Rapid Densification And Structure Optimization Of SiC_f/SiC Cladding Tube

Silicon carbide（SiC） is considered an ideal candidate material to replace the current zirconium-based alloy（zircaloy）used as accident-tolerant fuel cladding in light-water reactors.Highly pure,crystalline SiC exhibits excellent irradiation tolerance and good chemical stability at high temperatures.However,the brittle behavior of SiC monoliths makes them unsuitable for direct use as fuel cladding materials.Considering the operating conditions in light-water reactors,currently available SiC-based cladding materials have been mostly designed to be composed of multi-layered structures,with a continuous SiC fiber-reinforced SiC matrix（SiC_f/SiC）composite layer,enhancing their fracture toughness,and a monolithic SiC layer providing better hermeticity and corrosion resistance.The chemical vapor infiltration（CVI）method is a promising selection to fabricate the nuclear-grade SiC_f/SiC composites because it yields a high purity crystallineβ-SiC matrix at a low-temperature（1000℃）,which has the benefits of minimal process-induced damage to the fibers.However,inherent porosity（usually higher than 15%）of fabricated composites results in quite a low thermal conductivity.As LWR cladding materials,a high thermal conductivity is required to minimize the thermal stress and increase the thermal conversion efficiency.In order to overcome the inherent drawbacks of CVI and obtain a SiC_f/SiC cladding tube meeting the application requirements,we need to develop a suitable manufacturing technique to fabricate dense SiC cladding tubes.In this paper,SiC nanowires were introduced to change the valley-peak structure of the SiC fiber preform.The structure,thermal and mechincal properties of the SiC_f/SiC cladding tube were optimized to decrease its failure probability under service conditions.The main research contents and conclusions are listed as follows:（1）SiC nanowires were grown on the surface of SiC fabrics to optimize the structure of the 2D preform.The gradient distribution of SiC nanowires could not only decrease the closed pores inside the composite,but also narrow down of the size of pores,resulted in a density of 2.81 g/cm³ and total porosity of 2.7 vol.%.Due to high densification,the SiC_f/SiC composite exhibited outstanding thermal conductivity of26.7 W m^-1 K^-1 at RT.Moreover,the bending strength of SiC_f/SiC composite was up to 516±36 MPa.（2）A dense three-layer-NWs SiC cladding tube was obtained via the CVI process by introducing SiC nanowires on the graphite rod to change the valley-peak structure of tubular SiC preform.The SiC matrix could rapidly deposit on the inner layer SiC nanowires to form SiC sheath,voiding the delamination and valley-voids between layers,resulting in density of 2.85 g/cm³.The ability of three-layer-NWs SiC_f/SiC cladding tube to resist crack propagation increased due to tightly coupled structures of the three layers,leading to a damage mode of fiber/matrix debonding.This mode also increased the effective volume fraction of loading fibers and crack propagation path.As a result,the three-layer-NWs SiC_f/SiC cladding tube exhibited an average ultimate hoop strength of 290.6±53.6 MPa with a relatively high Weibull modulus of 11.55.（3）According to the service environment of the light water reactor cladding tube,the failure model of multi-layer SiC cladding tube was established.The finite element method was used to assess the failure probability of the cladding tube under the high burnup representative conditions,and the structure design of the low failure probability multi-layer cladding tube was proposed.The duplex SiC_f/SiC cladding consisting of an inner SiC_f/SiC composite,with a volume fraction of 35～45%,and an outer SiC monolith,with a volume fraction of 55～65%had lowest failure probability,which was～10^-8.The increase of the thermal conductivity of the cladding tube can reduce the overall stress,and resulted in decrease of the failure probability of the cladding tube.When the thermal conductivity of the inner composite material increased from 1.5 to3.0 W m^-1 K^-1,the failure probability of the cladding tube is only～10^-9.（4）The SiC_f/Ti₃SiC₂ composite was prepared by SPS process.Due to the existence of amorphous Si O_xCy,SiC fibers reacted with Ti₃SiC₂ matrix at 1250℃to form Ti Si₂ andβ-SiC.The infiltratedβ-SiC layer on the fibers prevented the interfacial reaction between the SiC fibers and Ti₃SiC₂ matrix and provided passage for the infiltration of Ti₃SiC₂ during the SPS process,resulting in a dense SiC_f/Ti₃SiC₂composite.By reducing the pore fraction,the SiC_f/Ti₃SiC₂ composite exhibited a thermal conductivity of 24.3 W m^-1 K^-1 at room temperature and 21.5 W m^-1 K^-1 at1000℃,only decreased by 12%.