|Boiling Point: 3508.15oK, 3235.0oC, 5855.0oF
Melting Point: 912.65oK, 639.5oC, 1183.1oF
Electrons Energy Level: 2, 8, 18, 32, 24, 8, 2
Isotopes: 20 + None Stable
Heat of Vaporization: 343.5 kJ/mol
Heat of Fusion: 2.84 kJ/mol
Density: 19.84 g/cm3 @ 300°K
Specific Heat: 0.13 J/gK
Atomic Radius: 131pm
Ionic Radius: 0.887Å
Electronegativity: 1.28 (Pauling), 1.22 (Allrod Rochow)
1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f6 6s2p6 7s2
Glenn T. Seabory at the Geiger Counter, 301 Gilman Hall, Berkeley, California, where he discovered plutonium.
The production of plutonium and neptunium by bombarding uranium-238 with neutrons was predicted in 1940 by two teams working independently: Edwin M. McMillan and Philip Abelson at Berkeley Radiaton Laboratory at the University of Berkeley, California and by Egon Bretscher and Norman Feather at the Cavendish Laboratory at the University of Cambridge. Coincidentally both teams proposed the same names to follow on from uranium, like the sequence of the outer planets.
Plutonium was first produced and isolated on February 23, 1941 by Dr. Glenn T. Seaborg, Dr. Michael Cefola, Edwin M. McMillan, J.W. Kennedy, and A.C. Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley. The discovery was kept secret due to the war. It was named after Pluto, having been discovered directly after neptunium (which itself was one higher on the periodic table than uranium), by analogy to solar system planet order as Pluto was considered to be a planet at the time (though technically it should have been "plutium", Seaborg said that he did not think it sounded as good as "plutonium"). Seaborg chose the letters "Pu" as a joke, which passed without notice into the periodic table. Originally, Seaborg and others thought about naming the element "ultinium" or "extremium" because they believed at the time that they had found the last possible element on the periodic table.
Edwin M. McMillan
Chemists at the University of Chicago began to study the newly manufactured radioactive element. The George Herbert Jones Laboratory at the university was the site where, for the first time, a trace quantity of this new element was isolated and measured in September 1942. This procedure enabled chemists to determine the new element's atomic weight. Room 405 of the building was named a National Historic Landmark in May 1967. During the Manhattan Project, the first production reactor was built at the Oak Ridge, Tennessee site that later became Oak Ridge National Laboratory. Later, large reactors were set up in Hanford, Washington, for the production of plutonium, which was used in the first atomic bomb used at the "Trinity" test at White Sands, New Mexico in July 1945. Plutonium was also used in the "Fat Man" bomb dropped on Nagasaki, Japan in August 1945. The "Little Boy" bomb dropped on Hiroshima utilized uranium-235, not plutonium.
Large stockpiles of plutonium were built up by both the Soviet Union and the United States during the Cold Warit was estimated that 300,000 kg of plutonium had been accumulated by 1982. Since the end of the Cold War, these stockpiles have become a focus of nuclear proliferation concerns. In 2002, the United States Department of Energy took possession of 34 metric tons of excess weapons-grade plutonium stockpiles from the United /States Department of Defense, and as of early 2003 was considering converting several nuclear power plants in the U.S. from enriched uranium fuel to MOX fuel as a means of disposing of plutonium stocks.
Hanford Site plutonium production reactors along the Columbia River during the Manhattan Project.
During the initial years after the discovery of plutonium, when its biological and physical properties were very poorly understood, a series of human radiation experiments were performed by the U.S. government and by private organizations acting on its behalf. During and after the end of World War II, scientists working on the Manhattan Project and other nuclear weapons research projects conducted studies of the effects of plutonium on laboratory animals and human subjects. In the case of human subjects, this involved injecting solutions containing (typically) five micrograms of plutonium into hospital patients thought to be either terminally ill, or to have a life expectancy of less than ten years either due to age or chronic disease condition. These eighteen injections were made without the informed consent of those patients and were not done with the belief that the injections would heal their conditions; rather, they were used to develop diagnostic tools for determining the uptake of plutonium in the body for use in developing safety standards for people working with plutonium during the course of developing nuclear weapons.
The episode is now considered to be a serious breach of medical ethics and of the Hippocratic Oath, and has been sharply criticised as failing "both the test of our national values and the test of humanity." More sympathetic commentators have noted that while it was definitely a breach in trust and ethics, "the effects of the plutonium injections were not as damaging to the subjects as the early news stories painted, nor were they so inconsequential as many scientists, then and now, believe.
Plutonium has been called "the most complex metal" and "a physicist's dream but an engineer's nightmare" for its peculiar physical and chemical properties. It has six allotopes normally and a seventh under pressure, each of which have very similar energy levels but with significantly varying densities, making it very sensitive to changes in temperature, pressure, or chemistry, and allowing for dramatic volume changes following phase transitions (in nuclear applications, it is usually alloyed with small amounts of gallium, which stabilizes it in the delta-phase.) Plutonium is silvery in pure form, but has a yellow tarnish when oxidized. It is also notable in that it possesses a low-symmetry structure causing it to become progressively more brittle over time. Because it self-irradiates, it ages both from the outside-in and the inside-out. However, self-irradiation can also lead to annealing which counteracts some of the aging effects. In general, the precise aging properties of plutonium are very complex and poorly understood, greatly complicating efforts to predict future reliability of weapons components.
The heat given off by alpha particle emission makes plutonium warm to the touch in reasonable quantities; larger amounts can boil water. It displays five ionic oxidation states in aqueous solution:
Note: The color shown by Pu solutions depends on both the oxidation state and the nature of the acid anion, which influences the degree of complexing of the Pu species by the acid anion.
Plutonium forms binary compounds with oxygen: PuO, PuO2, and intermediate oxides of variable composition; with the halides: PuF3, PuF4, PuCl3, PuBr3, PuI3; with carbon, nitrogen, and silicon: PuC, PuN, PuSi2. Oxyhalides are also well known: PuOCl, PuOBr, PuOI. The metal will take on a yellow tarnish when slightly oxidized. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid with formation of the Pu+3 ion. The six allotropic modifications have various crystalline structures that range from 16.00 to 19.86 g/cm3.
A piece of plutonium about the size of a softball would feel hot to the touch because of the high level of alpha particle radiation given off. A somewhat larger piece of the metal would boil water within minutes. Plutonium (238Pu) is occasionally used in deep-space probes as a source of energy (too far from the sun for effective solar power), the heat being directly converted into electricity by a special device.
While almost all plutonium is manufactured synthetically, extremely tiny trace amounts are found naturally in uranium ores. These come about by a process of neutron capture by 238U nuclei, initially forming 239U; two subsequent beta decays then form 239Pu (with a 239Np intermediary), which has a half-life of 24,110 years. This is also the process used to manufacture 239Pu in nuclear reactors. Some traces of 244Pu remain from the birth of the solar system from waste of supernovae, because its half-life (80 million yrs) is fairly long.
A relatively high concentration of plutonium was discovered at the natural nuclear fission reactor in Oklo, Gabon in 1972. Since 1945, approximately 7700 kg has been released onto Earth through nuclear explosions.
Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.
A ring of weapons-grade electrorefined plutonium, with 99.96% purity. This 5.3 kg ring is enough plutonium for use in a modern nuclear weapon.
There are small amounts of Pu-238 in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U-235 atom captures a neutron, it is converted to an excited state of U-236. Some of the excited U-236 nuclei undergo fission, but some decay to the ground state of U-236 by emitting gamma radiation. Further neutron capture creates U-237 which has a half-life of 7 days and thus quickly decays to Np-237. Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np-237 by chemical separation of neptunium. After this chemical separation, Np-237 is again irradiated by reactor neutrons to be converted to Np-238 which decays to Pu-238 with a half-life of 2 days.
The isotope 239Pu is a key fissile component in nuclear weapons, due to its ease of fissioning and availability. The critical mass for an unreflected sphere of plutonium is 16 kg, but through the use of a neutron-reflecting tamper the pit of plutonium in a fission bomb is reduced to 10 kg, which is a sphere with a diameter of 10 cm. The Manhattan Project "Fat Man" type plutonium bombs, using explosive compression of Pu to significantly higher densities than normal, were able to function with plutonium cores of only 6.2 kg. Complete detonation of plutonium will produce an explosion equivalent to the explosion of 20 kilotons of trinitrotoluene (TNT) per kilogram. However, complete detonation requires an additional neutron source (often from a small amount of fusion fuel), and primitive bombs may be far less efficient. For example, despite the 6.2 kg of plutonium, the Fat Man yield was only 21 kt.
Plutonium could also be used to manufacture radiological weaons or as a (not particularly deadly) chemical poison. In a number of instances damaged nuclear weapons have spread plutonium over a surrounding area, similar to the effect of a so-called "dirty bomb", and required extensive cleanup. On the other hand, 5 kg of plutonium was spread over the Nagasaki area (due to incomplete fission) and never cleaned up. Many of the more extreme claims about plutonium toxicity are inconsistent with the past and current habitability of the area and the health of the current residents.
238Pu, 240Pu, and 242Pu emit neutrons as a few of their nuclei spontaneously fission, albeit at a low rate. They also decay, and the decay heat of Pu-238 (0.56 W/g) make it well suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human lifetime. It is therefore used in radioisotope thermoelectric generators (RTGs) such as those powering the Cassini and Horizons (Pluto) space space probes. Plutonium-238 was used on the Apollo-14 lunar flight in 1971 to power seismic devices and other equipment left on the Moon, and it was also the power supply of the two Voyager supercraft launched in 1977. The Cassini spacecraft carries three generators providing 870 watts power as it orbits aroound Saturn. Plutonium has powered 24 US space vehicles and enabled operation for 20 years and may continue for another 20.
238Pu has been used successfully to power artificial heart pacemakers, to reduce the risk of repeated surgery. It has been largely replaced by Lithium based primary cells, but as of 2003 there were somewhere between 50 and 100 plutonium-powered pacemakers still implanted and functioning in living patients.
One kilogram is equivalent to about 22 million kilowatt hours of heat energy. The complete detonation of a kilogram of plutonium produces an explosion equal to about 20,000 tons of chemical explosive. Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world's nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals.
Plutonium is a key fissile component in modern nuclear weapons; care must be taken to avoid accumulation of amounts of plutonium which approach critical mass, the amount of plutonium which will self-generate a nuclear reaction. Despite not being confined by external pressure as is required for a nuclear weapon, it will nevertheless heat itself and break whatever confining environment it is in. Shape is relevant; compact shapes such as spheres are to be avoided.
Plutonium has assumed the position of dominant importance among the transuranium elements because of its successful use as an explosive ingredient in nuclear weapons and the place which it holds as a key material in the development of industrial use of nuclear 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 (2390° 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 reacts readily with oxygen, forming PuO andPuO2, as well as intermediate oxides. It reacts with the halides, giving rise to compounds such as PuX3 where X can be F, Cl, Br or I; PuF4 and PuF6 are also seen. The following oxyhalides are observed: PuOCl, PuOBr and PuOI. It will react with carbon to form PuC, nitrogen to form PuN and silicon to form PuSi2.
Plutonium like other actinides readily forms a dioxide plutonyl core (PuO2). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which can be readily mobile with low affinities to soil.
PuO2 formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO2 which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium.
Image showing colors of various oxidation states of Pu in solution on the left and colors of only one Pu oxidation state (IV) on the right in solutions containing different anions.
A diagram of the allotropes of plutonium at ambient pressure
Even at ambient pressure, plutonium occurs in a variety of allotropes. These allotropes differ widely in crystal structure and density; the a and d allotropes differ in density by more than 25% at constant pressure.
The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. The reasons for the complicated phase diagram are not entirely understood; recent research has focused on constructing accurate computer models of the phase transitions.
In weapons applications, plutonium is often alloyed with another metal (e.g., delta phase with a small percentage of gallium) to increase phase stability and thereby enhance workability and ease of handling. Interestingly, in fission weapons, the explosive shock waves used to compress a plutonium core will also cause a transition from the usual delta phase plutonium to the denser alpha phase, significantly helping to achieve supercriticality.
All 20 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:
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, others are capable of absorbing neutrons and becoming fissile.
The most important isotope of plutonium is 239Pu, with a half-life of 24,200 years. 239Pu is an alpha emitter (5 meV); 238Pu is also an alpha emitter.
It is produced in extensive quantities in nuclear reactors from natural uranium:
238U(n, gamma) 239U --(beta) --> 239Np--(beta) --> 239Pu.
The longest-lived isotope of plutonium is Pu-244 with a half-life of 82 million years. However the isotope of chief interest is Pu-239 which, like U-235, is fissionable. Most of the nuclear weapons built by the "great powers" today are based on Pu-239 which is derived from U-238 in special "breeder" reactors. Pu-239 is also a by-product of normal fission power reactors and accounts for a good deal of the concern over nuclear waste by-products since it is both highly radioactive and exceptionally toxic. Also, since the critical mass of plutonium is only about one third that of U-235, the possibility for terrorist diversion of the material is considered a serious matter.
Twenty plutonium radioisotopes have been characterized. The most stable are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states, though none are very stable (all have half-lives less than one second).
The isotopes of plutonium range in atomic weight from 228.0387 u (Pu-228) to 247.074 u (Pu-247). The primary decay modes before the most stable isotope, Pu-244, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before Pu-244 are uranium and neptunium isotopes (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are americium isotopes.
A pellet of plutonium-238, glowing under its own light, used for radioisotope thermoelectric generators.
Key isotopes for applications are Pu-239, which is suitable for use in nuclear weapons and nuclear reactors, and Pu-238, which is suitable for use in radioisotope thermoelectric generators. The isotope Pu-240 undergoes spontaneous fission very readily, and is produced when Pu-239 is exposed to neutrons. The presence of Pu-240 in a material limits its nuclear bomb potential since it emits neutrons randomly, increasing the difficulty of initiating accurately the chain reaction at the desired instant and thus reducing the bomb's reliability and power. Plutonium consisting of more than about 90% Pu-239 is called weapon-grade plutonium; plutonium obtained from commercial reactors generally contains at least 20% Pu-240 and is called reactor-grade plutonium.
Pu-240, while of little importance by itself, plays a crucial role as a contaminant in plutonium used in nuclear weapons. It spontaneously fissions at a high rate, and as a 1% impurity in Pu-239 will lead to unacceptably early initiation of a fission chain reaction in gun-type atomic weapons, blowing the weapon apart before much of its material can fission. Pu-240 contamination is the reason plutonium weapons must use an implosion design. A theoretical 100% pure Pu-239 weapon could be constructed as a gun type device, but achieving this level of purity is prohibitively difficult. Pu-240 contamination has proven a mixed blessing to weapons designers. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those very same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone toward accidental detonation than are gun-type weapons.
|239Pu||239.0521634||24.11 x 103 years|
|242Pu||242.0587426||3.75 x 105 years|
|244Pu||244.064204||8.00 x 107 years|
All isotopes and compounds of plutonium are toxic and radioactive. While plutonium is sometimes described in media reports as "the most toxic substance known to man", from the standpoint of literal toxicity this is incorrect. As of 2006, there has yet to be a single human death officially attributed to exposure to plutonium itself (with the exception of plutonium-related criticality accidents). Naturally-occurring radium is about 200 times more radiotoxic than plutonium, and some organic toxins like botulin toxin are still more toxic. Botulin toxin, in particular, has a lethal dose of 300 pg/kg, far less than the quantity of plutonium that poses a significant cancer risk. In addition, beta and gamma emitters (including the carbon-14 and potassium -40 in nearly all food) can cause cancer on casual contact, which alpha emitters cannot.
When taken in by mouth, plutonium is less poisonous (except for risk of causing cancer) than several common substances including caffeine, acetaminophen, some vitamins, pseudoephedrine, and any number of plants and fungi. It is perhaps somewhat more poisonous than pure ethanol, but less so than tobacco; and many illegal drugs. From a purely chemical standpoint, it is about as poisonous as lead and other heavy metals. Not surprisingly, it has a metallic taste.
Glowing hot bits of plutonium in a box, which have been set alight due to plutonium's pyrophoric nature.
That said, there is no doubt that plutonium may be extremely dangerous when handled incorrectly. The alpha radiation it emits does not penetrate the skin, but can irradiate internal organs when plutonium is inhaled or ingested. Particularly at risk are the skeleton, where it is likely to be absorbed by the bone surface, and the liver, where it will likely collect and become concentrated. Approximately 0.008 microcuries absorbed in bone marrow is the maximum withstandable dose. Anything more is considered toxic. Extremely fine particles of plutonium (on the order of micrograms) can cause lung cancer if inhaled.
Other substances including ricin, tetrodotoxin, botulinum toxin, and tetanus toxin are fatal in doses of (sometimes far) under one milligram, and others (the nerve agents, the amanita toxin) are in the range of a few milligrams. As such, plutonium is not unusual in terms of toxicity, even by inhalation. In addition, those substances are fatal in hours to days, whereas plutonium (and other cancer-causing radioactives) give an increased chance of illness decades in the future. Considerably larger amounts may cause acute radiation poisoning and death if ingested or inhaled; however, so far, no human is known to have immediately died because of inhaling or ingesting plutonium and many people have measurable amounts of plutonium in their bodies.
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 radiotoxicity (smoke detectors).
There are three principal routes by which plutonium can reach human beings:
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.
|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)|
Contamination of food, water, hands, or careless handling are the main causes of radionuclide ingestion. In the USA, about 17,000 persons are estimated to have worked with plutonium since 1943-1944. In France, the number in 1986 was about 1000. Among the major effluents from the use and processing of nuclear fuel are plutonium. Of these, only tritium and plutonium can possibly enter water supplies. The predominant form of plutonium release from nuclear power and processing plants is as an aerosol that will have little or no impact on drinking water. Although a single incident has occurred in which as much as 18,750 Ci of plutonium were released from liquid storage on a local basis, none apparently reached off site water supplies. The usual rate of release from liquid storage at a controlled sites is about 1 mCi/yr.
Trace amounts of plutonium are found naturally in uranium-rich ores. Humans produce most of the existing plutonium, in special nuclear reactors. Besides being naturally present in very small amounts, plutonium may also enter the environment from releases of nuclear reactors, weapons production plants, and research facilities. A major source of plutonium release is nuclear weapons testing. Annual world production of plutonium is probably in excess of 50 tons and there may be more than 1.000 tons of metal in storage, either as bombs or as metal rods.
Plutonium is sometimes described in media reports as the most toxic substance known to
man, although there is general agreement among experts in the field that this is
incorrect. As of 2003, there has yet to be a single human death officially attributed to
plutonium exposure. Naturally-occurring radium is about 200 times more radiotoxic than
plutonium, and some organic toxins like Botulism toxin are billions of times more toxic
The alpha radiation it emits does not penetrate the skin, but can irradiate internal organs when plutonium is inhaled or ingested. Extremely small particles of plutonium on the order of micrograms can cause lung cancer if inhaled into the lungs. Considerably larger amounts may cause acute radiation poisoning and death if ingested or inhaled; however, so far, no human is known to have died because of inhaling or ingesting plutonium and many people have measurable amounts of plutonium in their bodies. Plutonium is a dangerous substance that has been used in explosives for a long time. It is released into the atmosphere primarily by atmospheric testing of nuclear weapons and by accidents at weapon production sites. When plutonium is released into the atmosphere it will fall back onto earth eventually and end up in soils.
Exposure of humans to plutonium is not likely, but sometimes it takes
place as a result of accidental releases during use, transport or disposal.
Because plutonium has no gamma radiation, health effects are not likely to occur while working with plutonium, unless it is breathed in or swallowed somehow.
When people breathe it in, plutonium may remain in the lungs or move to the bones or organs. Generally it stays in the body for a long time and continually exposes body tissues to radiation. After a few years this could result in the development of cancer.
Furthermore, plutonium may affect the ability to resist disease and the radioactivity from plutonium may cause reproductive failure.
Plutonium may enter surface water from accidental releases and disposal of radioactive wastes. Soil can become contaminated with plutonium through fallout during nuclear weapons testing. Plutonium moves slowly downwards in the soil, into the groundwater.
Plants absorb low levels of plutonium, but these levels are not high enough to cause bio magnification of plutonium up the food chain, or accumulation in the bodies of animals.
Plutonium, both that routinely made in power reactors and that from dismantled nuclear weapons, is a valuable energy source in the nuclear fuel cycle. Over one third of the energy produced in most nuclear power plants comes from plutonium. It is created there as a by-product.
Because of the high rate of emission of alpha particles and the element being specifically absorbed by bone marrow and collected in the liver, plutonium, as well as all of the other transuranium elements except neptunium, are radiological poisons and must be handled with very special equipment and precautions. Plutonium is a very dangerous radiological hazard. Precautions must also be taken to prevent the unintentional formation of a critical mass . Plutonium in liquid solution is more likely to become critical than solid plutonium. The shape of the mass must also be considered where criticality is concerned. Plutonium-238 is available to authorized users from the O.R.N.L. at a cost of about $7.50/mg (97%) plus packing costs of $1250 per package.
On the basis of the measured and inferred plutonium concentration in the air of New York and a constant inhalation rate of 20 cu m/day, inhalation intake reflects the amt of radioactivity released by nuclear weapons tests. In 1960 the amt diminished & rose again in 1963 to a max of 450 mbecquerel following 1961-1962 nuclear weapons tests declined regularly after the Test Ban Treaty of 1963 to a value of about 7 mbecquerel/yr during the period from 1972-1974. At the end of 1973, it was estimated that 4.2 tons of 239plutonium & 240plutonium was dispersed in the atmosphere. This value should be compared to the est release of plutonium into the environment by the accident of the Chernobyl reactor. This release can be estimated to be in the maximum of 1 to 2% of the plutonium inside the reactor core, ie, 2.5-5 kg of 239plutonium & 240plutonium.
In contrast to naturally occurring radioisotopes such as radium or C-14, plutonium was manufactured, concentrated, and isolated in large amounts (hundreds of metric tons) during the Cold War for weapons production. These stockpiles, whether or not in weapons form, pose a significant problem because, unlike chemical or biological agents, no chemical process can destroy them. One proposal to dispose of surplus weapons-grade plutonium is to mix it with highly radioactive isotopes (e.g., spent reactor fuel) to deter handling by potential thieves or terrorists. Another is to mix it with uranium and use it to fuel nuclear power reactors (the Mixed Oxide or MOX approach). This would not only fission (and thereby destroy) much of the Pu-239, but also transmute a significant fraction of the remainder into Pu-240 and heavier isotopes that would make the resulting mixture useless for nuclear weapon.
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 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 for a LWR. 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 tons 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.
Plutonium-239 can also be used as a fuel in a new generation of fast-breeder nuclear weapons, which burn a mixed oxide (MOX) fuel consisting of uranium and plutonium. 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.
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 LWR.)
The 1.15% of plutonium in the spent fuel removed from a commercial LWR (Light Water Reactor) 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 & 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.
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.
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 which is important for the long-term sustainability of nuclear energy. 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 in spent fuel is some 60 tonnes per year. About 1300 tons have been produced so far, and most of this remains in the used fuel, with some 370 tons extracted. About one third of the separated Pu (130 t) has been used in MOX over the last 30 years. Currently 8-10 tons of Pu is used in MOX each year.
Three US reactors are able to run fully on MOX, as can Canadian heavy water (CANDU) 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 tons 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 tons of separated plutonium and this stockpile is expected to grow to 106 tons 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.
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. Weapons-grade Pu is made 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 (ie. if the fuel had only been used for a short time), but any significant proportions of Pu-240 in it 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).
* In 1962 a nuclear device using low-burnup plutonium from a UK power reactor was detonated in USA. The isotopic composition of this plutonium has not been officially disclosed, but it was evidently about 90% Pu-239.
Plutonium for weapons is made differently, in simple reactors (usually fueled 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 even of reactor-grade material.
Disarmament will give rise to some 150-200 tons 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:
The US Government has declared 38 tons of weapons-grade plutonium to be surplus, and planned to pursue the first two options above, though only the MOX one 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 used 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 tons of warhead plutonium per year. This would amount to burning 3000 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.
Recycling: The Use of "MOX" Fuel
With the transport and use of MOX (mixed oxide) fuels attracting increaed public attention, readers may find the following background information useful.
Plutonium is formed in uranium fuel during the operation of a reactor. Plutonium has substantial potential as a source of energy, and in fact is a significant contributor to the energy produced in a uranium-fueled reactor.
The use of MOX fuel reduces inventories of separated plutonium, and is likely to assume increasing importance for degrading weapons-grade plutonium released by disarmament.
The concept of plutonium recycling involves reprocessing of spent fuel from a reactor, in order to separate the plutonium produced in the fuel, fabricate it into fresh fuel, and use it for further energy production. When uranium is used to fuel a reactor, energy is produced primarily from the fissile isotope U-235, which constitutes only around 0.7% of natural uranium. Plutonium recycling offers substantially greater efficiency, because energy is produced from the most abundant uranium isotope, U-238 (which constitutes around 99.3% of natural uranium), through conversion of U-238 to plutonium. In theory therefore plutonium recycling offers some 150 times as much energy from a given quantity of uranium as the 'once-through' cycle (i.e. use of uranium without reprocessing). Practical factors prevent this theoretical maximum from being reached, but a very substantial increase appears to be practicably attainable. Plutonium recycling would therefore be extremely attractive if uranium were in short supply and high-priced.
Programs for the recycling of plutonium were developed in the 1970s when it appeared that uranium would be in scarce supply and would become increasingly expensive. It was proposed that plutonium would be recycled through fast breeder reactors, that is, fast neutron reactors with a uranium 'blanket', which would produce slightly more plutonium than they consume. Thus it was envisaged that the world's 'low cost' uranium resources, then estimated to be sufficient for about 50 years' consumption, could be extended for hundreds of years.
For a variety of reasons, high uranium prices have not eventuated, and future prices are uncertain. Some of the influences on this situation include:
At the moment the consumption of uranium in the world's nuclear energy programs substantially exceeds uranium production (by about 50%), and low cost uranium resources are still equivalent to only about 40 to 50 years' consumption at present levels. These factors might be expected to result in higher uranium prices, but prices remain depressed. In these circumstances there is no impetus to develop fast breeder reactors, particularly since these reactors present major engineering challenges which will be expensive to resolve. Meanwhile, however, around 30% of spent fuel arisings are covered by long-term reprocessing contracts, and the approach of plutonium recycling using light water reactors has been developed as a way of avoiding the accumulation of separated plutonium, and deriving an immediate economic return on this plutonium.
The term 'MOX' is derived from 'mixed oxides', and refers to reactor fuel made from a mixture of plutonium and uranium oxide. For use in a light water reactor, the proportion of plutonium is about 5%. This is a similar fissile content as low enriched uranium fuel. As is the case with uranium fuel, the MOX is formed into ceramic fuel pellets, which are extremely stable and durable, and which are sealed in metal (usually zirconium) tubes, which in turn are assembled into fuel elements. In most cases about a third of the reactor core can be loaded with MOX fuel elements without engineering or operational modifications to the reactor.
Contrary to suggestions from some commentators, there is nothing unusual in the presence of plutonium in light water reactors. Plutonium is produced during the operation of a reactor. The plutonium content of spent fuel from the normal operation of a light water reactor will be a little less than 1%, usually around 0.8%, when the fuel is unloaded. During the operation of the reactor, plutonium formed in the fuel will contribute an increasing proportion of the overall energy production of the reactor - towards the end of an operating cycle, a substantial proportion of the initial U-235 content of the fuel will have been consumed, and the energy produced by fission of plutonium will be very close to that produced by the remaining uranium.
Use of MOX fuel is expected to significantly reduce plutonium inventories. As an example, the Euratom Supply Agency estimates that the use of a single MOX fuel element consumes 9 kg of plutonium, and avoids the production of a further 5 kg (compared with the use of low enriched uranium fuel). Thus in this example each MOX fuel element used results in a net reduction of 14 kg of plutonium.
Currently plutonium is being recycled with 32 light water reactors in Europe, and this is shortly to commence in Japan. Use of MOX fuels in light water reactors will increase over the next decade. While this will involve mainly reprocessed civil plutonium, the use of MOX fuel to degrade weapons-grade plutonium*, transferred from military programs as part of the disarmament process, will assume increasing importance. By 2010 it is expected that MOX fuels will be used with 45 reactors in Europe, together with 16-18 in Japan, and possibly five in Russia and six in the US, that is, some 15-20% of the world's power reactors.
* There are two ways in which use of weapons-grade plutonium in MOX fuel degrades that plutonium: through the plutonium being associated with highly radioactive fission products in spent fuel (the 'spent fuel standard'); and through changes in isotopic composition during the irradiation process - in normal power reactor useage the plutonium would become reactor-grade.
As noted earlier, plutonium recycling programs were first developed with the breeder cycle in mind. There have been active fast breeder reactor research and demonstration programs in France, Japan and Russia. Future plans for fast breeder reactors are now uncertain, a major factor being economics, especially the price of uranium. At the moment the greatest interest appears to be in operating such reactors, not as breeders, but as net consumers or 'burners' of plutonium and of minor actinides. Clearly of crucial importance here is the future direction of nuclear energy, which will be determined by a complex range of political and economic considerations. If nuclear energy continues to make a significant contribution to world electricity production, and particularly if this contribution increases, plutonium could become an energy source as significant as uranium is today.
Are MOX Fuel Weapons-Useable?
Opponents of the use of MOX fuels commonly state that such fuels represent a proliferation risk because the plutonium in the fuel is said to be weapons-useable. This is a complex subject, where there is no consensus amongst experts, but the short answer is that there would be serious technical difficulties in attempting to make nuclear weapons from plutonium of the quality currently used for MOX (reactor-grade), and none of the countries possessing nuclear weapons has ever made weapons using plutonium of this quality.
Weapons-useable is not a technical term, and it is not clear what those using it mean, but if it is supposed to imply that reactor-grade plutonium is a material that could readily find its way into weapons, this overlooks two important facts: that there has been no practical demonstration of the use of such plutonium in nuclear weapons, and that rigorous IAEA safeguards apply to this material in non-nuclear-weapon States party to the NPT. It is misleading to conclude, because this material is subject to safeguards, that it is therefore weapons-useable.
To better understand this issue, it is necessary to appreciate that plutonium exists as several isotopes. As noted earlier, longer reactor irradiation times result in the formation of higher plutonium isotopes, Pu-240, Pu-241 and Pu-242 (and also the isotope Pu-238). The mix of isotopes (isotopic composition) of a particular quantity of plutonium will depend on how the plutonium was produced, that is, its irradiation history. The isotopic composition of plutonium affects its suitability for particular purposes, such as use in a reactor or use in nuclear weapons.
The plutonium isotope most suitable for weapons use is Pu-239. Plutonium used in nuclear weapons, weapons-grade plutonium, comprises at least 92%, usually more, Pu-239. This plutonium is produced in dedicated plutonium production reactors, specially designed and operated to produce plutonium of this quality by removal and reprocessing of fuel after short irradiation times.
The plutonium produced in the normal operation of light water reactors, from which MOX fuel is being made, is what is known as reactor grade plutonium. Because of the very long time fuel is irradiated in a power reactor (typically 3-4 years), reactor-grade plutonium has a substantial proportion of higher plutonium isotopes. Reactor-grade plutonium typically comprises less than 60% of the isotope Pu-239.
Reactor-grade plutonium contains a large proportion of isotopes which create serious technical difficulties for weapons use, namely Pu-238, Pu-240 and Pu-242. These difficulties include pre-initiation (a high spontaneous fission rate leading to the nuclear chain reaction starting too early), and radiation and heat levels which will adversely affect vital weapons components such as high explosives and electronics. While these difficulties could possibly be overcome, to some extent at least, by experienced weapons designers (e.g. from the nuclear-weapon States, with experience from hundreds of tests to draw upon), ASNO is not aware of any successful test explosion using reactor-grade plutonium, typical of light water reactor fuel*.
* There is some confusion over a 1962 test by the US using what was then described as reactor-grade plutonium, but at that time reactor-grade was much closer to weapons-grade than is currently the case. While the US has never revealed the quality of the plutonium used in that test, there are indications that it was of fuel-grade, an intermediate category between weapons-grade and reactor-grade, which has been recognised as a separate category since the 1970s.
IAEA Definition of Direct-Use Material
The confusion in the public mind regarding the suitability of reactor-grade plutonium for nuclear weapons appears to arise from the fact 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. In order to understand what this actually means, it is important to appreciate the following:
With respect to the use of MOX fuel, arguments about the weapons-useability of reactor-grade plutonium miss the point: as we have seen, MOX is a mixture of uranium and plutonium oxides, with the plutonium being very much in the minority. For light water reactor fuel, the plutonium content is typically around 5%. MOX cannot be used in nuclear weapons or nuclear explosives. To separate the plutonium content from MOX fuel elements would be a major undertaking, similar to reprocessing. IAEA safeguards measures would readily indicate if any attempt were made to process the fuel to separate plutonium.
Toxicity issues aside, care must be taken to avoid the accumulation of amounts of plutonium which approach critical mass, particularly because plutonium's critical mass is only a third of that of uranium-235's. Despite not being confined by external pressure as is required for a nuclear weapon, it will nevertheless heat itself and break whatever confining environment it is in. Shape is relevant; compact shapes such as spheres are to be avoided. Plutonium in solution is more likely to form a critical mass than the solid form (due to moderation by the hydrogen in water). A weapon-scale nuclear explosion cannot occur accidentally, since it requires a greatly supercritical mass in order to explode rather than simply melt or fragment. However, a marginally critical mass will cause a lethal dose of radiation and has in fact done so in the past on several occasions.
Criticality accidents have occurred in the past, some of them with lethal consequences. Careless handling of tungsten carbide bricks around a 6.2 kg plutonium sphere resulted in a lethal dose of radiation at Los Alamos on August 21, 1945, when scientist Harry K. Daghlian, Jr. received a dose estimated to be 510 rems (5.1 Sv) and died four weeks later. Nine months later, another Los Alamos scientist, Louis Slotin, died from a similar accident involving a beryllium reflector and the exact same plutonium core (the so-called "demon core") that had previously claimed the life of Daghlian. These incidents were fictionalized in the 1989 film Fat Man and Little Boy. In 1958, during a process of purifying plutonium at Los Alamos, a critical mass was formed in a mixing vessel, which resulted in the death of a crane operator. Other accidents of this sort have occurred in the Soviet Union, Japan, and many other countries. The 1986 Chernobyl accident caused a major release of plutonium.
Metallic plutonium is also a fire hazard, especially if the material is finely divided. It reacts chemically with oxygen and water which may result in an accumulation of plutonium hydride, a phrophoric substance; that is, a material that will ignite in air at room temperature. Plutonium expands considerably in size as it oxidizes and thus may break its container. The radioactivity of the burning material is an additional hazard. Magnesium oxide sand is the most effective material for extinguishing a plutonium fire. It cools the burning material, acting as a heat sink, and also blocks off oxygen. Water is also effective. There was a major plutonium-initiated fire at the Rocky Flats Plant near Boulder, Colorado in 1969. To avoid these problems, special precautions are necessary to store or handle plutonium in any form; generally a dry inert atmosphere is required.