多物理场耦合TRISO颗粒堆内行为研究

Research on In-pile Performance of TRISO Particle under Multi-physics Field Coupling Condition

三向同性燃料(TRISO)颗粒是高温气冷堆元件和弥散微封装燃料最核心的组成部分,在反应堆运行过程中,TRISO颗粒在辐照-热-力多物理场的作用下发生变形、产生温度梯度及颗粒内部裂变产物扩散等行为,为研究TRISO颗粒在高温气冷堆环境下的堆内行为,本文通过设置边界条件,定义燃料材料物性模型,建立了辐照-热-力耦合作用下TRISO颗粒的多物理场计算方法,应用三维有限元平台对TRISO颗粒的堆内行为进行分析。结果表明,TRISO颗粒核芯温度随核芯功率增大而增大,但相应的温度梯度绝对值变化较小;颗粒中疏松热解碳层(Buffer层)与内致密热解碳(IPyC)层产生间隙,且寿期末间隙尺寸随核芯功率增大而降低;TRISO颗粒中IPyC层受到较大拉应力,而SiC层只有在较高的核芯功率下,才会受到拉应力,且最大拉应力随核芯功率增大而增大,这导致高核芯功率下SiC层的失效概率达到2.2×10^(-6)。SiC层对^(110)Ag、^(90)Sr、^(137)Cs等裂变产物具有优良的包容能力,在寿期末,SiC层以外几乎不存在裂变产物,这验证了TRISO颗粒在堆内的安全性能。

The tristructural isotropic(TRISO)coated fuel particle is the most important part for high temperature gas reactors(HTGR)fuel element and dispersed microencapsulation fuel.Multi-layered TRISO particle possess excellent irradiation stability,fission production capacity and high temperature resistance.The in-pile performance of TRISO particle can affect the safety of reactor.Thus,the simulation of TRISO particle in-pile performance is important to forecast the safety of the particle.The performance of TRISO particle was calculated by employing the finite element software.1/8 sphere characteristic unit was established,the diameter of UO_(2)kernel was 800μm,and the thickness of buffer,inner pyrolytic carbon(IPyC)layer,SiC and outer pyrolytic carbon(OPyC)layers were set 100,30,40 and 30μm respectively.Three sides of the characteristic unit were set as symmetry condition,and the temperature was set on the outer side surface of the particle.The effect of the power on the performance of the TRISO particle was studied by defining the power of UO_(2)kernel.Deformation,temperature gradient and fission product diffusion occur in TRISO particle due to the irradiation-thermal-mechanical coupling condition during operation process.In order to investigate the in-pile performance in HTGR condition,the calculation method was established by setting boundary condition and defining the materials physical model.3-D finite element platform was employed to analyze the in-pile performance.The result indicates that kernel temperature increases with power,and the temperature gradient distinction among the TRISO particles with different power is small.The gap occurs between IPyC layer and buffer layer,and the gap size at the end of life decreases with the increase of kernel power.IPyC layer suffers relatively high tensile stress,which may cause the broken of IPyC layer.SiC layer suffers tensile stress when the kernel power reaches high value,and the maximum value of tensile stress increases with the kernel power.The failure probability of SiC layer reaches to 2.2×10^(-6).SiC layer can resist the diffusion of the fission product such as^(110)Ag,^(90)Sr and^(137)Cs.None of fission product appears out of SiC layer at the end of life which proves the safety of TRISO particles.PyC layers exert pressure on the SiC layer which decreases the tensile stress of SiC layer and the failure probability of SiC layer decreases significantly.As mentioned above,SiC layer possesses low failure probability and the excellent fission production capacity,and TRISO particle has preferable security features.