Abstract: A burnable poison rod for use in a nuclear reactor comprises a metallic tube with upper and lower closure means, a neutron absorber positioned within the cladding and a neutron moderating spacer means positioning the neutron absorber in spaced relation to the lower closure means. The neutron moderating spacing means can comprise a solid neutron moderating material, or a liquid coolant moderator contained within the lower section of the cladding with a sealing plug positioned between the neutron absorber and the liquid coolant moderator.

Abstract: A burnable poison rod for use in a nuclear reactor comprises a metallic tube with upper and lower closure means, a neutron absorber positioned within the cladding, and a neutron moderating spacer means positioning the neutron absorber in spaced relation to the lower closure means. The neutron moderating spacing means can comprise a solid neutron moderating material, or a liquid coolant moderator contained within the lower section of the cladding with a sealing plug positioned between the neutron absorber and the liquid coolant moderator.

Abstract: In a reconstitutable fuel assembly, a top nozzle attaching structure for mounting the top nozzle adapter plate in releasable locking engagement upon the guide thimble upper end portions includes several improved features. First, each locking tube is mounted within the guide thimble upper end portion for movement between an upper locking position wherein the adapter plate and guide thimble upper end portion are maintained in locking engagement and a lower unlocking position wherein the adapter plate is releasable from the guide thimble upper end portion. Also, cooperating means are defined on the locking tube and the guide thimble for retaining the locking tube at either of its upper or lower positions. The guide thimble cooperating means includes a pair of upper and lower circumferential bulges defined on the guide thimble upper end portion below the adapter plate.