uranium-238

Uranium-238 ( or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is ''fertile'', meaning it can be transmuted to fissile plutonium-239. ²³⁸U cannot 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 ²³⁸U'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's mass is uranium-238, which has a half-life of 1.41 seconds (4.468 years, or 4.468 billion years).
Due to its natural abundance and half-life relative to other radioactive elements, ²³⁸U produces ~40% of the radioactive heat produced within the Earth. The ²³⁸U decay chain contributes six electron anti-neutrinos per ²³⁸U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of ²³⁸U to daughter isotopes is extensively used in radiometric dating, particularly for material older than approximately 1 million years.

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.

Nuclear energy applications

In a fission nuclear reactor, uranium-238 can be used to generate plutonium-239, 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 comes from the fission of ²³⁹Pu, which is not supplied as a fuel to the reactor, but rather, produced from ²³⁸U. A certain amount of production of from is unavoidable wherever it is exposed to neutron radiation. Depending on burnup and neutron temperature, different shares of the are converted to , which determines the "grade" of produced plutonium, ranging from weapons grade, through reactor grade, to plutonium so high in that it cannot be used in current reactors operating with a thermal neutron spectrum. The latter usually involves used "recycled" MOX fuel which entered the reactor containing significant amounts of plutonium.

Breeder reactors

²³⁸U 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. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 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 ²³⁹Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of ²³⁸U for use in these power plants. Breeder technology has been used in several experimental nuclear reactors.

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 later built another unit, BN-800, at the Beloyarsk Nuclear Power Station which became fully operational in November 2016. Also, Japan's Monju breeder reactor, which has been inoperative for most of the time since it was originally built in 1986, was ordered for decommissioning in 2016, after safety and design hazards were uncovered, with a completion date set for 2047. Both China and India have announced plans to build nuclear breeder reactors.

The breeder reactor as its name implies creates even larger quantities of ²³⁹Pu or ²³³U 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 use ²³⁸U as fuel once the reactor is started with Low-enriched uranium (LEU) fuel. This design is still in the early stages of development.

CANDU reactors

Natural uranium, with 0.711% , is usable as nuclear fuel in reactors designed specifically to make use of naturally occurring uranium, such as CANDU reactors. By making use of non-enriched uranium, such reactor designs give a nation access to nuclear power for the purpose of electricity production without necessitating the development of fuel enrichment capabilities, which are often seen as a prelude to weapons production.

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 ²³⁹Pu 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 ²³⁹Pu 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 order in which each reaction takes place. 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.

Radium series (or uranium series)

The decay 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 of the decay products are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral. The decay proceeds as:

:$\begin\\ \ce \begin \ce \\ \ce \end \ce \\ \ce \end$

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

Thorium-234 has a mean lifetime of 3 seconds, so there is equilibrium if one mole of ²³⁸U contains 9 atoms of thorium-234, which is 1.5 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.

While ²³⁸U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are beta particle emitters with half-lives of 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 reach an equilibrium concentration for a very long time. When the two first isotopes in the decay chain reach their relatively small equilibrium concentrations, a sample of initially pure ²³⁸U will emit three times the radiation due to ²³⁸U itself, and most of this radiation is beta particles.

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 times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, a rate of 3 μW.

²³⁸U atom is itself a gamma emitter at 49.55 keV with probability 0.084%, but that is a very weak gamma line, so activity is measured through its daughter nuclides in its decay series.

Radioactive dating

²³⁸U abundance and its decay to daughter isotopes comprises multiple uranium dating techniques and is one of the most common radioactive isotopes used in radiometric dating. The most common dating method is uranium-lead dating, which is used to date rocks older than 1 million years old and has provided ages for the oldest rocks on Earth at 4.4 billion years old.

The relation between ²³⁸U and ²³⁴U gives an indication of the age of sediments and seawater that are between 100,000 years and 1,200,000 years in age.

The ²³⁸U daughter product, ²⁰⁶Pb, is an integral part of lead–lead dating, which is most famous for the determination of the age of the Earth.

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

Health concerns

Uranium emits alpha particles through the process of alpha decay. External exposure has limited effect. Significant internal exposure to tiny particles of uranium or its decay products, such as thorium-230, radium-226 and radon-222, can cause severe health effects, such as cancer of the bone or liver.

Uranium is also a toxic chemical, meaning that ingestion of uranium can cause kidney damage from its chemical properties much sooner than its radioactive properties would cause cancers of the bone or liver.''Uranium Mining in Virginia: Scientific, Technical, Environmental, Human Health and Safety, and Regulatory Aspects of Uranium Mining and Processing in Virginia''
Ch. 5. ''Potential Human Health Effects of Uranium Mining, Processing, and Reclamation''. National Academies Press (US); December 19, 2011.

See also

* Depleted uranium
* Uranium-lead dating

References

External links

NLM Hazardous Substances Databank – Uranium, Radioactive

{{Isotopes of uranium

Actinides
Fertile materials
Isotopes of uranium
Uranium
Radionuclides used in radiometric dating