核燃料元件包壳锆合金的LOCA行为及其机理研究

Zirconium alloys are an important structural material used for fuel cladding in nuclear pressurized water reactors. After the Fukushima nuclear power plants accident, the related behavior of zirconium alloys under loss of coolant accident (LOCA) condition is receiving more attention. During LOCA, the breakaway oxidation in high temperature steam and post-quenching brittleness of zirconium alloys are major issues. Now the corrosion resistance of commercial zirconium alloys under normal conditions has been improved greatly by adjusting chemical composition. So, the related performance of zirconium alloys under the LOCA condition can be also enhanced by adjusting chemical composition. In this project, the effect of chemical composition on the breakaway oxidation behavior in high temperature steam and post-quenching ductility of zirconium alloys to understand the relationship between the LOCA performance and chemical composition of zirconium alloys. The oxygen diffusion behavior during oxidation of zirconium alloys in high-temperature steam will be assessed by combining experiments and theoretical calculation to establish diffusion equation of oxygen. The effect of chemical composition on the microstructural evolution, β phase morphology and redistribution of alloying elements in zirconium alloys after quenching followed by oxidation will be also studied. Based on these investigations, the mechanism on the relationship between the LOCA performance and chemical composition of zirconium alloys will be illustrated. These results will provide theoretical evidence and guide in developing advanced zirconium alloys with excellent performance under both normal and LOCA conditions.

锆合金是水冷核反应堆中用作核燃料元件包壳的一种重要结构材料。福岛核事故后，锆合金包壳在失水事故（LOCA）下的行为引起了更大关注。在LOCA过程中，锆合金包壳在高温蒸汽中的失稳氧化和淬火脆性是导致其脆化的主要原因。现已商用的锆合金在正常工况下的耐腐蚀性能通过调整合金成分还有很大的改进空间。那么，通过调整合金成分也可以改进锆合金在LOCA条件下的性能。本申请项目拟通过研究锆合金在模拟LOCA条件下高温蒸汽中的失稳氧化行为和氧化淬火后的塑性，认识合金成分对锆合金LOCA行为的影响规律。通过实验和理论计算相结合研究锆合金在高温蒸汽氧化过程中氧的扩散行为，建立氧的扩散方程，结合合金成分影响氧化后淬火过程中显微组织演化、β相形态和合金元素再分布规律的研究，揭示合金成分影响锆合金LOCA行为的机制，为开发正常工况下和LOCA条件下均具有良好综合性能的新锆合金提供依据和指导。

锆合金包壳在失水事故（LOCA）下的失稳氧化和淬火脆性是重点关注的问题。本项目设计了Zr-xSn-0.35Fe-0.15Cr（x=0.5~1.5，wt%）、Zr-0.75Sn-0.35Fe-0.15Cr-xNb（x=0~1.0）、Zr-xNb（x=0.5~1.5）和Zr-Nb-xM（M=Fe、Cr、Cu、Bi、Ge、Si）合金，通过研究高温蒸汽氧化行为和氧化-淬火后的残余塑性，结合合金元素对O在锆中扩散行为的影响和在模拟LOCA过程中锆合金力学性能演化的模拟研究，探究了合金成分影响锆合金LOCA行为的机理。结果表明：1）失稳氧化主要发生在1000℃，Zr-Nb-xM合金除Si外均会发生失稳氧化，而其余合金没有发生失稳氧化，认为与合金成分影响Zr基体与氧化物的相转变和ZrO2膜的特性有关。2）发生失稳氧化时锆合金的残余塑性明显降低；氧化-淬火后， xSn系列合金的残余塑性随Sn含量的增加先增大后减小；xNb系列合金的残余塑性随Nb含量的增加而下降；Zr-xNb合金的残余塑性随Nb含量变化差别不大；添加Cr、Bi、Cu使Zr-1Nb合金的残余塑性降低，而添加Fe或Ge可提高Zr-1Nb合金的残余塑性；xSn（x=0.5~1.2）、Zr-xNb和Zr-1Nb-xM(M=Fe、Ge)合金的残余塑性与参比Zr-4和ZIRLO合金的相当，而其余合金的残余塑性低于参比合金。3）在hcp-Zr中，Nb、Fe、Cr、Bi会阻碍O的扩散，Sn、Cu、Ge不会阻碍O的扩散；在bcc-Zr中，所研究元素都会阻碍O的扩散。（4）氧化程度高的模型中，原β-Zr层中倾斜方向的板条晶存在阻碍拉伸变形；α-Zr(O)层指状侵入区在拉伸结束时存在网状应变场分布，阻止进一步变形。研究结果可以为开发正常工况下和LOCA条件下均具有良好综合性能的新锆合金提供依据和指导。研发出了2种综合性能优良的锆合金。