Another option.
The quantum nucleonic reactor.
Basically you bombard hafnium with x-rays and it releases energy at a rate of 60 to 1 for the energy input. Shut off the x-rays and the reactor goes cold. The ultimate fail safe system.
This is rumored to be the power source of a long endurance Globalhawk drone said to be flying for the US Air Force since 2008.
The design could prove useful for portable power for a host of uses from disaster recovery to powering subs or a moon base.