The goal of this work is to optimize the hybrid (γ, n) and (n, γ) transmutation system of Long-Lived Fission Products (LLFPs), particularly cesium and technetium, to stable or short-lived nuclei based on an electron accelerator by MCNPX code. Photons and neutrons produced by the interaction of electrons with high atomic number materials create a mixed field. In this study, design parameters such as geometry, materials, target dimensions, and configurations have been optimized to enhance the intensity of mixed field to increase the transmutation rate based on (γ, n) and (n, γ) reactions simultaneously. We have designed a system that uses the photons produced to transmute cesium based on the 135Cs (γ, n) 134Cs and 137Cs (γ, n) 136Cs reactions, and the neutrons produced to transmute the technetium based on the 99Tc (n, γ) 100Tc reaction. Furthermore, we have analyzed the temperature of the converter and heat transfer under Steady-State Thermal (SST) using ANSYS software. The designed system using a duct with a hydraulic diameter of 1.5 cm and 2 m/s for cooling water inlet velocity provides a suitable cooling process with a maximum converter temperature of about 337 ˚C.
