MODERATOR CONTROL APPARATUS FOR A NUCLEAR REACTOR FUEL ASSEMBLY

摘要

A moderator control apparatus is provided for use with a fuel assembly to improve fuel utilization in thereby reducing fuel cycle costs. The apparatus includes a plurality of displacer rods filled with a burnable poison gas which are inserted into the guide thimbles of the fuel assembly to displace a portion of the coolant in thus reducing the H/U ratio at the start of the cycle. The displacer rods are connected at their upper ends to a manifold which has a central opening with a plurality of inlet ports disposed about the central opening and in fluid flow communication with the rods. A rotatable valve is disposed in the central opening and operable to selectively open and close the inlet ports so as to either permit or obstruct the flow of coolant into the displacer rods in a predetermined controlled manner. During operation of the fuel cycle, the inlet ports are progressively opened in a sequential pattern to slowly release the poison gas from the rods and, at the same time, remove the displacement by filling the rods with the coolant in thus shifting the energy spectrum back to its normal H/U ratio.