Study on Control Rod Reactivity of Small Pebble Bed Reactor with Wallpaper Fuel Design

Abstract The control rod reactivity is a crucial reactor parameter designed to shut down the reactor and to control the safety of reactor core under any operation conditions. This paper discusses the control rod reactivity of small pebble bed reactor with wallpaper fuel design. The geometric configuration of RDE was selected for the reactor core model. A number of simulations with Monte Carlo n-particle transport code MCNP6 based on evaluated nuclear data library ENDF/B-VII were employed to investigate the total and individual control rod worth and reactivity in stuck rod condition. In the simulation model, the total amount of uranium per pebble was kept constant at 5 g by varying the hexagonal lattice pitch calculated for different radius of the central graphite zone. The simulation results show that the total control rod achieves the best effectiveness on the central graphite radius of 1.0 cm. The highest worth of individual rods in the wallpaper fueled core are almost greater than that of standard fueled core. The stuck rod criterion in all simulation results confirm that, the failure of one rod with highest worth will not prevent the control system from shutting down the reactor. These results conclude that selecting a central graphite radius of wallpaper fuel is one of the strategies to be carefully considered and a radius of 1.0 cm is the best one in controlling reactor safety and ensuring the control system could shut down the reactor safely.