A 3D CFD Model for the Study of Natural Circulation in the Pavia TRIGA Mark II Research Reactor

The TRIGA (Training, Research, Isotopes, General Atomics) MARK II reactor of the University of Pavia is a pool-type research reactor. One of the most challenging reactor feature to be considered in the modelling task is the natural circulation. In particular, there is no primary pump in the pool and the heat removal from the core is achieved by free convection. The buoyancy results from the difference in fluid density between the reactor core inlet and outlet. The characterisation of this phenomenon requires the modelling of the entire reactor including the pool in order to consider the total pressure drop along the core that establishes the equilibrium flow rate. In this work, we present a 3D thermal-hydraulic model of the TRIGA Mark II aimed at assessing the natural circulation capabilities of the reactor. In this light, a CFD (Computational Fluid Dynamics) approach is used, considering also the complexity and the asymmetric geometry of the reactor. The contribution of some components of the reactor core (i.e., the upper and the lower grids) to the pressure drop is modelled with a porous media approach, the latter tuned with a separate detailed model. As a major outcome, the model is able to provide reasonable values for the temperature within the core channels, which could be used to estimate the water mass flow rate induced by the natural circulation for each channel in the reactor.