The light water breeder reactor (LWBR) operated at the Shippingport Atomic Power Station from 1977 to 1982, serving the electric power grid for the Greater Pittsburgh area. The LWBR was a pressurized water reactor (PWR) with several unique features: It was designed and proved to be a breeder with an end-of-life fissile fuel content ∼1.3% greater than beginning of life; the reactor used the 233U-Th fuel system; and it had a large Doppler coefficient, low reactivity worth of transient xenon, and a significant reactivity effect from transient 233Pa. There were no control rods or soluble poison, and reactivity was controlled by movable fuel. Core operations went extremely well. The design lifetime of 18 000 effective full-power hours was exceeded by 60% by utilizing a gradual reduction in power level. The overall capacity factor was 65%. Physics experiments showed good agreement with predictions of movable fuel reactivity worth, most temperature coefficients, breeding, power distribution, and xenon stability. Unexpected results occurred in measurements of flow coefficient of reactivity, zero power temperature coefficients early in life, and bred fissile fuel distribution. The LWBR technology has demonstrated that water-cooled breeder reactors can operate in existing water power plants much like conventional PWRs.