The general idea is to have a few various sizes and power output ranges to use on long term, solar deprived, space probes, dark side of the moon sensory and observstional sites where solar panels would be ineffective. They could also be used for longer life probes where dust and micrometer impacts could impare the effectiveness over time. I would want them to be hyper safe and until activated, such as onboard the launch vehicle, the material would be sealed within a basically indestructible capsule. Then when it is powered up, the material is released into the reaction chamber. I am making basic assumptions that molten salt combined with the low ambient temperatures would be highly stable in keeping the reaction cool without the need for complex cooling systems. The more simple the design the better. Furthermore, once it has reached it's endlife, that a way could be designed to self arrest the reaction and permeantly seal the chamber as not to leave a potential hazard in the futur