Pebble Bed High Temperature Gas-cooled Reactor (PB-HTGR) is very different from other type of reactor in terms of its geometric structure, neutron characteristics, and mode of operation. Thus, it is imperative to develop a specialized analysis and calculation code for the PB-HTGR. A neutronics calculation module of the neutronics and thermal hydraulic calculation program suitable for PB-HTGR, named as NECP-Panda, has been independently developed by the Nuclear Engineering Computational Physics (NECP) Laboratory of Xi'an Jiaotong University.

This study aims to explore how the neutronics calculation module in NECP-Panda is achieved to improve the precision of core physics calculation of the PB-HTGR.

In the neutronics calculation, NECP-Panda adopted the two-step method. The first step was based on Monte Carlo method to calculate the component homogenized group constants while the homogenization of the pebble bed was performed using collision probability equations in the second step, and the whole-core diffusion calculation was subsequently completed using the three-dimensional cylindrical geometric Nodal Expansion Method (NEM). In order to accurately account for the influence of neutron leakage effect on the group constants of the model region, the diffusion calculation and neutron leakage correction were iterated until the group constants of the region converged. In addition, the neutron streaming effect of the porous structures was corrected in the iteration, and the cavity at the top of pebble bed was treated specially in the diffusion calculation. Finally, the NECP-Panda was verified by both the simplified mixed pebble bed reactor and the High Temperature Reactor Pebble-bed Module (HTR-PM).

The numerical results show that the eigenvalue calculation of the simplified mixed pebble bed reactor, calculated by NECP-Panda, is close to the Monte Carlo continuous energy result. The calculated HTR-PM critical loading height of the HTR-PM is highly consistent with the results of Monte Carlo continuous energy calculation, and the absorber value is also in good agreement.

Verification results in this study demonstrate that NECP-Panda possesses exceptional computational power and accuracy for the neutronics calculation of the PB-HTGR, establishing a solid foundation for the development of subsequent modules.