Uranium-238

Uranium-238, is the most common isotope of uranium found. When hit by a neutron, it becomes uranium-239, an unstable element which decays into neptunium-239, which then itself decays, with a half-life of 2.355 days, into plutonium-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.

Nuclear energy applications

In a nuclear reactor, uranium-238 can be used to breed plutonium, 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 (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 U-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 [http://www.world-nuclear.org/info/inf08.htm.

As of December 2005, the only breeder reactor producing power is BN-600 reactor in Beloyarsk, Russia. The electricity output of BN-600 is 600 megawatts. 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

U-238 is also used as a radiation shield — 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 radiation 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 to turn it into low enriched uranium and thus suitable for use in commercial nuclear fuel.

U-238 from depleted 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 boosted fission weapon, the high flux of very energetic neutrons from the resulting fusion reaction causes the U-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 U-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 DU tamper. Because DU 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 U-238 final stage was replaced with lead. Had U-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

While uranium-238 is minimally radioactive, its decay products—thorium 234 and protactinium 234—are beta particle emitters with half-lives about 20 days and one minute respectively (Pa 234 decays to uranium 234, which has a half-life of hundreds of millennia, and this isotope does not build to equilibrium concentration for a very long time). When the two first isotopes in the decay chain reach their (tiny) equilibrium concentrations, a sample of initially pure uranium-238 will emit three times the radiation due to uranium-238 itself, and most of this will be beta radiation. After all the beta radiation is almost over, the by-product of uranium-238 would be (Pb) lead.

The mean lifetime of U-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 U-238 emits 3 × 10⁶ alpha particles per second, producing the same number of Th-234 atoms. In a closed system an equilibrium would be reached, with all amounts except Pb-206 in fixed ratios, in slowly decreasing amounts, and an accordingly increasing amount of Pb-206; all steps in the decay chain have this same rate of 3 × 10⁶ decayed particles per second per mole U-238.

Th-234 has a mean lifetime of 3 × 10⁶ seconds, so there is equilibrium if 1 mole of U-238 contains 9 × 10¹² atoms of Th-234, which is 1.5 × 10^-11 mole (the ratio of the two half-lifes). 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 U-238, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, per mole of U-238, 3 × 10⁶ times per second one alpha and two beta particles and gamma radiation 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.