용융물-냉각수 상호작용에서 제트 분열에 관한 수치해석적 연구

Abstract

The potential risk of fuel coolant interaction (FCI, steam explosions) in light water reactor severe accidents has drawn substantial attention since the investigation of TMI-2 accident was reported. Although there was no sign of steam explosion occurrence in TMI-2, the contact of molten fuel and coolant heightened the concern on the potential risk of energetic steam explosions. Steam explosion is considered a critical issue in the reactor safety analysis because it can lead to serious damage to the reactor structures. During LWR's severe accident, molten core may fall in water pool in the form of jet thus the jet breakup is an important process during the FCI. The number of particles and sizes of particles produced in jet breakup are the main physical parameters because the strength of the vapor explosion depends greatly on the size of the particles produced in jet breakup. As for the jet breakup mechanism, it is generally acknowledged that the jet leading edge breaks up by the boundary layer stripping and the jet lateral surface breaks up by the Kelvin-Helmholtz instability. Many computational studies have been conducted over the years to analyze the two jet breakup mechanisms and they have succeeded in producing general results such as breakup length and qualitative shape changes as a function of jet diameter and jet injection velocity. But previous studies failed to explain the apparent role of the mechanisms. In this study, numerical simulations of the jet breakup mechanisms have been carried out using ANSYS Fluent code. The boundary layer stripping was calculated in three dimensional domain and the Kelvin-Helmholtz instability in two dimensions. Two mechanisms are mainly analyzed with quantitative information such as effects of grid size, particle size, and length of wave. Qualitative information such as boundary layer thickness and hydrodynamic characteristics are also discussed. The size of particles produced by boundary layer stripping is 1-3 mm and the size of particles produced by the Kelvin-Helmholtz instability is less than 1 mm when the relative velocity is 4 m/s. Through this study, qualitative and quantitative results of the jet breakup that were difficult to observe through experiments was possible, and the reasons why the theoretical relations that were commonly used were different from the experimental observations were explained. The particle sizes obtained in the simulations were generally agreed with experimental data.