Reactor fuel assembly

Purpose: To arrange at least half in number of fuel rods containing burnable poisons in the proximities of water rods, thereby to make uniform the core output distribution through burnup and reduce the maximum linear output density. Constitution: Gd ruds are arranged adjacent to the water rods arranged in the center of the fuel assembly. That is, if the difference between the upper part and lower part of each of fuel assemblies having an effective percentage of voids in the peripheries of Gd rods is small, the difference in the spectra of thermal neutron energies flowing in the Gd rods becomes small. As a result, the difference in the infinite multiplication factors at the upper part and lower part of the assembly becomes small in the variation due to the burnup, and flattening of the output distribution is easy to be materialized even at the time point where the burnup is advanced. (Yoshihara, H.)