What is Uranium

Uranium is a naturally-occurring chemical element with atomic number 92 which means there are 92 protons and 92 electrons in the atomic structure. The chemical symbol for uranium is U. Uranium was discovered in 1789 by Martin Klaproth in the mineral called pitchblende (uraninite). He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier. It was first isolated as a metal in 1841 by Eugene-Melchior Peligot. Henri Becquerel discovered uranium to be radioactive in 1896. He discovered that uranium minerals could expose a photographic plate through another material. In fact he was the first to discover the process of radioactivity.

Uranium is commonly found  at low levels (a few ppm – parts per million) in all rocks, soil, water, plants, and animals (including humans). Uranium occurs also in seawater, and can be recovered from the ocean water. Significant concentrations of uranium occur in some substances such as uraninite (the most common uranium ore), phosphate rock deposits, and other minerals.

Natural uranium consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (238.03u).  Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). The abundance of  isotopes in the nature is caused by difference in the half-lifes. All three naturally-occurring isotopes of uranium (238U, 235U and 234U)  are unstable. On the other hand these isotopes (except 234U) belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.5×109 years for 238U).

In nuclear reactors we have to consider three artificial isotopes, 236U, 233U and 232U. These are produced by transmutation in nuclear reactors from 235U and 232Th.

Uraninite - the most common uranium ore.Uraninite – the most common uranium ore.
Uranium 235. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 235. Comparison of total fission cross-section and cross-section for radiative capture.

Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1

Isotopes of Uranium

The main isotopes, which have to be considered in the fuel cycle of all commercial light water reactors, are:

Naturally-occurring isotopes

  • 238U. 238U belongs to the group of fertile isotopes. 238U decays via alpha decay to 234Th with half-life of ~4.5×109 years. 238U occasionally decays by spontaneous fission with probability of 0.000055%. Its specific activity is very low ~3.4×10-7 Ci/g.
  • 235U. 235U belongs to the group of fissile isotopes. In fact, 235U is the only existing fissile nucleus from naturally-occurring isotopes and therefore it is a highly strategic material. 235U decays via alpha decay (by way of thorium-231) into 231Pa with half-life of ~7×108 years. 235U occasionally decays by spontaneous fission with very low probability of 0.0000000072%. Its specific activity is very low ~2.2×10-6 Ci/g.
  • 234U. 234U belongs to the group of fertile isotopes. 234U decays via alpha decay to 230Th with half-life of 246 000 years. 234U occasionally decays by spontaneous fission with very low probability of 0.0000000017%. Its specific activity is much higher ~0.0063 Ci/g.

Artificial isotopes

  • 233U. 233U belongs to the group of fissile isotopes. It is produced by neutron radiative capture in nuclear reactors containing thorium fuel. 233U decays via alpha decay into 229Th with half-life of 159 200 years. 233U occasionally decays by spontaneous fission with very low probability of 0.000000006%. Its specific activity is ~0.0098 Ci/g.
  • 236U. 236U is neither a fissile isotope, nor a fertile isotope. 236U is fissionable only by fast neutrons. Isotope 236U is formed in a nuclear reactor from fissile isotope 235U. 236U decays via alpha decay to 232Th with half-life of ~2.3×107 years. 236U occasionally decays by spontaneous fission with very low probability of 0.00000009%. Its specific activity is ~6.5×10-5 Ci/g.
  • 232U. 232U belongs to the group of fertile isotopes. 232U is a side product in the thorium fuel cycle and also this isotope is a decay product of 236Pu in the uranium fuel. 232U decays via alpha decay to 228Th with half-life of 68.9 years. 232U very rarely decays by spontaneous fission. Its specific activity is very high ~22 Ci/g and its decay chain produces very penetrating gamma rays.

Uranium in the Environment

All three naturally-occurring isotopes of uranium (238U, 235U and 234U) have very long half-life (e.g. 4.47×109 years for 238U). Because of this very long half-life uranium is weakly radioactive and contributes to low levels of natural background radiation in the environment. These isotopes are alpha radioactive (emitting alpha particle), but they can also rarely undergo a spontaneous fission.
All naturally-occurring isotopes belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.54×109 years). Uranium has the second highest atomic mass of these primordial nuclides, lighter only than plutonium. Moreover the decay heat of uranium and its decay products (e.g. radon, radium etc.) contributes to heating of Earth’s core. Together with thorium and potassium-40 in the Earth’s mantle is thought that these elements are the main source of heat that keeps the Earth’s core liquid.
Major heat-producing isotopes. Uranium and its decay products (e.g. radon, radium etc.) contributes to heating of Earth's core.Share of major heat-producing isotopes on the heating of Earth’s core. Uranium 238 has important share of 39%.

Uranium consumption in a nuclear reactor

Consumption of a 3000MWth (~1000MWe) reactor (12-months fuel cycle)

It is an illustrative example, following data do not correspond to any reactor design.

  • Typical reactor may contain about 165 tonnes of fuel (including structural material)
  • Typical reactor may contain about 100 tonnes of enriched uranium (i.e. about 113 tonnes of uranium dioxide).
  • This fuel is loaded within, for example, 157 fuel assemblies composed of over 45,000 fuel rods.
  • A common fuel assembly contain energy for approximately 4 years of operation at full power.
  • Therefore about one quarter of the core is yearly removed to spent fuel pool (i.e. about 40 fuel assemblies), while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment (see Power Distribution).
  • The removed fuel (spent nuclear fuel) still contains about 96% of reusable material (it must be removed due to decreasing kinf of an assembly).
  • Annual natural uranium consumption of this reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
  • Annual enriched uranium consumption of this reactor is about 25 tonnes of enriched uranium.
  • Annual fissile material consumption of this reactor is about 1 005 kg.
  • Annual matter consumption of this reactor is about 1.051 kg.
  • But it corresponds to about 3 200 000 tons of coal burned in coal-fired power plant per year.

See also: Fuel Consumption

Naturally-occurring Isotopes of Uranium

Uranium 238 decay.

Uranium 238 decays via alpha decay (by way of thorium-234 and protactinium-234) into 234U.
Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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.

Fissile / Fertile Material Cross-sections

Fissile / Fertile Material Cross-sections. Uranium 238.
Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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 decay.

Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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.

Fissile / Fertile Material Cross-sections

Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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.

235U content as a function of burnup level of a PWR fuel.

235U content as a function of burnup level of a PWR fuel.
Source: IAEA-TECDOC-1529 Management of
Reprocessed Uranium

See also: Uranium 235

See also: Neutron Cross-section

Fissile / Fertile Material Cross-sections

Source: JANIS (Java-based Nuclear Data Information Software)
http://www.oecd-nea.org/janis/

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.

Residual 234U content as function of burnup level of PWR fuel.

Residual 234U content as function of burnup level of PWR fuel.
Source: IAEA-TECDOC-1529 Management of
Reprocessed 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

Artificial Isotopes of Uranium

Fissile / Fertile Material Cross-sections

Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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.

Uranium absorption reaction

236U content as a function of burnup level of a PWR fuel.

236U content as a function of burnup level of a PWR fuel.
Source: IAEA-TECDOC-1529 Management of
Reprocessed Uranium

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%.

Fissile / Fertile Material Cross-sections

Source: JANIS (Java-based Nuclear Data Information Software)
http://www.oecd-nea.org/janis/

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.

Uranium 232 production

Fissile / Fertile Material Cross-sections

Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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

See previous:

See above:

See next:

Click edit button to change this text.