乏燃料棒M5锆合金包壳的透射电镜分析

TEM Examination of M5 Zirconium Alloy Cladding of Spent Fuel Rod

压水堆燃料元件的锆合金包壳,在服役期间会经受高中子注量辐照,其微观组织将发生很大变化,从而影响其宏观性能,因此锆合金包壳的中子辐照行为研究一直是核领域的研究重点。但由于材料经中子辐照后具有较强的放射性,相关的实验必须在热室内进行,因此针对辐照后燃料包壳微观组织的研究也一直是工作的难点。本文在中国原子能科学研究院热室设施上,通过透射电镜分析手段,研究了M5锆合金包壳材料中子辐照后的微观组织。样品来源于国内商业压水堆AFA3G型乏燃料棒,其燃耗分别为14 GW·d/tU和41 GW·d/tU。从燃料棒上截取长度约10 mm的包壳样品,在热室内完成去芯块与化学清洗,获得空包壳样品,然后通过机械制样方法,制备出?3 mm薄片状包壳基体样品,最后采用电解双喷减薄方法,制备出包壳透射电镜观察分析样品。另外,为对比锆包壳辐照后的组织变化,采用同样方法制备了相同材料的冷态观察分析样品。冷态样品与辐照样品的观察分析结果表明:冷态Zr合金包壳基体组织内部存在原生的第二相粒子,基体内部整体较为干净,纳米析出相稀少,未观察到明显的位错结构;辐照后,基体内原生的第二相粒子尺寸和分布与冷态样品差异不明显,但出现了明显的纳米析出相和高密度位错组织;随着燃耗的增加,纳米析出相尺寸有增加的现象;低燃耗与高燃耗样品位错组织具有相似性,表明在14 GW·d/tU燃耗下,锆合金包壳内由辐照产生的位错组织已基本趋于饱和状态;电子选取衍射结果表明,辐照后,基体内原生的第二相粒子虽存在一些非晶组织,但仍以bcc晶体结构为主,表明在41 GW·d/tU燃耗下,第二相粒子保持了一定的辐照稳定性;另外,第二相的EDS结果表明,随着燃耗的增加,Nb元素的含量有贫化趋势;分析认为,Zr合金经中子辐照,第二相粒子中的Nb原子扩展至Zr基体内,将促进Nb元素以纳米富Nb相形式在Zr基体中析出。

The zirconium alloy cladding of PWR fuel rods which undergo high neutron irradiation during service,will cause significant changes in its microstructure,thereby affecting its macroscopic performance.Therefore,the study of neutron irradiation behavior of zirconium alloy cladding is a focus of nuclear field.However,due to the strong radioactivity of materials after neutron irradiation,relevant experiments must be conducted in a hot cell.Therefore,research on the microstructure of irradiated fuel cladding is a difficult task.In this study,the microstructure of M5 TM zirconium alloy cladding material after neutron irradiation was studied by means of transmission electron microscope in the hot cell facility of China Institute of Atomic Energy.The samples were from commercial pressurized water reactor AFA3G type spent fuel rods with burnup of 14 GW·d/tU and 41 GW·d/tU,respectively.A cladding sample with a length of about 10 mm from the fuel rod was cut,and the defueling and chemical cleaning in the hot cell were carried out to obtain a clean cladding sample.Then,mechanical sampling methods was used to prepare a thin slice sample of the cladding with 3 mm diameter.Finally,the electrolytic twin-jet thinning method was used to prepare the cladding transmission electron microscopy observation and analysis sample.In addition,to compare the structural changes of zirconium alloy cladding before and after irradiation,the same method was used to prepare un-irradiated observation and analysis samples of the same material.The observation and analysis results of the un-irradiated and irradiated samples reveal that there are native second phase particles(SPPs)inside the matrix structure of the un-irradiated zirconium alloy cladding,and the overall interior of the matrix is with few nano precipitates and no obvious dislocation structure observed.After irradiation,there is no significant difference in the size and distribution of the native SPPs in the matrix compared to the un-irradiated sample,but significant nano precipitates and high-density dislocation structures appear.As the fuel burnup increases,the size of nano precipitates increases.The similarity of dislocation structures between low and high burnup samples indicates that under the burnup of 14 GW·d/tU,the dislocation structures generated by irradiation in the zirconium alloy cladding basically reach saturation state.The results of selected area electron diffraction(SAED)indicate that although there are some amorphous structures in the native SPPs in the matrix after irradiation,the bcc crystal structure is still the main structure,indicating that the SPPs maintain certain irradiation stability at the burnup of 41 GW·d/tU.In addition,the EDS results of the SPPs indicate that with the increase of fuel burnup,the content of Nb element tends to be depleted.Analysis suggests that after neutron irradiation,the Nb atoms in the SPPs of zirconium alloy expand into the Zr matrix,promoting the precipitation of Nb elements in the form of nano Nb rich phases in the Zr matrix.