Study On Hydrothermal Corrosion Behavior Of Silicon Carbide For Nuclear Reactors

Fully ceramic microencapsulated fuel(FCM)combines the advantages of the tristructural isotropic(TRISO)fuel particles and liquid phase sintering SiC(LPS-SiC),so that it has excellent safety and reliability,which has attracted wide attention in PWR applications.TRISO particles have a multilayer structure(a porous carbon layer,an inner pyrolytic carbon layer,a silicon carbide layer(SiC)and an outer pyrolytic carbon layer)to provide multiple barriers for the reactor.As a key component of the entire fuel particle,the behavior of SiC layer is related to the safety of the reactor.In addition,the LPS-SiC matrix plays an important role in thermal transfer and blocking fission products,so its reliability is also critical.Especially,under accident conditions(loss of coolant accident,cladding tube rupture accident,etc.),the hydrothermal corrosion of SiC layer of TRISO particles,the hydrothermal corrosion of LPS-SiC matrix,and the effect of hydrothermal corrosion on the fracture strength of SiC layer of TRISO particles,which will reduce their reliability.However,the research on the hydrothermal corrosion behavior of SiC material is limited,and the effect of hydrothermal corrosion on the fracture strength of SiC layer of TRISO particles has not been reported.Therefore,it is necessary to conduct a comprehensive research on it to provide guidance for the application of FCM fuel pellets in PWR.With this work,the corrosion behavior of SiC layer of TRISO particles in the high temperature and pressure water and its influence on the fracture strength of SiC layer were studied.Besides,the effect of sintering additives on the hydrothermal corrosion behavior of LPS-SiC bulks was investigated.Finally,the effect of grain sizes on the hydrothermal corrosion behavior of LPS-SiC bulks was investigated.The main contents and progress of this study are as follows:(1)Under the high temperature and pressure water(400?/10.3 MPa),the hydrothermal corrosion behavior of SiC layer of TRISO fuel particles and its influence on the fracture strength of SiC layer were studied.The results showed that carbon was formed on the surface,and the thickness of SiC layer decreased linearly with the increase of the corrosion time,indicating that the corrosion process of SiC layer was controlled by the chemical reaction(the reaction of Si O₂ formation).In addition,the hydrothermal corrosion was found preferentially occurred at the grain boundary of SiC,leading to the grain detachment and pit formation.The fracture strength of SiC layer after corrosion was evaluated using the crush test.The fracture strength decreased slightly with the increase of corrosion time,mainly due to the reduction of the thickness of SiC layer after corrosion.The results of this study demonstrated that the SiC in TRISO particles has good corrosion resistance in the hydrothermal environment.(2)Fully dense LPS-SiC bulks with Al₂O₃ and Al₂O₃+Y₂O₃ sintering additives were prepared by spark plasma sintering(SPS)and the effect of sintering additives on the hydrothermal corrosion behavior of LPS-SiC bulks was investigated in the high temperature and pressure water at 400?/10.3 MPa.The LPS-SiC specimen with Al₂O₃sintering additive exhibited a higher weight loss and followed a linear law.However,the LPS-SiC specimen with Al₂O₃+Y₂O₃ sintering additive exhibited a lower weight loss and followed a parabolic law.In addition,a deposited layer was formed on the surface of LPS-SiC specimen with Al₂O₃+Y₂O₃ sintering additive after corrosion,which can effectively protect the LPS-SiC specimen from further corrosion,and thereby improved the corrosion resistance of the LPS-SiC specimen with Al₂O₃+Y₂O₃ sintering additive.(3)LPS-SiC bulks with different grain sizes(40 nm?800 nm and 3?m)were prepared by SPS and the effect of grain size on the hydrothermal corrosion behavior of LPS-SiC specimen was investigated in the high temperature and pressure water at400?/10.3 MPa.The results revealed that the hydrothermal corrosion behavior of LPS-SiC specimen was strongly influenced by the grain size,and the corrosion resistance of LPS-SiC specimen increased with an increase of grain size.The LPS-SiC specimen with the smallest grain size(40 nm)lost all sintering additive Y₃Al₅O₁₂ and resulted in the formation of Al OOH and Y₂Si O₅ precipitates.The LPS-SiC specimen(800 nm)lost all Al element from the sintering additive Y₃Al₅O₁₂ and the Y element reformed into Y₂O₃blocky precipitate.The LPS-SiC specimen with the largest grain size(3?m)exhibited the best corrosion resistance,this can be attributed to the fact that the larger SiC grain had a more stable grain boundary and a lower grain boundary energy,this made them less susceptible to reaction with water.In addition,the sintering additive Y₃Al₅O₁₂was almost unchanged after corrosion,indicating that the larger SiC grain can improve the corrosion resistance of the sintering additive Y₃Al₅O₁₂.