10.58. Boiling-water reactors are operated so that the fuel economy is enhanced by shifting the neutron spectrum to control reactivity. During the first half, or so, of the operating cycle, the coolant flow rate is reduced so that the core coolant void fraction is relatively high. Thus, the core is somewhat undermoderated. The resulting increased resonance region neu­tron flux results in the increased conversion of uranium-238 to fissile plu­tonium. Since this undermoderation acts in the direction of reducing the reactivity at the beginning of the operating cycle, when compensation for the reactivity introduced by fresh fuel is needed, the neutrons used for conversion would otherwise be lost by absorption in control poisons.

10.59. As reactor operation proceeds, fuel is consumed and fission prod­uct poisons accumulate. The coolant flow is then increased, thus reducing the core coolant void fraction and softening the neutron spectrum. This acts in the direction of increasing the reactivity, as necessary to maintain operation. The fissile atoms formed by the previous conversion process now contribute to energy production.