A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal) [REF].
A nuclear reactor coolant - usually water but sometimes a gas or a liquid metal (like liquid sodium) or molten salt - is circulated past the reactor core to absorb the heat that it generates.