Uranium-238 (U-238), is the most common isotope of uranium found in nature. When hit by a neutron, it becomes uranium-239 (U-239), an unstable isotope which decays into neptunium-239 (Np-239), which then itself decays, with a half-life of 2.355 days, into plutonium-239 (Pu-239).
Around 99.284% of natural uranium is uranium-238, which has a half-life of 1.41 × 1017 seconds (4.46 × 109 years, or 4.46 billion years). Depleted uranium consists mainly of the 238 isotope, and enriched uranium has a higher-than-natural quantity of the uranium-235 isotope. Reprocessed uranium is also mainly U-238, but contains significant quantities of uranium-236, and in fact all the isotopes of uranium between uranium-232 and uranium-238 except uranium-237.
As of December 2005, the only breeder reactor producing power is the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned for a re-start, having been shut down since 1995, and both China and India have announced intentions to build breeder reactors.
The breeder reactor as its name implies creates even larger quantities of plutonium-239 than the fission nuclear reactor.
The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will potentially be able to burn uranium-238 as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development.
Uranium-238 from depleted uranium and natural uranium is also used with recycled plutonium from weapons stockpiles for making mixed oxide fuel (MOX) which is now being redirected to become reactor fuel. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the very expensive and complex enrichment and separation processes before assembling a weapon.
The larger portion of the total explosive yield in this design comes from the final fission stage fueled by uranium-238, producing enormous amounts of radioactive fission products. For example, 77% of the 10.4 megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper. Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The 1961 Soviet test of Tsar Bomba produced "only" 50 megatons, over 90% from fusion, because the uranium-238 final stage was replaced with lead. Had uranium-238 been used, the yield could have been as much as 100 megatons, and would have produced fallout equivalent to one third of the global total at that time.
The mean lifetime of uranium-238 is 1.41 × 1017 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2 × 1017 seconds, so 1 mole of uranium-238 emits 3 × 106 alpha particles per second, producing the same number of thorium-234 (Th-234) atoms. In a closed system an equilibrium would be reached, with all amounts except lead-206 and uranium-238 in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while U-238 decreases; all steps in the decay chain have this same rate of 3 × 106 decayed particles per second per mole uranium-238.
Thorium-234 has a mean lifetime of 3 × 106 seconds, so there is equilibrium if 1 mole of uranium-238 contains 9 × 1012 atoms of thorium-234, which is 1.5 × 10-11 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life.
As already touched upon above, when starting with pure uranium-238, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, per mole of uranium-238, 3 × 106 times per second one alpha and two beta particles and gamma ray are produced, together 6.7 MeV, a rate of 3 µW. Extrapolated over 2 × 1017 seconds this is 600 GJ, the total energy released in the first three steps in the decay chain