Preparation And Thermal Behavior Of Sacrificial Concrete With Graphene Nanosheets And Molten Core Concrete Interaction

Nuclear power is a kind of low-carbon energy,and is also an important foundation for sustainable development of China’s future energy.Sacrificial concrete is a key component of European Pressurized Water Reactor which is the tepical representative of the third generation nuclear technique.The significant innovation of this technique is severe accident mitigation and security improvements of nuclear power plant by utilization of sacrificial concrete.Firstly,the effect of graphene sulfonate nanosheets on mechanical strengths,thermal expansion,and ablation behaviour of sacificial cement paste and mortar was studied,according to the initial mix design of sacrificial concrete.Based on these findings,the mixture of sacrificial concrete containing graphene sulfonate nanosheets was designed.Secondly,the thermal properties of sacrificial concrete with and without graphene sulfonate nanosheets were comprehensively investigated.Two new experimental apparatuses were developed and used to measure mechanical sterngths(compressive strength and splitting tensile strength)of sacrificial concrete during elevated temperatues exposure.Thermal physical parameters(specific heat,thermal conductivtiy,and thermal diffusivity)of sacrificial concrete with and without graphene sulfonate nanosheets were measured,and the variation mecanisms for the thermal physical parameters of sacrificial concrete subjected to elevated temperatures were exposed.Using ultrasonic testing technique,variation of ultrasonic pulse velocity propagation in sacrificial concrete subjected to different high temperatures was detected.According to definition of damage,the relationship between damage of sacrificial concrete and ultrasonic pulse velocity was derived,eventually concluding a correlation between the damage of sacrificial concrete and elevated temperatures that sacrificial concrete subjected to.In addition,the microstructure,porosity,thermal analysis,thermal expansion,ablation behaviour,and damage evolution of sacrificial concrete with and without graphene sulfonate nanosheets before and after exposure to various temperatures were systematically investigated.Thirdly,an experimental setup was designed to measure the real-time temperature and pore vapour pressure of sacrificial concrete during high temperature exposure,and real-time temperature and pore vapour pressure of sacrificial concrete exposure to high temperatures were determined simultaneously.According to those results,a visco-elastic thermal expansion model of sacrificial concrete was established by using a micromechanics approach.Finally,the effects of thermal conductivity and decomposition enthalpy of sacrificial concrete on the process of MCCI were studied throuth numerical simulation.The main conclusions are summarized as follows,1)The addition of graphene sulfonate nanosheets to sacrificial concrete could improve many properties of the material,and the mixtures of sacrificial concrete containing graphene sulfonate nanosheets were designed in the paper.The optimal amount of graphene sulfonate nanosheets was O.lwt.%(with respect to weight of binders),considering the mechanical strength,microstructure,and ablation velocity of sacrificial cement paste and mortar.2)New experimental apparatuses were developed and used to measure mechanical sterngths(compressive strength and splitting tensile strength)of sacrificial concrete during elevated temperatues exposure.The results suggested that the damage induced by high temperature in sacrificial concrete was recoverd slightly during cooling,and the mechanical sterngths(compressive strength and splitting tensile strength)after elevated temperature exposure could not represent the actual situation of sacrificial concrete.Consequently,the mechanical sterngths obtained in the paper could provide a better guidance for strength design of sacrificial concrete.3)Variation mechanism for specific heat of sacrificial concrete subjected to elevated temperatues was revealed via Debye theory,and the specific heat of sacrificial concrete subjected to elevated temperatures could be accurately predicted by the model established in the work.Variation mechanism for thermal conductivity and thermal diffusivity of sacrificial concrete subjected to high temperatues was exposed by heat transfer theory.In the range of 25 ℃～1000 ℃,there were two different heat transfer mechanisms for sacrificial concrete.During 25 ℃～600 ℃,heat transfer of sacrificial concrete was mainly due to the phonon,while the heat transfer of sacrificial concrete was by means of photon in the range of 600 ℃～1000 ℃.4)An experimental setup was designed to measure the real-time temperature and pore vapour pressure of sacrificial concrete during high temperature exposure,and real-time temperature and pore vapour pressure of sacrificial concrete exposure to high temperatures were determined simultaneously.The temperature and pore vapour pressure of different sacrificial concretes exposure to high temperatures were consistent,and the temperature-time and pore vapour pressure-time relationships were quadratic polynomial form and piecewise function,respectively.5)A visco-elastic thermal expansion model of sacrificial concrete was established by using a micromechanics approach.The results of model prediction agreed well with the experimental measurement,which indicated that the thermal expansion of sacrificial concrete under uniform temperature field could be accurately predicted by the model.6)An MCCI analytical model in severe nuclear accident was set up,and the effects of thermal conductivity and decomposition enthalpy of sacrificial concrete on the process of MCCI were studied throuth numerical simulation.With the increase of thermal conductivity or decomposition enthalpy of sacrificial concrete,the axial ablation rate calculated by the mumerical model was always decreased.Thus,the melt-through time of basemat should be extended,and the safety nuclear power plant could be improved in severe nuclear accident by increasing the thermal conductivity or decomposition enthalpy of sacrificial concrete.