Development and verification of the higher-order mode neutron flux calculation code HARMONY2.0

Higher-order modes of the neutron diffusion/transport equation can be used to study the temporal behavior of nuclear reactors and can be applied in modal analysis, transient analysis, and online monitoring of the reactor core. Both the deterministic method and the Monte Carlo (MC) method can be used to solve the higher-order modes. However, MC method, compared to the deterministic method, faces challenges in terms of computational efficiency and \(\alpha\) $\alpha$ mode calculation stability, whereas the deterministic method encounters issues arising from homogenization-related geometric and energy spectra adaptation. Based on the higher-order mode diffusion calculation code HARMONY, we developed a new higher-order mode calculation code, HARMONY2.0, which retains the functionality of computing \(\lambda\) $\lambda$ and \(\alpha\) $\alpha$ higher-order modes from HARMONY1.0, but enhances the ability to treat complex geometries and arbitrary energy spectra using the MC-deterministic hybrid two-step strategy. In HARMONY2.0, the mesh homogenized multigroup constants were obtained using OpenMC in the first step, and higher-order modes were then calculated with the mesh homogenized core diffusion model using the implicitly restarted Arnoldi method (IRAM), which was also adopted in the HARMONY1.0 code. In addition, to improve the calculation efficiency, particularly in large higher-order modes, event-driven parallelization/domain decomposition methods are embedded in the HARMONY2.0 code to accelerate the inner iteration of \(\lambda /\alpha\) $\lambda /\alpha$ mode using OpenMP. Furthermore, the higher-order modes of complex geometric models, such as Hoogenboom and ATR reactors for \(\lambda\) $\lambda$ mode and the MUSE-4 experiment facility for the prompt \(\alpha\) $\alpha$ mode, were computed using diffusion theory.