The materials for creating a nuclear warhead (fisson or fusion) really aren't unstable at all. The only problem with those materials is the radiation, but even that is easily shielded against even with our present level of technology. Really, the only unstable part (with present technology) is the detonator which is a small amount of a conventional explosive and even that will only detonate if given the proper impulse. Take that detonator out of the warhead and you have a very small unstable thing and a big, very heavy thing that you could hit repeatedly with a sledgehammer and not cause a KABOOM.
TL;DR
Nuclear materials are not gunpowder. They won't spontaneously go "KABOOM" on you as you fly around the sector.
On the other hand, antimatter is the most reactive stuff possible. It takes a great deal of effort and energy to initiate a fission or fusion reaction, while it takes a monumental amount of effort and energy to not initiate matter/antimatter annihilation.
Comments
On the other hand, antimatter is the most reactive stuff possible. It takes a great deal of effort and energy to initiate a fission or fusion reaction, while it takes a monumental amount of effort and energy to not initiate matter/antimatter annihilation.