The influence of the coolant flow fluctuations on the consequences of a hypothetical boron dilution event

The flow field in the pressure vessel of a PWR is of turbulent nature. Statistical fluctuations occurring under quasi-stationary and transient flow conditions influence the mixing of coolant with different quality. It can be assumed, that the resulting boron concentrations and coolant temperatures at the inlet of the fuel assemblies are influenced by these fluctuations, too. Experiments at the ROCOM test facility for a hypothetical boron dilution event following the inadvertent start-up of the first main coolant pump were carried out with the goal to provide boundary conditions for core calculations. The core behaviour is simulated with the reactor dynamic code DYN3D. The influence of statistical fluctuations of the coolant and its consequences are investigated. Several simulations are performed with different core boundary conditions based on average values, a single measurement and values which are the boundaries of given confidence intervals of the measurements. Considering the different simulations the influence of the turbulent fluctuations on the reactor power and its distribution cannot be neglected. It can be important for similar transients, if safety parameters are close to the given limits.