Uhm, firstly, reactors do not produce electric power. Reactors produce heat from radioactive decay. It’s a matter of converting heat into electricity.
Full size reactors like the ones found at power plants, submarines, ships, or even planes (yes) use the most common way, that is, by boiling water and using steam to turn turbines. This construction is utterly massive even for most vehicles, so fitting it into human is like fitting a rocket engine into your finger. Not going to work.
There is, however, another way of getting energy from radioactive decay through the usage of thermoelectric generators. Resulting arrays are hugely unefficient but can be very compact. Problem is, a human-sized construction only gives you just about enough energy to run a PC, and two of them may let you run a microwave. Another problem is that the material (plutonium, strontium, and such) heats up to about a thousand degrees celsius and you steel need some shielding to protect everything from radiation and heat.
You can see these RITEGs used in the ‘Martian’ movie by the main character to heat his rover, for instance.
All things considered, both vehicle-mounted minireactors and implanted reactors are completely unrealistic. Of course, it’s future and so on, but still.
Radiation is not always a concern, as different materials emit different particles when decaying. By choosing correct materials, you may lower the radiation levels to safe-ish levels, at least not the levels you’d get when devouring plutonium.
You don’t need explosive power for the reactor to function. Critical mass is about creating a chain reaction, which, believe me, you woudn’t want happening anywhere close to you.
You don’t. In fact, Voyager spacecrafts still function by sucking power from the same batteries as 40 years ago. That is quite a lot of time. Sure the power output is falling, but for a human 40 years is more than enough.
