Patent Number: 051788250
Section: abstract

In a nuclear fuel bundle for a boiling water reactor, the prior art suggested concept of tapered fuel rods forming the bundle array is disclosed in a practical design including a fuel bundle and an improved associated spacer utilized in the upper two phase region of the fuel bundle. A tapered fuel rod is utilized having a large diameter bottom tube for holding corresponding large diameter fuel pellets at the fuel rod bottom, a smaller diameter top tube for holding correspondingly smaller diameter pellets at the top of the fuel rod, and a bell reducer situated between the large diameter lower fuel tube and the smaller diameter upper fuel tube. This bell reducer tapers from the large outside diameter of the lower fuel tube and the smaller outside fuel diameter of the upper fuel tube and forms a smooth transition between the large and small diameter sections of the fuel rod. An improved fuel rod spacer is disclosed for use with the upper and smaller tube portions of the fuel rods. This upper spacer defines a matrix of rod holding positions having sufficient dimension for enabling both the upper and smaller diameter tube and the lower large diameter tube to pass through the cells defined for each fuel rod. A spring having a relatively wide flexure range protrudes into each cell. When the fuel bundle is conventionally assembled, the spring in each cell biases the upper smaller diameter tubes in their designed side-by-side relation against stops defined at each cell. At the same time, the spring is provided with sufficient flexibility to allow the lower large diameter tube and the transition bell reducer to pass through the individual cells of the upper spacer. With the improved upper spacer, a fuel rod can be lifted vertically upward and out of the fuel bundle or replaced vertically downward into the fuel bundle.