How it works
Boiling water reactors

In the PWR, on demand for power from the grid, the local primary control opens the moderator valve; steam flows in and saturation pressure and temperature drop in the steam generator secondary, the primary water returns a little cooler to the core inlet, increasing reactivity, and the required power level is naturally reached.

With the same demand on the BWR, the pressure and saturation temperature in the reactor fall; bubble volume increases, reactivity drops and, instead of meeting the demand for increased power, the reactor suffocates. To prevent this from happening, the network's demand for additional power controls the acceleration of circulation pumps or the extraction of control rods. The turbine moderator valve is short-circuited...