Basic Plutonium facts:
Plutonium has occurred naturally, but except for trace quantities it is not now found in the earth's crust. It was first isolated in 1941.
A typical large nuclear power reactor creates about 230 kilograms of plutonium per year. Worldwide this amounts to about 75 tones per year.
There are several tones of plutonium in our biosphere, a legacy of atmospheric weapons testing in the 1950s and 1960s.
Plutonium is radiologically hazardous, particularly if inhaled, so must be handled with appropriate precautions.
Plutonium, both from reactors and from dismantled nuclear weapons, is a valuable energy source when integrated into the nuclear fuel cycle.
Half-life is the time it takes for a radionuclide to lose half of its own radioactivity. The fissile isotopes can be used as fuel in a nuclear reactor, the fertile ones are capable of absorbing neutrons and becoming fissile.
All plutonium isotopes are radioactive, and most emit relatively weak alpha radiation which can be blocked even by a sheet of paper (but which is hazardous if within the body - see below). Plutonium-238, Pu-240 and Pu-242 emit neutrons as their nuclei spontaneously fission, albeit at a low rate.
The decay heat of Pu-238 (0.56 W/g) enables its use as an electricity source in the thermo-electric generators of some cardiac pacemakers, space satellites, navigation beacons, etc.
Plutonium has powered 24 US space vehicles and enabled the Voyager spacecraft to send back pictures of distant planets.
These spacecraft have operated for 20 years and may continue for another 20. In 1998 the Cassini probe to Saturn was launched, with three thermoelectric generators providing all its 870 watts of power.
In commercial power-plants and research applications plutonium generally exists as plutonium oxide (PuO2), a stable ceramic material with an extremely low solubility in water or body fluids and with a high melting point (2 390 degrees C).
In pure form plutonium is a hard and brittle metal, like cast iron, but which spontaneously ignites in air to form PuO2.
Apart from its formation in today's nuclear reactors, plutonium was formed by the operation of the natural reactors in a uranium deposit at Oklo in west Africa some two billion years ago.
Plutonium as an Energy Source for Fission:
Plutonium is a by-product of the fission process in nuclear reactors, due to neutron capture by uranium-238 in particular.
When operating, a typical nuclear reactor contains within its uranium fuel load about 325 kilograms of plutonium, with plutonium-239 being the most common isotope.
In a power reactor, much of the energy from the fission process is due to the plutonium, complementing that from the uranium-235.
Well over half of the plutonium created in the reactor core is "burned" in situ and is responsible for about one third of the total heat output.
Of the rest, one sixth through neutron capture becomes Pu-240 (and Pu-241), the rest emerges as Pu-239 in the spent fuel.
An ordinary large nuclear power reactor (1000 MWe LWR) gives rise to about 20-25 tones of spent fuel a year, containing 200-250 kilograms of plutonium.
Plutonium, like uranium, is an immense energy source. In the plutonium extracted from used reactor fuel, the Pu-239 can be used as a direct substitute for U-235 in the usual fuel.
If the spent fuel is reprocessed, the recovered plutonium oxide is mixed with uranium oxide to produce mixed-oxide (MOX) fuel, with about 5 percent Pu-239.
Plutonium can be used on its own in fast neutron reactors, where the Pu-240 also fissions, and so functions as a fuel (along with U-238). It is thus said to be "fissionable", as distinct from fissile.
One kilogram of Pu-239 being slowly consumed over three years in a conventional nuclear reactor can produce sufficient heat to generate nearly 10 million kilowatt-hours of electricity - sufficient to meet the needs of over 1000 Australian households.
Plutonium-240 is the second most common isotope, and its concentration in nuclear fuel builds up steadily, since it does not undergo fission to produce energy in the same way as Pu-239. (In a fast neutron reactor it is fissile, which means that such a reactor can utilize recycled LWR plutonium more effectively than a LWR.)
The 1.15 percent of plutonium in the spent fuel removed from a commercial power reactor (burn-up of 42 GWd/t) consists of about 55 percent Pu-239, 23 percent Pu-240, 12 percent Pu-241 and lesser quantities of the other isotopes, including 2 percent of Pu-238 which is the main source of heat and radioactivity.
Reactor-grade plutonium is defined as that with 19 percent or more of Pu-240.
Plutonium stored over several years becomes contaminated with the Pu-241 decay product Americium, which interferes with normal fuel fabrication procedures.
After long storage, Am-241 must be removed before the Pu can be used in a normal MOX plant.
While of a different order of magnitude to the fission occurring within a nuclear reactor, Pu-240 has a relatively high rate of spontaneous fission with consequent neutron emissions.
This and its inability to sustain a chain reaction make reactor-grade plutonium entirely unsuitable for use in a bomb (see below).
Recovered plutonium can only be recycled through a light water reactor once or twice, as the isotopic quality deteriorates.
However, fast neutron reactors can then use this material and complete its consumption. Such reactors can also be configured to be net breeders of plutonium (as originally envisaged), but the need for this is remote.
Meanwhile research on fast neutron reactors is focused on maximising consumption of plutonium and incineration of actinides formed in the light water reactors.
Total world generation of reactor-grade plutonium is some 50 tones per year. About 850 tones have been produced so far, and most of this remains in the spent fuel.
About one third of the separated Pu has been used in MOX over the last 30 years: seven French reactors use 30 percent MOX in their fuel load for instance. Currently 8-10 tones of Pu is used in MOX each year.
Three US reactors are able to run fully on MOX, as can Canadian heavy water reactors. All Western and the later Soviet light water reactors can use 30 percent MOX in their fuel.
Some 32 European reactors are licensed to use MOX fuel, and six in France are using it as 30 percent of their fuel.
Some 22 tones of reactor-grade plutonium is separated by reprocessing plants in the OECD each year and this is set to double by 2003, by which time its usage in MOX is expected to outstrip this level of production so that stockpiles diminish.
See also Appendix on plutonium recycling from 1999 ASNO Annual Report.
Plutonium and Weapons: It takes about 10 kilograms of nearly pure Pu-239 to make a bomb. This would require 30 megawatt-years of reactor operation, with frequent fuel changes and reprocessing the 'hot' fuel.
For weapons use, Pu-240 is considered a serious contaminant and it is not feasible to separate Pu-240 from Pu-239. An explosive device could be made from plutonium extracted from low burn-up reactor fuel (ie. if the fuel had only been used for a short time), but any significant proportions of Pu-240 in it would make it hazardous to the bomb makers, as well as unreliable and unpredictable.
Plutonium for weapons is made differently, in simple reactors (usually fuelled with natural uranium) run for that purpose, with frequent fuel changes (ie. low burn-up).
This, coupled with the application of international safeguards, effectively rules out the use of commercial nuclear power plants.
International safeguards arrangements applied to traded uranium extend to the plutonium arising from it, ensuring constant audits.
Disarmament will give rise to some 150-200 tones of weapons-grade plutonium, over half of it in former USSR.
Discussions are progressing as to what should be done with it.
The main options for the disposal of weapons-grade plutonium are:
Vitrification with high-level waste - treating plutonium as waste,
Fabrication with uranium oxide as a mixed oxide (MOX) fuel for burning in existing reactors,
Fuelling fast-neutron reactors.
The US Government has declared 38 tones of weapons-grade plutonium to be surplus, and will pursue the first two options above.
There is wide support for burning it as a mixed oxide fuel in conventional reactors, but the novelty of this for the US will mean regulatory and technical delays.
Meanwhile the US has developed a "spent fuel standard", which means that plutonium, including that from weapons, should never be more accessible than if it is incorporated in spent fuel.
However, Europe has a well-developed MOX capacity and this suggests that weapons plutonium could be disposed of relatively quickly.
Input plutonium would need to be about half reactor grade and half weapons grade, but using such MOX as 30 percent of the fuel in one third of the world's reactor capacity would remove about 15 tones of warhead plutonium per year.
This would amount to burning 3000 warheads per year to produce 110 billion kWh of electricity, - enough for two thirds of Australia's needs.
Canada is promoting the use of its CANDU heavy water reactors as having very flexible fuel requirements and hence as suitable for disposing of military plutonium.
Various mixed oxide fuels have been tested in these reactors, which can be operated economically with a full MOX core.
Russia intends to use its plutonium in mixed-oxide fuel, burning it in both late-model conventional reactors and BN series fast neutron reactors.
Toxicity and Health Effects:
Despite being toxic both chemically and because of its ionizing radiation, plutonium is far from being 'the most toxic substance on earth' or so hazardous that 'a speck can kill'.
On both counts there are substances in daily use that, per unit of mass, have equal or greater chemical toxicity (arsenic, cyanide, caffeine) and radio toxicity (smoke detectors).
There are three principal routes by which plutonium can reach human beings:
ingestion,
contamination of open wounds,
inhalation.
Ingestion is not a significant hazard, because plutonium passing through the gastro-intestinal tract is poorly absorbed and is expelled from the body before it can do harm.
Contamination of wounds has rarely occurred although thousands of people have worked with plutonium. Their health has been protected by the use of remote handling, protective clothing and extensive health monitoring procedures.
Note: Originally published on February 1999.
Source: www.uic.com.au
© 2001 Mena Report (www.menareport.com)
