Doesn't work like that. The plutonium core of a nuclear weapon, I read, is not even warm to the touch.Having never personally handled one, and having seen where nuclear HLW (High Level Waste e.g. Uranium reactor fuel) materal can produce heat at the rate of 2kW/m3 I'll have to have a second opinion on this ...
Reference: www.uic.com.au/nip51.htm
OK, 2nd Opinion: I've haven't "handled" the core, but I certainly have "handled" the stuff around it. They ain't hot (temperature wise).
Remember, radioactive decay (and associated heat) can be many things other than neutron emission. But alpha and beta particles are not going to set off a chain reaction.
What you say about high level nuclear reactor waste is of course true. One recollects the Russian processing accidents.
Nuclear weapons use carefully separated isotopes.