Uranium

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. 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 also occurs in seawater and can be recovered from 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 the uranium element is close to the atomic mass of the 238U isotope (238.03u).  Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). Differences in the half-lives cause the abundance of isotopes in nature. 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.
Fissile / Fertile Material Cross-sectionsUranium 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 a half-life of ~4.5×109 years. 238U occasionally decays by spontaneous fission with the 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. 235U is the only existing fissile nucleus from naturally occurring isotopes, and therefore it is a highly strategic material. 235U decays via alpha decay (thorium-231) into 231Pa with a half-life of ~7×108 years. 235U occasionally decays by spontaneous fission with a 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 a half-life of 246 000 years. 234U occasionally decays by spontaneous fission with a 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 radiative neutron capture in nuclear reactors containing thorium fuel. 233U decays via alpha decay into 229Th with a half-life of 159 200 years. 233U occasionally decays by spontaneous fission with a 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 a half-life of ~2.3×107 years. 236U occasionally decays by spontaneous fission with a 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 a 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 uranium isotopes (238U, 235U, and 234U) have a 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 natural background radiation levels in the environment. These isotopes are alpha radioactive (emitting alpha particle), but they can also rarely spontaneously 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 the Earth’s core. Together with thorium and potassium-40 in the Earth’s mantle, these elements are the main source of heat that keeps the Earth’s core liquid.

Major heat-producing isotopes.Share of major heat-producing isotopes on the heating of Earth’s core. Uranium 238 has an 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, and the following data do not correspond to any reactor design.

  • A typical reactor may contain about 165 tonnes of fuel (including structural material)
  • A 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 contains energy for approximately 4 years of operation at full power.
  • Therefore about one-quarter of the core is yearly removed to the spent fuel pool (i.e., about 40 fuel assemblies). At the same time, 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).
  • This reactor’s annual natural uranium consumption is about 250 tons of natural uranium (to produce about 25 tons of enriched uranium).
  • The annual enriched uranium consumption of this reactor is about 25 tonnes of enriched uranium.
  • The annual fissile material consumption of this reactor is about 1 005 kg.
  • The 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
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 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 the probability of 0.000055%.

238U is a fissionable isotope but is not a fissile isotope. 238U is not capable of undergoing a fission reaction after absorbing a thermal neutron. On the other hand, 238U can be fissioned by fast neutron with energy higher than >1MeV. 238U does not also meet the alternative requirement to fissile materials. 238U cannot sustain a nuclear fission chain reaction because too many neutrons produced by the fission of 238U have lower energies than the 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 also belongs to the group of fertile isotopes. Radiative capture of a neutron leads to the formation of fissile 239Pu. This is how 238U contributes to the operation of nuclear reactors and the 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
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 nature. This isotope has a 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 a very low probability of 0.0000000072%.
235U is a fissile isotope, which means 235U can undergo a fission reaction after absorbing a thermal neutron.

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

Moreover, 235U also meets the 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. 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 is only a residue caused by the difference in the half-lives of 235U and 238U. If humankind 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

Uranium 234
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 a half-life of only 2.46×105 years, and therefore it does 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 because 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 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 a function of burnup level of PWR fuel.
Source: IAEA-TECDOC-1529 Management of
Reprocessed Uranium

Since uranium enrichment separates light isotopes from heavy isotopes, uranium enrichment also results in the enrichment of 234U. Therefore enriched uranium also contains more isotope 234U than natural uranium. On the other hand, enrichment tails or also depleted uranium contain much less than 234U. In nuclear reactors, which use enriched uranium as a fuel, such increased content of 234U is acceptable. 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 cannot undergo a fission reaction after absorbing a thermal neutron. 234U belongs to the group of fertile isotopes.  Radiative capture of a neutron leads to 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. 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 compared to the 239Pu breeding.

See also: Uranium 234

See also: Neutron Cross-section

Artificial Isotopes of Uranium

 
Uranium 233
Fissile / Fertile Material Cross-sections
Source: JANIS (Java-based nuclear information software)
http://www.oecd-nea.org/janis/

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

233U is a fissile isotope, which means 233U can undergo a fission reaction after absorbing a thermal neutron. Moreover, 233U also meets the alternative requirement that the amount (~2.48 per one fission by thermal neutron) of neutrons produced by 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 be 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
Uranium 236 is not naturally-occurring isotope of uranium. It is a manufactured isotope found in spent nuclear fuel or 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 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 neutrons. 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 a half-life of 2.34×107 years and has a longer half-life than any other artificial actinide or fission product produced in nuclear reactors. 236U has about 190 times higher specific activity than the isotope 238U. This is due to the fact 238U has a half-life about 190 times as long as 236U. This results in a high contribution to the radioactivity of reprocessed uranium. 236U decays via alpha decay to 232Th. 236U occasionally decays by spontaneous fission with a 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 isotope 237U, which quickly beta decays to the isotope 237Np. 237Np may absorb another neutron, thus resulting in 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
Uranium 232 is not naturally-occurring isotope of uranium. It is a manufactured 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 the 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 a fission reaction after absorbing a thermal neutron. This feature plays no significant role in nuclear reactors because the amount of 232U is negligible in sustaining a nuclear fission chain reaction. 232U has a significant fission cross-section (75 barns for thermal neutrons) and 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 the 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, which emits gamma rays of 2.6 MeV, which are very energetic and highly penetrating. These intense radiations make the 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:

Nuclear Fuel

See next: