Burnup characteristics of a peu a peu fuel loading scheme in a 110 MWt simplified pebble bed reactor

The efficient use of nuclear fuel is one of the important issues in the current development of nuclear reactors due to the limitation of natural uranium resources and the need for overall economy. Simplicity in the reactor design could further increase its economy while making it easier to operate. A pebble bed reactor is one of the most promising reactor systems to fulfill these criteria. The purpose of this study was to design a simplified pebble bed reactor by removing the unloading devices from the system and then optimizing the fuel composition and reactor configuration so that the system could achieve better burnup all use scarce uranium resources more effectively. A computer code based on the Monte Carlo method was developed and used in this study in order to obtain precise calculation results due to the weakness of the diffusion method in treating the large cavity region in the core during most of the reactor operation. With this code, analysis and optimization were performed for a 110 MW simplified pebble bed reactor using peu a peu fuel loading scheme. An optimized design using 12% uranium enrichment and 7% packing fraction was the result, calculated to achieve high burnup of 135 GWD/T for more than 20 years' operation time. Neutronic analysis, steady-state thermal hydraulic analysis, and fuel economic analysis for this optimized design are discussed in this study. (author)