Preliminary Study of Burnup Characteristics for a Simplified Small Pebble Bed Reactor

Simplification of the pebble bed reactor by removing the unloading device from the system was peformed. For this reactor design, a suitable fuel‐loading scheme is the Peu à Peu (little by little) fueling scheme. In the Peu à Peu modus, there is no unloading device; as such, the fuels are never discharged and remain at the bottom of the core during reactor operation. This means that the burnup cycle and reactivity is controlled by the addition of fuel. The objectives of the the present study were to find a means of carrying out the exact calculations needed to analyze the Peu à Peu fuel‐loading scheme and to optimize the fuel composition, and fuel‐loading scheme to achieve better burnup characteristics. The Monte Carlo method is used to perform calculations with high accuracy. Before the calculation of the whole core, the analysis for the infinite geometry was performed. The power generated per mass consumed for each combination of the uranium enrichment and packing fraction was analyzed from the parametric survey. By using the optimal value obtained, a whole‐core calculation for the small 20 MWth reactor was performed and the criticality and burnup of this design was analyzed.