Not sure what you're getting at. The spent rods from nuclear reactors contain Plutonium, which is quite trivial to separate chemically. If Japan wants to do tha

Not sure what you're getting at. The spent rods from nuclear reactors contain Plutonium, which is quite trivial to separate chemically. If Japan wants to do that it can certainly can do it (and it's likely they already stockpiled some Plutonium for a rainy day). Plutonium can then be used to make a Nagasaki-type bomb. Nothing to do with fusion. If someone wants to go further and make a hydrogen bomb (i.e. fusion-based), that's also possible, but fission-based atomic bombs are good enough to provide deterrence.