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 exhibi ...
(
94Pu) is an
artificial element, except for trace quantities resulting from
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, ...
by uranium, and thus a
standard atomic weight cannot be given. Like all artificial elements, it 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 ...
s. 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 numb ...
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
Plutonium-244 (244Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium ...
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
Plutonium-242 (242Pu or Pu-242) is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years.
The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the la ...
with a half-life of 373,300 years, and
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 ...
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 ...
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 giv ...
from 228.0387
u (
228Pu) to 247.074 u (
247Pu). The primary
decay modes before the most stable isotope,
244Pu, are
spontaneous fission and
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 ...
; the primary mode after is
beta emission
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 exam ...
. The primary
decay products before
244Pu are
isotopes of uranium
Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The d ...
and
neptunium
Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it bein ...
(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
In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move clos ...
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,
239Pu[fissile nuclide][Most 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)×10
4 y
, α
, ''
235U''
, rowspan=2, 1/2+
, rowspan=2, Trace
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, ...
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)×10
3 y
, α
,
236U
, rowspan=3, 0+
, rowspan=3, Trace
[Intermediate 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)×10
5 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)×10
7 y
, α (99.88%)
,
240U
, rowspan=3, 0+
, rowspan=3, Trace
[Interstellar, some may also be primordial 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
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 ...
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 pr ...
s. Pure
238Pu for
radioisotope thermoelectric generator
A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s that power some
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
is produced by neutron capture on
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 ...
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 and ...
can contain as much as a few percent
238Pu, originating from
237Np,
alpha decay
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 ...
of
242Cm, or (n,2n) reactions.
*
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 ...
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 t ...
, meaning that the nuclei of their atoms can
break apart by being bombarded by
slow moving thermal neutrons, releasing energy,
gamma radiation and
more neutrons. It can therefore sustain a
nuclear chain reaction, 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 fr ...
s.
239Pu is synthesized by irradiating
uranium-238 with neutrons in a nuclear reactor, then recovered via
nuclear reprocessing of the fuel. Further
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, ...
produces successively heavier isotopes.
*
Plutonium-240
Plutonium-240 ( or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
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
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
(< 7%),
fuel grade (7–19%) and
reactor grade (> 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 struct ...
s but can fuel
fast reactor
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as oppose ...
s.
*
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 ...
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 ...
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 com ...
.
*
Plutonium-242
Plutonium-242 (242Pu or Pu-242) is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years.
The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the la ...
is not fissile, not very fertile (requiring 3 more neutron captures to become fissile), has a low 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 ...
, and a longer half-life than any of the lighter isotopes.
*
Plutonium-244
Plutonium-244 (244Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium ...
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 in nuclear reactors after
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 ...
, and the most used fuel in the
fission portion of
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, is produced from
uranium-238 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
fission on capture of a
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 ...
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 beh ...
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 occurring ...
but not fissile and also have a lower overall probability (
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 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
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an al ...
,
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 info ...
s, however, are
fissionable
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 ...
with
fast 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.
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
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 and ...
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
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 com ...
, one of the
minor actinide
The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium (element 93), americium (element 95), curium (element 96), berkeliu ...
s, 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-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
Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding ...
, 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
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as oppose ...
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 nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
emissions are also weaker than those of the other isotopes.
243Pu has a half-life of only 5 hours, beta decaying to
americium-243
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 ...
. 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
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 ...
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 generator
A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...
s), by the (n,2n) reaction of fast neutrons on
239Pu, or by alpha decay of
curium-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
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 ...
for
239Pu is 747.9
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, 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
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
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
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 ...
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 exp ...
. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding
uranium-238 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-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
Plutonium-240 ( or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
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, 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 sys ...
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
Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nucle ...
; 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 and ...
from commercial power reactors generally contains at least 20%
240Pu and is called
reactor-grade plutonium 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 der ...
. However, modern nuclear weapons use
fusion boosting
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released d ...
, which mitigates the predetonation problem; if the
pit 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
Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another. Although this is someti ...
, but achieving this level of purity is prohibitively difficult.
240Pu contamination has proven a mixed blessing to
nuclear weapons design
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three existing basic design types:
* pure fission weapons, the simplest and least technically ...
. While it created delays and headaches during the
Manhattan Project
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
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
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 exhibi ...