Boiling water reactor

A boiling water reactor is a nuclear reactor in which water boils inside the reactor vessel and the resulting steam directly drives a turbine. Water serves as both coolant and neutron moderator. Moisture separators and dryers remove entrained liquid before steam reaches the turbine; a condenser and feedwater system return it to the vessel.^[ref]

The heat balance makes its purpose concrete: in a simplified steady model, $\dot{Q}\simeq \dot{m}(h_{\mathrm{steam}}-h_{\mathrm{feedwater}})$, where $\dot{m}$ is the water mass flow and $h$ is specific enthalpy. With an illustrative enthalpy increase of $2MJ/kg$, transporting $2\mathrm{GW}$ of heat requires approximately $1000kg/s$. This is a thermal balance, not the electrical output, because the turbine cycle rejects heat to the condenser.

A boiling water reactor differs from a pressurized water reactor, which keeps its primary coolant liquid and transfers heat to a separate steam-producing circuit. Shutting down the chain reaction does not remove decay heat, so heat removal remains necessary after shutdown.