Uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

(₉₂U) is a naturally occurring

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

element that has no stable isotope. It has two

primordial isotope In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...

uranium-238 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 ...

and

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...

, that have long

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 at ...

and are found in appreciable quantity in the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

. The

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...

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 ...

is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from ²¹⁴U to ²⁴²U (with the exceptions of ²²⁰U and ²⁴¹U). The

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, is ...

natural uranium Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes fr ...

is . Naturally occurring uranium is composed of three major

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

(99.2739–99.2752%

natural abundance In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the atomic ...

(0.7198–0.7202%), and

(0.0050–0.0059%). All three isotopes are

(i.e., they are radioisotopes), and the most abundant and stable is uranium-238, with a

half-life 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 (close to the

age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years This age may represent the age of Earth's accretion, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of ...

). Uranium-238 is an

alpha emitter Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...

, decaying through the 18-member uranium series into lead-206. The decay series of uranium-235 (historically called actino-uranium) has 15 members and ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent-to-daughter elements useful in

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...

. Uranium-233 is made from

thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay ...

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

bombardment. Uranium-235 is important for both

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...

s (energy production) and

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

s because it is the only isotope existing in nature to any appreciable extent that is fissile in response to

thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

s, i.e., thermal

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

has a high probability of inducing fission. A chain reaction can be sustained with a sufficiently large (

critical Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine *Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing in ...

) mass of uranium-235. Uranium-238 is also important because it is

fertile Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertilit ...

: it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

List of isotopes

, - , ²¹⁴U , , style="text-align:right" , 92 , style="text-align:right" , 122 , , 0.52(+0.95−0.21) ms , α , ²¹⁰Th , 0+ , , , - , ²¹⁵U , , style="text-align:right" , 92 , style="text-align:right" , 123 , 215.026760(90) , 1.4(0.9) ms , α , ²¹¹Th , 5/2−# , , , - , ²¹⁶U , , style="text-align:right" , 92 , style="text-align:right" , 124 , 216.024760(30) , 6.9(2.9) ms , α , ²¹²Th , 0+ , , , - , style="text-indent:1em" , ^216mU , , colspan="3" style="text-indent:2em" , , 1.4(0.9) ms , , , 8+ , , , - , ²¹⁷U , , style="text-align:right" , 92 , style="text-align:right" , 125 , 217.02437(9) , 0.85(0.71) ms , α , ²¹³Th , 1/2−# , , , - , ²¹⁸U , , style="text-align:right" , 92 , style="text-align:right" , 126 , 218.02354(3) , 0.35(0.09) ms , α , ²¹⁴Th , 0+ , , , - , ²¹⁹U , , style="text-align:right" , 92 , style="text-align:right" , 127 , 219.02492(6) , 60(7) μs , α , ²¹⁵Th , 9/2+# , , , - , ²²¹U , , style="text-align:right" , 92 , style="text-align:right" , 129 , 221.02640(11)# , 0.66(14) μs , α , ²¹⁷Th , (9/2+) , , , - , rowspan=2, ²²²U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 130 , rowspan=2, 222.02609(11)# , rowspan=2, 4.7(0.7) μs , α , ²¹⁸Th , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁺ (10⁻⁶%) , ²²²Pa , - , ²²³U , , style="text-align:right" , 92 , style="text-align:right" , 131 , 223.02774(8) , 65(12) μs , α , ²¹⁹Th , 7/2+# , , , - , ²²⁴U , , style="text-align:right" , 92 , style="text-align:right" , 132 , 224.027605(27) , 396(17) μs , α , ²²⁰Th , 0+ , , , - , ²²⁵U , , style="text-align:right" , 92 , style="text-align:right" , 133 , 225.02939# , 62(4) ms , α , ²²¹Th , (5/2+)# , , , - , ²²⁶U , , style="text-align:right" , 92 , style="text-align:right" , 134 , 226.029339(14) , 269(6) ms , α , ²²²Th , 0+ , , , - , rowspan=2, ²²⁷U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 135 , rowspan=2, 227.031156(18) , rowspan=2, 1.1(0.1) min , α , ²²³Th , rowspan=2, (3/2+) , rowspan=2, , rowspan=2, , - , β⁺ (.001%) , ²²⁷Pa , - , rowspan=2, ²²⁸U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 228.031374(16) , rowspan=2, 9.1(0.2) min , α (95%) , ²²⁴Th , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , EC (5%) , ²²⁸Pa , - , rowspan=2, ²²⁹U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 137 , rowspan=2, 229.033506(6) , rowspan=2, 57.8(0.5) min , β⁺ (80%) , ²²⁹Pa , rowspan=2, (3/2+) , rowspan=2, , rowspan=2, , - , α (20%) , ²²⁵Th , - , rowspan=3, ²³⁰U , rowspan=3, , rowspan=3 style="text-align:right" , 92 , rowspan=3 style="text-align:right" , 138 , rowspan=3, 230.033940(5) , rowspan=3, 20.23(0.02) d , α , ²²⁶Th , rowspan=3, 0+ , rowspan=3, , rowspan=3, , - , SF (1.4×10⁻¹⁰%) , (various) , - , β⁺β⁺ (rare) , ²³⁰Th , - , rowspan=2, ²³¹U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 139 , rowspan=2, 231.036294(3) , rowspan=2, 4.2(0.1) d , EC , ²³¹Pa , rowspan=2, (5/2)(+#) , rowspan=2, , rowspan=2, , - , α (.004%) , ²²⁷Th , - , rowspan=4, ²³²U , rowspan=4, , rowspan=4 style="text-align:right" , 92 , rowspan=4 style="text-align:right" , 140 , rowspan=4, 232.0371562(24) , rowspan=4, 68.9(0.4) y , α , ²²⁸Th , rowspan=4, 0+ , rowspan=4, , rowspan=4, , - , CD (8.9×10⁻¹⁰%) , ²⁰⁸Pb
²⁴Ne , - , CD (5×10⁻¹²%) , ²⁰⁴Hg
²⁸Mg , - , SF (10⁻¹²%) , (various) , - , rowspan=4, ²³³U , rowspan=4, , rowspan=4 style="text-align:right" , 92 , rowspan=4 style="text-align:right" , 141 , rowspan=4, 233.0396352(29) , rowspan=4, 1.592(2)×10⁵ y , α , ²²⁹Th , rowspan=4, 5/2+ , rowspan=4, TraceIntermediate decay product of ²³⁷Np , rowspan=4, , - , SF (6×10⁻⁹%) , (various) , - , CD (7.2×10⁻¹¹%) , ²⁰⁹Pb
²⁴Ne , - , CD (1.3×10⁻¹³%) , ²⁰⁵Hg
²⁸Mg , - , rowspan=4, ²³⁴UUsed in uranium–thorium datingUsed in uranium–uranium dating , rowspan=4, Uranium II , rowspan=4 style="text-align:right" , 92 , rowspan=4 style="text-align:right" , 142 , rowspan=4, 234.0409521(20) , rowspan=4, 2.455(6)×10⁵ y , α , ²³⁰Th , rowspan=4, 0+ , rowspan=4, .000054(5)ref group="n">Intermediate

of ²³⁸U , rowspan=4, 0.000050–
0.000059 , - , SF (1.73×10⁻⁹%) , (various) , - , CD (1.4×10⁻¹¹%) , ²⁰⁶Hg
²⁸Mg , - , CD (9×10⁻¹²%) , ¹⁸⁴Hf
²⁶Ne
²⁴Ne , - , style="text-indent:1em" , ^234mU , , colspan="3" style="text-indent:2em" , 1421.32(10) keV , 33.5(2.0) ms , , , 6− , , , - , rowspan=3, ²³⁵U Primordial

radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...

Used in Uranium–lead datingImportant in nuclear reactors , rowspan=3, Actin Uranium
Actino-Uranium , rowspan=3 style="text-align:right" , 92 , rowspan=3 style="text-align:right" , 143 , rowspan=3, 235.0439299(20) , rowspan=3, 7.038(1)×10⁸ y , α , ²³¹Th , rowspan=3, 7/2− , rowspan=3, .007204(6), rowspan=3, 0.007198–
0.007207 , - , SF (7×10⁻⁹%) , (various) , - , CD (8×10⁻¹⁰%) , ¹⁸⁶Hf
²⁵Ne
²⁴Ne , - , style="text-indent:1em" , ^235mU , , colspan="3" style="text-indent:2em" , 0.0765(4) keV , ~26 min , IT , ²³⁵U , 1/2+ , , , - , rowspan=2, ²³⁶U , rowspan=2, Thoruranium , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 144 , rowspan=2, 236.045568 , rowspan=2, 2.342(3)×10⁷ y , α , ''²³²Th'' , rowspan=2, 0+ , rowspan=2, TraceIntermediate decay product of ²⁴⁴Pu, also produced by

of ²³⁵U , rowspan=2, , - , SF (9.6×10⁻⁸%) , (various) , - , style="text-indent:1em" , ^236m1U , , colspan="3" style="text-indent:2em" , 1052.89(19) keV , 100(4) ns , , , (4)− , , , - , style="text-indent:1em" , ^236m2U , , colspan="3" style="text-indent:2em" , 2750(10) keV , 120(2) ns , , , (0+) , , , - , ²³⁷U , , style="text-align:right" , 92 , style="text-align:right" , 145 , 237.0487302(20) , 6.752(0.002) d , β⁻ , ²³⁷Np , 1/2+ , TraceNeutron capture product, parent of trace quantities of ²³⁷Np , , - , rowspan=3, ²³⁸U , rowspan=3, Uranium I , rowspan=3 style="text-align:right" , 92 , rowspan=3 style="text-align:right" , 146 , rowspan=3, 238.0507882(20) , rowspan=3, 4.468(3)×10⁹ y , α , ²³⁴Th , rowspan=3, 0+ , rowspan=3, .992742(10), rowspan=3, 0.992739–
0.992752 , - , SF (5.45×10⁻⁵%) , (various) , - , β⁻β⁻ (2.19×10⁻¹⁰%) , ²³⁸Pu , - , style="text-indent:1em" , ^238mU , , colspan="3" style="text-indent:2em" , 2557.9(5) keV , 280(6) ns , , , 0+ , , , - , ²³⁹U , , style="text-align:right" , 92 , style="text-align:right" , 147 , 239.0542933(21) , 23.45(0.02) min , β⁻ , ²³⁹Np , 5/2+ , , , - , style="text-indent:1em" , ^239m1U , , colspan="3" style="text-indent:2em" , 20(20)# keV , >250 ns , , , (5/2+) , , , - , style="text-indent:1em" , ^239m2U , , colspan="3" style="text-indent:2em" , 133.7990(10) keV , 780(40) ns , , , 1/2+ , , , - , rowspan=2, ²⁴⁰U , rowspan=2, , rowspan=2 style="text-align:right" , 92 , rowspan=2 style="text-align:right" , 148 , rowspan=2, 240.056592(6) , rowspan=2, 14.1(0.1) h , β⁻ , ²⁴⁰Np , rowspan=2, 0+ , rowspan=2, TraceIntermediate decay product of ²⁴⁴Pu , rowspan=2, , - , α (10⁻¹⁰%) , ²³⁶Th , - , ²⁴²U , , style="text-align:right" , 92 , style="text-align:right" , 150 , 242.06293(22)# , 16.8(0.5) min , β⁻ , ²⁴²Np , 0+ , ,

Actinides vs fission products

Uranium-214

Uranium-214 is the lightest known isotope of uranium. It was discovered in 2021 at the Spectrometer for Heavy Atoms and Nuclear Structure (SHANS) at the Heavy Ion Research Facility in

Lanzhou Lanzhou (, ; ) is the capital and largest city of Gansu Province in Northwest China. Located on the banks of the Yellow River, it is a key regional transportation hub, connecting areas further west by rail to the eastern half of the country. H ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

in 2021, produced by firing argon-36 at tungsten-182. It undergoes alpha decay with a half-life of .

Uranium-232

Uranium-232 has a half-life of 68.9 years and is a side product in the

thorium cycle The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural tho ...

. It has been cited as an obstacle to

nuclear proliferation Nuclear proliferation is the spread of nuclear weapons, fissionable material, and weapons-applicable nuclear technology and information to nations not recognized as " Nuclear Weapon States" by the Treaty on the Non-Proliferation of Nuclear Wea ...

using ²³³U as the fissile material, because the intense

gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...

emitted by ²⁰⁸Tl (a daughter of ²³²U, produced relatively quickly) makes the ²³³U contaminated with it more difficult to handle. Uranium-232 is a rare example of an even-even isotope that is fissile with both thermal and fast neutrons.

Uranium-233

Uranium-233 is a fissile isotope of uranium that is bred from

as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It was occasionally tested but never deployed in nuclear weapons and has not been used commercially as a nuclear fuel. It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of around 160,000 years. Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a

, it becomes

thorium-233 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the gen ...

, which has a half-life of only 22 minutes. Thorium-233 decays into

protactinium-233 Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32, ...

through

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur. Uranium-233 usually fissions on neutron absorption but sometimes retains the neutron, becoming

. The capture-to-fission ratio is smaller than the other two major fissile fuels

and plutonium-239; it is also lower than that of short-lived plutonium-241, but bested by very difficult-to-produce

neptunium-236 Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be sy ...

Uranium-234

Uranium-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 One million (1,000,000), or one thousand thousand, is the natural number following 999,999 and preceding 1,000,001. The word is derived from the early Italian ''millione'' (''milione'' in modern Italian), from ''mille'', "thousand", plus the au ...

) of the raw

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

because its

of just 245,500 years is only about 1/18,000 as long as that of ²³⁸U. The path of production of ²³⁴U via nuclear decay is as follows: ²³⁸U nuclei emit an

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...

to become

thorium-234 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the gene ...

. Next, with a short

, a ²³⁴Th nucleus emits a

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...

to become protactinium-234. Finally, ²³⁴Pa nuclei each emit another beta particle to become ²³⁴U nuclei. ²³⁴U nuclei usually last for hundreds of thousands of years, but then they decay by alpha emission to thorium-230, except for the small percentage of nuclei that undergo

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

. Extraction of rather small amounts of ²³⁴U from natural uranium would be feasible using

isotope separation Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...

, similar to that used for regular uranium-enrichment. However, there is no real demand in

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, or engineering for isolating ²³⁴U. Very small pure samples of ²³⁴U can be extracted via the chemical

ion-exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

process—from samples of

plutonium-238 Plutonium-238 (238Pu or Pu-238) is a fissile, radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitab ...

that have been aged somewhat to allow some decay to ²³⁴U via alpha emission. Enriched uranium contains more ²³⁴U than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining

, which concentrates lighter isotopes even more strongly than it does ²³⁵U. The increased percentage of ²³⁴U in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched) reprocessed uranium might contain even higher fractions of ²³⁴U, which is undesirable. This is because ²³⁴U is not fissile, and tends to absorb slow

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

in a

—becoming ²³⁵U. ²³⁴U has a

cross section of about 100

barns A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. N ...

for

thermal neutrons The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

, and about 700 barns for its

resonance integral Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

—the average over neutrons having various intermediate energies. In a nuclear reactor, non-fissile isotopes capture a neutron breeding fissile isotopes. ²³⁴U is converted to ²³⁵U more easily and therefore at a greater rate than

is to plutonium-239 (via neptunium-239), because ²³⁸U has a much smaller neutron-capture

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

of just 2.7 barns.

Uranium-235

Uranium-235 is an

of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope

, it is fissile, i.e., it can sustain a fission chain reaction. It is the only

fissile isotope In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typi ...

that is a

primordial nuclide In geochemistry, geophysics and nuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on Earth that have existed in their current form since before Earth was formed. Primordial nuclides were present in the ...

or found in significant quantity in nature. Uranium-235 has a

of 703.8

million years The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...

. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear

for slow

is about 504.81 barns. For fast

it is on the order of 1 barn. At thermal energy levels, about 5 of 6 neutron absorptions result in fission and 1 of 6 result in neutron capture forming

uranium-236 Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the rep ...

. The fission-to-capture ratio improves for faster neutrons.

Uranium-236

Uranium-236 is an

with a half-life of about 23 million years that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived

radioactive waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...

. It is found in spent

nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...

and in the reprocessed uranium made from spent nuclear fuel.

Uranium-237

Uranium-237 is an isotope of

. It has a half life of about 6.75(1) days. It decays into neptunium-237 by

Uranium-238

Uranium-238 (²³⁸U or U-238) is the most common

found in nature. It is not fissile, but is a fertile material: it can capture a slow

and after two

beta decays In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

become fissile plutonium-239. Uranium-238 is fissionable by fast neutrons, but 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 is uranium-238, which has a half-life of 1.41×10¹⁷ seconds (4.468×10⁹ years, or 4.468 billion 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-233, and

uranium-232 Uranium-232 () is an isotope of uranium. It has a half-life of around 69 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamm ...

Uranium-239

Uranium-239 is an isotope of uranium. It is usually produced by exposing ²³⁸U to neutron radiation in a nuclear reactor. ²³⁹U has a half-life of about 23.45 minutes and decays into neptunium-239 through

, with a total decay energy of about 1.29 MeV. The most common gamma decay at 74.660 keV accounts for the difference in the two major channels of beta emission energy, at 1.28 and 1.21 MeV.''CRC Handbook of Chemistry and Physics'', 57th Ed. p. B-423 ²³⁹Np further decays to plutonium-239 also through

(²³⁹Np has a half-life of about 2.356 days), in a second important step that ultimately produces fissile ²³⁹Pu (used in weapons and for nuclear power), from ²³⁸U in reactors.

References

{{Authority control Uranium