Fuel element having variable orifice

A gas-cooled nuclear reactor having disposed therein a plurality of generally vertically extending nuclear fuel elements and wherein a downward flow of gaseous coolant is maintained throughout the region wherein said fuel elements are disposed, each of said fuel elements comprising a plurality of elongated parallel fuel pins containing fissionable material, a tubular housing surrounding said fuel pins and having an inlet opening at the upper end thereof and an outlet opening at the lower end thereof and constituting a connecting passageway therebetween for directing coolant flow over said fuel pins, said housing having a convex annular constriction in said fuel element passageway, a movable plug disposed within said housing adjacent said convex constriction, a stationary baffle supported within said passageway downstream of said convex constriction, said plug having a concave surface facing upstream and mating with said baffle to form a streamlined composite of inverted teardrop shape with respect to the downward direction of coolant flow, said plug and said convex annular constriction forming a generally annular orifice through which the gaseous coolant flows, the size of which orifice is variable in accordance with the vertical position of said plug, and means for moving said plug vertically between a lower, fully open position wherein it mates with said baffle and an upper closed position wherein said plug resides in its closest proximity to said convex constriction.