Neutronic effects of the pebble bed model on tritium breeding calculations for fusion blankets

In the design of fusion blankets, the tritium breeding region is typically filled with a large number of lithium ceramic particles, forming a pebble bed with a complex porous structure. This configuration facilitates fast tritium release and online extraction while mitigating thermal stress in the lithium ceramics. Due to the complexity of the pebble bed structure, it is difficult to directly model the pebble bed structure in neutronic calculations. The homogenized model is usually used to macroscopically approximate the pebble bed structure in neutronic calculations. However, the deviation of neutronic performance introduced by homogenization has not been fully evaluated. In this paper, two regular packing pebble bed models and a random packing pebble bed model were constructed by using OpenMC. The accuracy of the random packing pebble bed was verified by comparing the calculated results of different models with the experimental results of the Li₂O blanket mockup. This comparison demonstrated that the random packing pebble bed model better represents the spatial distribution of tritium production rate in the real pebble bed structure, with the C/E value deviation from 1.0 reduced by 20 %. Subsequently, the pebble bed models were applied to the solid breeder pebble bed blanket. Take the homogenized model as a reference, a systematic evaluation was conducted to assess the impact of the pebble bed model on tritium breeding neutronic calculations. Local variation of the packing factor was found to cause significant differences in neutronic characteristics in the near-wall regions, while the overall tritium generation rate deviation in the entire region is relatively small. In the thermal neutron energy range, the pebble bed model exhibits significant differences compared to the homogenized model.