the little rods to the left are the 'fuel', which is inert until it contacts with the panels in the chamber, then it releases an intense heat and eventually burns off cleanly.
isn't science fiction great?
(only have the evaluation version of the render where I made this, excuse the lines ;D)
This kind of reactor was discovered in 1822 (the thermoelectric effect). It is in use in space probes for about 30 years now.