Study On Molten Core-concrete Interactions Under Severe Accident In Nuclear Power Plant

The melts falls into the reactor cavity,thus causing a series of new accidents phenomenon such as Molten Core-Concrete Interaction(MCCI)and external cooling of the melts.MCCI phenomenon is the main factor that threatens the integrity of containment in accidents.In recent years,domestic and foreign scholars have carried out a lot of experimental studies on short-term effect of scaling-down tests,conducted mechanical analysis on experimental phenomena and developed relevant numerical models.At present,numerical simulation such as integrated severe accident analysis system program is one of the most effective means to study the MCCI phenomenon and its threats to the containment.In this paper,three numerical models of classical MCCI experimental models were established by MELCOR 2.2.The coolability of the melts without considering decay heat,the contrast of the melting rate of typical limestone and silicate concrete in radial and axial directions,and the periodic formation and breaking of the hard shell on the upper surface of melts were studied.Combined with the experimental data,it is believed that the simulation prediction results have regularity and conservativeness,and the prediction results of melt rate,melt penetration depth and heat transfer between melts and cooling water are reliable.Then,the numerical model of AP1000 nuclear power plant was established,and the accident sequence of 3BR-1 superimposed with the failure of In-containment refueling water storage tank was set up,to study the three aspects of explosion risk,overpressure risk and direct melting risk,through which MCCI phenomenon threats the integrity of the containment vessel during the accident process.Finally,the influence of changing concrete type and cavity size on accident is studied.The results shows that after the melts fell into the cavity,the reaction generated CO and hydrogen.Under normal circumstances,hydrogen in the containment reached 4.0% of the lower limit of the ignition limit firstly and triggered the ignition device,causing instantaneous high temperature and high pressure.The production of combustible gas is closely related to the content of metal components.In this study,all metals were oxidized at 56,100 seconds,from when no more combustible gas was generated.At theWhen a serious accident occurs in nuclear power plant,where the core is melted,and the safety facilities fail to timely and effectively limit the molten material in the pressure vessel.moment of the breach,the pressure inside the containment reached a peak value of 0.4 MPa.After more than 20,000 seconds of decreasing pressure,it finally reached a stable value and maintained at 0.2 MPa.During this period,the partial pressure of water vapor decreased from0.271 MPa to 0.073 MPa,at a decrease of 70%,making a major contribution to the pressure drop in the containment vessel.At the cavity scale,the phenomenon of concrete ablation is mainly about axial.At 100,000 seconds,the melts melted through the concrete floor at a depth of 1.01 meters,posing no threat to the integrity of the containment vessel.By changing the design of the cavity,it is found that the difference between different concrete is mainly reflected in the amount of gas produced,and the melting penetration depth is relatively close.With other conditions being the same,it is found the larger radius of the reactor cavity,the greater heat transfer between the melts and cooling water,and the lower the penetration depth in the radial and axial directions.