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Plutonium
Nuclear Issues Briefing Paper 18
June 2002

Over one third of the energy produced in most nuclear power plants comes from plutonium. It is created there as a by-product.
Plutonium has occurred naturally, but except for trace quantities it is not now found in the earth's crust.
There are several tonnes 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.
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).

The main isotopes of plutonium are:

Pu-238, (half-life 88 years)
Pu-239, fissile (half-life 24 000 yrs)
Pu-240, fertile (half-life 6 500 yrs)
Pu-241, fissile (half-life 14 years)
Pu-242, (half-life 37 600 yrs)
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 others are capable of absorbing neutrons and becoming fissile.

Plutonium-238, Pu-240 and Pu-242 emit neutrons as their nuclei spontaneously fission, albeit at a low rate. They also decay, and the decay heat of Pu-238 (0.56 W/g) enables its use as an electricity source in the radioisotope thermoelectric generators (RTGs) 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. The Cassini spacecraft carries three generators providing 870 watts power en route to Saturn.

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° 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: a fission energy source
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. Pu-239 is fissile, yielding much the same energy as the fission of a U-235 atom, and complementing it.

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 balance emerges as Pu-239 in the spent fuel.

An ordinary large nuclear power reactor (1000 MWe LWR) gives rise to about 25 tonnes of spent fuel a year, containing up to 290 kilograms of plutonium. Plutonium, like uranium, is an immense energy source. The plutonium extracted from used reactor fuel can be used as a direct substitute for U-235 in the usual fuel, the Pu-239 being the main fissile part but Pu-241 also contributing.

If the spent fuel is reprocessed, the recovered plutonium oxide is mixed with depleted uranium oxide to produce mixed-oxide (MOX) fuel, with about 5% 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 typical households.

Plutonium-240 is the second most common isotope, formed by occasional neutron capture by Pu-239. 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 fissionable, which means that such a reactor can utilise recycled LWR plutonium more effectively than a LWR.)

The 1.15% of plutonium in the spent fuel removed from a commercial power reactor (burn-up of 42 GWd/t) consists of about 55% Pu-239, 23% Pu-240, 12% Pu-241 and lesser quantities of the other isotopes, including 2% of Pu-238 which is the main source of heat and radioactivity. Reactor-grade plutonium is defined as that with 19% or more of Pu-240.

Plutonium stored over several years becomes contaminated with the Pu-241 decay product Americium (see paper on Smoke detectors & 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 makes 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.

Resources of plutonium
Total world generation of reactor-grade plutonium is some 50 tonnes per year. About 900 tonnes 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% MOX in their fuel load for instance. Currently 8-10 tonnes 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% MOX in their fuel.

Some 32 European reactors are licensed to use MOX fuel, and several in France are using it as 30% of their fuel.

About 22 tonnes 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.

The UK has 65 tonnes of separated plutonium and this stockpile is expected to grow to 106 tonnes by 2012 - some 81t from Magnox fuel and 25t from AGR fuel. Using it all in MOX fuel rather than immobilising it as waste is expected to yield a £700-1200 million resource cost saving to UK, along with 300 billion kWh of electricity (about one year's UK supply). The 106t Pu could be consumed in two 1000 MWe light water reactors using 100% MOX fuel over 35 years.

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. Producing 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 tonnes 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 tonnes of weapons-grade plutonium to be surplus, and planned to pursue the first two options above, though only the MOX is proceeding. Meanwhile the US has developed a "spent fuel standard", which means that plutonium, including weapons Pu, should never be more accessible than if it is incorporated in spent fuel.

Europe has a well-developed MOX capacity and this suggests that weapons plutonium could be disposed of relatively quickly. Input plutonium in facilities such as Sellafield's new MOX plant would need to be about half reactor grade and half weapons grade, but using such MOX as 30% of the fuel in one third of the world's reactor capacity would remove about 15 tonnes of warhead plutonium per year. This would amount to burning 3000 warheads per year to produce 110 billion kWh of electricity.

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 is strongly committed to using 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 ionising 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 radiotoxicity (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.

The main threat to humans comes from inhalation. While it is very difficult to create airborne dispersion of a heavy metal like plutonium, certain forms, including the insoluble plutonium oxide, at a particle size less than 10 microns, are a hazard.

If inhaled, much of the material is immediately exhaled or is expelled by mucous flow from the bronchial system into the gastro-intestinal tract, as with any particulate matter. Some however will be trapped and readily transferred, first to the blood or lymph system and later to other parts of the body, notably the liver and bones. It is here that the deposited plutonium's alpha radiation may eventually cause cancer.

However, the hazard from Pu-239 is similar to that from any other alpha-emitting radionuclides which might be inhaled. It is less hazardous than those which are short-lived and hence more radioactive, such as radon daughters, the decay products of radon gas, which (albeit in low concentrations) are naturally common and widespread in the environment.

Over the past 50 years, there have been several incidents where workers at US nuclear weapons facilities came in contact with or inhaled plutonium. To date, intensive health checks of these people have revealed no serious consequence and no fatalities that could be attributed to the exposure.

Plutonium is one among many toxic materials that have to be handled with great care to minimise the associated but well understood risks.