Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
Curium
About 20 radioisotopes and 7 nuclear isomers between Cm and Cm are known for curium, and no stable isotopes. The longest half-lives have been reported for Cm (15.6 million years) and Cm (348,000 years). Other long-lived isotopes are Cm (half-life 8500 years), Cm (8,300 years) and Cm (4,760 years). Curium-250 is unusual in that it predominantly (about 86%) decays via spontaneous fission. The most commonly used curium isotopes are Cm and Cm with the half-lives of 162.8 days and 18.1 years, respectively.
All isotopes between Cm and Cm, as well as Cm, undergo a self-sustaining nuclear chain reaction and thus in principle can act as a nuclear fuel in a reactor. As in most transuranic elements, the nuclear fission cross section is especially high for the odd-mass curium isotopes Cm, Cm and Cm. These can be used in thermal-neutron reactors, whereas a mixture of curium isotopes is only suitable for fast breeder reactors since the even-mass isotopes are not fissile in a thermal reactor and accumulate as burn-up increases. The mixed-oxide (MOX) fuel, which is to be used in power reactors, should contain little or no curium because the neutron activation of Cm will create californium. Californium is a strong neutron emitter, and would pollute the back end of the fuel cycle and increase the dose to reactor personnel. Hence, if the minor actinides are to be used as fuel in a thermal neutron reactor, the curium should be excluded from the fuel or placed in special fuel rods where it is the only actinide present.