Fuel assembly for nuclear boiling-water reactors

#The invention relates to a fuel assembly for nuclear boiling-water reactors and, more particularly, to such a fuel assembly formed of a cluster of fuel rods held by a base and a head piece, respectively, and, with the aid of a fastening angle surrounded by a casing preferably of square cross section for guiding the coolant flow, the fastening angle being provided with leaf springs for mutual spacing and bracing of a multiplicity of such fuel assemblies forming a reactor core. Such fuel assemblies are traversed by cooling water upwardly from the bottom thereof, the cooling water transforming into steam in the process. For control purposes, conventional cruciform control rods are provided, which are generally introduced from below into the reactor core and into intermediate spaces located between groups of four adjacent fuel assemblies. Such four adjacent fuel assemblies form a so-called core cell.

#During operation of the reactor, the structural material of the fuel assemblies is subjected to an extremely heavy neutron flux, which results in a permanent growth in length of the individual fuel assemblies. Since, on the one hand, the fuel assemblies in the various zones of the reactor core are subjected to a varying neutron flux and, on the other hand, new fuel assemblies are continuously exchanged for previously spent ones during fuel assembly exchanges, it is unavoidable that, after a given operating period of the reactor, fuel assemblies of varying length will be present adjacent one another in the core. This means, however, that the engagement or contact locations between the spring elements of adjacent fuel assemblies will be at different axial levels, so that the mutual holding forces become different and can thus lead to difficulties when inserting the control rods.

#It is accordingly an object of the invention to provide a construction for the spring elements, which affords constant spring action even for varying mutual bracing locations, and therewith, permit no change in the clamping conditions of the fuel assemblies forming the core.

#The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

#FIG. 1 is a longitudinal view, partly broken away and in section, of a conventional fuel assembly for a boiling water nuclear reactor;

#FIG. 2 is a top plan view of FIG. 1;

#Referring now to the drawing and first, particularly to FIGS. 1 and 2 thereof, there are shown in an elevational view and a top view, respectively, a boiling-water fuel assembly of conventional design. This assembly is formed of a multiplicity of fuel rods 3, which are disposed between a head piece 2 and a base 5. They are held at a mutual spacing by spacers 4. A fuel-assembly can or box 6 surrounds the entire fuel assembly and serves for guiding the coolant, which flows through this fuel assembly from the bottom to the top thereof. For handling the fuel assembly, as a whole, a lifting handle 1 is provided, which is connected to the head piece 2.

#In accordance with FIG. 4, groups of four fuel assemblies 11 of the type shown in FIG. 1 are arranged to form a core cell, cruciform control rods 12, shown diagrammatically in FIG. 4, being disposed between the individual core cells. Spring elements 10 are provided for mutually bracing or clamping the fuel assemblies 11 in the combined or compound structure of the reactor core (note also the enlarged view of the fuel assembly head in FIG. 3) and, in addition, solid or rigid contact bosses or knobs 9 are provided, both the spring element 10 and the boss 9 together with a fastening angle 7 and a fastening screw 8, and simultaneously also holding the fuel-element casing or can 6, being connected to the head piece 2 of the fuel assembly 11.

#As can be seen from FIGS. 2, 4 and 5, a spring element 10 is disposed on the two sides of each fuel assembly located at one corner of the fuel assembly. This spring element 10 is formed of two leaf springs which are disposed at an angle to one another and are bent in accordance with FIG. 3. This construction of the spring elements 10 is also visible in the side view of FIG. 6 wherein the mutual spatial relationship of two fuel assemblies 11 is further represented, namely, at the left-hand side, a fuel assembly 11 that has already been in operation for a very long time and has accordingly grown in length and, at the right-hand side, a new fuel assembly 11. From the view of FIG. 6, it can be seen that the points of contact of the two spring elements 10 are shifted relative to one another and that, consequently, changes in the clamping or bracing forces are of necessity also produced (note also FIG. 4).