Uranium-238 (
238U 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 6
electron anti-neutrinos per
238U nucleus (1 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 ~ 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-234 (234U or U-234) is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million) of the raw uranium because its half-life ...
,
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
The BN-600 reactor is a sodium-cooled fast breeder reactor, built at the Beloyarsk Nuclear Power Station, in Zarechny, Sverdlovsk Oblast, Russia. Designed to generate electrical power of 600 MW in total, the plant dispatches 560 MW ...
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 neutron
In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event, by one of the fission products (or actually, a fission product daughter after beta decay), any time from a few milliseconds to a few minutes after the ...
s, 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 0.7% , 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 DUCRETE (Depleted Uranium Concrete) is a high density concrete alternative investigated for use in construction of casks for storage of radioactive waste. It is a composite material containing depleted uranium dioxide aggregate instead of convent ...
, a concrete made with uranium dioxide
aggregate
Aggregate or aggregates may refer to:
Computing and mathematics
* collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
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
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U ...
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
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
'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
Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-live ...
,
bismuth,
lead,
polonium,
protactinium,
radium,
radon
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains th ...
,
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:
:
The
mean lifetime of
238U is 1.41 seconds divided by 0.693 (or multiplied by 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
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
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. Extrapolated over 2 seconds this is 600 gigajoules, the total energy released in the first three steps in the decay chain.
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
Lead–lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium–lead dating (U–Pb dating), but in certain special ...
, 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