Plutonium-239 ( or Pu-239) is an
isotope of plutonium. Plutonium-239 is the primary
fissile
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal Nuclear chain reaction#Fission chain reaction, chain reaction can only be achieved with fissil ...
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 or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
s, although
uranium-235
Uranium-235 ( 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 exists in nat ...
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 Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
s, along with
uranium-235
Uranium-235 ( 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 exists in nat ...
and
uranium-233
Uranium-233 ( 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 Nuclear fuel, reactor fuel. It has been used successfully ...
. Plutonium-239 has a
half-life Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''Half Life: ...
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 uranium-235, led to its use in nuclear weapons and nuclear power plants. 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 isotopes. 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. 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 = 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.
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, its nucleus will capture a
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
, changing it to
239U. This happens more often 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 in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
and an
anti-neutrino (
), leaving a proton in the nucleus — the first β
− decay transforming the
239U into
neptunium-239, 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. 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. Pure
239Pu also has a reasonably low rate of neutron emission due to
spontaneous fission (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 sys ...
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. 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 would be necessary to separate them. Weapons-grade plutonium 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 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 technology, nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was ...
inspects licensed reactors often. A few commercial power reactor designs, such as the ''reaktor bolshoy moshchnosti kanalniy'' (
RBMK) and pressurized heavy water reactor (
PHWR), do permit refueling without shutdowns, and they may pose a proliferation risk. By contrast, the Canadian
CANDU 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 (Integral Fast Reactor) can also be operated in an incineration mode, having some advantages in not accumulating the
plutonium-242 isotope or the long-lived
actinide
The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...
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 reactors or plutonium production reactors called
breeder reactors 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 reactors, since
fast neutrons 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. Both plutonium-239 and uranium-235 are obtained from
Natural uranium, which primarily consists of uranium-238 but contains traces of other isotopes of uranium such as uranium-235. The process of
enriching uranium, i.e. increasing the ratio of
235U to
238U to weapons grade, 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 nuclear weapons in place of the conventional plutonium used in the Air Force's versions. "Supergrade" is industry parlance for plutonium alloy bearing an exceptionally high fraction of
239Pu (>95%), leaving a very low amount of
240Pu, which is a high spontaneous fission isotope (see above). Such plutonium is produced from fuel rods that have been irradiated a very short time as measured in MW-day/ton
burnup. Such low irradiation times limit the amount of additional neutron capture and therefore buildup of alternate isotope products such as
240Pu in the rod, and also by consequence is considerably more expensive to produce, needing far more rods irradiated and processed for a given amount of plutonium.
Plutonium-240, in addition to being a neutron emitter after fission, is a
gamma
Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...
emitter, and so is responsible for a large fraction of the radiation from stored nuclear weapons. Whether out on patrol or in port, submarine crew members routinely live and work in very close proximity to nuclear weapons stored in torpedo rooms and missile tubes, unlike Air Force missiles where exposures are relatively brief. The need to reduce radiation exposure justifies the additional costs of the premium supergrade alloy used on many naval nuclear weapons. Supergrade plutonium is used in
W80 warheads.
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 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 and ...
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 (UO
2) and plutonium dioxide (PuO
2). The addition of plutonium-239 reduces the need to enrich the uranium in the fuel.
Hazards
Plutonium-239 emits
alpha particles to become uranium-235. 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 agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...
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
Nuclear 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
*
Isotopes of plutonium
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