## 1 Introduction  
Tritium is the primary nuclear fuel for deuterium-tritium fusion reactors. However, tritium's reserves in nature are so small, they are almost negligible. Therefore, to maintain the continuity of the fusion reactor, production of tritium through the tritium-breeding blanket in a fusion reactor is required [1]. In the tritium-breeding blanket, tritium is bred via the Li (n, \(\alpha\)) \({}^{3}\)H section between lithium atoms and neutrons in tritium breeder materials [2, 3]. The lithium ceramic pebbles are always selected as breeder materials, such as Li\({}_{4}\)SiO\({}_{4}\) and Li\({}_{2}\)TiO\({}_{3}\) pebbles, which are packed in cavities of tritium-breeding blankets and formed fixed beds [4, 5]. For example, the Li\({}_{4}\)SiO\({}_{4}\) pebbles with diameters of \(\sim\)1 mm are utilized in the helium-cooled ceramic breeder (HCCCB) blanket [2, 6]. In the tritium-breeding blanket, when the produced tritium is released into the pores of the bed, the tritium needs to be carried out of the bed by the purge gas and then into the tritium extraction system (TES) for further processing [7, 8]. However, the efficiency of tritium extraction is affected by the flow characteristics of purging gas. Additionally, the gas flow behaviors are influenced by the pebble packing structure and the operating conditions of the fluid. Therefore, it is crucial to comprehensively investigate flow behaviors and characteristics of the tritium-purging gas for the design and operation of the tritium-breeding blanket in a fusion reactor. The flow characteristics of the tritium-purging gases are primarily characterized by pressure drop and flow rate at the macroscopic level. [PERSON] et al. [9; 10] experimentally measured the helium gas pressure drops in cylindrical and rectangular pebble beds at the macroscopic level and analyzed the influences of the pebble size, bed length, and inlet velocity on the pressure drop in pebble beds. [PERSON] et al. [11] and [PERSON] et al. [12] measured the pressure drop of helium gas in beds packed with stainless steel pebbles of 0.5-2 mm with focus on the influence of pebble size. The results show that reducing the particle size can reduce the porosity and increase the pressure drop. [PERSON] et al. [13] evaluated the pressure drop of nitrogen gas in particle beds with different materials and pebble sizes. Their findings indicated that the pressure drop increases as pebble diameter decreases and velocity increases, which are consistent with the Ergun model with modified Ergun constant. The aforementioned experimental results are crucial in the design of solid tritium-breeding blanket. However, experimental measurements can only provide the macroscopic pressure drops and cannot discern the intricate flow characteristics of the localized pebble bed. At the fine-particle-scale level, flow behaviors are manifested as local pressure, flow and velocity distribution, along with local flow characteristics, which are always explored by numerical simulation. [PERSON] et al. [14] conducted numerical studies on the properties of helium gas flow in unary-sized and binary-sized pebble beds, providing a detailed analysis of pressure distribution and velocity variation, and obtained the porosity distribution and velocity and pressure field distributions in binary-sized pebble beds. [PERSON] et al. [15] explored the impact of bed scale on the flow characteristics of purge helium gas in small pebble beds with various column-to-pebble diameter ratios. The results indicate that the wall effect on the packing structures and flow characteristics cannot be neglected in small pebble beds. For different-order and randomly pebble packed beds, [PERSON] et al. [16] examined the effects of the order packing structures and random packing structures on the pressure distribution and velocity distribution, and obtained the loss coefficient of helium through the pebble bed with different packing structures. Meanwhile, [PERSON] et al.