"Now, I agree about the temps and pressures being unnecessary. The reason for these is about efficiency of scale. In a power grid like America's where we need 1

"Now, I agree about the temps and pressures being unnecessary. The reason for these is about efficiency of scale. In a power grid like America's where we need 1 gigawatt and greater plants, plus with licensing a plant being so difficult, you build the biggest baddest plant you can which can output the most power. This means you go with the highest temps and pressures while still being ultra safe to create a more power efficient reactor. [P] Smaller reactors which would be better for the power grids of the world. Like 250 mega watts would not need these extreme environments." This isn't actually accurate. Reactor core water is pressurized to raise the boiling point -- at 0.1 MPa (atmospheric) it's 100 ºC, at 15 MPa (reactor coolant) it's 342 ºC, so they can push water to around 300 ºC and still keep it liquid in the core. The higher the temperature, the higher (in general) the efficiency of converting heat to work (in the case of nuclear plants, efficiency of the steam turbine). This is pretty much independent of the size of the reactor. (Why liquid water? One huge reason is neutronics (the nuclear part): a very high density of hydrogen nuclei (H in H2O) is useful for scattering neutrons, which slows them down to speeds where they get absorbed by heavy nuclei (reactor fuel) and start fission reactions. [This isn't necessary: in fact "fast reactors" work with neutrons flying at relativistic speeds. But it's much easier.]) 300 ºC is actually pretty cool; the steam from coal power plants gets up to around 600 ºC [1], and internal-combustion gas turbines can reach temperatures of even 1,600 ºC [2]. Water-cooled reactors are held back in efficiency by the need to keep water liquid at core temperatures. Conceptually they can get a bit further by pressurizing water to supercritical conditions [3], at about 510-550 ºC/25 MPa; these aren't being built. (These are fast reactors; the density of this supercritical water is very low, about 0.1 kg/L, so it's a weaker moderator). [1] http://www.ge-energy.com/products_and_services/products/steam_turbines/fossil_g_series.jsp http://www.ge-energy.com/products_and_services/products/stea... [2] http://www.mhi.co.jp/en/news/story/1105261435.html http://www.mhi.co.jp/en/news/story/1105261435.html [3] http://www.gen-4.org/Technology/systems/scwr.htm http://www.gen-4.org/Technology/systems/scwr.htm