【作者】 林建波；

【导师】 余笑寒；

【作者基本信息】 中国科学院研究生院（上海应用物理研究所） ， 核技术及应用， 2014， 博士

【摘要】 熔盐堆是第四代核反应堆国际研讨会提出的六个―候选‖开发的核能系统之一，同时也是中国科学院战略性先导科技专项《未来先进核裂变能》的重要部分，具有核燃料可持续利用、热转化效率高、核废料少、固有安全性等多种优点，钍增值熔盐堆技术是解决未来核燃料短缺的重要途径。熔盐堆的主冷却剂是一种熔融态的氟化盐，众所周知，高温熔融氟化盐具有极强的腐蚀性。Hastelloy N合金作为熔盐反应堆中堆芯容器、熔盐回路管道和换热器等结构材料，与液态氟化盐燃料直接接触，高温、辐照以及强腐蚀所引起的Hastelloy N合金的失效问题是影响熔盐堆堆结构材料使用寿命的关键问题。本论文采用常规正电子湮灭寿命谱、扫描电镜、失重法等多种传统研究手段，结合近年来刚刚兴起的具有高探测灵敏度、高空间分辨本领和样品低损伤分析的第三代同步辐射光源的X射线分析手段，研究He+离子辐照后的Hastelloy N合金在熔盐（FLiNaK）中的耐腐蚀性。Hastelloy N合金的耐腐蚀性能与其微观结构关系紧密，研究Hastelloy N合金在反应堆中中子辐照下的微观结构变化可以对合金在反应堆中的失效行为给出评价，预测材料的使用寿命。由于反应堆内中子辐照实验研究具有周期长、耗费大、感生放射性和实验条件苛刻等不利因素，自1960年起，研究者采用其它粒子，如电子、质子或离子辐照，模拟中子辐照，研究合金的相关性能或机理，这种方法周期短、效率高、低耗费并且实验方案容易实施，近来越来越受到人们的关注。因此本文用He+离子模拟中子辐照，研究Hastelloy N合金辐照后的微观结构变化与在熔融氟化盐中的耐腐蚀性，从而模拟熔盐堆内中子辐照下的合金熔盐腐蚀，进而推测Hastelloy N合金在中子辐照环境下的耐腐蚀性能。采用4.5MeV的He+离子辐照Hastelloy N合金，用SRIM2008程序计算对应辐照剂量0He+cm2、1×1015He+cm2、1×1016He+cm2和5×1016He+cm2的峰值损伤分别约为0dpa、0.05dpa、0.5dpa和2.5dpa，用常规正电子湮没寿命谱仪研究辐照剂量对合金空位大小和浓度的影响。实验结果表明：随着辐照剂量的增加，短寿命谱参数1基本保持在100ps，辐照产生了很多空位型缺陷，导致晶格的不稳定性增加，从而引起空位不停地迁移和聚集，使得空位型缺陷的浓度参数I1不断地减小；随着辐照剂量的增加，间隙型位错的形成使得长寿命谱参数2有不断减小的趋势，而间隙型位错的密度参数I2不断增加，表明新产生的空位不断增加。纳米压痕实验表明：随着辐照剂量的增加合金表面硬度增加，并且随着深度的增加，硬度减小。采用浸入法将4.5MeV He+离子辐照后的Hastelloy N合金在700℃高温下进行500h的腐蚀实验，在上海光源硬X射线微聚焦线站进行微束X射线荧光元素分布分析，测得对应辐照剂量0He+cm2、1×1015He+cm2、1×1016He+cm2和5×1016He+cm2的腐蚀深度分别为24μm、34μm、43μm和55μm，说明辐照对合金的耐腐蚀性能产生了比较严重的影响，随着辐照剂量的增加，合金的耐腐性有下降的趋势，这主要由于晶界和位错作为元素扩散的快速通道，随着He+离子注入剂量的增大，位错逐渐增多，高温下元素从合金基体扩散到表面和熔盐中的速度加快，从而导致合金的耐腐蚀性变差。若腐蚀过程中混入了过多的氧气和水蒸气，腐蚀将进一步加剧。同时发现合金中元素Cr的腐蚀情况最为严重，而其他元素Ni和Mo不易流失，说明合金的腐蚀主要表现为元素Cr的流失。最后，总结了镍基合金的高温熔盐腐蚀机理，对合金组分、温度等影响因素对熔盐腐蚀的影响机制进行了阐述，并对He+离子辐照引起的Cr元素的扩散影响机制进行了研究。更多还原

【Abstract】 Molten salt reactor (MSR) is one of the six most promising Generation IVreactors in the International symposium on fourth generation nuclear reactor. Also itis an important part of the future advanced nuclear fission belonging to the strategicleading science and technology projects of Chinese academy of sciences. It hasincomparable advantages: fission fuel sustainable utilization, excellent heat transfercharacteristics, producing less long-lived wastes, inherent safety and so on.Thorium-based Molten Salt Reactor technology is an important way to supplylong-term nuclear energy for the future. It is well known that the primary coolant ofmolten salt reactor is a kind of molten salt with severe corrosion. Hastelloy N alloywas developed for high temperature structural materials, such as the core container ofmolten salt reactor, molten salt return piping and the heat exchanger. The structuralmaterials are directly contact with liquid fuel salt. The invalidation problems ofirradiated Hastelloy N alloy caused by high temperature, intense irradiation andsevere corrosion are the key issues for the service life of structural materials of MSR.In this study, we adopted a variety of traditional research methods, such aspositron annihilation lifetime spectroscopy, scanning electron microscopy,weight-loss method and so on. As the development of the third generationsynchrotron radiation source, synchrotron radiation X-ray analytic techniquesgradually grew up with some advantages, such as samples of low damage analysis,high detection sensitivity and high spatial resolution. It is well-known that thecorrosion resistances of Hastelloy N alloy is related to its microstructure, theinvestigation of the microstructure changes under neutron irradiation in the reactor onHastelloy N alloy can evaluate the invalid problems of alloy in the reactor, predictingthe service life of materials. Direct neutron irradiation testing is vitally important, butthe experiment condition is restricted due to some unfavorable factors, for instance,long cycle, high cost, harsh experimental condition and so on. Since the1960s,simulation irradiation researches have turned up by using the other particles, such aselectrons, protons and ions instead of neutrons to investigate the related performanceand mechanism of the alloy, in recent years, this method got more and more attentionfrom people with short cycle, high efficiency, low cost and easily implementedexperiment scheme. Therefore, this paper presents some investigation results on thecorrosion of Hastelloy N alloy after4.5MeV He+ion irradiation performed in moltenfluoride salt at700℃for500h to simulate the corrosion irradiation in reactors. Hastelloy N samples were irradiated by4.5MeV He+ions. The irradiation doseis0He+cm2、1×1015He+cm2、1×1016He+cm2、5×1016He+cm2，thecorresponding peak damage is0,0.05,0.5,2.5dpa calculated by SRIM2008code.The results showed that with dose accumulation I1constantly decreased, and1wasabout100ps from beginning to end. The helium irradiation produced many vacancytype defects. The instability in crystal lattice resulted in that the vacancies incessantlymigrated and gathered, which brought about the decrease of I1unceasingly. Withincreasing irradiation dose, I2increased, which implied that the new generatedvacancies increased constantly, which led I2to increase. The generated vacancieswould form voids. The decrease of2suggested the formation of interstitial clusterswhich kept the trend of decrease of2. The nano indentation experiments show thatwith the increasing irradiation dose the hardness of the alloy surface increases, and thehardness decrease with the increase of depth.Hastelloy N alloy irradiated by4.5MeV He+ions was performed in moltenfluoride salts at700℃for500h. Then the samples were analyzed using synchrotronradiation microbeam hard X-ray fluorescence analysis at BL15U1beamline station ofShanghai Synchrotron Radiation Facility. The results show that with the doseincreasing from0,1×1015He+cm2,1×1016He+cm2,5×1016He+cm2, significantdepletion of Cr rose from the depth of24μm,34μm,43μm and55μm, which showsthat the irradiation has a serious impact on the corrosion resistance of alloy. That isbecause that grain boundaries and dislocations act as quick paths for Cr elementdiffusion, the dislocations of Hastelloy N alloy increased with increasing irradiationdose, so that the Cr element diffusing out of matrix became fast, and then thecorrosion resistance became weak. If in the process of corrosion mixed with too muchoxygen and water vapor, and the corrosion will be further intensified. The element Crin the matrix suffered from the most serious corrosion, and other element Ni and Modidn‘t drain easily, which illustrated that the corrosion of ally was mainly due to theloss of element Cr.Finally, we summarized the corrosion mechanism of Ni-based alloys under hightemperature molten salt, and expounded the molten salt corrosion mechanism of thecomposition, temperature and other influence factors. Furthermore, we studied theinfluence mechanism of the diffusion of the element Cr caused by the He+ionirradiation.更多还原