System Description of a Boron Dilution Test Facility (BOMIX-II) and Preliminary Test and Analysis Results

In the present study, a test facility called BOMIX-II that has 1/5 scale was constructed to investigate a boron mixing phenomena in a reactor vessel downcomer. The BOMIX-II test facility was designed to be adequate to examine a concentration distribution at a reactor inlet and turbulent characteristics of a mixing flow in the downcomer. Using the BOMIX-II test facility, preliminary experiments were performed on the single phase mixing phenomena in the downcomer, and the velocity profile of the mixing flow in the downcomer was investigated. Using a thermal-hydraulic safety analysis code, MARS, a mixing characteristics in the downcomer under the APR1400 MSLB accident was investigated for a two-channel downcomer model and a six-channel downcomer model. In addition, a reactivity response and a delayed arrival of a borated water to the reactor core were examined in accordance with the injection modes i.e., a cold leg injection mode vs. a DVI injection mode. A three-dimensional CFD analysis was carried out with the purpose of mutual comparison with experimental results. The CFD analysis was performed for the downcomer that has a scale ratio of 1/5 same to the BOMIX-II. The initial and boundary conditions were adjusted to the flow conditions of the preliminary experiments. Through the CFD analysis, the followings were investigated: a velocity profile and a three-dimensional flow structure of the mixing flow in the downcomer and a borated water concentration distribution at the reactor core inlet