Uncertainty analysis of keff due to the random packing of pebbles in the pebble bed HTR

Abstract

There has been an increasing demand for the uncertainty analysis for the numerical results from complicated High Temperature Gas Cooled Reactors (HTGRs) system with the continued development of HTGR. In the pebble bed HTR, not only the fuel and moderator pebbles are randomly packed, but also the packing fraction of fuel pebbles has some uncertainty. A new grid-based Expanding System Method (ESM) is proposed to generate an explicit pebble bed core model with random packing of pebbles with different packing fractions. Even more important, the keff uncertainty due to the random packing and uncertainty of packing fraction are quantified based on this high-fidelity model. The numerical results indicate that the grid-based ESM can efficiently generate a realistic pebble bed core model and the uncertainty of keff due to the random packing of pebbles is relatively smaller than the contribution of nuclear data.