Patent Number: 047088460
Section: summary

INTRODUCTION The design of fuel assemblies for boiling water nuclear reactors presents special problems not present in pressurized water reactors. In a BWR, a portion of the core is necessarily occupied by a mixture of water and steam, which, because of its lower density and, therefore, lower spacial concentration of hydrogen atoms, possesses a lesser moderating effect on neutrons than does a core completely filled with liquid water, such as is present in a pressurized water reactor. It has, therefore, become the practice to include in each assembly for a BWR one or more "water rods," i.e., tubes through which liquid water flows. These enhance the moderating effect of the assembly. Another special problem for boiling water reactors resides in the lesser cooling power of the mixture of water and steam present in the upper portion of the reactor core. The coefficient of heat transfer between the fuel elements and steam is significantly less than that between the fuel elements and boiling water. This gives rise to a critical heat transfer zone in the upper portion, which determines the maximum power that can safely be generated by the reactor, termed the "critical power." When the "water rods" are in their simplest form, i.e., open at the bottom and the top, the water passing through them, while effective as a moderator, is relatively ineffective as a coolant. This bypass of a significant portion of the moderator-coolant restricts the number of water rods that may be used in a given fuel assembly. It has been proposed, e.g. in U.S. Pat. No. 3,802,995 granted Apr. 9, 1974, to James R. Fritz and Frederick R. Channon, to provide outlet holes in the water rods to permit flow from them into the space between the fuel rods to enhance the cooling power of the assembly. However, a significant fraction of the water emerges from the top of the rod, and of the assembly, and thus bypasses the zone where maximum cooling is needed. SUMMARY OF THE INVENTION This invention involves an assembly provided with "water rods," in which the total length of the rod is used for moderating purposes, and the total flow through the rod is used for cooling, the coolant being introduced particularly in the upper zone, where the critical fuel rod heat transfer region exists. This critical region extends from about 60% to 80% of the height of the assembly, measured from the bottom. To this end, the water rod, which is preferably made larger than the fuel rods of the assembly, has a closed upper end and an inner tube reaching nearly to that end. Surrounding the inner tube is a reverse-flow annulus, through which water flows downwardly to an exit hole provided in the upper portion of the assembly, and from which water flows and mingles with the coolant surrounding the fuel rods. This increases the number of water rods that may be used in an assembly, and therefore, achieves optimum moderation and also optimum cooling of the assembly, raising the critical power level, i.e. the power level at which boiling adjacent some fuel rods changes from nucleate to film, with possible damage to those rods. Injection of subcooled water rod coolant, i.e., water below its boiling point, into the critical heat transfer region of the assembly improves the heat transfer. Tests show that for a fixed total coolant flow through a fuel bundle, the critical power of the bundle is increased with increasing injection flow. In other words, the best heat transfer performance is achieved by diverting the maximum amount of inlet bundle coolant flow into the water rods and later injecting it into the critical heat transfer region of the core. Other considerations limit the number of fuel rods which may be replaced by water rods, so that there is an optimum number of water rods, which will vary with the particular reactor. With our type of water rods, optimum number of water rods per assembly and the maximum bundle power output are greater than for prior designs.