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Plutonium-239 (239Pu or Pu-239) is an
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
of
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 ...
. Plutonium-239 is the primary
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 ...
isotope used for the production 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, although
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum
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, along with
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
and
uranium-233 Uranium-233 (233U or U-233) is a fissile Isotopes of uranium, 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 Nuclear fuel, reactor fuel. It ha ...
. Plutonium-239 has a
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of 24,110 years.


Nuclear properties

The nuclear properties of plutonium-239, as well as the ability to produce large amounts of nearly pure 239Pu more cheaply than highly enriched
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 ...
uranium-235, led to its use in
nuclear weapons 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 bomb ...
and
nuclear power plant A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a electric generator, generato ...
s. The fissioning of an atom of uranium-235 in the reactor of a nuclear power plant produces two to three neutrons, and these neutrons can be absorbed by uranium-238 to produce plutonium-239 and other
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
s. Plutonium-239 can also absorb neutrons and fission along with the uranium-235 in a reactor. Of all the common nuclear fuels, 239Pu has the smallest
critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissi ...
. A spherical untamped critical mass is about 11 kg (24.2 lbs), 10.2 cm (4") in diameter. Using appropriate triggers, neutron reflectors, implosion geometry and tampers, the critical mass can be less than half of that. The fission of one atom of 239Pu generates 207.1
MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an Voltage, electric potential difference of one volt i ...
= 3.318 × 10−11 J, i.e. 19.98 TJ/ mol = 83.61 TJ/kg, or about 23 gigawatt hours/kg.


Production

Plutonium is made from
uranium-238 Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it ...
. 239Pu is normally created in nuclear reactors by transmutation of individual atoms of one of the isotopes of uranium present in the fuel rods. Occasionally, when an atom of 238U is exposed 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 Neutron capture, react with Atomic nucleus, nuclei of other ...
, its nucleus will capture a
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 ...
, changing it to 239U. This happens more easily with lower kinetic energy (as 238U fission activation is 6.6MeV). The 239U then rapidly undergoes two β decays — an emission of an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
and an anti-neutrino (\bar_e), leaving a proton — the first β decay transforming the 239U into
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 ...
, and the second β decay transforming the 239Np into 239Pu: :^_U + ^_n -> ^_U -> beta^-23.5\ \ce] ^_Np -> beta^-2.356\ \ce] ^_Pu Fission activity is relatively rare, so even after significant exposure, the 239Pu is still mixed with a great deal of 238U (and possibly other isotopes of uranium), oxygen, other components of the original material, and
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
. Only if the fuel has been exposed for a few days in the reactor, can the 239Pu be chemically separated from the rest of the material to yield high-purity 239Pu metal. 239Pu has a higher probability for fission than 235U and a larger number of neutrons produced per fission event, so it has a smaller
critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissi ...
. Pure 239Pu also has a reasonably low rate of neutron emission due to
spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...
(10 fission/s·kg), making it feasible to assemble a mass that is highly supercritical before a detonation
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 syst ...
begins. In practice, however, reactor-bred plutonium will invariably contain a certain amount of 240Pu due to the tendency of 239Pu to absorb an additional neutron during production. 240Pu has a high rate of spontaneous fission events (415,000 fission/s-kg), making it an undesirable contaminant. As a result, plutonium containing a significant fraction of 240Pu is not well-suited to use in nuclear weapons; it emits neutron radiation, making handling more difficult, and its presence can lead to a " fizzle" in which a small explosion occurs, destroying the weapon but not causing fission of a significant fraction of the fuel. (However, in modern nuclear weapons using neutron generators for initiation and
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 ...
to supply extra neutrons, fizzling is not an issue.) It is because of this limitation that plutonium-based weapons must be implosion-type, rather than gun-type. Moreover, 239Pu and 240Pu cannot be chemically distinguished, so expensive and difficult
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 ...
would be necessary to separate them.
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 Uranium is a chemical eleme ...
is defined as containing no more than 7% 240Pu; this is achieved by only exposing 238U to neutron sources for short periods of time to minimize the 240Pu produced. Plutonium is classified according to the percentage of the contaminant plutonium-240 that it contains: * Supergrade 2–3% * Weapons grade 3–7% * Fuel grade 7–18% * Reactor grade 18% or more A nuclear reactor that is used to produce plutonium for weapons therefore generally has a means for exposing 238U to neutron radiation and for frequently replacing the irradiated 238U with new 238U. A reactor running on unenriched or moderately enriched uranium contains a great deal of 238U. However, most commercial
nuclear power 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 ...
designs require the entire reactor to shut down, often for weeks, in order to change the fuel elements. They therefore produce plutonium in a mix of isotopes that is not well-suited to weapon construction. Such a reactor could have machinery added that would permit 238U slugs to be placed near the core and changed frequently, or it could be shut down frequently, so proliferation is a concern; for this reason, the
International Atomic Energy Agency The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 1957 ...
inspects licensed reactors often. A few commercial power reactor designs, such as the ''reaktor bolshoy moshchnosti kanalniy'' (
RBMK The RBMK (russian: реактор большой мощности канальный, РБМК; ''reaktor bolshoy moshchnosti kanalnyy'', "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and buil ...
) and pressurized heavy water reactor (
PHWR A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water ( deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The ...
), do permit refueling without shutdowns, and they may pose a proliferation risk. (In fact, the RBMK was built by the Soviet Union during the Cold War, so despite their ostensibly peaceful purpose, it is likely that plutonium production was a design criterion.) By contrast, the Canadian
CANDU The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
heavy-water moderated natural-uranium fueled reactor can also be refueled while operating, but it normally consumes most of the 239Pu it produces ''in situ;'' thus, it is not only inherently less proliferative than most reactors, but can even be operated as an " actinide incinerator". The American
IFR In aviation, instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR). The U.S. Federal Aviation Administration's (FAA) ''Instrument Fly ...
(Integral Fast Reactor) can also be operated in an " incineration mode", having some advantages in not accumulating the
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 ...
isotope or the long-lived
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 inform ...
s, which cannot be easily burned except in a fast reactor. Also IFR fuel has a high proportion of burnable isotopes, while in CANDU an inert material is needed to dilute the fuel; this means the IFR can burn a higher fraction of its fuel before needing reprocessing. Most plutonium is produced in
research reactor Research reactors are nuclear fission-based nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used for electricity production, heat generation, or maritim ...
s or plutonium production reactors called
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 because they produce more plutonium than they consume fuel; in principle, such reactors make extremely efficient use of natural uranium. In practice, their construction and operation is sufficiently difficult that they are generally only used to produce plutonium. Breeder reactors are generally (but not always)
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 opposed t ...
s, since
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 are somewhat more efficient at plutonium production. Plutonium-239 is more frequently used in nuclear weapons than uranium-235, as it is easier to obtain in a quantity of
critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissi ...
. Both plutonium-239 and uranium-235 are obtained from
Natural uranium Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes fr ...
, which primarily consists of uranium-238 but contains traces of other isotopes of uranium such as
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
. The process of enriching uranium, i.e. increasing the ratio of 235U to 238U to
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 ...
, is generally a more lengthy and costly process than the production of plutonium-239 from 238U and subsequent reprocessing.


Supergrade plutonium

The "supergrade" fission fuel, which has less radioactivity, is used in the primary stage of
US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage of ...
nuclear weapons in place of the conventional
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 ...
used in the Air Force's versions. "Supergrade" is industry
parlance {{Short pages monitor


In nuclear power reactors

In any operating nuclear reactor containing 238U, some plutonium-239 will accumulate in the nuclear fuel. Unlike reactors used to produce weapons-grade plutonium, commercial nuclear power reactors typically operate at a high
burnup In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per init ...
that allows a significant amount of plutonium to build up in irradiated reactor fuel. Plutonium-239 will be present both in the reactor core during operation and 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 an ...
that has been removed from the reactor at the end of the fuel assembly's service life (typically several years). Spent nuclear fuel commonly contains about 0.8% plutonium-239. Plutonium-239 present in reactor fuel can absorb neutrons and fission just as uranium-235 can. Since plutonium-239 is constantly being created in the reactor core during operation, the use of plutonium-239 as nuclear fuel in power plants can occur without reprocessing of spent fuel; the plutonium-239 is fissioned in the same fuel rods in which it is produced. Fissioning of plutonium-239 provides more than one-third of the total energy produced in a typical commercial nuclear power plant. Reactor fuel would accumulate much more than 0.8% plutonium-239 during its service life if some plutonium-239 were not constantly being "burned off" by fissioning. A small percentage of plutonium-239 can be deliberately added to fresh nuclear fuel. Such fuel is called MOX (mixed oxide) fuel, as it contains a mixture of uranium dioxide (UO2) and plutonium dioxide (PuO2). The addition of plutonium-239 reduces the need to enrich the uranium in the fuel.


Hazards

Plutonium-239 emits
alpha particles 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 prod ...
to become
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
. As an alpha emitter, plutonium-239 is not particularly dangerous as an external radiation source, but if it is ingested or breathed in as dust it is very dangerous and
carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...
ic. It has been estimated that a pound (454 grams) of plutonium inhaled as plutonium oxide dust could give cancer to two million people. However, ingested plutonium is by far less dangerous as only a tiny fraction is absorbed in gastrointestinal tract. 800 mg would be unlikely to cause a major health risk as far as radiation is concerned. As a heavy metal, plutonium is also chemically toxic. See also Plutonium#Precautions. Weapons grade plutonium (with greater than 90% 239Pu) is used to make nuclear weapons and has many advantages over other fissile material for that purpose. Lower proportions of 239Pu would make a reliable weapon design difficult or impossible; this is due to the spontaneous fission (and thus neutron production) of the undesirable 240Pu.


See also

*
Teller-Ulam design A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lowe ...


References


External links


NLM Hazardous Substances Databank – Plutonium, Radioactive


* ttp://www.nucleide.org/DDEP_WG/Nuclides/Pu-239_tables.pdf Half-life of Plutonium-239 {{Isotopes of plutonium Actinides Fissile materials Isotopes of plutonium Special nuclear materials Radioactive contamination