Plutonium(IV) nitrate

Plutonium(IV) nitrate. It is a key chemical compound in the nuclear fuel cycle, primarily encountered in the processing of spent nuclear fuel and the purification of plutonium metal. This inorganic salt consists of plutonium cations in the +4 oxidation state coordinated by nitrate anions, forming various hydrates. Its behavior and handling are dominated by the intense radioactivity and chemical toxicity of its central plutonium atom, requiring stringent safety protocols in all laboratory and industrial settings.

Properties

Plutonium(IV) nitrate typically exists as a series of hydrated crystalline solids, with the hexahydrate, Pu(NO₃)₄·6H₂O, being a common form. The compound is highly soluble in water and polar organic solvents like nitric acid, which is the medium for its formation and primary use. Solutions exhibit colors ranging from green to brown, depending on concentration, acidity, and the presence of other species like neptunium or americium. It is a strong oxidizing agent due to the plutonium(IV) ion and is both radiologically and chemically hazardous, sharing handling characteristics with other transuranium element compounds. The solid decomposes upon heating, ultimately yielding plutonium dioxide, a common ceramic fuel form.

Preparation

The compound is not prepared from the elements but is invariably generated during aqueous reprocessing of irradiated nuclear fuel. The primary route involves dissolving spent fuel rods, typically composed of uranium dioxide or mixed oxide fuel, in hot concentrated nitric acid. During this PUREX process, plutonium is oxidized to the +4 state, often using agents like nitrogen dioxide or sodium nitrite, to form the soluble plutonium(IV) nitrate complex. Subsequent liquid-liquid extraction using tributyl phosphate in kerosene separates it from uranyl nitrate and fission products. Isolation of the solid salt is achieved by evaporation of the aqueous nitric acid solution, but this is rarely done outside of specialized research due to the extreme hazards.

Structure

In the solid state, the structure involves the plutonium(IV) ion, which has a high charge density, coordinated by oxygen atoms from both nitrate ions and water molecules. Crystallographic studies of related actinide compounds, such as thorium(IV) nitrate, suggest a coordination number of 10 or 12 for the central metal ion. The nitrate ions typically act as bidentate ligands. In concentrated nitric acid solutions, the plutonium(IV) exists as a complex array of species, including cationic complexes like [Pu(NO₃)_n]^(4-n)+ and neutral Pu(NO₃)₄, which are the species extracted into the organic phase during the PUREX process.

Reactions

The most significant reactions are those employed in nuclear fuel reprocessing. In acidic nitrate media, plutonium(IV) can be reduced to plutonium(III) using agents like ferrous sulfamate or hydroxylammonium nitrate, a step critical for partitioning in some process variants. Conversely, it can be oxidized to plutonyl(VI) (PuO₂²⁺) with strong oxidants like silver(II) oxide or ozone. Upon strong heating, it undergoes thermal decomposition, losing nitrogen dioxide and oxygen to form plutonium dioxide. It also participates in precipitation reactions; for example, adding hydrofluoric acid yields insoluble plutonium(IV) fluoride, a precursor to plutonium metal via calcium reduction.

Applications

Its sole major application is within the nuclear industry, specifically in the plutonium purification and fuel recycling stages of the nuclear fuel cycle. As the key soluble intermediate in the PUREX process, it is the form in which plutonium is separated from uranium and highly radioactive fission products like cesium-137 and strontium-90. After purification, solutions of plutonium(IV) nitrate are further processed, often through precipitation as plutonium(IV) oxalate or plutonium(IV) peroxide, which are then calcined to produce plutonium dioxide for use in mixed oxide fuel for nuclear reactors or other purposes.

Safety

Handling requires extensive precautions due to the compound's extreme radiotoxicity and potential for criticality. It is an alpha emitter, with principal isotopes being plutonium-239 and plutonium-240, posing a severe internal hazard if ingested or inhaled. Operations are conducted in gloveboxes or within hot cells with heavy shielding, often using remote manipulators. Chemical safety concerns include its strong oxidizing nature and reactivity with organic materials. Facilities like the Savannah River Site, Hanford Site, and Sellafield have historically managed large quantities under strict protocols governed by agencies such as the Nuclear Regulatory Commission and the International Atomic Energy Agency. Category:Plutonium compounds Category:Nitrates Category:Actinide compounds