Numerical analysis of fragmentation mechanisms in vapor explosions

Fragmentation of molten metal is the key process in vapor explosions. Howev er, this process is so rapid that the mechanisms have not yet been clarifie d in experimental studies. In addition, numerical simulation is difficult b ecause we have to analyze water, steam and molten metal simultaneously with boiling and fragmentation. The authors have been developing a new numerica l method, the moving particle semi-implicit (MPS) method, based on moving p articles and their interactions. Grids are not necessary. Incompressible fl ows with fragmentation on free surfaces have been calculated successfully u sing the MPS method. In the present study, numerical simulation of the frag mentation processes using the MPS method is carried out to investigate the mechanisms. A numerical model to calculate boiling from water to steam is d eveloped. In this model, new particles are generated on water-steam interfa ces. A two-step pressure calculation algorithm is also developed. Pressure fields are separately calculated in both heavy and light fluids to maintain numerical stability with the water and steam system. The new model and alg orithm are added to the MPS code. Water jet impingement on a molten tin poo l is calculated using the MPS code as a simulation of collapse of a vapor f ilm around a melt drop. Penetration of the water jet, which is assumed in K im-Corradini's model, is not observed. If the jet fluid density is hypothet ically larger, the penetration appears. Next, impingement of two water jets is calculated. A filament of the molten metal is observed between the two water jets as assumed in Ciccarelli-Frost's model. If the water density is hypothetically larger, the filament does not ay,pear. The critical value of the density ratio of the jet fluid over the pool fluid is rho(jet)/rho(poo l) = 0.7 in this study. The density ratios of tin-water and UO2-water are i n the region of filament generation, Ciccarelli-Frost's model. The effect o f boiling is also investigated. Growth of the filament is not accelerated w hen the normal boiling is considered. This is because normal boiling requir es more time than that of the jet impingement, although the filament growth is governed by an instant of the jet impingement. Next, rapid boiling base d on spontaneous nucleation is considered. The filament growth is markedly accelerated. This result is consistent with the experimental fact that the spontaneous nucleation temperature is a necessary condition of vapor explos ions. (C) 1999 Elsevier Science S.A. All rights reserved.