Don't we have them already?

https://en.wikipedia.org/wiki/Pressurized_water_reactor

> PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms.

When people say "fission reactor" do they mean something else?