IMPACT OF CONTROL ROD HISTORY ON PHYSICS PARAMETERS IN A SOLUBLE-BORON-FREE SMALL MODULAR REACTOR

The standard neutronics model used for Pressurized Water Reactor (PWR) is a two-level computational scheme where a detailed 2D assembly modeling is first used to generate homogenized and collapsed neutron data for a 3D core calculation performed on a coarse space and energy mesh. In this context, the 2D assembly depletion calculation is performed assuming nominal conditions for the assembly depletion (i.e. nominal temperature and neutron spectrum). This assumption is considered to be valid for a PWR in which control rods are mostly extracted during fuel depletion. However, it is incorrect in the case of a boron-free Small Modular Reactor where the neutron spectrum is very dependent on the control rod insertion history. This deficiency of the current neutronics modeling to account for history effects results in an increase of safety margins and therefore a decrease of the core performance. The magnitude of the so-called control rod history effects is investigated in the present paper. For this purpose, the neutronics model has been modified to include an additional parameter to capture history effects at the core level. Current and improved neutronics models have been compared on a 3D core test case. The impact of control rod history on physics parameters such as core cycle length, 3D power distribution and local isotopic production and depletion has been evaluated and analyzed.