Moderator control apparatus for a nuclear reactor fuel assembly

This patent describes a fuel assembly for a nuclear reactor including an organized array of upstanding fuel rods, a number of elongated guide thimbles strategically located within the fuel rod array, and a moderator/coolant flowing upwardly along the fuel rods to above the fuel assembly. An apparatus to control the nuclear reactivity is described for improving fuel utilization and thereby reducing fuel cycle costs. The control apparatus comprises: (a) hollow displacer rods inserted into respective ones of the guide thimbles, displacing a predetermined volume of the moderator/coolant associated with the fuel rods to decrease the hydrogen/uranium ratio from a given level; (b) a manifold disposed on the top of the fuel assembly in fluid flow communication with the moderator/coolant and having inlet ports and exit ports connected to and in fluid flow communication with respective ones of the displacer rods. Each inlet port is in fluid flow communication with at least one of the exit ports; and (c) valve means operably associated with the manifold inlet ports for controlling the flow and non-flow of the moderator/coolant through the inlet ports and into the displacer rods via the exit ports of the manifold. The flow of the moderator/coolant through the inlet ports and into the displacer rods increases the moderator/coolant volume in thereby shifting the hdyrogen/uranium ratio back to its given level