Patent Number: 050911465
Section: abstract

In a fuel bundle for a boiling water reactor having one or more part length rods in the two phase region, a steam vent tube is introduced overlying the part length rods. The fuel bundle includes a lower tie plate for admitting water moderator and supporting a plurality of fuel rods in upstanding side-by-side relation, an upper tie plate for permitting water and steam to be discharged from the top of the fuel bundle and maintaining the fuel rods in upstanding side-by-side relation, a surrounding fuel channel for confining moderator flow along a path over the fuel rods and between the tie plates, and dispersed vertically intermittent spacers for maintaining the fuel rods in their designed side by side relation. One or more fuel rods extends from the lower tie plate vertically less than the full length to the upper tie plate ending interior of the fuel bundle at a disposition where the upper end of the part length rods is braced in the vertical position by a spacer. At least one of these partial length rods is provided with an overlying steam vent tube. This steam vent tube has openings and devices distributed along its length to encourage steam flow interior of the tube and remove liquid flow from the interior of the tube. The steam flow within the vent tube eliminates the interface drag between the steam interior of the tube and the surrounding water steam mixture. The presence of the steam vent tube as a high velocity steam escape path enables remaining portions of the fuel bundle to contain a higher liquid moderator fraction with flatter axial voids and power distributions during the operating state of the fuel bundle. The presence of high velocity escaping steam combined with distributed apertures along the length of the steam vent tube promotes resistance to fluid oscillations within the fuel bundles. Variations of steam vent tubes are disclosed including steam vent tubes overlying multiple clustered part length rods. There results a fuel bundle design in which the entire upper cross section of the fuel bundle is devoted to steam generation and coolant outflow.