PIRT and V&V associated to an uncertainty quantification methodology for fluid thermal mixing

The operating conditions of nuclear reactor vessels do not allow an easy access to thermal-hydraulics parameters. Thus, only macroscopic parameters can give information on the thermal hydraulic state of the vessel. Developments and validations of Computational Fluid Dynamics (CFD) tools become crucial to have a local view of the flow corresponding to the measured macroscopic parameters. In order to ensure accurate simulations of such flows, a well-known step-by-step procedure has been applied to a fluid thermal mixing case. A first step, called the Phenomena Identification and Ranking Table (PIRT) approach, consists of identifying all the potential physical phenomena that contribute to the target flow. These phenomena are then ordered according to two main criteria: their contribution level and their degree of maturity in terms of modelling. Based on this step, verification test cases are identified for each selected physical phenomenon. They are simulated with the Computational Fluid Dynamics (CFD) tool ANSYS Fluent. Mesh convergence are systematically done on these test cases. A specific effort is made on phenomena that have been considered with a high contribution and a low maturity level in the PIRT step. Then, the objective of the Validation step is the improvement of the modelling and meshing choices by considering single and multiple phenomena effects in the same flow. For this step, experimental studies are conducted by researchers from the University of Poitiers* (France). An experimental facility has been designed with an increasing level of flow complexity. Hydraulic and concentrations of passive tracer measurements are planned. Finally, uncertainty quantifications methods are explored to quantify the discretization errors and the sensibility of the simulations on some parameters (geometry, boundary conditions, etc.). This paper gives an overview of the step-by-step methodology applied to a specific fluid thermal mixing problem with illustrated examples at each step.