Feasibility of using Gd2O3 particles in VVER-1000 fuel assembly for controlling excess reactivity

Neutronics feasibility of using Gd2O3 particles for controlling excess reactivity and pin power peaking factor of the VVER-1000 fuel assembly has been investigated. The motivation is that the use of Gd2O3 in form of micro-particles would increase the thermal conductivity of the Gd2O3 bearing fuel pellet which is one of the desirable characteristics for designing future high burnup fuel. Neutronics calculations have been conducted for the fuel assembly with the Gd2O3 particles distributed randomly using the Monte Carlo neutron transport MVP code. The results show that the Gd2O3 particles with the diameter of 60 µm could control the reactivity similarly to the homogeneous distribution of Gd2O3 with the same total amount. The power densities at the fuel rods with Gd2O3 particles increase by about 11%, leading to the decrease of the power peak and a slightly flatter power distribution. The power peak appears at the periphery fuel pins at the beginning of burnup which decreases slightly by 0.9%. Investigation has been performed to reduce the pin power peaking factor by increasing the number of Gd2O3-dispersed fuel rods and optimizing the particle diameter. The results show that by using 18 Gd2O3-dispersed fuel rods (instead of 12 Gd2O3-bearing fuel rods) with the same total amount of Gd2O3 and the particle diameter of 300 µm, similar reactivity curve can be obtained as the reference one while the pin power peaking factor at the beginning of burnup is decreased by about 5%.