The fast boron shutdown injection system in the CNA II pressurized heavy water reactor consists of a set of jets flowing through a very large moderator tank that contains an array of cylindrical coolant channels. The prediction of the turbulent jet mixing is required to determine an accurate distribution of boron inside the moderator tank. The boron distribution is used to calculate the multidimensional insertion of negative reactivity into the reactor during fast shutdown in a PARCS/RELAP5 model of CNA II.

A computational fluid dynamics (CFD) code is used to determine the distribution of boron in the moderator tank. The flow is analyzed with a porous-medium model based on volume-averaged momentum, turbulent kinetic energy, and turbulence dissipation equations. The additional source terms that arise due to the averaging must be constituted. The constitutive relations for the additional source terms that are implemented in the present model are (a) the drag force on an array of cylinders for the momentum equations and (b) the additional mixing effect of the cylinders, which results in the sources of turbulent kinetic energy and turbulence dissipation transport equations.

The CFD analysis is performed on a porous, axisymmetric domain. The CFD results are compared with data for the boron concentration distribution obtained in a scaled geometrically similar experiment, demonstrating the validity of the approach. Finally, based on the similarity of turbulent jets, the validated model is scaled up to prototypic conditions and inserted into the PARCS/RELAP5 model.