Numerical Simulation Of Steam Explosion Under The Condition Of Severe Accidents Of Nuclear Power Plants

Under the condition of severe accidents of nuclear power plants, interaction between melting fuel and coolant may lead to the phenomenon of steam explosion, which may threaten the integrity of containment structure after the generation of huge energy and pressure wave and cause the leak of radioactive fission product. As a result, it is necessary to study the phenomenon of steam explosion under the condition of nuclear power plant severe accidents and evaluate the negative effects of this accident scenario. Additional, due to the lack of relevant analysis codes developed by domestic institutes, it is indispensable to carry out the development of codes focus on the simulation of fuel-coolant interaction.The mechanisms involved in the study of steam explosion are numerical simulation. Firstly, three dimensional multiphase CFD code MC3D developed by IRSN (France) is applied to evaluate the negative effects of steam explosion accidents of 1000MW level nuclear power plant. Secondly, modification and development of code have been carried out based on the source program of two dimensional multiphase CFD code JASMINE-PRO. Finally, applicability analysis has been validated based on the modified code.Because of the stability, robustness and application of the relatively mature steam explosion physical model, MC3D is applied to simulate the process of steam explosion and evaluate the negative effects of 1000MW level nuclear power plant. A two dimensional geometry model with a relevant fine grid density is established firstly. The simulation results based on this model indicate that the process of premixing has a low level pressure variation with a long duration. On the contrary, explosion stage creates a high level and sharp peak pressure which threatens the integrity of the reactor cavity structure. In addition, the mass of the melting drops contacts the coolant all over the calculating cells has a significant effect regarding the magnitude of the impulse created by steam explosion.Considering the In Vessel Retention Ex-Reactor Vessel Cooling (IVR-ERVC) mitigation measurement of nuclear power plant severe accidents, sensitivity analysis concerning parameters such as geometry structure has been carried out based on a three dimensional model with a relevant rough grid density, which is simplified from the two dimensional model. The results indicate the influence of different sensitivity parameters impose on the negative effects of steam explosion. According to the sensitivity analysis, an input data set assumed to be under the worst condition has been organized to predict the maximum negative effects of steam explosion. Under this condition, the peak pressure created by the steam explosion is higher than normal conditions and its duration is longer, the impulse load on the containment structure is much higher. Consequently, the integrity of the reactor pit is more likely to be fail under this condition. Furthermore, a sampling analysis based on the three dimensional model indicates the probability distribution of the pressure load and impulse load.The modification and development of the code JASMINE-PRO is from the aspects of conservation equations and numerical methods. Firstly, the pressure correction equations are modified with first order upwind scheme, in order that pressure correction equations are matched with continuity equations. Secondly, the numerical method has been modified with PISO method, which uses the pressure correction equation twice and with this method the calculations can get a convergence much sooner. Finally, fragmentation model of melting drops has been modified according to the model introduced into MC3D. After these modifications, the applicability analysis indicates that the applicability of the code has been expanded, the stability and robustness have also been improved.