They are known as fermions, after Enrico Fermi, the physicist who built the first nuclear reactor, and who also gave his name to Fermilab, a particle-physics laboratory near Chicago.
David Sanderson, a physicist at the Scottish universities' research and reactor centre, says that while this would need heavy investment (and a lot of workers), plutonium can be mixed with waste and vitrified into storable glass blocks, so making it unusable in bombs if any terrorists managed to steal some.