Uranium-238

Uranium-238 (U-238), is the most common plutonium-239 (Pu-239).

Around 99.284% of uranium-236, and in fact all the isotopes of uranium between uranium-232 and uranium-238 except uranium-237. ^[2]

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

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 lead, so a shield with the same effectivity can be packed into a thinner layer.

radioactive waste.

Downblending

The opposite of enriching is 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 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 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 fallout equivalent to one third of the global total at that time.

Radioactivity and decay

Uranium-238's decay product uranium-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. ^[3]

Radium series

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

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 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

References

See also

• Depleted uranium