Contribution to the assessment of CFD codes for in-vessel flow investigation

The present research aims at contributing to the CFD code assessment process for nuclear reactor applications, and particularly for the predictive analysis of the fluid dynamic phenomena occurring inside the reactor pressure vessel of a pressurized water reactor. The importance of such phenomena relies, for instance, on the influence that they can have on the spatial and temporal distribution of coolant properties (such as temperature or boron concentration) at the core inlet during certain accident transients involving perturbations of such properties with respect to nominal conditions; furthermore, in-vessel mixing phenomena can also affect the thermal interaction between coolant and pressure vessel during pressurized thermal shock scenarios. The contribution provided by this work consists in the proposal of a general and systematic methodology to be applied in the CFD code assessment for in-vessel flow investigations. Within the proposed approach, the relevant modelling issues are identified and discussed, so as to point out the main capabilities and limitations in the present state-of-the-art tools and methods. Then, the main steps of the code application procedure are described and discussed analytically, thus providing guidance for a quality-oriented use of the codes, and complementing the existing best practice guidelines for this specific problem. Furthermore, the research addresses the problem of the quantification of the accuracy for numerical predictions (both from CFD and integral codes) about coolant properties perturbations at the core inlet. As a result, a methodology is proposed based on a set of accuracy indicators, which can represent a means for judging whether the code results are sufficiently close to experimental data, once acceptance thresholds have been defined and the method has been completely assessed. The work is supported by extensive CFD code validation and application results obtained in the frame of several international research projects and co-operations, and by a continuous interaction with the involved International scientific community.