Pre and Post Test Analysis of LBLOCA Late Reflood Phase in ATLAS Using RELAP5/MOD3.3

The code simulation capability for the KAERI (Korea Atomic Energy Research Institute) integral effect test facility, ATLAS (Advanced Thermal hydraulic Test Loop for Accidents Simulaion), has been assessed for LBLOCA (Large Break Loss Of Coolant Accidents) reflood test. IETs (Integral Effect Tests) using ATLAS facility for directing simulation of the prototype plant APR1400 LBLOCA reflood with the same control logics and accident scenarios have been performed in 2007. The experimental results show that Maximum temperature of heater rods is 722°C and the reflooding rate and quenching speed is ∼6.2cm/sec and ∼0.3cm/sec, respectively. Also, quenching time is about 650 sec from the beginning of reflood. The best-estimate code, RELAP5/MOD3.3 is chosen to examine the code prediction capability. In basecase analysis, initial safety injection water is not nearly injected into core through downcomer. As the result, the maximum rod temperature, 1400°C is nearly twice as high as the experimental data. And the quenching time is similar to the experimental data. We adjusted a hydraulic behavior in core and downcemer through the improvement of the adequate nodalization. However, the thermal behavior for maximum heater rod temperature in core still included the limitation. To adjust the thermal behavior of core heater, we have performed the several sensitivity analyses for heat transfer region in core. The results of sensitivity analyses showed that the dominant factor is the wall heat transfer model in film boiling region. Therefore, we developed and assessed the sensitivity analysis model to examine the effect of void fraction for thermal behaviors. As the result, void fraction within film coefficient correlation had a considerable effect on the temperature behavior of heater rods.