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 consi ...

element that has no

stable isotope The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

. 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 ...

s, uranium-238 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 exi ...

, 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. The decay product

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-lif ...

is also found. Other isotopes such as

uranium-233 Uranium-233 (233U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in exp ...

have been produced in

breeder reactor A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile mate ...

s. 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 of natural uranium 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 numb ...

s, uranium-238 (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 atomi ...

(0.7198–0.7202%), and

(0.0050–0.0059%). All three isotopes are

(i.e., they are

radioisotope 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; transferr ...

s), 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 at ...

of (close to the age of the Earth). 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 In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay direct ...

into

lead-206 Lead (82Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the urani ...

. 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 ...

Uranium-233 Uranium-233 (233U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in exp ...

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 fr ...

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 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 t ...

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 wi ...

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 A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events. Chain reactions are one way that sys ...

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: it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope

plutonium-239 Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three mai ...

, 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 dating Uranium–thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique established in the 1960s which has been used since the 1970s to determine the age of calciu ...

Used in

uranium–uranium dating Uranium–uranium dating is a radiometric dating technique which compares two isotopes of uranium (U) in a sample: uranium-234 (234U) and uranium-238 (238U). It is one of several radiometric dating techniques exploiting the uranium radioactive dec ...

, 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 decay product 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 radionuclideUsed in

Uranium–lead dating Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routi ...

Important 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, China 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 t ...

. It has been cited as an obstacle to nuclear proliferation using ²³³U as the

material, because the intense gamma radiation 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

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 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

; it is also lower than that of short-lived

plutonium-241 Plutonium-241 (241Pu or Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes (especially 239Pu), 241Pu is fissile, with a neutron absorption cross section about one-third greater t ...

, but bested by very difficult-to-produce neptunium-236.

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) 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 pr ...

to become thorium-234. Next, with a short

, a ²³⁴Th nucleus emits a beta particle to become

protactinium-234 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, ...

. 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 Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

thorium-230 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 ...

, except for the small percentage of nuclei that undergo spontaneous fission. 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,

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 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 suita ...

that have been aged somewhat to allow some decay to ²³⁴U via

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 ...

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 Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material s ...

might contain even higher fractions of ²³⁴U, which is undesirable. This is because ²³⁴U is not

, and tends to absorb slow neutrons 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. ...

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 uranium-238 is to

(via

neptunium-239 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 s ...

), 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 uranium-238, it is

, i.e., it can sustain a fission

. 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 ty ...

that is a primordial nuclide or found in significant quantity in nature. Uranium-235 has a

of 703.8 million years. It was discovered in 1935 by

Arthur Jeffrey Dempster Arthur Jeffrey Dempster (August 14, 1886 – March 11, 1950) was a Canadian-American physicist best known for his work in mass spectrometry and his discovery in 1935 of the uranium isotope 235U. Early life and education Dempster was born i ...

. Its (fission) nuclear

for slow

is about 504.81

barn 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.Alle ...

s. For fast neutrons 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 re ...

. 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 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 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-238

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

found in nature. It is not

, but is a

fertile material Fertile material is a material that, although not itself fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions. Naturally occurring fertile materials Naturally occurring ...

: it can capture a slow

and after two beta decays become fissile

. Uranium-238 is fissionable by fast neutrons, but cannot support a chain reaction because inelastic scattering reduces

neutron energy 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 ...

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

, 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 Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new isotopes— ...

in a nuclear reactor. ²³⁹U has a half-life of about 23.45 minutes and decays into

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

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