MARS best-estimate evaluation using ATLAS test data on 8.5″ cold leg break LOCA

Abstract

A MARS best-estimate evaluation was performed for 8.5″ cold leg break loss-of-coolant accident (LOCA) in the advanced thermal–hydraulic test loop for accident simulation (ATLAS) facility. The ATLAS system was modeled in one-dimensional manner and the break nozzle was modeled by a break valve using the Henry-Fauske critical flow. First, a best-fitted critical flow discharge coefficient was obtained by comparing the test data and the calculations especially on the pressurizer (PZR) pressure and integrated break flow. Then, for a trial calculation related to the counter-current flow limit (CCFL) phenomena for the three locations, all the same options adopted in the LSTF (large scale test facility) evaluation were used. No good agreements were obtained between the test data and the calculations on the core collapsed water level and core cladding temperature. Another approach for the CCFL options was selected: Wallis model for inlets of the SG U-tube and the SG inlet plenum bottom; Bankoff model for the fuel alignment plate (FAP). From trial calculations, best-fitted CCFL options were obtained by comparing the test data and the calculations on the core collapsed water level and core cladding temperature.