranium. It is a man-made isotope and is key fissile isotope in the thorium fuel cycle. This isotope has half-life of 159,200 years. ²³³U is produced by neutron radiative capture in nuclear reactors containing thorium 232.

²³³U is a fissile isotope, which means ²³³U is capable of undergoing fission reaction after absorbing thermal neutron. Moreover ²³³U meets also alternative requirement that the amount (~2.48 per one fission by thermal neutron) of neutrons produced by fission of ²³³U is sufficient to sustain a nuclear fission chain reaction.

For the ²³³U nuclei, the number of fission neutrons produced per absorption in the fuel (the reproduction factor – η) is greater than 2.0 over a wide range of thermal neutron spectrum, unlike ²³⁵U and ²³⁹Pu. Therefore breeding can obtained with fast, epithermal or thermal spectra.

²³³U decays via alpha decay into ²²⁹Th. The decay chain of ²³³U itself is in the neptunium series.

See also: Uranium 233

See also: Neutron Cross-section

Uranium 236 is not naturally-occurring isotope of uranium. It is a man-made isotope, which can be found in spent nuclear fuel or in the reprocessed uranium. The presence of this isotope in a sample of uranium is evidence that the sample has been in a nuclear reactor. Isotope ²³⁶U is formed in a nuclear reactor from fissile isotope ²³⁵U. Most of absorption reactions result in fission reaction, but a minority results in radiative capture forming ²³⁶U. The cross-section for radiative capture for thermal neutrons is about 99 barns (for 0.0253 eV neutron). Therefore about 15% of all absorption reactions result in radiative capture of neutron. About 85% of all absorption reactions result in fission.

This isotope has the half-life of 2.34×10⁷ years and has longer half-life than any other artificial actinide or fission product produced in nuclear reactors. ²³⁶U have about 190 times higher specific activity than the isotope ²³⁸U. This due to the fact ²³⁸U has the half-life about 190 times as long as ²³⁶U. This results in high contribution to radioactivity of reprocessed uranium. ²³⁶U decays via alpha decay to ²³²Th. ²³⁶U occasionally decays by spontaneous fission with very low probability of 0.00000009%.

²³⁶U is neither a fissile isotope, nor a fertile isotope. ²³⁶U is fissionable only by fast neutrons. Radiative capture of a neutron leads to the formation of the isotope ²³⁷U, which quickly beta decays to the isotope ²³⁷Np. ²³⁷Np may absorb another neutron, thus results in formation of ²³⁸Np, which quickly beta decay to ²³⁸Pu. The presence of this isotope certainly does not contribute to the neutron economy. On the other hand the radiative capture cross-section for ²³⁶U is very low and this process does not happen quickly in a thermal reactor.

See also: Uranium 236

See also: Neutron Cross-section

Uranium 232 is not naturally-occurring isotope of uranium. It is a man-made isotope and is a side product in the thorium fuel cycle and also this isotope is a decay product of ²³⁶Pu in the uranium fuel. ²³²U is produced from ²³⁵U and ²³²Th via two of the reaction chains shown below. The formation of this isotope in both reactions results from specific (n,2n) reactions in which an incoming neutron knocks two neutrons out of a target nucleus. ²³²U can also be produced by two successive single radiative captures of neutron starting with naturally-occurring isotope ²³⁰Th. ²³⁰Th is a decay product of ²³⁴U, which is in turn a decay product of ²³⁸U.

It is unusual, but ²³²U is a fissile isotope and it is therefore capable of undergoing fission reaction after absorbing thermal neutron. This feature plays no significant role in nuclear reactors, because the amount of ²³²U is negligible in terms of sustaining a nuclear fission chain reaction. ²³²U has a significant fission cross-section (75 barns for thermal neutrons) as well as radiative capture cross-section (73 barns for thermal neutrons). Therefore ²³²U also belongs to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile ²³³U.

The isotope ²³²U has another very important feature. ²³²U has a relatively short half-life of 68.9 years, and therefore the specific activity of ²³²U is much higher than specific activity of the isotope ²³⁸U. In addition the decay chain of ²³²U produces very penetrating gamma rays. The most important gamma emitter, accounting for about 85 percent of the total dose from ²³²U after 2 years, is thallium 208, that emits gamma rays of 2.6 MeV which are very energetic and highly penetrating. These intense radiations make handling of fissile ²³³U or reprocessed uranium contaminated with ²³²U far more dangerous than conventional fuels.

See also: Uranium 232

See also: Neutron Cross-section

Reference: Kang, J.; Von Hippel, F. N. (2001). “U‐232 and the proliferation‐resistance of U‐233 in spent fuel”. Science & Global Security