Neptunium(IV) oxide, or neptunium dioxide, is a radioactive, olive green cubic crystalline solid with the formula NpO2. It emits both α- and γ-particles.
Production
Industrially, neptunium dioxide is formed by precipitation of neptunium(IV) oxalate, followed by calcination to neptunium dioxide.
Production starts with a nitric acid 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):
- 2Np5 C6H8O6 → 2Np4 C6H6O6 2H
- Np6 C6H8O6 → Np4 C6H6O6 2H
Addition of oxalic acid precipitates hydrated neptunium oxalate...
- Np4 2H2C2O4 6H2O → Np(C2O4)2.6H2O(v) 4H
...which pyrolyzes when heated:
- Np(C2O4)2.6H2O Δ
→ Np(C2O4)2 Δ
→ NpO2 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 fission reactors, neptunium dioxide can be purified by fluorination, 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 by an untested but appreciable amount. The compound has a low specific heat capacity (900 K, compared with uranium dioxide'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 spent nuclear fuel will commonly be treated so that various AnO2 (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 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.
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