Study On The Interaction Between Sacrificial Concrete And Melt Corium (MCCI) In Nuclear Power Plants

With respect to the ever-increasing energy and environmental crisis, the nuclear power displays huge advances being more clean, stable and efficient compared to fossil fuels. In history, three severe accidents used to take place which brought the safety of nuclear power plants (NPP) into public focus. At present, the third generation NPPs have been developed that substantially reduce the leakage potential of radioactive materials in a severe accident. The corium can be stabilized in two ways:in-vessel stabilization or ex-vessel stabilization. In particular, the ex-vessel stabilization introduces a core catcher, such as the third generation EPR plant. The sacrificial concrete is an important component in the core catcher. For EPR, two types of sacrificial concretes are employed:the "FeSi" concrete and the siliceous concrete. The former one contains both siliceous and hematite aggregates, while the latter one contains siliceous aggregates. In the current study, specific "FeSi" and silicous sacrificial concretes are designed towards the investigation of MCCI in the third generation EPR.Firstly, tests were performed about the evolution of mechanical properties exposed to high temperature in conjunction with DSC, SEM and MIP. Secondly, the morphology and ablation behavior were examined for the sacrificial concretes that interacted with simulated high temperature melt. Moreover, to reveal the mixing and reactions in the melt pool, the XRF and XRD analysis were implemented on the solidified melt. Last but not the least, based on the computer program MELCOR, the theoretical analysis was applied to determine whether the concrete decomposition oxides were involved in the chemical reactions, including concrete ablation enthalpy, Fe2O3 and H2O contents. Some main conclusions could be drawn as follows:(1) As the temperature increased, the sacrificial concrete constantly decomposed and lost water. The increase of internal cracks and porosity resulted in the decrease of concrete compressive strength and ultrasonic velocity. In particular, the minor decrease was detected below 400℃, while the sharp decrease was detected above 400℃ due to the decomposition of Ca(OH)2 and C-S-H. At 800℃, around 30% compressive strength remained for F and S concretes. When the temperature approached to 1000℃, the concrete almost lost fully the load-bearing capacity. In addition, it was found that pathways emerged from the melting of PP fibers at 160℃ that facilitated to the release of water vapor and CO2. As a result, for concretes with PP fibers, the internal pressure was decreased and the residual compressive strength was higher about 10% compared to the concrete without PP fibers at 400～800℃. The concretes with fibers had less cracks and did not spall.(2) In the MCCI simulating experiments, the concrete crucible quickly cracked with gas released after the igniting of thermite powder. Due to the lower water to binder ratio and higher fly ash content, the siliceous concrete was more compact and less permeable than the "FeSi" concrete. Compared to the the "FeSi" concrete, the higher SiO2 content resulted in a larger volume expansion for the siliceous concrete, i.e., the crystal transition of SiO2 aggregates. Thus, the amount and maximum width of cracks in the siliceous concrete were both larger than those in the "FeSi" concrete. Compared with concrete without PP fibers, smaller values were detected for concrete with PP fibers, i.e., the amount and maximum width of cracks. Moreover, the cracks were tiny and not connected, which enabled the crucibles with fibers to maintain the integrity. It was concluded that one can enhance the concrete performance by increasing water to binder ratio within a reasonable range, controlling the content of fly ash and incorporating PP fibers.(3) The maximum ablation depth of the siliceous concrete was slightly larger than that of the "FeSi" concrete. The ablation rate of two concretes showed little difference with respect to the limited experimental time and specimens. Results from the XRF and XRD analysis on the solidified melt showed that the concrete decomposition products and the melt not only mixed well but also interacted that produced hercynite as well as other components.(4) If the oxides from the concrete decomposition took part in the oxidation-reduction reactions with the melt, the ablation rate and hydrogen generation rate were much larger than those in the case without the oxidation-reduction reactions. In particular, the mass of ablated concrete increased by 59.2%. The time to melt through the basemat concrete decreased by 14.46%. The density decreased by 4.97%. The temperature of the melt decreased by 60K. The heat released by chemical reactions played an important role during MCCI.(5) The higher the concrete ablation enthalpy resulted in the lower concrete ablation. In other words, with a higher concrete ablation enthaply, the time to melt through basemat concrete would be longer. Moreover, the hydrogen generation rate and the mass of ablated concrete mass were smaller while the density and temperature of the melt were higher. However, there was little difference in the mass of generated hydrogen.(6) Compared to the siliceous aggregates, the hematite aggregates have the lower enthalpy that would release more heat by oxidizing Zr. The concrete ablation rate and hydrogen generation rate increased while the melt temperature decreased faster when the Fe2O3 content increased. The time to melt the basemat concrete decreased with the increase of Fe2O3 content.(7)The effect of H2O content in the concrete on MCCI was closely related to the configuration of the melt pool. When LOX appeared in the melt, the concrete ablation rate, the hydrogen generation rate and the temperature of the MET increases rapidly because of the sharp increase of heat transmitted to concrete resulting from the quick decrease of heat to the surface. In most of the time melting through the basemat concrete, the concrete ablation rate decreased with the increase of H2O content. Compared with the concrete containing 2% water, the melt-through time and total hydrogen mass of the concrete containing 8% water were 0.48 and 2.6 times larger, respectively.