[7] Production starts with a nitric acid feed solution containing neptunium ions in various oxidation states.

[7] 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.

[4] However, the aforementioned synthesis yields a quite pure solid, with less than 0.3% mass fraction of impurities.

[7] Neptunium dioxide contributes to the α-decay of 241Am, reducing its usual half-life an untested but appreciable amount.

Mentioned above, it references an abnormal level of order for an actinitde dioxide complex at low temperature.