Burnup Computation for HTR-10 Using Layer-to-Layer Movement

This study proposes a layer-to-layer movement model using a once-through fuel cycle strategy to dynamically simulate the on-line refueling process employed in HTR-10. The MCNPX 2.6.0 computer code and continuous energy data library ENDF/B-VII were used in performing all of the computations. In this study, the pebble bed in the core was equally divided into five layers in the axial direction, and the volume fractions of the fuel and graphite pebbles in the initial core were 0.57 and 0.43, respectively. After each fuel cycle, the bottom layer was discharged from the core and discarded while a new layer containing only fuel pebbles was added to the top layer of the core. Hence, the volume fraction of the fuel pebbles increased with greater operation time. This study further proposes that each fuel cycle is stopped to initiate the refueling process for next fuel cycle whenever the effective multiplication factor (k_eff) reaches approximately 1.005. The results revealed that spikes in the k_eff versus reactor operation time are the result of burnup and refueling. The fuel cycle tends to approach an equilibrium cycle after refueling five times. In addition, the axial power distribution tends to change from a bottom-peaked to a top-peaked phenomenon as the fuel cycle number increases. In essence, the axial power distribution is nearly un-changed once the reactor core reaches an equilibrium cycle. This phenomenon is also verified by the corresponding axial burnup distribution, average burnup, and mass of special nuclides as a function of operation time.