Patent Number: 055330789
Section: summary

FIELD OF THE INVENTION The present invention relates generally to nuclear fuel assemblies for pressurized water nuclear rectors, and more particularly to decreasing the pressure drop across the fuel assembly and increasing the amount of power generated by the fuel assembly. Background of the Invention As is well known, improvements in fuel cycle costs of nuclear power plants may be achieved by increasing the net amount of fuel in the fuel assemblies in the reactor core. Although increasing the diameter of the fuel rods would produce such an increase, it would also result in the concomitant increase in the resistance to coolant flow within the assembly and an increase in pressure drop. Spacer grids which assist in maintaining the fuel rods in fixed positions in the fuel assembly also contribute significantly to the resistance to coolant flow and increase in pressure drop across the fuel assembly. Since reactor power levels are limited by the amount of coolant flowing through the assembly as well as by local heat transfer conditions present at the surface of the fuel rods, it is highly desirable that spacer grids offer as little resistance to coolant flow as is possible. Furthermore, since there are several spacer grids which are located at selected intervals along the length of the fuel assembly, their total contribution to resistance to coolant flow and pressure drop affects the maximum quantity of nuclear fuel that may be utilized in a particular fuel assembly design. It would thus be an advantage over prior art designs if a spacer grid offered lower resistance to coolant flow, or one or more of the spacer grids could be eliminated from the assembly resulting in a decrease in the pressure drop and thereby permitting an increase in the total amount of power generated by the nuclear fuel assembly. Further improvements in nuclear reactor operations could be achieved if the amount of power that could be safely produced within the fuel assembly was increased. Spacer grids which provide lateral bracing and rod-to-rod spacing are typically designed to allow differential axial expansion of the fuel rods. Springs incorporated in the spacer grids are most frequently used to permit some sliding of the fuel rods with respect to the spacer grids. In some of the designs, the spacer grid is free to move axially a small amount to accommodate minor changes in the axial length of the fuel rods during irradiation. If spacer grids were to be rigidly connected to the fuel rods as well as to structural members of the fuel assembly, then relative axial movement due to rod growth and thermal expansion of adjacent rods can cause local fuel rod skewing and bowing. As is well known, spacer grids are generally built up from a relatively large number of different intricately shaped strips that are fitted together to form spacer cells and subsequently welded. Each spacer cell includes dimples and/or springs to maintain the desired rod-to-rod spacing. Thus, the springs and dimples keep the fuel rods in their proper lateral positions. But, under the influence of radiation, the springs are prone to relax and this can lead to undesirable changes in fuel rod pitch (i.e. rod-to-rod spacing) or it may cause gaps or spaces to develop between fuel rods and the springs and dimples, and increases the likelihood that the rods and/or spacer grids will vibrate. Such gaps, changes in fuel rod pitch, and vibration may lead to fuel rod fretting and failure. Furthermore, as the fuel is irradiated, the fuel rods undergo a shrinkage or diameter reduction known as "creepdown" which can result in gaps between the fuel rod cladding and the springs or dimples which in turn can cause or contribute to fuel rod fretting. Thus, it would be an advantage over prior art nuclear fuel assemblies to: (a) eliminate at least one spacer from the assembly and thereby decrease the pressure drop across the assembly and increase the power generated by the assembly; and (b) further increase the power generated by the assembly by replacing the eliminated spacer(s) with a securing means which permits additional fuel to be placed within the fuel rods but without extending the overall length of the fuel assembly and without increasing the pressure drop across the fuel assembly. SUMMARY OF THE INVENTION A nuclear fuel assembly having a predetermined overall length for a pressurized water reactor, comprising a lower tie plate having at least one aperture, a guide tube having an upper end and a lower end connected to the lower tie plate, spacer grids spaced along the guide tube, an upper tie plate which is attached to the upper end of the guide tube, an instrumentation tube attached at one end to the lower tie plate and attached at an opposite end to the upper tie plate, extended fuel rods which extend to the lower tie plate and which are spaced radially and supported along the guide tube by the spacer grids, and at least one of the extended fuel rods has at a lower end a fuel rod lower end cap which is secured within the aperture in the lower tie plate.