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Uranium 236

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. The amount of 236U present in spent nuclear fuel is directly proportional to the burnup levels and is dependent on initial ²³⁵U enrichment. For discharged fuel around 50 MWd/tU HM and enrichment of 4% of ²³⁵U it is typically around 0.5 wt%.

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: Neutron Cross-section

Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data LibraryUranium 236. Comparison of total fission cross-section and cross-section for radiative capture.236U content as a function of burnup level of a PWR fuel.
Source: IAEA-TECDOC-1529 Management of Reprocessed Uranium

See also:

Uranium 233

See also:

Uranium

See also:

Uranium 232

Uranium 233

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. 

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, that is 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.

Fission fragments after a nucleus fission. Fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize.Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1Uranium 233. Comparison of total fission cross-section and cross-section for radiative capture.Neutron production per one fast fission of uranium 233.Fission fragment yield for different nuclei. The most probable fragment masses are around mass 93 (Strontium) and 137 (Xenon).

See also:

Uranium 234

See also:

Uranium

See also:

Uranium 236

Uranium 235

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.

Uranium 235 Fission

Uranium 235 is a fissile isotope and its fission cross-section for thermal neutrons is about 585 barns (for 0.0253 eV neutron). For fast neutrons its fission cross-section is on the order of barns. 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.

Typically, when uranium 235 nucleus undergoes fission, the nucleus splits into two smaller nuclei (triple fission can also rarely occur), along with a few neutrons (the average is 2.43 neutrons per fission by thermal neutron) and release of energy in the form of heat and gamma rays. The average of the fragment atomic mass is about 118, but very few fragments near that average are found. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass 95 (Krypton) and 137 (Barium).

Most of the fission fragments are highly unstable (radioactive) nuclei and undergo further radioactive decays to stabilize itself. Therefore part of the released energy is radiated away from the reactor (See also: Reactor antineutrinos). On the other hand most of the energy released by one fission (~170MeV of total ~200MeV) appears as kinetic energy of these fission fragments. The fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. Creation of ion pairs requires energy, which is lost from the kinetic energy of the charged fission fragment causing it to decelerate. The positive ions and free electrons created by the passage of the charged fission fragment will then reunite, releasing energy in the form of heat (e.g. vibrational energy or rotational energy of atoms). This is the principle how fission fragments heat up fuel in the reactor core.

Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1Fission fragments after a nucleus fission. Fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize.Uranium 235 decays via alpha decay into 231Pa.Uranium 235. Comparison of total fission cross-section and cross-section for radiative capture.235U content as a function of burnup level of a PWR fuel.
Source: IAEA-TECDOC-1529 Management of
Reprocessed UraniumNeutron production per one fast fission of uranium 235.Fission fragment yield for different nuclei. The most probable fragment masses are around mass 95 (Krypton) and 137 (Barium).

See also:

Uranium 238

See also:

Uranium

See also:

Uranium 234

Uranium 234

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: Comparison of cross-sections

Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1Uranium 238 decays via alpha decay (by way of thorium-234 and protactinium-234) into 234U.Uranium 234. Comparison of total fission cross-section and cross-section for radiative capture.Residual 234U content as function of burnup level of PWR fuel.
Source: IAEA-TECDOC-1529 Management of Reprocessed Uranium

See also:

Uranium 235

See also:

Uranium

See also:

Uranium 233

Uranium 238

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: Comparison of cross-sections

Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1Uranium 238 decays via alpha decay (by way of thorium-234 and protactinium-234) into 234U.Uranium 238. Comparison of total fission cross-section and cross-section for radiative capture.Neutron production per one fast fission of uranium 238.

See also:

Isotopes of Uranium

See also:

Uranium

See also:

Uranium 235

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Naturally-occurring Isotopes of Uranium

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

• ²³⁸U. ²³⁸U belongs to the group of fertile isotopes.
• 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.
• ²³⁴U. ²³⁴U belongs to the group of fertile isotopes.

Artificial Isotopes of Uranium

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

• ²³³U. ²³³U belongs to the group of fissile isotopes. It is produced by neutron radiative capture in nuclear reactors containing thorium fuel. ²
• 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.
• 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.

Cross-sections of Naturally-occurring Isotopes

Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

Uranium 238. 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.

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Uranium 234. Comparison of total fission cross-section and cross-section for radiative capture.

Cross-sections of Artificial Isotopes

Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

Uranium 233. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 236. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 232. Comparison of total fission cross-section and cross-section for radiative capture.

Fissile / Fertile Material Cross-sections. Comparison of total fission cross-sections.

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Half-life of Isotopes of Uranium

See also:

Uranium

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Uranium in the Environment

Natural Uranium

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.

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.

Share of major heat-producing isotopes on the heating of Earth’s core. Uranium 238 has important share of 39%.

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Cross-sections of Uranium

See also:

Uranium

Decay mode and Half-lifes of Isotopes of Uranium

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.

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

Naturally-occurring isotopes

• ²³⁸U. 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 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 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 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 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 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 238 decay. Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data LibraryUranium 235 decay. Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

See also:

Uranium 232

See also:

Uranium

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Cross-sections of Uranium

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.

Comparison of total fission cross-section and cross-section for radiative capture.

Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data LibraryUranium 238. 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.Uranium 234. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 233. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 236. Comparison of total fission cross-section and cross-section for radiative capture.Uranium 232. Comparison of total fission cross-section and cross-section for radiative capture.

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What is Uranium?

See also:

Uranium

See also:

Uranium 238

What is 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. 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 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.

Share of major heat-producing isotopes on the heating of Earth’s core. Uranium 238 has important share of 39%.

Uranium 235. Comparison of total fission cross-section and cross-section for radiative capture.Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data LibraryUranium 238. Comparison of total fission cross-section and cross-section for radiative capture.Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

See also:

Uranium

See also:

Isotopes of Uranium