Complex sentence: The third and final UKAEA-operated reactor to be built on the Dounreay site was the Prototype Fast Reactor (PFR), which achieved criticality in 1974 and began supplying National Grid power in January 1975.
Split into simpler sentences: The third and final UKAEA-operated reactor to be built on the Dounreay site was the Prototype Fast Reactor (PFR). It achieved criticality in 1974 and began supplying National Grid power in January 1975.