Control rod history effect assessment in a soluble boron-free S-PWR

The standard deterministic neutronics model used for Pressurized Water Reactor (PWR) calculations is a two-level computational scheme. A detailed 2D assembly modeling is first used to generate homogenized and condensed neutron data for use in a 3D core calculation performed on coarse space and energy meshes. In a soluble boron-free small-sized PWR (SPWR), the control of the reactivity relies mainly on the insertion or withdrawal of the control rods during fuel depletion. However, the current computational scheme at CEA does not model the history effects due to control rods movement with burnup. Especially, an ARO (All control Rods Out) configuration is generally assumed to generate the fuel assembly cross section library. In this paper, the lattice capabilities of APOLLO3 code are used to compute the history effects induced by the control rod for soluble boron free S-PWR fuel assemblies. To account for the impact of burnable poisons on the history effects, two different fuel assemblies are studied. A decomposition methodology has been developed to analyze the macroscopic cross section biases. The results show some discrepancies in the evaluation of physical parameters such as reactivity, power peaking factors (F_xy), rod worth, isotopic concentrations for different insertion profile configurations of the control rods. For a control rod history of 20 GWd/t, the typical biases at rod withdrawal are about 1100 pcm (8%) on reactivity, -2400 pcm (7.5%) on rod worth and 6% on F_xy. (authors)