Jet impingement model for system analysis code to enhance mixing behavior prediction in downcomer during DVI line break accident

Abstract

Advanced Power Reactor of 1400 MWe (APR1400) adopts a Direct Vessel Injection (DVI) system that injects Safety Injection (SI) water directly into the reactor vessel downcomer during accidents. As the DVI nozzles are directly attached to the reactor vessel downcomer, complex thermal-hydraulic phenomena occur in the downcomer region. Recently, in the International Standard Problem (ISP) No. 50 exercise, the phenomena of Emergency Core Cooling (ECC) water mixing in the upper downcomer during the DVI line break accident observed in Advanced Thermal-hydraulic Test Loop for Accident Simulation (ATLAS) test were highlighted in terms of the prediction capability of the system analysis codes. Thus, to validate the results from the ATLAS study independently, an additional experimental study was performed with an integral effect test facility, Seoul National University Facility (SNUF), to observe the mixing behavior in the downcomer during a DVI line break accident. According to the SNUF test results, the ECC water mixed vigorously in the downcomer annulus. However, the temperature difference in the azimuthal direction was predicted by a system analysis code, Multi-dimensional Analysis Reactor Safety (MARS). In the MARS calculation, the momentum flux terms are set to zero for the junction between the one-dimensional volume and three-dimensional cell of the MultiD component because the axial and radial velocities are marginal in the large three-dimensional region. However, if the nozzles are attached to the downcomer with a thin gap, the axial and radial velocities are significant when the incoming orthogonal flow through the nozzles impinges against the downcomer wall. It was necessary to consider the momentum flux terms induced by the impinging flow, and to do so, an appropriate jet impingement model, for incorporation in the system analysis code, MARS, was developed in this study. To develop the jet impingement model, Computational Fluid Dynamics (CFD) calculations were carried out, and the jet impingement model was formulated based on the CFD calculations under various conditions. The momentum flux term resulting from the jet impingement phenomenon was correlated with the diameter of the nozzle, downcomer gap size, and incoming flow velocity. This model was applied to MARS by considering the momentum flux term for the junctions connected to the cell of the MultiD component. The modified MARS incorporating the jet impingement model was validated with the test results from the SNUF and ATLAS, and the analysis results exhibited reasonable agreement with the test data.