Uranium-238

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10 gram sample of uranium-238 next to a dime for size comparison.

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 element 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.

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[edit] 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.

[edit] 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 [1]. Breeder technology has been used in several reactors [2].

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 (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.

[edit] 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.

[edit] 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 (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.

[edit] 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.

[edit] 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 (Pa-234) decays to uranium-234 (U-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 lead-206 (Pb-206).

[edit] Radium series

The 4n+2 chain of U-238 is commonly called the "radium series".

nuclide decay mode half life MeV product of decay
U 238 α 4.468·109 a 4.270 Th 234
Th 234 β- 24.10 d 0.273 Pa 234
Pa 234 β- 6.70 h 2.197 U 234
U 234 α 245500 a 4.859 Th 230
Th 230 α 75380 a 4.770 Ra 226
Ra 226 α 1602 a 4.871 Rn 222
Rn 222 α 3.8235 d 5.590 Po 218
Po 218 α 99.98 %
β- 0.02 %
3.10 min 6.615
0.265
Pb 214
At 218
At 218 α 99.90 %
β- 0.10 %
1.5 s 6.874
2.883
Bi 214
Rn 218
Rn 218 α 35 ms 7.263 Po 214
Pb 214 β- 26.8 min 1.024 Bi 214
Bi 214 β- 99.98 %
α 0.02 %
19.9 min 3.272
5.617
Po 214
Tl 210
Po 214 α 0.1643 ms 7.883 Pb 210
Tl 210 β- 1.30 min 5.484 Pb 210
Pb 210 β- 22.3 a 0.064 Bi 210
Bi 210 β- 99.99987%
α 0.00013%
5.013 d 1.426
5.982
Po 210
Tl 206
Po 210 α 138.376 d 5.407 Pb 206
Tl 206 β- 4.199 min 1.533 Pb 206
Pb 206 . stable . .

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-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 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


Uranium-237 Isotopes of Uranium Uranium-239
Produced from:
Plutonium-242 (α)
Protactinium-238 (β-)
Decay chain Decays to:
Thorium-234 (α)

[edit] See also

ja:ウラン238

Uranium-238

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