(A) study on very-low-boron APR1400 core with 24-month cycle length

Abstract

A 24-month APR1400 core with a very low boron (VLB) concentration has been investigated for inherent safety and high-performance of large PWR in this study. The VLB operation is an essential feature for high-performance APR1400 which is able to do the passive frequency control operation. In this dissertation, the CSBA (Centrally-Shielded Burnable Absorber) is applied to realize a VLB operation for 24-month APR1400 core with a substantially-reduced reactivity swing during the cycle. This innovative design of the VLB APR1400 core includes the optimization of BA and fuel loading patterns as well as axial cutback for a 24-month cycle operation. In addition to CSBA, a secondary burnable absorber such as Er-doped guide thimble is also introduced for partial management of the excess reactivity and for control of local power peaking in fuel assemblies. In order to improve the neutron economy of the 24-month APR1400 core, two alternative radial reflectors are adopted in this study, which are stainless steel (SS) and zirconium dioxide ($ZrO_2$). The core reactivity and power distributions for a 2-batch equilibrium cycle are analyzed and compared for each reflector design. Numerical results show that the APR1400-VLB core can be successfully designed with an extended cycle length up to 24 months and the passive frequency operation is feasible with its very low boron feature. In addition to the core design with 24 months cycle, another APR1400-VLB core having a 22-month cycle is also proposed in this work. The 22-month APR1400-VLB core, which replaces 121 fuel assemblies for each refueling, can prevent discharge of once-burned spent fuel and satisfy current criterion on fuel rod maximum burnup limit. All multi-physics analyses are performed using the Monte Carlo Serpent 2 code and 3-D diffusion nodal code COREDAX with the ENDF/B-VII.1 library.