Not sure you need to consider that much of a volume; we're talking about surface temperature, so you can just take an approximation of heat capacity of the surface of Earth.

First I would check the number that I gave you and make sure it is correct. Then you put it into perspective; e.g. how many millennia would it take for a couple of hundred nuclear reactors to produce that much of energy.

Play with the numbers and see what you can learn about them.