A Monte Carlo-Diffusion Two-step Analysis of a Soluble-Boron-Free SMR Using Centrally-Shielded Burnable Absorber

In light water reactors, there are three types of reactivity control methods: SB, burnable absorber (BA), and control rods (CRs). Without SB, the excess reactivity is mostly compensated by BA since the CR insertion can cause a massive axial power distortion and increase possibility of rod ejection accident. Therefore, an SBF system is only achievable with an effective BA design. To eliminate SB, a new BA concept, centrally-shielded burnable absorber (CSBA), was introduced and optimized in the center for autonomous small modular reactor research (CASMRR). In this paper, an SBF SMR loaded with the CSBA has been designed and analyzed with both the Monte Carlo (MC) Serpent code and a two-step Serpent-diffusion nodal procedure. In the two-step nodal analysis, the Monte Carlo Serpent code was used for the FA spatial homogenization and the COREDAX nodal diffusion code was used for 3D nodal calculation. In addition, thermal hydraulic (TH)-coupled neutronic calculation is also investigated using the 2-step Serpent-COREDAX procedure. This paper presents a preliminary investigation on the neutronic feasibility of an SBF ATOM core loaded with CSBA by using the MC-diffusion hybrid two-step procedure. The numerical results show a good agreement between Serpent and Serpent-COREDAX in terms of core depletion reactivity, power profiles, and the control rod worth.