Fans of 60s television may fondly recall Robin calling out "Nuclear batteries to power, turbines to speed" every time the Batmobile went into action. Well, nuclear batteries have now become available to many of the rest of us, promising MCU designs decades of continuous power. It's just not the kind of power needed to fire up an engine.
The batteries I'm talking about are betavoltaic. When a beta particle from a radioactive source enters a p-n junction and collides with atoms, it generates an electron-hole pair. Those loose electrons are free to flow through a circuit, generating a small amount of current.
The open-circuit voltage of a single cell is about 0.8V, and the current generated is in the tens of nano-amperes. It's not a whole lot of power, admittedly, but using a low-risk radioactive material like tritium -- a hydrogen isotope with a half-life a bit longer than twelve years -- allows a betavoltaic battery to supply that power for decades. Further, because the electron-generation is a mechanical and not a chemical process, it is relatively insensitive to environmental factors like temperature and pressure.
Betavoltaic batteries are now available to the industry without the need for any type of regulatory paperwork or end-user radiologic training. (In the US, at least.) City Labs has a General License from the US Nuclear Regulatory Commission to manufacture and sell such batteries and has just released its first commercial product -- the NanoTritium P100a battery. It is available in 0.8V, 1.6V, and 2.4V configurations with maximum currents of 50 to 350 nano-amperes.
Ultra-low-power MCU designs could use the battery to directly power the entire system, but there are other uses, even if you have more demanding requirements. One possibility is to use the nuclear battery to trickle-charge a more conventional battery or a super-capacitor that can then power the system for short bursts of activity. Another design might use the nuclear battery for SRAM backup power in applications where Flash or EEPROM are not feasible.
Nuclear batteries do not supply a lot of power, but what they provide is consistent and enduring. What would you be able to do if you had a battery that could run for twenty years?