Professor Davis explains why neptunium could be used in much the same way as uranium or plutonium to make a nuke, and also why it probably hasn't just yet.

More on nukes from Professor Davis
   • Do Nukes Expire?  The benefits and dr...  

More on nuclear radiation from Professor Davis
   • How does a nucleus eject an electron ...  
   • A Brief Introduction to Alpha, Beta a...  

1996 ISIS report (chapter 5)
https://isis-online.org/uploads/books...

NuDat webpage
https://www.nndc.bnl.gov/nudat3/