Plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhib ...

(₉₄Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a

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, ...

cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first

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 number ...

synthesized being ²³⁸Pu in 1940. Twenty plutonium

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 have been characterized. The most stable are plutonium-244 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 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining

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 ...

isotopes have half-lives that are less than 7,000 years. This element also has eight

meta state A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have h ...

s; all have half-lives of less than one second. The isotopes of plutonium range in

atomic weight Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a give ...

from 228.0387 u (²²⁸Pu) to 247.074 u (²⁴⁷Pu). The primary

decay mode 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 ...

s before the most stable isotope, ²⁴⁴Pu, are

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 breakd ...

and alpha emission; the primary mode after is beta emission. The primary decay products before ²⁴⁴Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are

isotopes of americium Americium (95Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was 241Am in 1944. The artificial element decays by ...

List of isotopes

, - , rowspan=2, ²²⁸Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 134 , rowspan=2, 228.03874(3) , rowspan=2, 1.1(+20−5) s , α (99.9%) , ²²⁴U , rowspan=2, 0+ , rowspan=2, , - , β⁺ (.1%) , ²²⁸Np , - , ²²⁹Pu , style="text-align:right" , 94 , style="text-align:right" , 135 , 229.04015(6) , 120(50) s , α , ²²⁵U , 3/2+# , , - , rowspan=2, ²³⁰Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 230.039650(16) , rowspan=2, 1.70(17) min , α , ²²⁶U , rowspan=2, 0+ , rowspan=2, , - , β⁺ (rare) , ²³⁰Np , - , rowspan=2, ²³¹Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 137 , rowspan=2, 231.041101(28) , rowspan=2, 8.6(5) min , β⁺ , ²³¹Np , rowspan=2, 3/2+# , rowspan=2, , - , α (rare) , ²²⁷U , - , rowspan=2, ²³²Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 232.041187(19) , rowspan=2, 33.7(5) min , EC (89%) , ²³²Np , rowspan=2, 0+ , rowspan=2, , - , α (11%) , ²²⁸U , - , rowspan=2, ²³³Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 139 , rowspan=2, 233.04300(5) , rowspan=2, 20.9(4) min , β⁺ (99.88%) , ²³³Np , rowspan=2, 5/2+# , rowspan=2, , - , α (.12%) , ²²⁹U , - , rowspan=2, ²³⁴Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 140 , rowspan=2, 234.043317(7) , rowspan=2, 8.8(1) h , EC (94%) , ²³⁴Np , rowspan=2, 0+ , rowspan=2, , - , α (6%) , ²³⁰U , - , rowspan=2, ²³⁵Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 235.045286(22) , rowspan=2, 25.3(5) min , β⁺ (99.99%) , ²³⁵Np , rowspan=2, (5/2+) , rowspan=2, , - , α (.0027%) , ²³¹U , - , rowspan=4, ²³⁶Pu , rowspan=4 style="text-align:right" , 94 , rowspan=4 style="text-align:right" , 142 , rowspan=4, 236.0460580(24) , rowspan=4, 2.858(8) y , α , ²³²U , rowspan=4, 0+ , rowspan=4, , - , SF (1.37×10⁻⁷%) , (various) , - , CD (2×10⁻¹²%) , ²⁰⁸Pb
²⁸Mg , - , β⁺β⁺ (rare) , ²³⁶U , - , rowspan=2, ²³⁷Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 143 , rowspan=2, 237.0484097(24) , rowspan=2, 45.2(1) d , EC , ²³⁷Np , rowspan=2, 7/2− , rowspan=2, , - , α (.0042%) , ²³³U , - , style="text-indent:1em" , ^237m1Pu , colspan="3" style="text-indent:2em" , 145.544(10)2 keV , 180(20) ms , IT , ²³⁷Pu , 1/2+ , , - , style="text-indent:1em" , ^237m2Pu , colspan="3" style="text-indent:2em" , 2900(250) keV , 1.1(1) μs , , , , , - , rowspan=4, ²³⁸Pu , rowspan=4 style="text-align:right" , 94 , rowspan=4 style="text-align:right" , 144 , rowspan=4, 238.0495599(20) , rowspan=4, 87.7(1) y , α , ²³⁴U , rowspan=4, 0+ , rowspan=4, Trace Double beta decay product of ²³⁸U , - , SF (1.9×10⁻⁷%) , (various) , - , CD (1.4×10⁻¹⁴%) , ²⁰⁶Hg
³²Si , - , CD (6×10⁻¹⁵%) , ¹⁸⁰Yb
³⁰Mg
²⁸Mg , - , rowspan=2, ²³⁹Pufissile nuclideMost useful isotope for nuclear weapons , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 145 , rowspan=2, 239.0521634(20) , rowspan=2, 2.411(3)×10⁴ y , α , ''²³⁵U'' , rowspan=2, 1/2+ , rowspan=2, Trace Neutron capture product of ²³⁸U , - , SF (3.1×10⁻¹⁰%) , (various) , - , style="text-indent:1em" , ^239m1Pu , colspan="3" style="text-indent:2em" , 391.584(3) keV , 193(4) ns , , , 7/2− , , - , style="text-indent:1em" , ^239m2Pu , colspan="3" style="text-indent:2em" , 3100(200) keV , 7.5(10) μs , , , (5/2+) , , - , rowspan=3, ²⁴⁰Pu , rowspan=3 style="text-align:right" , 94 , rowspan=3 style="text-align:right" , 146 , rowspan=3, 240.0538135(20) , rowspan=3, 6.561(7)×10³ y , α , ²³⁶U , rowspan=3, 0+ , rowspan=3, TraceIntermediate decay product of ²⁴⁴Pu , - , SF (5.7×10⁻⁶%) , (various) , - , CD (1.3×10⁻¹³%) , ²⁰⁶Hg
³⁴Si , - , rowspan=3, ²⁴¹Pu , rowspan=3 style="text-align:right" , 94 , rowspan=3 style="text-align:right" , 147 , rowspan=3, 241.0568515(20) , rowspan=3, 14.290(6) y , β⁻ (99.99%) , ²⁴¹Am , rowspan=3, 5/2+ , rowspan=3, , - , α (.00245%) , ²³⁷U , - , SF (2.4×10⁻¹⁴%) , (various) , - , style="text-indent:1em" , ^241m1Pu , colspan="3" style="text-indent:2em" , 161.6(1) keV , 0.88(5) μs , , , 1/2+ , , - , style="text-indent:1em" , ^241m2Pu , colspan="3" style="text-indent:2em" , 2200(200) keV , 21(3) μs , , , , , - , rowspan=2, ²⁴²Pu , rowspan=2 style="text-align:right" , 94 , rowspan=2 style="text-align:right" , 148 , rowspan=2, 242.0587426(20) , rowspan=2, 3.75(2)×10⁵ y , α , ''²³⁸U'' , rowspan=2, 0+ , rowspan=2, , - , SF (5.5×10⁻⁴%) , (various) , - , ²⁴³Pu , style="text-align:right" , 94 , style="text-align:right" , 149 , 243.062003(3) , 4.956(3) h , β⁻ , ²⁴³Am , 7/2+ , , - , style="text-indent:1em" , ^243mPu , colspan="3" style="text-indent:2em" , 383.6(4) keV , 330(30) ns , , , (1/2+) , , - , rowspan=3, ²⁴⁴Pu , rowspan=3 style="text-align:right" , 94 , rowspan=3 style="text-align:right" , 150 , rowspan=3, 244.064204(5) , rowspan=3, 8.00(9)×10⁷ y , α (99.88%) , ²⁴⁰U , rowspan=3, 0+ , rowspan=3, TraceInterstellar, some may also be

primordial Primordial may refer to: * Primordial era, an era after the Big Bang. See Chronology of the universe * Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis * Primordial nuclide, nuclides, a few radioactive, that formed before ...

but such claims are disputed , - , SF (.123%) , (various) , - , β⁻β⁻ (7.3×10⁻⁹%) , ²⁴⁴Cm , - , ²⁴⁵Pu , style="text-align:right" , 94 , style="text-align:right" , 151 , 245.067747(15) , 10.5(1) h , β⁻ , ²⁴⁵Am , (9/2−) , , - , ²⁴⁶Pu , style="text-align:right" , 94 , style="text-align:right" , 152 , 246.070205(16) , 10.84(2) d , β⁻ , ^246mAm , 0+ , , - , ²⁴⁷Pu , style="text-align:right" , 94 , style="text-align:right" , 153 , 247.07407(32)# , 2.27(23) d , β⁻ , ²⁴⁷Am , 1/2+# ,

Actinides vs fission products

Notable isotopes

* Plutonium-238 has a half-life of 87.74 years and emits

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 pro ...

s. Pure ²³⁸Pu for radioisotope thermoelectric generators that power some

spacecraft A spacecraft is a vehicle or machine designed to spaceflight, fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth ...

is produced by neutron capture on neptunium-237 but plutonium from

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...

can contain as much as a few percent ²³⁸Pu, originating from ²³⁷Np, alpha decay of ²⁴²Cm, or (n,2n) reactions. * Plutonium-239 is the most important isotope of plutonium, with a half-life of 24,100 years. ²³⁹Pu and ²⁴¹Pu are

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 typ ...

, meaning that the nuclei of their atoms can break apart by being bombarded by slow moving thermal neutrons, releasing energy,

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 ...

and more neutrons. It can therefore sustain a

nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. The specific nu ...

, leading to applications in

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 and

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. ²³⁹Pu is synthesized by irradiating

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 ...

with neutrons in a nuclear reactor, then recovered via nuclear reprocessing of the fuel. Further neutron capture produces successively heavier isotopes. * Plutonium-240 has a high rate of spontaneous fission, raising the background

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� ...

of plutonium containing it. Plutonium is graded by proportion of ²⁴⁰Pu: weapons grade (< 7%),

fuel grade A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...

(7–19%) and

reactor grade Reactor-grade plutonium (RGPu) is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up. The uranium-238 from which most of the plutonium isotopes d ...

(> 19%). Lower grades are less suited for nuclear weapons and

thermal reactor A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and stru ...

s but can fuel fast reactors. * Plutonium-241 is fissile, but also

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 e ...

s with a half-life of 14 years to

americium-241 Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is c ...

. * Plutonium-242 is not fissile, not very fertile (requiring 3 more neutron captures to become fissile), has a low neutron capture cross section, and a longer half-life than any of the lighter isotopes. * Plutonium-244 is the most stable isotope of plutonium, with a half-life of about 80 million years. It is not significantly produced in nuclear reactors because ²⁴³Pu has a short half-life, but some is produced in nuclear explosions. Plutonium-244 has been found in interstellar space and is has the longest half-life of any non-primordial radioisotope.

Production and uses

²³⁹Pu, a fissile isotope that is the second most used

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 undergoi ...

in nuclear reactors after

uranium-235 Uranium-235 (235U or U-235) is an Isotopes of uranium, 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 ...

, and the most used fuel in the

fission Fission, a splitting of something into two or more parts, may refer to: * Fission (biology), the division of a single entity into two or more parts and the regeneration of those parts into separate entities resembling the original * Nuclear fissio ...

portion of

s, is produced from

by neutron capture followed by two beta decays. ²⁴⁰Pu, ²⁴¹Pu, and ²⁴²Pu are produced by further neutron capture. The odd-mass isotopes ²³⁹Pu and ²⁴¹Pu have about a 3/4 chance of undergoing

on capture of a thermal neutron and about a 1/4 chance of retaining the

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 behav ...

and becoming the next heavier isotope. The even-mass isotopes are

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 occurrin ...

but not fissile and also have a lower overall probability ( cross section) of neutron capture; therefore, they tend to accumulate in nuclear fuel used in a thermal reactor, the design of nearly all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, ²⁴⁰Pu may even be the most common isotope. All plutonium isotopes and other

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inf ...

s, however, are fissionable with fast neutrons. ²⁴⁰Pu does have a moderate thermal neutron absorption cross section, so that ²⁴¹Pu production in a thermal reactor becomes a significant fraction as large as ²³⁹Pu production. ²⁴¹Pu has a half-life of 14 years, and has slightly higher thermal neutron cross sections than ²³⁹Pu for both fission and absorption. While nuclear fuel is being used in a reactor, a ²⁴¹Pu nucleus is much more likely to fission or to capture a neutron than to decay. ²⁴¹Pu accounts for a significant proportion of fissions in thermal reactor fuel that has been used for some time. However, in

that does not quickly undergo nuclear reprocessing but instead is cooled for years after use, much or most of the ²⁴¹Pu will beta decay to

, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors. ²⁴²Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either

curium Curium is a transuranium element, transuranic, radioactive decay, radioactive chemical element with the chemical symbol, symbol Cm and atomic number 96. This actinide element was named after eminent scientists Marie Curie, Marie and Pierre Curie ...

-245 or ²⁴¹Pu) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb a fourth neutron, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming ²⁴²Pu again; so the mean number of neutrons absorbed before fission is even higher than 3. Therefore, ²⁴²Pu is particularly unsuited to recycling in a thermal reactor and would be better used in a fast reactor where it can be fissioned directly. However, ²⁴²Pu's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. ²⁴²Pu's half-life is about 15 times as long as ²³⁹Pu's half-life; therefore, it is 1/15 as radioactive and not one of the larger contributors to

nuclear 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 ...

radioactivity. ²⁴²Pu's

gamma ray 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 nucleus, atomic nuclei. It consists of the shortest wavelength electromagnetic wav ...

emissions are also weaker than those of the other isotopes. ²⁴³Pu has a half-life of only 5 hours, beta decaying to americium-243. Because ²⁴³Pu has little opportunity to capture an additional neutron before decay, the

nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in w ...

does not produce the long-lived ²⁴⁴Pu in significant quantity. ²³⁸Pu is not normally produced in as large quantity by the nuclear fuel cycle, but some is produced from neptunium-237 by neutron capture (this reaction can also be used with purified neptunium to produce ²³⁸Pu relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on ²³⁹Pu, or by alpha decay of

-242, which is produced by neutron capture from ²⁴¹Am. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become ²³⁹Pu.

Manufacture

Plutonium-240, -241 and -242

The fission cross section for ²³⁹Pu is 747.9 barns for thermal neutrons, while the activation cross section is 270.7 barns (the ratio approximates to 11 fissions for every 4 neutron captures). The higher plutonium isotopes are created when the uranium fuel is used for a long time. For high burnup used fuel, the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel that is reprocessed to obtain weapons grade plutonium.

Plutonium-239

Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are

and

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 ex ...

. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding

with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor. Plutonium ring

Plutonium-238

There are small amounts of ²³⁸Pu in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a ²³⁵U atom captures a neutron, it is converted to an excited state of ²³⁶U. Some of the excited ²³⁶U nuclei undergo fission, but some decay to the ground state of ²³⁶U by emitting gamma radiation. Further neutron capture creates ²³⁷U, which has a half-life of 7 days and thus quickly decays to ²³⁷Np. Since nearly all neptunium is produced in this way or consists of isotopes that decay quickly, one gets nearly pure ²³⁷Np by chemical separation of neptunium. After this chemical separation, ²³⁷Np is again irradiated by reactor neutrons to be converted to ²³⁸Np, which decays to ²³⁸Pu with a half-life of 2 days.

²⁴⁰Pu as an obstacle to nuclear weapons

Plutonium-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of ²⁴⁰Pu limits the plutonium's use in a

nuclear bomb 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 bo ...

, because the neutron flux from spontaneous fission initiates the

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 sy ...

prematurely, causing an early release of energy that physically disperses the core before full implosion is reached. This prevents most of the core from participation in the chain reaction and reduces the bomb's power. Plutonium consisting of more than about 90% ²³⁹Pu is called weapons-grade plutonium; plutonium from

from commercial power reactors generally contains at least 20% ²⁴⁰Pu and is called reactor-grade plutonium. However, modern nuclear weapons use fusion boosting, which mitigates the predetonation problem; if the

pit Pit or PIT may refer to: Structure * Ball pit, a recreation structure * Casino pit, the part of a casino which holds gaming tables * Trapping pit, pits used for hunting * Pit (motor racing), an area of a racetrack where pit stops are conducted * ...

can generate a nuclear weapon yield of even a fraction of a

kiloton TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a ...

, which is enough to start deuterium-tritium fusion, the resulting burst of neutrons will fission enough plutonium to ensure a yield of tens of kilotons. ²⁴⁰Pu contamination is the reason plutonium weapons must use the implosion method. Theoretically, pure ²³⁹Pu could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. ²⁴⁰Pu contamination has proven a mixed blessing to nuclear weapons design. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone to accidental detonation than are gun-type weapons.

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Authority control Plutonium