Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary
fissile isotope used for the production of nuclear weapons, although uranium-235
has also been used. Plutonium-239 is also one of the three main isotopes
demonstrated usable as fuel in thermal spectrum nuclear reactors, along with
uranium-235 and uranium-233. Plutonium-239 has a 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 Pu-239 more cheaply than highly enriched
weapons-grade uranium-235, led to its use in nuclear weapons and nuclear power
stations. 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, Pu-239
has the smallest critical mass. A spherical untamped critical mass is
about 11 kg, 10.2 cm in diameter. Using appropriate triggers, neutron
reflectors, implosion geometry and tampers, this critical mass can be
reduced by more than twofold. This optimization usually requires a large
nuclear development organization supported by a sovereign nation.
The fission of one atom of Pu-239 generates 207.1 MeV = 3.318 × 10−11 J,
i.e. 19.98 TJ/mol = 83.61 TJ/kg. Manufacturing
Plutonium is made from U-238. Pu-239 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 U-238 is exposed to neutron
radiation, its nucleus will capture a neutron, changing it to U-239. This
happens more easily with lower kinetic energy. The U-239 then rapidly undergoes
two beta decays, becoming Pu-239. After the 238U absorbs a neutron to become
239U, it then emits an electron and an anti-neutrino by β− decay to become
neptunium-239 and then emits another electron and anti-neutrino by a second
β− decay to become 239Pu: Fission activity is relatively rare, so
even after significant exposure, the Pu-239 is still mixed with a great deal
of U-238, 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 Pu-239 be chemically separated from the rest of the material to yield
high-purity Pu-239 metal. Pu-239 has a higher probability for
fission than U-235 and a larger number of neutrons produced per fission event,
so it has a smaller critical mass. Pure Pu-239 also has a reasonably low rate of
neutron emission due to spontaneous fission, making it feasible to assemble
a mass that is highly supercritical before a detonation chain reaction
begins. In practice, however, reactor-bred
plutonium will invariably contain a certain amount of Pu-240 due to the
tendency of Pu-239 to absorb an additional neutron during production.
Pu-240 has a high rate of spontaneous fission events, making it an undesirable
contaminant. As a result, plutonium containing a significant fraction of
Pu-240 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, Pu-239 and Pu-240 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% Pu-240; this is achieved by only exposing U-238 to
neutron sources for short periods of time to minimize the Pu-240 produced.
Pu-240 exposed to alpha particles will incite a nuclear fission.
Plutonium is classified according to the percentage of the contaminant
plutonium-240 that it contains: Supergrade 2–3%
Weapons grade less than 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 U-238
to neutron radiation and for frequently replacing the irradiated U-238 with new
U-238. A reactor running on unenriched or moderately enriched uranium contains
a great deal of U-238. 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 U-238 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
inspects licensed reactors often. A few commercial power reactor designs, such
as the reaktor bolshoy moshchnosti kanalniy and pressurized heavy water
reactor, 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 Pu-239 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 can also be operated in an "incineration
mode," having some advantages in not building up the Pu-242 isotope or the
long-lived actinides, either of 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 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 such as 235. The process of
enriching Uranium, ie increasing the ratio of U-235 to U-238 to weapons
grade, is generally a more lengthy and costly process than the production of
Plutonium 239 from U-238 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 Pu-239, leaving a very low amount of Pu-240 which is a high
spontaneous fission isotope. 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 Pu-240 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 emitter in that process as well, and so is responsible for a large
fraction of the radiation from stored nuclear weapons. Submarine crew members
routinely operate in close proximity to stored weapons in torpedo rooms, unlike
Air Force missiles where exposures are relatively brief—hence justifying the
additional costs of the premium supergrade alloy used on many naval
nuclear torpedo weapons. Supergrade plutonium is used in W80 warheads.
In nuclear power reactors In any operating nuclear reactor
containing U-238, 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 that has been removed from the reactor at the end of the fuel
assembly’s service life. 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 about 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 fuel, as
it contains a mixture of uranium oxide and plutonium oxide. The addition of
plutonium-239 reduces or eliminates the need to enrich the uranium in the fuel.
Hazards Plutonium-239 emits alpha particles to
become the fairly harmless 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 carcinogenic. It has
been estimated that a pound of plutonium inhaled as plutonium oxide dust could
give cancer to two million people. Therefore as little as a milligram would
be quite likely to cause cancer in a person. As a heavy metal, plutonium is
also toxic. See also Plutonium#Precautions.
Weapons grade Plutonium can be used to make nuclear weapons, and there is
danger of it falling into the wrong hands. Its storage must be very secure.
See also Teller-Ulam design
References ^ "Physical, Nuclear, and Chemical
Properties of Plutonium". Institute for Energy and Environmental Research.
Retrieved 20 November 2015. ^ FAS Nuclear Wepons Design FAQ,
Accessed 2010-9-2 ^ a b "Table of Physical and Chemical
Constants, Sec 4.7.1: Nuclear Fission". Kaye & Laby Online.
^ Jeremy J. Whitlock. "The Evolution of CANDU Fuel Cycles and their Potential
Contribution to World Peace". ^ Hala, Jiri; James D. Navratil.
Radioactivity, Ionizing Radiation, and Nuclear Energy. Brno: Konvoj. p. 102.
ISBN 80-7302-053-X. ^ Bernard L. Cohen. "Chapter 13,
Plutonium and bombs". The Nuclear Energy Option. Plenum Press. ISBN
978-0306435676. Archived from the original on July 21, 2013.
External links NLM Hazardous Substances Databank –
Plutonium, Radioactive Table of nuclides with Pu-239 data at
Kaye and Laby Online Half-life of Plutinium-239