An approach to minimize reactivity penalty of Gd2O3 burnable absorber at the early stage of fuel burnup in Pressurized Water Reactor

Abstract

The high capture cross-section (σc) of Gadolinium (Gd-155 and Gd-157) causes reactivity penalty and swing at the initial stage of fuel burnup in Pressurized Water Reactor (PWR). The present study is concerned with the feasibility of the combination of mixed burnable poison with both low and high σc as an approach to minimize these effects. Two considered reference designs are fuel assemblies with 24 IBA rods of Gd2O3 and Er2O3 respectively. Models comprise nuclear fuel with a homogeneous mixture of Er2O3, AmO2, SmO2, and HfO2 with Gd2O3 as well as the coating of PaO2 and ZrB2 on the Gd2O3 pellet's outer surface. The infinite multiplication factor was determined and reactivity was calculated considering 3% neutron leakage rate. All models except Er2O3 and SmO2 showed expected results namely higher values of these parameters than the reference design of Gd2O3 at the early burnup period. The highest value was found for the model of PaO2 and Gd2O3 followed by ZrB2 and HfO2. The cycle burnup, discharge burnup, and cycle length for three batch refueling were calculated using Linear Reactivity Model (LRM). The pin power distribution, energy-dependent neutron flux and Fuel Temperature Coefficient (FTC) were also studied. An optimization of model 1 was carried out to investigate effects of different isotopic compositions of Gd2O3 and absorber coating thickness.