Plutonium Totally Explained

Everything about Plutonium totally explained

:"Pu" redirects here. For the abbreviation, see PU. For Chinese and other personal and place names starting with "Pu", see .

Production

During the Manhattan Project, the first production reactor, the X-10 Graphite Reactor, was built at the Oak Ridge, Tennessee site that became Oak Ridge National Laboratory. Later, large (200MWt) reactors were set up at the Hanford Site (near Richland, Washington), for the production of plutonium, which was used in the first atomic bomb used at the "Trinity" test 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 used uranium-235, not plutonium.
Large stockpiles of "weapons-grade" plutonium were built up by both the Soviet Union and the United States during the Cold War. The U.S. reactors at Hanford and the Savannah River Site in South Carolina produced 103,000 kg; It was estimated there are another 170,000 kg of military plutonium in Russia, with 300,000 kg accumulated worldwide. 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 US from enriched uranium fuel to MOX fuel as a means of disposing of plutonium stocks.

Medical experimentation

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."

Occurrence

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 the waste of supernovae, because its half-life of 80 million years 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.

Manufacture

Pu-240, Pu-241 and Pu-242

The activation cross section for ²³⁹Pu is 270 barns, while the fission cross section is 747 barns for thermal neutrons. The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain bomb grade plutonium. 240Pu, ²⁴¹Pu and ²⁴²Pu from ²³⁸U>

Element	Isotope	Thermal neutron cross section	decay mode	halflife
U	238	2.7	α	4.47 x 10⁹ years
U	239	-	β	23 minutes
Np	239	-	β	2.36 days
Pu	239	270 (capture)	α	24,110 years
Pu	240	289 (capture)	α	6,564 years
Pu	241	362 (capture)	β	14.35 years
Pu	242	18.8	α	373,300 years

Pu-239

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. 239Pu from ²³⁸U>

Element	Isotope	Thermal neutron cross section	decay mode	halflife
U	238	2.7	α	4.47 x 10⁹ years
U	239	-	β	23 minutes
Np	239	-	β	2.36 days
Pu	239	-	α	24,110 years

Pu-238

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's 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. 238Pu from ²³⁵U>

Element	Isotope	Thermal neutron cross section	decay mode	halflife
U	235	99	α	703,800,000 years
U	236	5.3	α	23,420,000 years
U	237	-	β	6.75 days
Np	237	165 (capture)	α	2,144,000 years
Np	238	-	β	2.11 days
Pu	238	-	α	87.7 years

Compounds

Plutonium reacts readily with oxygen, forming PuO and PuO₂, as well as intermediate oxides. It reacts with the halogens, 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(CO₃)₁^-2

PuO₂(CO₃)₂^-4

PuO₂(CO₃)₃^-6 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.

Allotropes

Even at ambient pressure, plutonium occurs in a variety of allotropes. These allotropes differ widely in crystal structure and density; the α and δ 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 (for example, 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.

Isotopes

Twenty-one 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. Because of its comparatively large half-life, minute amounts of Pu-244 can be found in nature, 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.
   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; see above for more details. 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 weapons-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 a 1% impurity in Pu-239 will lead to unacceptably early initiation of a fission chain reaction in gun-type atomic weapons (for example the proposed Thin Man bomb), 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.

Precautions

Toxicity

Isotopes and compounds of plutonium are toxic due to its radioactivity While plutonium is sometimes described in media reports as "the most toxic substance known to man", from the standpoint of actual chemical or radiological toxicity this is incorrect. When taken in by mouth, plutonium is less poisonous than if inhaled, since it isn't absorbed into the body efficiently when ingested. The U.S. Department of Energy estimates the increase in lifetime cancer risk for inhaled plutonium as 3×10⁻⁸ pCi⁻¹. (this means that inhaling 1 μCi, or about 2.5 μg of reactor-grade plutonium is estimated to increase one's lifetime risk of developing cancer as a result of the exposure to 3%). When plutonium is absorbed into the body, it's excreted very slowly, with a biological half-life of 200 years. From a purely chemical standpoint, it's about as poisonous as lead and other heavy metals. Not surprisingly, it has a metallic taste.
Plutonium may be extremely dangerous when handled incorrectly. The alpha radiation it emits doesn't penetrate the skin, but can irradiate internal organs when plutonium is inhaled or ingested. Particularly at risk are the skeleton, where it's likely to be absorbed by the bone surface, and the liver, where it'll 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 isn't 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.

Criticality potential

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'll nevertheless heat itself and break whatever confining environment it's 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 can't 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 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. (See List of nuclear accidents.) The 1986 Chernobyl accident caused a minor release of plutonium.

Flammability

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 pyrophoric 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. 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.

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