Instead of completely consuming the fuel, burning fuel in a nuclear reactor results in used up uranium fuel cells. These used up cells can be reprocessed in a centrifuge to get back some of the uranium used to create the fuel cells.
Reactors receive an adjacency bonus for adjacent operating reactors, which increases their effective thermal output by 100% per link. This means that 2 reactors operating next to each other will output a total of 160MW thermal, with each reactor producing 40MW base and receiving 40MW of adjacency bonus. Un-fueled reactors do not confer any bonus.