Encyclopedia
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 × 10
17 seconds .
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-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 BN-600 reactor at the Beloyarsk Nuclear Power Station in
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 , 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 — 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 and thus 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 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 . 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
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. Protactinium-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 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 lead-206 .
The mean lifetime of uranium-238 is 1.41 × 10
17 seconds divided by 0.693 , i.e. ca. 2 × 10
17 seconds, so 1 mole of uranium-238 emits 3 × 10
6 alpha particles per second, producing the same number of thorium-234
atoms. In a closed system an equilibrium would be reached, with all amounts except lead-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 × 10
6 decayed particles per second per mole uranium-238.
Thorium-234 has a mean lifetime of 3 × 10
6 seconds, so there is equilibrium if 1 mole of uranium-238 contains 9 × 10
12 atoms of thorium-234, which is 1.5 × 10
-11 mole . 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 × 10
6 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 × 10
17 seconds this is 600 GJ, the total energy released in the first three steps in the decay chain.
See also
