Neptunium(IV) oxide, or neptunium dioxide, is a
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
, olive green cubic crystalline solid with the formula NpO
2. It emits both α- and γ-particles.
Production
Industrially, neptunium dioxide is formed by
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
of neptunium(IV) oxalate, followed by
calcination
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...
to neptunium dioxide.
Production starts with a
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
feed solution containing neptunium ions in various oxidation states. First, a
hydrazine inhibitor is added to slow any oxidation from standing in air. Then
ascorbic acid reduces the feed solution to predominantly neptunium(IV):
:2Np
5+ + C
6H
8O
6 → 2Np
4+ + C
6H
6O
6 + 2H
+
:Np
6+ + C
6H
8O
6 → Np
4+ + C
6H
6O
6 + 2H
+
Addition of
oxalic acid precipitates hydrated
neptunium oxalate…
:Np
4+ + 2H
2C
2O
4 + 6H
2O → Np(C
2O
4)
2.6H
2O(v) + 4H
+
…which pyrolyzes when heated:
[ Equations extrapolated from verbal description.]
:Np(C
2O
4)
2.6H
2O Np(C
2O
4)
2 NpO
2 + 2CO(g)
Neptunium dioxide can also be formed from precipitation of
neptunium(IV) peroxide, but the process is much more sensitive.
Purification
As a byproduct of nuclear waste, neptunium dioxide can be purified by
fluorination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
, followed by reduction with excess calcium in the presence of iodine.
However, the aforementioned synthesis yields a quite pure solid, with less than 0.3%
mass fraction of impurities. Generally, further purification is unnecessary.
Other properties
Neptunium dioxide contributes to the α-decay of
241Am, reducing its usual
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...
an untested but appreciable amount. The compound has a low
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
(900 K, compared with
uranium dioxide
Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear re ...
's specific heat capacity of 1400 K), an abnormality theorized to stem from its 5f electron count. Another unique trait of neptunium dioxide is its "mysterious low-temperature ordered phase". Mentioned above, it references an abnormal level of order for an actinitde dioxide complex at low temperature. Further discussion of such topics could indicate useful physical trends in the actinoides.
Uses
The neptunium dioxide complex is used as a means of stabilizing, and decreasing the "long term environmental burden"
of neptunium as a nuclear fission byproduct. Actinoide-containing nuclear waste will commonly be treated so that various AnO
2 (where An = U, Np, Pu, Am, etc.) complexes form. In neptunium dioxide, neptunium is of reduced radio toxicity compared with elemental neptunium and is thus more desirable for storage and disposal. Neptunium dioxide has also been show to contribute to increased decay rates of radioactive elements, an application which is currently being explored.
Neptunium dioxide is also used experimentally for research into nuclear chemistry and physics, and it is speculated that it could be used to make efficient nuclear weapons. In nuclear reactors, neptunium dioxide can also be used as the target for plutonium bombardment.
Furthermore, a patent for a rocket powered by neptunium dioxide is held by Shirakawa Toshihisa,
but there is little information available into research and production associated with such a product.
References
{{DEFAULTSORT:Neptunium(Iv) Oxide
Neptunium compounds
Oxides