Effects of temperature gradient and nonlinear neutron irradiation on the stress in nuclear graphite reflector

Nuclear graphite is an ideal material for neutron moderators and reflectors of nuclear power systems that bear a severe environment of high temperature (up to 1000 ℃) and accumulated neutron irradiation (up to 10²⁷n/m²). In service, the mechanical properties of the nuclear graphite considerably evolve with the unsteady coupled thermal-irradiation field, bringing out undesired internal stresses and deformations that potentially imperil the structural integrity and reliability. Although there is a large temperature gradient and nonlinear irradiation distribution in real working conditions, the majority of the open literature does not take these effects into consideration during the stress analysis. Herein, in order to provide a safe assessment of the structural integrity, we take the cylindrical IG-110 nuclear graphite reflector as a representative to numerically investigate the effects of temperature and irradiation gradient on the temporal and spatial variations of the stress field. Numerical analysis indicates that regardless of the magnitude of the temperature gradient and irradiation gradient, the maximum tensile stress of the whole structure is always achieved after fixed periods of operation and is located at the inner surface of the cylinder. However, a greater maximum tensile stress can be induced under an inhomogeneous temperature field of larger gradients, or a nonlinear irradiation field of smaller gradient factors. Compared with conventional analyses that ignore the effect of the thermal-irradiation gradient, our analysis renders a safe and conservative design for nuclear graphite structures.