The Study On Oxidation Behavior And Thermal Conductivity Of SiC Encapsulated Nuclear Feul Particle

Tristructural-isotropic?TRISO?particle was developed for the application in high-temperature gas-cooled reactor,which is being widely studied in many countries throughout the world due to its intrinsic safety and thermal efficiency.TRISO particle of¹ mm diameter consists of a spherical fuel kernel and four coated layers,namely a porous carbon layer?Buffer?,an inner pyrolytic carbon layer?IPyC?,a silicon carbide layer?SiC?and an outer pyrolytic carbon layer?OPyC?.TRISO particle is the basic component of fuel element,which is designed not only to contain the nuclear fuel and prevent the release of radioactive fission products,but also to transfer thermal effectively.Therefore,the behavior of TRISO particle under normal operating conditions or in case of accidents is of great importance for the safety of the reactors.The high-temperature gas-cooled reactor can operate at a high temperature with fuel temperature at around 1200? in normal conditions.However,under complex in-service conditions,especially air/steam ingress accidents,the oxidation behavior and fracture strength of TRISO particle would influence its ability to restrain fission products.Besides,for high operating temperature environment,the temperature distribution within TRISO particle is determined by the thermal conductivity of the coated layers in TRISO particle,which affects the thermal transfer efficiency and the failure of TRISO particle so on.Furthermore,it is also a particularly significant challenge to enable TRISO particle with a new fuel system design to achieve higher temperature and fuel burnup in further advanced reactor.The main content and progress of this work is shown below:Firstly,to study the safety of TRISO particle in case of air/steam ingress accidents,the oxidation of the SiC layer of TRISO particle at 900?¹400? in air and steam environment was investigated.Compared with air environment,the presence of water vapor promotes the crystallization of the amorphous SiO₂.The formation of cracks in the SiO₂ and even spallation of the SiO₂ could occur due to the crystallization or phase transition of SiO₂.The damaged integrity of SiO₂ resulted in an acceleration of the oxidation rate with the formation of pores and carbon.The oxidation of SiC fellow the linear-parabolic law in air environment.However,the linear-parabolic oxidation law of SiC transferred to parabolic law at 1200?in steam environment.It was proposed that the rate-controlling step of the oxidation was the diffusion of oxygen or water vapor molecules in the oxide scale.Both the fracture strength and Weibull modulus decreased with the increase of the oxidation temperature,which were related with the outer radius to thickness ratio of SiC layer.Secondly,to analyze the thermal efficiency and safety of TRISO particle under high operating temperature,Raman spectroscopy was first proposed to determine the thermal conductivity of each layer in TRISO particle.Based on the laser heating theory,the 3D-model of Raman method related with thermal conductivity was established,in which laser acted simultaneously as an excitation source and a heating source.The laser spot,the laser probing volume and the absorptivity of laser were evaluated.The calculated thermal conductivities were 8.9±0.2 W/m·?,13.9±1.5W/m·?,and 11.9±0.9 W/m·? for the Buffer,IPyC and OPyC layer,respectively.The differences in the thermal conductivity of PyC layers with low texture were originated from the microstructure,such as porosity,particle size and defects.The 3D-simulations of temperature distribution in materials induced by laser irradiation were performed using Matlab.It indicated that the temperature distribution area in SiC material exceeded the area of SiC layer in TRISO particle.Therefore,it can be assumed that the neighboring PyC layers would constrain the thermal conduction in the SiC layer.Besides,both the limited localized temperature rise and the larger laser probing volume influenced the thermal conductivity measurement of SiC layer.Finally,to further ensure the safety of TRISO particles under more complex in-service conditions and higher operating temperature,the fabrication of the fully ceramic microencapsulated pellet?FCM?with TRISO particles embedded in SiC matrix was proposed.It takes the advantages of SiC,such as excellent thermal conductivity and irradiation resistance.FCM was fabricated using spark plasma sintering?SPS?method.The effects of SPS conditions on the microstructure,mechanical property and thermal conductivity of SiC and FCM was systematicly studied.SPS technique can achieve high densification at a much shorter time,thus minimizing the microstructural degradation in TRISO particle induced by the thermal experience.During the fabrication of FCM,the reactions at the interface of the OPyC layer of TRISO particle and the SiC matrix occured,resulting the formation of porous SiC due to the trapped gas products.Based on Maxwell-Eucken model,the effective thermal conductivity of TRISO particle with/without OPyC layers was calculated to be 14.4 W/m·? and 25.2 W/m·?,respectively.The distribution of heat flows was also simulated using Comsol multiphysic,suggesting the SiC layer plays a dominant role on the effective thermal conductivity of TRISO particle.However,both the OPyC layers and the defects at OPyC/SiC interface hindered the heat flows through the SiC layer,resulting in a lower thermal conductivity of FCM.In contrast,the FCM material that consisted of TRISO particles without OPyC layers exhibited good contact at the interface between the SiC matrix and the SiC layer,showing a higher thermal conductivity.Within this work,the oxidation mechanism of SiC layer in air/steam environment and the evolution of fracture strength of SiC layer was revealed.The 3D-model of Raman spectroscopy related with thermal conductivity was proposed,by which the thermal conductivity of each layer in TRISO particle was determined.This work provides the guidance not only for assessing the safety of the TRISO particle in case of air/steam ingress accidents,but also for predicting the thermal efficiency of TRISO particles.Furthermore,FCM was fabricated using SPS method,which takes the advantages of both TRISO particle and SiC with the high thermal conductivity,irradiation resistance and intrinsic safety.The thermal conductivities of both FCM and TRISO particle were investigated.It's also of great significance for the accident tolerant FCM fuel to promote TRISO particles to be applied in advanced reactor systems for higher temperature and higher fuel burnup.