Plutonium

Source: ATSDR

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).

Previous Element: Neptunium

Next Element: Americium
94

Pu

239.1
Physical Properties
Color silvery
Phase at Room Temp. solid
Density (g/cm3) 19.86
Hardness (Mohs) ---
Melting Point (K) 914.2
Boiling Point (K) 3503
Heat of Fusion (kJ/mol) 2.8
Heat of Vaporization (kJ/mol) 343.5
Heat of Atomization (kJ/mol) 352
Thermal Conductivity (J/m sec K) 6.3
Electrical Conductivity (1/mohm cm) 7.072
Source Synthetic
Atomic Properties
Electron Configuration [Rn]7s25f6
Number of Isotopes 24 (0 natural)
Electron Affinity (kJ/mol) ---
First Ionization Energy (kJ/mol) 585
Second Ionization Energy (kJ/mol) ---
Third Ionization Energy (kJ/mol) ---
Electronegativity 1.3
Polarizability (Å3) 24.5
Atomic Weight 239.1
Atomic Volume (cm3/mol) 12.3
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 159
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) 114
Common Oxidation Numbers +4
Other Oxid. Numbers +3, +5, +6, +7
Abundance
In Earth's Crust (mg/kg) ---
In Earth's Ocean (mg/L) ---
In Human Body (%) ---
Regulatory / Health
CAS Number 7440-07-5
OSHA Permissible Exposure Limit (PEL) No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit (REL) No limits
Sources:
University of Wisconsin General Chemistry
Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com
 

Introduction

In practical terms, there are two different kinds of plutonium to be considered: reactor-grade and weapons-grade.  The first is recovered from typical used fuel from a nuclear reactor, which has been irradiated ("burned") for about three years.  It is a by-product of electricity generation.  The second is made specially for the military purpose, and is recovered from uranium fuel which has been irradiated for only 2-3 months in a production reactor.  The two kinds differ in their isotopic composition.

Plutonium, both that routinely made in power reactors and that from dismantled nuclear weapons, is a major energy source in the nuclear fuel cycle. Over one-third of the energy produced in most nuclear power plants comes from plutonium which is created there as a by-product and burned before the used fuel is discharged.

Like all other heavy elements, plutonium has a number of isotopes, differing in the number of neutrons in the nucleus.  All 15 plutonium isotopes are radioactive, because they are to some degree unstable and therefore decay, emitting particles and some gamma radiation as they do so.  Most decays emit relatively weak alpha radiation that can be blocked even by a sheet of paper (but can be hazardous if within the body - see below).

The main isotopes of plutonium are:

  • Pu-238, (half-life 88 years, alpha decay)
  • Pu-239, fissile (half-life 24,000 years, alpha decay)
  • Pu-240, (half-life 6,500 years, alpha decay)
  • Pu-241, fissile (half-life 14 years, beta decay)
  • Pu-242, (half-life 37,600 years, alpha decay)

The half-life is the time it takes for a radionuclide to lose half of its own radioactivity. The two fissile isotopes can be used as fuel in a nuclear reactor, others are capable of absorbing neutrons and becoming fissile (ie they are "fertile").  Alpha decays are generally accompanied by gamma radiation.

caption Buttons of plutonium metal. Photo: DOE

Pu-238, Pu-240, and Pu-242 emit neutrons as their nuclei spontaneously fission, albeit at a low rate. They also decay, emitting alpha particles and heat.  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 of power as it orbits around 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 a high melting point (2,390° C).

In pure form, plutonium exists in six allotropic forms or crystal structure - more than any other element. As temperature changes, it switches forms - each has significantly different mechanical and electrical properties. One is nearly twice the density of lead (19.8 g/cm3). It melts at 640°C into a very corrosive liquid. The alpha phase is hard and brittle, like cast iron, and if finely divided it spontaneously ignites in air to form PuO2. Beta, gamma and delta phases are all less dense. Alloyed with gallium, plutonium becomes more workable.

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 LWR nuclear reactor contains within its uranium fuel-load several hundred 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 a LWR. Of the rest, one-sixth becomes Pu-240 and Pu-241 through neutron capture, and the balance emerges as Pu-239 in the spent fuel.

An ordinary, large nuclear power reactor (1,000 MWe light water reactor (LWR)) gives rise to about 25 tonnes of used fuel per 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, with Pu-239 being the main fissile part and Pu-241 also contributing.

In order to extract it for recycle, the used fuel is reprocessed, and 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). Isotopes of uranium and plutonium which fission in a fast neutron environment are said to be "fissionable", as distinct from fissile. As with uranium, the energy potential of plutonium is more fully realised in a fast reactor.

Type Composition Origin Use
Reactor-grade from high-burnup fuel 55-70% Pu-239, >19% Pu-240, typically about 30% non-fissile Comprises about 1% of spent fuel from normal operation of civil nuclear reactors used for electricity generation As ingredient (c5%) of MOX fuel for normal reactor
Weapons-grade Pu-239 with <7% Pu-240 From military "production" reactors specifically designed and operated for production of low burn-up Pu. Nuclear weapons (can be recycled as fuel in fast neutron reactor or as ingredient of MOX)

One kilogram of Pu-239 being slowly consumed over three years in a conventional nuclear reactor can produce enough heat to generate nearly 10 million kilowatt-hours of electricity.

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 utilize recycled LWR plutonium more effectively than a typical LWR.

The approximately 1.15% of plutonium in the used fuel removed from a commercial LWR power reactor (burn-up of 42 GWd/t) consists of about 53% Pu-239, 25% Pu-240, 15% Pu-241, 5% Pu-242 and 2% of Pu-238 which is the main source of heat & radioactivity. Comparable isotopic ratios are found in the spent fuel of CANDU heavy-water reactors at much lower burnups (8 GWd/t), due to their use of natural uranium fuel and high thermal neutron spectrum. 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, 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 because it emits intense gamma radiation (in the course of its alpha decay to Np-237).

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 use this material and complete its consumption. Such reactors can also be configured to be net breeders of plutonium (as originally envisaged), which is important for the long-term sustainability of nuclear energy. Meanwhile, research on fast neutron reactors is focused on maximizing consumption of plutonium and incineration of actinides formed in the light water reactors.

Resources of plutonium

Total world generation of reactor-grade plutonium in spent fuel is some 60 tonnes per year. About 1,300 tonnes have been produced thus far, most of which remains in the spent fuel, with some 370 tonnes extracted. About one-third of the separated plutonium (130 t) has been used in MOX over the last 30 years. Currently 8-10 tonnes of plutonium is used in MOX each year.

Three US reactors are able to run fully on MOX, as can Canadian CANDU heavy water reactors. All Western and the later Soviet light water reactors can with little modification 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. Areva's new EPR is capable of running a full core load of MOX.

About 22 tonnes of reactor-grade plutonium is separated by reprocessing plants in OECD nations each year, and this is set to increase.  Eventually its usage in MOX is expected to outstrip this level of production so that stockpiles diminish.

At the end of 2007 the UK had 77 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.  It also held 27 tonnes on behalf of foreign utilities which had had their used fuel reprocessed at Sellafield.  Using all of UK's plutonium in MOX fuel rather than immobilizing it as waste is expected to yield a  GBP 700-1,200 million resource cost saving to the UK, along with 300 billion kWh of electricity (about one year's supply for the UK). The 106t Pu could be consumed in two 1,000 MWe light water reactors using 100% MOX fuel over 35 years.

At the end of 2006 France held nearly 50 tonnes of separated plutonium and Russia 41 tonnes. Worldwide stocks were estimated as just over 250 tonnes.

Plutonium and Weapons

It takes about 10 kilograms of nearly pure Pu-239 to make a bomb. Production of this amount of Pu-239 requires about 30 megawatt-years of reactor operation, with frequent fuel changes and reprocessing of the 'hot' fuel. Hence weapons-grade plutonium is made in special production reactors by burning natural uranium fuel to the extent of only about 100 MWd/t (effectively 3 months), instead of the 45,000 MWd/t typical of LWR power reactors (or even the 7000 - 10,000 MWd/t in CANDU or Magnox reactors used for power).

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 (i.e., if the fuel had only been used for a short time), but any significant proportions of Pu-240 in the fuel would make it extremely hazardous to the bomb makers, as well as unreliable and unpredictable. Typical plutonium recovered from reprocessing used power reactor fuel has about one-third non-fissile isotopes (mainly Pu-240). This is known as reactor-grade plutonium.

(In 1962, a nuclear device using low-burnup plutonium from a UK power reactor was detonated in the US. The isotopic composition of this plutonium has not been officially disclosed, but it was evidently about 85% Pu-239 – what would since 1971 have been called "fuel-grade" plutonium.)

Plutonium for weapons is made differently, in simple reactors (usually fuelled with natural unenriched uranium) operated for that purpose, with frequent fuel changes (i.e., low burn-up). This, coupled with the application of international safeguards, effectively rules out the use of commercial nuclear power plants for the production of weapons-grade plutonium.

International safeguards arrangements applied to traded uranium extend to the plutonium arising from it, ensuring constant audits even of reactor-grade material. This addresses uncertainty as to the explosive potential of reactor-grade plutonium - some authorities say that it could be used for an explosive device in the one kiloton range, though others disagree.  (All we know for sure is that it has never been made to explode.)

The International Atomic Energy Agency (IAEA) is conservative on this matter so that, for the purpose of applying IAEA safeguards measures, all plutonium (other than plutonium comprising 80% or more of the isotope Pu-238) is defined by the IAEA as a 'direct-use' material, that is, 'nuclear material that can be used for the manufacture of nuclear explosives components without transmutation or further enrichment'.  The IAEA is not saying that all plutonium is suitable for making weapons, simply that on the basis of calculations and under certain technically-demanding conditions it might be made to explode.  The 'direct use' definition applies also to plutonium which has been incorporated into commercial MOX fuel, which certainly could not be made to explode.

Disarmament will give rise to some 150-200 tonnes of weapons-grade plutonium, over half of it from the former USSR, and 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; and
  • Fueling 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 one is proceeding. The first trials of MOX made with weapons plutonium are under way in South Carolina.  Developments under the Global Nuclear Energy partnership (GNEP) make it very likely that the some military plutonium will be used in fast reactors in USA.  Meanwhile, the US has developed a "spent fuel standard", which means that plutonium, including weapons Pu, should never be more accessible than if it were incorporated in used fuel.

Europe has a well-developed MOX capacity, which suggests that weapons plutonium could be disposed of relatively quickly. Input-plutonium to a MOX plant would need to be about half reactor-grade and half weapons-grade. 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 3,000 warheads per year to produce 110 billion kWh of electricity.

Canada was 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.

The only use for "reactor grade" plutonium is as a nuclear fuel, after it is separated from the high-level wastes by reprocessing. It is not and has never been used for weapons, due to the relatively high rate of spontaneous fission and radiation from the heavier isotopes such as Pu-240 making any such attempted use fraught with great uncertainties.

Toxicity and Health Effects

The Health effects of plutonium are determined by the dose (how much), the duration (how long), the route or pathway by which a person might be exposed (breathing, eating, drinking, or skin contact), the other chemicals to which the person is exposed, and individual characteristics such as age, sex, nutritional status, family traits, life style, and state of health.

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 get into those very few human beings who may be exposed to it: • 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.

In the 1940s some 26 workers at US nuclear weapons facilities became contaminated with plutonium. Intensive health checks of these people have revealed no serious consequence and no fatalities that could be attributed to the exposure. In the 1990s plutonium was injected into and inhaled by some volunteers, without adverse effects.

Plutonium is one among many toxic materials that have to be handled with great care to minimise the associated but well understood risks. In the 1950s Queen Elizabeth was visiting Harwell and was handed a lump of plutonium (presumably Pu-239) in a plastic bag and invited to feel how warm it was.

Exposure to plutonium

Plutonium has been released to the environment primarily by atmospheric testing of nuclear weapons and by accidents at weapons production and utilization facilities. In addition, accidents involve weapons transport, satellite reentry, and nuclear reactors have also released much smaller amounts of plutonium into the atmosphere. When plutonium was released to the atmosphere by weapons tests up to the mid 1960s, it returned to the Earth's surface as fallout. Average fallout levels in soils in the United States are infinitessimal - about 2 millicuries (mCi)/square kilometer (about 0.4 square miles) for plutonium-230 and 0.05 mi/square kilometer for plutonium-238. A millicurie is a unit used to measure the amount of radioactivity; 1 mCi of plutonium-239 weighs 0.016 gm, while 1 mCi of plutonium-238 weighs 0.00006 gm.

Measurements in air have been made at a few locations. For example, air levels of plutonium-239 in New York in the 1970s were reported to be 0.00003 picocuries (pCi) per cubic meter of air. One pCi is one billionth of a mCi. Persons who work at nuclear plants using plutonium have a greater chance of being exposed than individuals in the general population. However, a peson could be exposed to plutonium if there was an accidental release of plutonium during use, transport, or disposal. Because plutonium does not release very much gamma radiation, harmful health effects are not likely to occur from being near plutonium unless it is breathed in or swallowed.

Health effects

Plutonium may remain in the lungs or move to the bones, liver, or other body organs. Any that is actually absorbed rather than expelled generally stays in the body for decades and continues to expose the surrounding tissues to radiation. This may eventually increase a person's chance of developing cancer, but it would be several years before such cancer effects became apparent. The experimental evidence is inconclusive, and studies of some human populations who have been exposed to low levels of plutonium have not definitely shown an increase in cancer. However, plutonium has been shown to cause both cancers and other damage in laboratory animals, and might affect the ability to resist disease (immune system).

There is no information from studies in humans or animals to identify the specific levels of exposures to plutonium in air, food, or water that have resulted in harmful effects. However, it is generally assumed that any amount of absorbed radiation, no matter how small, may cause some damage. When expressed as the amount of radioactivity deposited in the body per kilogram of body weight (kg bw) as a result of breathing in plutonium, studies in dogs report that 100,000 pCi plutonium/kg bw caused serious lung damage within a few months, 1,700 pCi/kg bw caused harm to the immune system, and 1,400 pCi/kg bw caused bone cancer after 4 years. In each of these cases the dogs were exposed to the plutonium in air for one day.

Further Reading



Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Agency for Toxic Substances and Disease Registry. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Agency for Toxic Substances and Disease Registry should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

Glossary

Citation

Hore-Lacy, I., & Association, W. (2009). Plutonium. Retrieved from http://www.eoearth.org/view/article/155272

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