Numerical investigation of potential elimination of ‘hot streaking’ and stratification in the VHTR lower plenum using helicoid inserts

The helium-cooled, high temperature Next Generation Nuclear Plant (NGNP) and Very High Temperature Reactor (VHTR) with prismatic type cores are being designed to operate at reactor exit temperatures ranging from 873 to 923 K and 1123 to 1223 K, respectively. The helium flow velocity in the coolant channels of the core is ∼70 m/s. The high-temperature helium jets exiting the coolant channels impinge onto the bottom plate in the lower plenum (LP), causing “hot spots” (“hot streaking”) and stratification due to the absence of proper mixing and the obstruction caused by the graphite support columns. In order to minimize or eliminate hot streaking and enhance helium mixing in the LP, this work investigates using static, quadruple helicoid inserts at the exit of the coolant channels. These inserts introduce radial and azimuthal momentum flow components, which result in extensive entrainment and mixing of the surrounding gas in the LP, significantly reducing the impingement onto the bottom plate, thereby minimizing hot streaking and stratification. Results of parametric analyses and a comparison of the flow fields of helium free conventional and swirling jets, and of a convectional jet in cross flow are presented and discussed. The analyses with helium at 1273 K and the dynamic Smagorinsky turbulence model are conducted using Fuego, a 3D, finite element, incompressible, reactive flow, massively parallel code with state-of-the-art turbulence models developed at Sandia National Laboratories. The calculations are benchmarked successfully by comparing the numerical results with experimental data and semi-empirical analytical expressions.