Patent Number: 047042486
Section: summary

The present invention relates to fuel elements used in nuclear reactors and, more particularly, to a high performance fuel element including components for forming a peripheral seal and additional coolant passages aligned with, but not communicating coolant to, the sealing components. BACKGROUND OF THE INVENTION It is conventional in the nuclear reactor art, and in particular in high temperature gas cooled reactors, to provide a reactor core made up of a number of core blocks or elements which are stacked in columns. The core elements may include fuel elements and control rod elements. An example of such a fuel block or element is disclosed in U.S. Pat. No. 4,060,450 which illustrates and describes a fuel element or block comprised substantially of graphite and having generally cylindrical passages or channels therethrough for receiving fuel rods or rod segments and facilitating passage of coolant through the fuel element. Other blocks or elements are for accommodating control rods and are similar in exterior shape but generally include channels for control rods, reserve shutdown pellets or power rods which include a neutron absorbing material, such as boron carbide, and function to control operation of the reactor core. In high temperature gas cooled reactors employing prismatic fuel and control elements having generally planar end surfaces, gaps may form at the interface between stacked elements allowing coolant leakage flow into and out of the element coolant channels. Should a gap be created between adjacent fuel elements, the coolant leakage into and out of the nearest coolant channels can result in undesirable maldistribution of coolant within the fuel elements. In order to prevent or at least substantially reduce such coolant leakage, it has been proposed to provide a dependent peripheral flange on one end surface of the element and a mating recess on the other to align the coolant passages of the stacked elements and form a peripheral seal. For further information regarding the structure and operation of such fuel elements, reference may be made to U.S. Pat. No. 3,413,196, particularly to FIG. 5. The effectiveness of the seal is, in part, a function of the thickness of the flange. However, as flange thickness increases, the relative core power density decreases because a continuation of the normal fuel hole and coolant passage pattern is not possible in the region underlying the flange without degrading the operation of the seal. SUMMARY OF THE INVENTION Among the several objects of the present invention may be noted the provision of an improved high performance fuel element for a high temperature gas cooled reactor. The fuel element has increased core power density by including fuel holes and accompanying coolant passages in the region underlying the flange, but without compromising the sealing function and without mechanically weakening the flange. Stacked fuel elements are proportioned to define plenums therebetween so that a single blockage in one coolant passage in one element does not render inoperative vertically aligned passages in other elements because the presence of the plenums enables communication of coolant through all passages opening onto it to shunt around the blockage. Other aspects and features of the present invention will be, in part, apparent and, in part, pointed out in the following specification and in the accompanying claims and drawings. Briefly, the fuel element of the present invention includes an elongate block having substantially parallel, spaced, first and second end surfaces. The first end surface has a peripheral sealing flange while the second end surface has a peripheral sealing recess sized to receive the flange. The block has a plurality of first coolant passages disposed inwardly of the flange and recess and a plurality of elongate fuel holes disposed inwardly of the flange and recess. The block also includes a plurality of peripheral longitudinal second coolant passages extending intermediate the end surfaces and in general alignment with the flange and the recess. The block includes two bypasses for each peripheral coolant passage. One bypass intersects the peripheral passage adjacent the first end surface and intersects a first passage, and the other bypass intersects the second passage adjacent the second end surface and intersects a first passage so that coolant flowing through the second passages enters and exits the block through the first coolant passages.