I know we have physics men on this forum that could provide a much better answer, but the reality now is that such compact "power packs" are fiction. We do have some very cool devices that can be called nuclear batteries for lack of a batter term.
Atomic battery - Wikipedia, the free encyclopedia
Just about any space probe that goes really deep (much past Mars) needs something like this, as the solar energy is not adequate to keep normal batteries charged via solar panels. Voyager (which has left the solar system) comes to mind.
There is also a concept for a miniature
sealed nuclear power plant that could power a town. It is the size of a truck; can be dropped off anywhere, and can provide power for a couple of decades. At the end of its life, it is taken back to the factory for refurbishment. They run on a similar process, using heat to create electricity, no moving parts.
The problem I think is energy density. Getting hundreds of amperes at higher voltages is just not possible out of something that can fit in the palm of your hand. They might be scaled down, but the smaller they get, the less power they can provide.
Maybe some pf the physics guys can explain it better. BTW I thought Iron Man was cool!