Complete Thermal-Hydraulic Modelling of the Pavia TRIGA Mark II Research Reactor for the Study of the Natural Circulation Regime

The TRIGA Mark II reactor at University of Pavia is a pool-type reactor mainly used for training and research purposes. Since its first criticality in 1965, several studies have been performed on it, mainly focused on neutronic analysis, and its coupling with a thermos-hydraulic one. However, little attention has been given to the latter so far. Indeed, a complete simulation of the reactor, including the non-active regions, the supporting grids and the reactor pool, has not been done yet. In this, one of the most challenging features that must be considered is the treatment of natural circulation, that is, the primary cooling mechanism of the reactor. In order to fully characterize the natural circulation phenomenon, the entire system, including the reactor pool, has to be modelled, to consider the overall pressure drop and thus to correctly evaluate the equilibrium flow rate. With this in mind, this work extends the study performed in [1], in which a 3D thermal-hydraulic model of the reactor core was presented. In order to fully assess the natural circulation capabilities of the system, the reactor pool is added to the previous model. A CFD (Computational Fluid-Dynamics) approach is used, considering the asymmetric geometry of the reactor. To simplify the model whilst retaining a faithful representation, the contribution to the overall pressure drop of the core supporting grids is modelled with a porous media approach, preliminary tuned with a separated detailed model of the two components. As a major outcome, such a model is able to provide reasonable values for the temperature field within the core channels, the overall mass flow rate within the reactor core, and to correctly simulate the natural circulation regime within the pool.