It depends on how easy it is to release the energy. A full petrol tank already contains a scary amount of energy, but you can't release it instantanously since it is bottlenecked by oxygen intake for combustion. Explosives bring their own compact oxidizer to get around that problem.
A chunk of uranium-238 contains a lot of energy but it is only fissionable but not fissile, so you need an external fast neutron source to extract that energy. Comparatively it is a fairly safe energy source for its density. Similarly there are some metastable nuclear isomers that can theoretically be stimulated to release their energy with high energy photons with a precise frequency and would otherwise release their energy relatively slowly through their natural decay modes.
And I think the difficulty of turning a hypothetical portable fusion power plant into an explosive device should be evident from the difficulties of keeping current experiments running for even a few minutes.
I just though of an interesting twist on subcritical reactor. What about smelting a californium neutron source together with the fuel? We get a much cheaper RTG alternative with near no need for regulation.
A chunk of uranium-238 contains a lot of energy but it is only fissionable but not fissile, so you need an external fast neutron source to extract that energy. Comparatively it is a fairly safe energy source for its density. Similarly there are some metastable nuclear isomers that can theoretically be stimulated to release their energy with high energy photons with a precise frequency and would otherwise release their energy relatively slowly through their natural decay modes.
And I think the difficulty of turning a hypothetical portable fusion power plant into an explosive device should be evident from the difficulties of keeping current experiments running for even a few minutes.