Effects Of Post-irradiation Annealing And Re-irradiation On Microstructure In Highly Irradiated RPV Steel Studied By Positron Annihilation Spectroscopy

Reactor pressure vessel（RPV）is an irreplaceable key component in the primary circuit of a nuclear power plant（NPP）.It will be affected by high temperature,high pressure and neutron irradiation during long-term operation.So far,the first-generation nuclear power plants（Qinshan and Daya Bay NPPs）of China have been operated for more than 20 years.Their service time is getting closer to their design lifetime（40 years）.How to extend the lifetime of NPPs to 60 years or even more（80 years）has become one of the most realistic problems in the field of nuclear power engineering in China and in the world.The lifetime extension of nuclear power plants makes the RPV steel subjected to high-dose neutron irradiation.On the other hand,the high-dose irradiated RPV steel will restore the toughness after thermal annealing which can alleviate the irradiation brittleness of the RPV steel and provide an important practical method for the lifetime extension of nuclear power plants.At present,a large number of studies have shown that the reason for the irradiation-induced embrittlements of RPV steel with high-Cu and low-Cu content under normal service conditions mainly includes solute-atoms clusters,matrix defects and segregation of impurity elements at the grain boundary,carbide and matrix interface.Due to the strict control of the content of elements such as Cu and P in modern RPV steel,the effects of Cu-enrich precipitates and P segregations become weak,and the effects of irradiation-induced matrix damages will be prominent.Moreover,recent studies have confirmed that low-Cu content of RPV steels exposed to high-dose irradiation may lead to new sources of embrittlements,such as Mn-Ni-Si-enriched clusters or precipitates（i.e.,late blooming phase,LBP）.Once the LBP phases appear,they will cause a second hardening,which may lead to accelerated embrittlement of the RPV materials（i.e.,LBP effects）.Therefore,it is important practical significance to study the evolution of microstructures and mechanical properties of post-irradiation annealed and re-irradiated domestic RPV steel under high-dose irradiation conditions.In this dissertation,the combination of slow positron beam Doppler broadening,TEM,three-dimensional atomic probe tomography（3D-APT）and nanoindentation techniques were used to study the evolution of microstructures and hardening properties of high-dose proton and Fe¹³⁺ions irradiated domestic RPV steel（A508-3steel,Cu content:0.01 wt.%）at room temperature and post-irradiation annealed and re-irradiated A508-3 steel and Fe-Cu alloys（Cu content:0.05 wt.%and 0.1 wt.%）with high-dose proton irradiation under high temperature（290°C-the actual working temperature around RPV in reactors）,respectively.The main conclusions obtained are as follows:1.Slow positron beam Doppler broadening measurement showed that the same type of defects,i.e.,vacancy-type defects,including vacancies,vacancy clusters,H-vacancy complexes,and dislocation loops,are generated in the RPV steel irradiated by 110 keV,240 keV protons and 3 MeV Fe¹³⁺ions at room temperature,.The number density and size of vacancy-type defects in 110 keV and 240 keV proton-irradiated RPV steels increase with increasing dose,while number density and size of vacancy-type defects in 3 MeV Fe¹³⁺ions irradiated RPV steel are easy to reach saturation as the dose increases further.The TEM results showed that the number of the dislocation loops generated in 110 keV proton-irradiated RPV steel increases with the increase dose at room temperature,but the average size of the dislocation loops is basically unchanged.2.The nanoindentation measurement confirmed that hardening occur in RPV steel irradiated by 110 keV,240 keV protons and 3 MeV Fe¹³⁺ions at room temperature.Combined with the results of slow positron beam Doppler broadening and TEM,the vacancy-type defects are the main reason for the hardening of 110 keV,240 keV protons and 3 MeV Fe¹³⁺ions irradiated RPV steel at room temperature.The hardness of 110 keV and 240 keV proton-irradiated RPV steels increases with the increase dose;when the RPV steel irradiated by 3 MeV Fe¹³⁺ions,the hardness（less than 0.35 dpa in the experiments）increase with the increase dose under low-dose conditions,but the hardness is basically unchanged under the high-dose conditions,which is mainly attributed to the fact that the vacancy-type defects generating in the high-dose Fe¹³⁺ions irradiated RPV steel are substantially saturated.3.Combined with the results of slow positron beam Doppler broadening,TEM and 3D-APT,two types of defects are produced in highly proton（240 keV）irradiated RPV steel under high temperature,these two types defects include vacancy-type defects（vacancies,vacancy clusters,H-vacancy complexes,micro-voids and dislocation loops）and Mn-Ni-Si-enriched clusters.After post-irradiation annealing（500°C）,the vacancy-type defects and some Mn-Ni-Si-enriched clusters recovered substantially,but a small amount of stable Mn-Ni-Si-enriched clusters remained.The types of defects generated in highly re-irradiated RPV steel are essentially the same as those of the initial irradiation.The TEM results showed that the average size of the dislocation loops generated in the initial and re-irradiated RPV steels is substantially the same,and the number of dislocation loops increases as the increasing of re-irradiation dose.The hardening behavior of high-dose irradiated（initial irradiation and re-irradiation）domestic RPV steel under high temperature is attributed to vacancy-type defects and Mn-Ni-Si-enriched clusters.4.Vacancy-type defects（including vacancies,vacancy clusters,Cu/H-vacancy complexes and dislocation loops）and Cu-rich clusters are produced in high-dose proton-irradiated（including initial irradiation and re-irradiation）Fe-0.05Cu and Fe-0.1Cu alloys under high temperature.The number density and size of the Cu-rich clusters increase as the increasing Cu content in the Fe-Cu alloys.Under post-irradiation annealing at 500°C,the defects in the Fe-0.05Cu alloy recovered substantially,while a small amount of Cu-enriched clusters still existed in the Fe-0.1Cu alloy.After re-irradiation,the S parameter decreases with the increasing dose,indicating that the vacancies inducing by 240 keV protons re-irradiation under high temperature are combined with Cu and H atoms to form a large number of Cu/H-vacancy complexes in the Fe-Cu alloys,resulting in a decrease in the annihilation probability of positrons in the vacancies,thus reducing the S parameter.The nanoindentation results showed that the vacancy-type defects and Cu-enriched clusters lead to the hardening of high-dose proton-irradiated Fe-Cu alloys under high temperature.5.Defects in RPV steel irradiated with high-dose are basically recovered after annealing,which is to make its toughness restore,and the re-irradiation behavior is similar to the initial irradiation.Therefore,RPV steel recovery annealing is planned as one of the methods to extend the service lifetime of Chinese domestic nuclear power plants.