Analysis of double-ended guillotine break at a direct vessel Injection line of ATLAS

Abstract

A double-ended guillotine break at a direct vessel injection line of the ATALS facility has been evaluated with the MARS-KS thermal hydraulic system analysis code. The results are validated against the ATLAS experimental data provided under the First Domestic Standard Problem for Code Assessment (DSP-01) project, indicating that the calculation can generally explain the important phenomena observed in the experiment. The major difference is that the simulation predicts a 2nd peak for the peak cladding temperature, which was not measured during the experiment. Further investigation for the core and downcomer liquid levels reveals that the 2nd peak happens because of incorrect prediction of the downcomer wall temperature caused by the lack of information for heat loss through the downcomer wall in the experiment. This is confirmed by a calculation with the downcomer wall temperature measured at the experiment modeled as a boundary condition. The results with modified downcomer wall boundary conditions show a good prediction of the PCT behavior, indicating that additional heat loss measurements at the downcomer are required for a better understanding of the complex phenomena occurring in the ATLAS downcomer.