Researchers made compounds of neptunium — which is difficult to work with owing to its high radioactivity — and examined them to learn more about its properties and behaviour. (sciencedaily.com)
[2] Minor actinides (MAs): Elements synthesized in nuclear fuel other than uranium and plutonium, such as neptunium, americium and curium. (sciencedaily.com)
Neutrons were fired at an aluminium target filled with neptunium - 237, of which there is a larger stockpile. (newscientist.com)