Uranium-238

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 &times; 1017 seconds (4.46 &times; 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.

Nuclear energy applications
In a nuclear reactor, uranium-238 can be used to breed plutonium-239, which itself can be used in a nuclear weapon or as a reactor fuel source. In fact, in a typical nuclear reactor, up to a third of the generated power does come from the fission of plutonium-239, which is not supplied as a fuel to the reactor, but transmuted from uranium-238.

Breeder reactors
Uranium-238 is not usable directly as nuclear fuel; however, it can be used as a source material for creating the element plutonium. Breeder reactors carry out such a process of transmutation to convert fertile isotopes such as uranium-238 into fissile plutonium. It has been estimated that there is anywhere from 10,000 to five billion years worth of uranium-238 for use in these power plants. Breeder technology has been used in several reactors.

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 Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned for restart, having been shut down since 1995, and both China and India have announced intentions to build breeder reactors.

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.

Radiation shielding
Uranium-238 is also used as a radiation shield &mdash; its alpha radiation is easily stopped by the non-radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons is highly effective in absorbing gamma rays and x-rays. However, it is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxide are being used as materials for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectivity can be packed into a thinner layer.

DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste.

Downblending
The opposite of enriching is downblending. Surplus highly-enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial nuclear fuel.

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.

Nuclear weapons
Most modern nuclear weapons utilize uranium-238 as a "tamper" material (see nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and add inertia to the compression of the plutonium charge. As such, it increases the efficiency of the weapon and reduces the amount of critical mass required. In the case of a thermonuclear weapon uranium-238 can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes the uranium-238 to fission and adds energy to the yield of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the three consecutive stages of the explosion.

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.

Radioactivity and decay
Uranium-238's decay product thorium-234 has a halflife of 246,000 years and so is useful for determining the age of sediments that are between 100,000 years and 1,200,000 years in age.

The mean lifetime of uranium-238 is 1.41 &times; 1017 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2 &times; 1017 seconds, so 1 mole of uranium-238 emits 3 &times; 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 &times; 106 decayed particles per second per mole uranium-238.

Thorium-234 has a mean lifetime of 3 &times; 106 seconds, so there is equilibrium if 1 mole of uranium-238 contains 9 &times; 1012 atoms of thorium-234, which is 1.5 &times; 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 &times; 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 &times; 1017 seconds this is 600 GJ, the total energy released in the first three steps in the decay chain