A lot less than in a submarine or electrical reactor. Heck McMaster U has a swimming pool sized 5 MW.

There's no pressure vessel and a massive thermal inertia. Not child's play, but very safe.

The big issue is the waste.

No reactor is a calm place. The heat capacity of water changes with pressure, but not by orders of magnitude. So any 400 MW reactor has to pump similar amount of water to keep the T constant.

So the pumps have to be similarly sized, except you no longer need a pressure vessel.

This.

When The Coca Cola Company (the folks who own the secret sauce) bought Coca Cola North America (N. America's largest bottler) there was an entire floor that TCCC empoyees weren't allowed into. CCNA had bottling agreements with TCCC's competitors.

The restrictions were strict!

I keep an HP48 in the office and an HP28S at home.

I might buy another HP28 for the car. :D