Crack Propagation Mechanism And Failure Probability Evaluation Of SiC Composites Cladding Under LOCA Accidents

Silicon carbide is one of the alternative accident-resistant fuel claddings due to its steam oxidation resistance,radiation resistance,high temperature resistance and corrosion resistance.The research at home and abroad usually evaluates the safety of silicon carbide cladding under conventional working conditions.However,in fact,after the loss of cooland accident,the core of the reactor will heat up rapidly in a short time,and great thermal stress will be generated inside the cladding,resulting in the growth of internal defects in the cladding material to form cracks.Once the crack penetrates inside and outside the cladding,the radioactive fission product will be released,and the cladding will lose its due role.Therefore,from the perspective of reactor safety,the research on the crack propagation mechanism of silicon carbide composite cladding under the loss of cooland accident and the failure probability evaluation of cladding materials at this time have more reference value.In this paper,the crack propagation of the silicon carbide composite cladding structure with the inner layer of silicon carbide ceramic matrix composite and the outer layer of monolithic silicon carbide is simulated under the loss of cooland accident.Firstly,the distribution of temperature field and stress field inside cladding under the loss of cooland accident is simulated by the finite element simulation software.Then,the crack initiation and propagation of cladding under the loss of cooland accident is obtained based on the extended finite element method,and the causes of crack growth are analyzed.Then,the influence of linear power density,internal pressure and initial temperature of the fuel on crack growth was studied,and the mechanism of the effect on crack growth was analyzed.Finally,combined with the improved Weibull failure probability calculation model,the failure probability of silicon carbide composite cladding under the condition of crack is calculated,and the influence of the above three factors on the failure probability is also studied.The results show that the thermal stress of cladding is concentrated at the interface between the two layers after the loss of cooland accident,and because the tensile property of the outer silicon carbide material is slightly lower than that of the inner composite material,cracks will appear from the outer material and expand to the outer side of the cladding.At the same time,with the temperature inside the cladding increasing first and then decreasing,the crack propagation mainly occurred during the temperature increasing period.After the temperature decreasing,the length and depth of crack propagation did not change.According to the research on the influence of three related factors on crack growth,it is found that the linear power density has a more obvious influence on crack growth.With the increase of linear power density,the number of cracks and the depth of crack growth increase.The increase of cladding initial temperature increases the number of cracks,which is because the thermal stress caused by the increase of cladding temperature increases after the initial temperature increases.At a low initial temperature,cracks were generated and expanded outwards,but the crack growth depth was basically the same.The change of internal pressure had little influence on the number of cracks and the depth of crack growth.This was mainly because,in the early stage of the water-loss accident,the thermal stress inside the cladding accounted for a relatively high proportion and became the maximum principal stress.The change of internal pressure mainly affects the mechanical stress inside the cladding,so the contribution to crack propagation is small.The study on the failure probability of cladding with cracks shows that the existence of cracks can significantly improve the failure probability of cladding at the initial stage of loss of cooland accident,but with the decrease of temperature,the influence of cracks on the failure probability also weakens.The results show that the linear power density has a significant influence on the failure probability through the thermal stress field,the number of cracks and the depth of propagation.The internal pressure has a slight influence on the failure probability in the initial stage of the loss of cooland accident,and becomes the main factor in the later stage.The initial temperature of cladding affects the stress field by changing the number of cracks.And then the failure probability is affected.