Re: SNAP
SNAP = Systems for Nuclear Auxiliary Power.
Basically, it converts heat given off by fission product from nuclear reactors. The radioactive breakdown of an isotope is used to rise the temperature of one end of a thermocouple and produces electricity. As the half-life of some of the fission products is >20years one may expect that such a battery will operate for long time.
Nowadays they are more often called "radioisotope thermoelectric generators" and mor info you can find here:
**broken link removed**
Regards,
IanP