## Decay mode and Half-lifes of Isotopes of Uranium

**Natural uranium** consists primarily of isotope ^{238}U (99.28%). Therefore the atomic mass of the uranium element is close to the atomic mass of the ^{238}U isotope (238.03u). Natural uranium also consists of two other isotopes: ^{235}U (0.71%) and ^{234}U (0.0054%). Differences in the half-lives cause the abundance of isotopes in nature. All three naturally occurring isotopes of uranium (^{238}U, ^{235}U, and ^{234}U) are unstable. On the other hand, these isotopes (except ^{234}U) belong to primordial nuclides because their half-life is comparable to the age of the Earth (~4.5×10^{9} years for ^{238}U). In nuclear reactors, we have to consider three **artificial isotopes**, ^{236}U, ^{233}U, and ^{232}U. These are produced by transmutation in nuclear reactors from ^{235}U and ^{232}Th**.**

**Naturally-occurring isotopes**

.^{238}U^{238}U decays via alpha decay to^{234}Th with a half-life of ~4.5×10^{9}years.^{238}U occasionally decays by spontaneous fission with the probability of 0.000055%. Its specific activity is very low ~3.4×10^{-7}Ci/g..^{235}U^{235}U decays via alpha decay (thorium-231) into^{231}Pa with a half-life of ~7×10^{8}years.^{235}U 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..^{234}U^{234}U decays via alpha decay to^{230}Th with a half-life of 246 000 years.^{234}U 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**

.^{233}U^{233}U decays via alpha decay into^{229}Th with a half-life of 159 200 years.^{233}U occasionally decays by spontaneous fission with a very low probability of 0.000000006%. Its specific activity is ~0.0098 Ci/g..^{236}U^{236}U decays via alpha decay to^{232}Th with a half-life of ~2.3×10^{7}years.^{236}U occasionally decays by spontaneous fission with a very low probability of 0.00000009%. Its specific activity is ~6.5×10^{-5}Ci/g..^{232}U^{232}U decays via alpha decay to^{228}Th with a half-life of 68.9 years.^{232}U very rarely decays by spontaneous fission. Its specific activity is very high, ~22 Ci/g, and its decay chain produces very penetrating gamma rays.

Isotope | Specific Activity |
---|---|

Uranium 232 | 22 Ci/g |

Uranium 233 | 0.0098 Ci/g |

Uranium 234 | 0.0063 Ci/g |

Uranium 236 | 0.000065 Ci/g |

Uranium 235 | 0.0000022 Ci/g |

Uranium 238 | 0.00000034 Ci/g |

Table of specific activities of isotopes of uranium.