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.