Uranium-238

Uranium-238
10 gram sample Full table
General
Name, symbol	Uranium-238,238U
Neutrons	146
Protons	92
Nuclide data
Natural abundance	99.2745%
Half-life	4.468 billion years
Parent isotopes	242Pu (α) 238Pa (β−)
Decay products	234Th
Isotope mass	238.05078826 u
Decay mode	Decay energy
Alpha decay	4.267 MeV

Uranium-238 (²³⁸U or U-238) is the most common isotope of uranium found in nature and is used as a fuel in nuclear power plants. It can capture a slow neutron and after two beta decays become fissile plutonium-239. ²³⁸U is fissionable by fast neutrons,and can not support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U-238's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Around 99.284% of natural uranium is uranium-238, which has a half-life of 1.41×10¹⁷ seconds (4.468×10⁹ years, or 4.468 billion years).^[1] Depleted uranium has an even higher concentration of the ²³⁸U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly ²³⁸U. Reprocessed uranium is also mainly ²³⁸U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, and much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, and uranium-232.^[2]

Nuclear energy applications

In a fission nuclear reactor, uranium-238 can be used to breed ²³⁹Pu, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power does come from the fission of ²³⁹Pu, which is not supplied as a fuel to the reactor, but rather, produced from ²³⁸U.

Breeder reactors

²³⁸U is not usable directly as nuclear fuel, though it can produce energy via "fast" fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of ²³⁸U to split in two. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the about 1.7 neutrons produced in each fission have enough speed to continue a chain reaction.

²³⁸U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope ²³⁸U into fissile Pu-239. It has been estimated that there is anywhere from 10,000 to five billion years worth of ²³⁸U for use in these power plants.^[3] Breeder technology has been used in several experimental nuclear reactors.^[4]

By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned to be started, having been shut down since 1995, and both China and India have announced plans to build nuclear breeder reactors.

The breeder reactor as its name implies creates even larger quantities of Pu-239 than the fission nuclear reactor.

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 ²³⁸U as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development.

Radiation shielding

²³⁸U 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 are highly effective in absorbing gamma rays and x-rays. It is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxide are used for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectiveness 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.

²³⁸U from depleted uranium and natural uranium is also used with recycled Pu-239 from nuclear weapons stockpiles for making mixed oxide fuel (MOX), which is now being redirected to become fuel for nuclear reactors. 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 chemical separation of uranium and plutonium process before assembling a weapon.

Nuclear weapons

Most modern nuclear weapons utilize ²³⁸U as a "tamper" material (see nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and to add inertia to the compression of the Pu-239 charge. As such, it increases the efficiency of the weapon and reduces the critical mass required. In the case of a thermonuclear weapon ²³⁸U can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes ²³⁸U nuclei to split and adds more 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. An example of such a weapon is Castle Bravo.

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by ²³⁸U, producing enormous amounts of radioactive fission products. For example, an estimated 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 Soviet Union's test of the "Tsar Bomba" in 1961 produced "only" 50 megatons of explosive power, over 90% of which came from fusion, because the ²³⁸U final stage had been replaced with lead. Had ²³⁸U been used instead, the yield of the "Tsar Bomba" could have been well-above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time.

Radioactivity and decay

Alpha-decay of Uranium accompanies with low energy gamma-radiation – 49.5 keV ^[5]

²³⁸U radiates alpha-particles and decays (by way of thorium-234 and protactinium-234) into uranium-234. ²³⁴U has a half-life of 245,500 years. The relation between ²³⁸U and ²³⁴U gives an indication of the age of sediments that are between 100,000 years and 1,200,000 years in age.^[6]

The Voyager spacecrafts carry small amounts of initially pure ²³⁸U on the covers of their golden records to facilitate dating in the same manner.

²³⁸U occasionally decays by spontaneous fission or double beta decay with probabilities of 5×10⁻⁷ and 2×10⁻¹² per alpha decay, respectively.^[7]

Radium series (or uranium series)

The 4n+2 chain of ²³⁸U is commonly called the "radium series" (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. All are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral.

The mean lifetime of ²³⁸U is 1.41×10¹⁷ seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2×10¹⁷ seconds, so 1 mole of ²³⁸U 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 for lead-206 and ²³⁸U in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while that of ²³⁸U decreases; all steps in the decay chain have this same rate of 3×10⁶ decayed particles per second per mole ²³⁸U.

Thorium-234 has a mean lifetime of 3×10⁶ seconds, so there is equilibrium if one mole of ²³⁸U 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 ²³⁸U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of ²³⁸U, 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 gigajoules, the total energy released in the first three steps in the decay chain.

See also

• Depleted uranium

References

External links

• NLM Hazardous Substances Databank – Uranium, Radioactive
• Simulation of U238 using the Monte Carlo method