Analysis of an ATLAS 6-in. cold-leg break simulation with MARS code

A transient analysis of an ATLAS (Advanced Thermal-Hydraulic Test Loop for Accident Simulation) 6-in. cold-leg break was carried out with the MARS (Multi-dimensional Analysis for Reactor Safety) safety analysis code in the frame of the Domestic Standard Problem exercise in Korea. The calculated major sequence of events of the 6-in. cold-leg break simulation showed good agreement with the measured data. The calculated break mass flowrate was predicted well, whereas accumulated mass of the break outflow was underestimated due to the underestimation of the break mass flowrate in a later phase. The general trends of the collapsed water level were well predicted in the core and the downcomer region. The loop seal clearing phenomena were observed at about 400 s in the 1-A and 2-B intermediate legs in the calculation results which is identical to the experiment. To investigate the emergency core coolant (ECC) bypass phenomena, variation of the boron concentration in safety injection water was adopted. In the loop seal clearing phase of the 6-in. cold-leg break, about 40-50% of the ECC bypass fraction was calculated and after the loop seal clearance, 20-30% of ECC water bypassed, that is, not participating in core cooling directly. The boron tracking approach is considered to be a feasible methodology with which quantify the ECC bypass flow in the early phase of a small-break loss-of-coolant accident (SBLOCA).