The core of a conventional fission reactor, such as a PWR has around 3500MW being produced in a volume of around 3 cubic meters. A fast breeder reactor is even smaller – hence they need to use a more efficient coolant (liquid sodium). But you can get that order of head out of a small volume. It’s just a case of pumping through a large enough volume of coolant fast enough.
A PWR for example pumps around 11tonnes of water through it ever second!
