Understanding Plutonium
Plutonium, denoted by the symbol Pu, is a transuranic radioactive chemical element with atomic number 94. It is an actinide and one of the heaviest elements that can be produced in macroscopic quantities. While trace amounts of plutonium can occur naturally in uranium ores, it is primarily produced synthetically in nuclear reactors. Its most common isotope, Plutonium-239, is known for its fissile properties, meaning it can undergo nuclear fission when struck by a neutron, releasing a significant amount of energy.
Reactivity with Common Substances
Plutonium exhibits significant chemical reactivity, characteristic of the actinide series. Its reactivity is influenced by its multiple possible oxidation states, ranging from +3 to +7, with +3, +4, and +6 being the most common in solution.
Interaction with Air
Plutonium metal readily reacts with oxygen in the air. Upon exposure, its surface quickly tarnishes, forming various oxides such as $PuO$ and $PuO_2$ (plutonium dioxide). This oxidation process is visible as the bright, silvery metal darkens to a dull gray or yellowish-green. In finely divided forms, such as powder or thin foils, plutonium is pyrophoric, meaning it can spontaneously ignite in air at room temperature due to its rapid reaction with oxygen. This poses a significant handling challenge. Additionally, plutonium reacts with water vapor, nitrogen, and carbon in the air at elevated temperatures, forming hydrides, nitrides, and carbides, respectively.
Interaction with Water
Plutonium metal reacts with water and steam. In cold water, the reaction is slow, forming plutonium(III) hydroxide, $Pu(OH)_3$, and releasing hydrogen gas. The reaction becomes more vigorous with increasing temperature, especially with steam, forming plutonium dioxide, $PuO_2$, and hydrogen gas. The generation of hydrogen gas in confined spaces can create an explosion risk.
Key Properties
Beyond its chemical reactivity, plutonium possesses several critical properties that define its handling and applications.
Toxicity
Plutonium is chemically toxic, similar to other heavy metals, if ingested or absorbed into the body. However, its chemical toxicity is generally overshadowed by its much more significant radiological hazard.
Radioactivity
All isotopes of plutonium are radioactive. The most common isotope, Plutonium-239, primarily decays by alpha emission, with a half-life of approximately 24,100 years. Alpha particles are highly ionizing but have a short range, meaning they cannot penetrate the skin. The primary danger from plutonium’s radioactivity arises if it is ingested, inhaled, or enters the bloodstream through a wound. Once inside the body, it can deposit in bone marrow, liver, and other organs, continuously irradiating tissues and significantly increasing the risk of cancer. The alpha decay also generates heat, which can be considerable in larger quantities, causing the metal to feel warm to the touch and requiring specific cooling in storage or transport.
Flammability
As mentioned previously, finely divided plutonium is highly flammable (pyrophoric) in air. Bulk plutonium metal is less prone to spontaneous ignition but will burn if heated sufficiently in air or oxygen. A plutonium fire is extremely hazardous due to the dispersal of radioactive plutonium oxide particles, which can be easily inhaled.
A Historical Chemical Reaction Example
One famous example of chemical reactions involving plutonium is found in its initial large-scale production and separation during the Manhattan Project at facilities like the Hanford Site in the US. The Bismuth Phosphate Process was the first industrial method used to chemically separate plutonium from irradiated uranium fuel and fission products. This multi-step process relied on the different chemical properties of plutonium in its various oxidation states.
In this process, irradiated uranium fuel (containing plutonium, unreacted uranium, and fission products) was dissolved in nitric acid. Plutonium was then chemically adjusted to its +4 oxidation state ($Pu^{4+}$). Bismuth phosphate ($BiPO_4$) was added, which precipitated along with $Pu^{4+}$ as bismuth phosphate-plutonium phosphate solid solution. Uranium and most fission products remained in solution. The precipitate was then dissolved, and the plutonium was oxidized to the +6 oxidation state ($Pu^{6+}$), which does not co-precipitate with bismuth phosphate. This allowed for the removal of remaining fission products by re-precipitating bismuth phosphate. Finally, plutonium was reduced back to the +4 oxidation state and precipitated as plutonium phosphate or further processed to yield a purified plutonium compound. This cycle of oxidation, reduction, and precipitation was a series of critical chemical reactions essential for isolating plutonium.