Thorium utilization in a small long-life HTR. Part II: Seed-and-blanket fuel blocks

In order to utilize thorium in high temperature gas-cooled reactors (HTRs), the concept of seed-and-blanket (S&B) fuel block is introduced into the U-Battery, which is a long-life block-type HTR with a thermal power of 20 MWth. A S&B fuel block consists of a seed region with uranium in the center, and a blanket region with thorium. The neutronic performance, such as the multiplication factor, conversion ratio and reactivity swing, of a typical S&B fuel block was investigated by SCALE 6.0 by parametric analysis of the composition parameters and geometric parameters of the fuel block for the U-Battery application. Since the purpose of U-235 in the S&B fuel block is to ignite the fission reactions in the fuel block, 20% enriched uranium is recommended for the S&B fuel block. When the ratio of the number of carbon to heavy metal atoms changes with the geometric parameters of the fuel block in the range of 200-250, the reactivity swing reaches very small values. Furthermore, for a reactivity swing of 0.1 Δk  during 10 effective full power years, three configurations with 36, 54 and 78 UO2 fuel rods are recommended for the application of the U-Battery. The comparison analysis of the S&B fuel block with the Th/U MOX fuel block shows that the former has a longer lifetime and a lower reactivity swing.