Page "A2W reactor" Paragraph 6

Much of the reactor power control during steady state operation comes as a result of the coolant water's negative temperature coefficient.

The power of the reactor is determined by the number of fission events that takes place in the fuel at any given moment.

As the water heats up, it expands and becomes less dense which provides fewer molecules per volume to moderate the neutrons, hence fewer neutrons are slowed to the required thermal energies to sustain thermal fission.

Conversely, when the coolant water temperature decreases, its density increases and a greater number of neutrons reach the required thermal energy, increasing the number of fissions per unit of time, creating more heat.

This has the effect of allowing " steam demand " to control reactor power, requiring little intervention by the Reactor Operator for changes in the power demanded by the ship's operations.