TRIO_U analysis of natural convection in the upper plenum of the MONJU reactor

The IAEA is coordinating a benchmark project on natural convection phenomena in the Upper Plenum of the MONJU reactor. The JAEA has provided both detailed geometrical data of the plant and complete thermalhydraulic boundary conditions describing a pump trip transient, accomplished during the start-up experiments. For the initial conditions of the pump trip transient, extensive sensitivity analyses have been made with the CFD code Trio_U. These calculations show a high sensitivity of the global flow pattern in the MONJU Upper Plenum depending on the order of the numerical scheme and the modeling of the geometrically complex Upper Core Structure. During the pump trip, the formation of a thermal stratification within the plenum has been observed which persists for more than one hour. All calculations with Trio_U have shown a homogenization of the temperature in the plenum after about 15 minutes. A slight reduction of the mixing in the Upper Plenum could have been achieved by modifying the form of the flow holes in the Inner Barrel (fillets instead of sharp edges) in order to reduce their axial pressure loss. (author)