a nuclear reactor coolant pump impeller / shaft assembly , including an impeller nut with a plurality of conduits formed therein in fluid communication with both ends of a passageway formed in the impeller nut in contact with a portion of the shaft , the fluid inlet and fluid outlet of the pump . due to this conduit / passageway combination , coolant is effectively pumped into the passageway to heat the shaft to a temperature approximately equal to that of the impeller , thus creating thermal balance of the pump &# 39 ; s component parts and providing improved pump efficiency .