In this study, the most suitable diameter and pitch of fuel rods, according to neutron parameters, were determined by MCNPX code. Due to the fact that by decreasing the temperature in the fuel rod, neutron production increases in the fission process, a light water cell is created in the center of the fuel rod. The temperature reduction in the center of the fuel rod increases effective multiplication factor keff and reactor&#۳۹;s ultimate energy Efis. Temperature changes in the light water cell and in the fuel rod are analyzed theoretically and simulated by Ansys software. Finally, the infinite multiplication factor k∞, keff, the sum of the produced neutrons in the reactor, the total released fission energy per each fusion neutron and Efis examined in the selected design at temperatures of ۲۹۳ K, ۶۰۰ K and ۹۰۰ K for fuel and temperatures of ۲۹۳ K and ۶۰۰ K for light water cell.