Patent Number: 046845046
Section: abstract

A fuel assembly for use at non-control rod locations of a nuclear reactor core includes top and bottom nozzles and longitudinal structural members extending between and attached to the nozzles for forming an integral unitary structure. One or more of the structural members includes an elongated hollow cladding tube extending between the top and bottom nozzles and end plugs secured to opposite ends of the tube for hermetically sealing and attaching the tube to the top and bottom nozzles. The improvements in the structural member relate to features for reducing fuel assembly bow. Such features relate to a quantity of thermal or irradiation-induced creep resistant material and pretensioning means positioned within the tube. The creep resistant material is a ceramic material in stacked pellet form and coated with a burnable adsorber material. The pretensioning means applies a predetermined compressive load to the ceramic pellet stack and reacts the load so as to preload the tube in a state of pretension having a magnitude sufficient to substantially counteract an axial load typically transmitted through the unitary structure of the fuel assembly and thereby greatly reduce the compressive stress in the structural member tube. There are two embodiments of the pretensioning means. In one embodiment, it is an elongated bellows type device positioned within the tube between the ceramic pellet stack and one of the tube ends, with the interior of the bellows type device being pressurized to create a predetermined axial force therein which places the ceramic pellet stack in compression and the tube in the state of pretension. In the other embodiment, it is an arrangement of belleville springs positioned within the tube between the ceramic pellet stack and one of the tube ends and stacked both in series and in parallel.