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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.

A nuclear power plant converts energy from controlled nuclear fission into electrical energy. A nuclear reactor produces heat; a coolant carries it to the working cycle; a turbine drives an electrical generator. A pressurized water reactor transfers heat to a separate steam circuit, whereas a boiling water reactor generates steam in the reactor vessel.^[ref]

If thermal input is $P_{\mathrm{th}}$ and conversion efficiency is $\eta$, gross electrical output is approximately $\eta P_{\mathrm{th}}$; the remaining thermal power must be rejected. The plant also needs its own electrical supply, so gross output differs from net export. After the chain reaction stops, decay heat still requires removal.