Toward The Performance Regeneration Of Aged Nuclear Reactor Pressure Vessel Steel By Pulsed Electric Current

Energy crisis and environmental degradation are the bottlenecks restricting global economic development.Nuclear energy is widely used in military,industrial,medical and other fields,and it is the most promising future energy for sustainable development of human.Nuclear power plant is the most complex energy supply system,and the operation safety is the lifeline of nuclear power development.The reaction pressure vessel is the only key component that cannot be replaced in the whole service period of nuclear island due to its huge volume.Therefore,its design life determines the operation time of nuclear power plant.For the reactor pressure vessel in its service period,high flux neutron irradiation,high temperature and high-pressure environment will introduce some defects such as dislocation loops and nanoclusters into the materials,which will seriously damage the performance.If the comprehensive mechanical properties cannot satisfy the safety threshold of nuclear reactor operation,it means nuclear power plant shutdown.The radiation damage of reactor pressure vessel is the most important factor to limit the operation time of nuclear power plant,and the microstructure modification of damaged materials is an effective pathway to solve this problem.As an important method to improve material properties by adjusting microstructure,electropulsing has been paid more and more attention because of its advantages including high efficiency and energy saving.In this study,the damaged materials were obtained by ion irradiation and thermal aging to simulate the service environment of real reactor pressure vessel.The irradiation-induced dislocation loops and thermal aged nanoclusters were introduced into commercial reactor pressure vessel steel and copper-rich simulated steel,respectively.Electropulsing was used to control the dislocation loops and nanoclusters to repair properties.Based on the structure characterization,mechanical properties test,numerical calculation and Molecular Dynamics simulation,the microcosmic regulation mechanisms of multi-type,multi-scale and multi-configuration defects under electropulsing and their correlation with performance repair were studied,so as to realize synchronous improvement of microstructure and performance of damaged materials.In the early service period of reactor pressure vessel(the first 20 years),the generation of dislocation loops is the main cause for performance deterioration.In this study,the high-density dislocation loops were introduced into the commercial pressure vessel steel by heavy ion irradiation(Au2+ 6MeV),and then electropulsing was performed on the irradiated material.The results shown the hardening can be repaired rapidly(10min)at a lower temperature(450℃)for about 70%.More than 80%of dislocation loops were removed.Based on Laplace equation,Ohm’s law,electromigration and electrical free energy theory,a mathematical model was established for dislocation loop.The change of electrical free energy and the current distribution around the loop were calculated.The results shown that the additional electric free energy introduced by electropulsing can help the vacancies break through the repulsive force without a certain scale around the dislocation loop(>10b,where b is Burgers vector),making it easier for the vacancies to contact the dislocation loops.Meanwhile,electropulsing can reduce the activation barrier that used for combination of dislocation loops and vacancies,making it easier for dislocation loops to absorb vacancies.Under the electron-wind force,the random moving vacancies in the material will form a directional high-speed vacancy flow,and thereby increasing the collision probability between the dislocation loops and vacancies.Therefore,the dislocation loops can efficiently absorb more vacancies,resulting in rapid elimination of dislocation loops.In the middle and late service period of reactor pressure vessel(the last 20 years),the generation of nanocluster is another main reason for performance deterioration.In this study,the thermal aging embrittlement materials were obtained by smelting copper-rich simulated steel and increasing the thermal aging temperature to accelerate the precipitation of nanoclusters,and then electropulsing was carried out on them.The results shown that more than 90%of nanoclusters were quickly dissolved,and 60%～80%of damaged performance can be repaired at 430℃～450℃,while the processing time was only 3min～10min.Based on the distribution characteristics of nanocluster and electrical conductivity difference between nanocluster and matrix,a mathematical model was established to calculate the current density distribution around nanoclusters in different arrangement states.And then the dissolution behaviour of nanoclusters under electropulsing was investigated.The results shown that the current density is the highest in the gap between the closely agminated nanoclusters,which can accelerate the element diffusion between the adjacent nanoclusters,leading to interface blurring and eventual merger.However,solute elements in dispersed nanoclusters can only diffuse into the matrix.Therefore,the dispersed nanoclusters can only dissolve gradually under electropulsing until they disappear.To prove the sustainability and reliability of electropulsing as a performance repair method,the cyclic aging-pulsing process was carried out on the copper-rich simulated steel,and the performance evolution was clarified.Combined with structure characterizations,the relationship between the microstructure and the performance during cyclic aging-pulsing process was explored.The results shown that the ductile-brittle transition temperature of the 1000h+electropulsing sample is basically the same as that of the 100h sample,indicating the"performance clock" has reversed by almost 90%.Meanwhile,the hard embrittlement rate of the pulsed materials was delayed in the subsequent aging process,because the electropulsing reduced the density of dislocation lines,which are the preferred nucleation sites of nanoclusters.To clarify the correlation between performance and the interaction of electropulsing,nanocluster and dislocation line,pulsed tensile tests were performed on thermal aging samples.The results shown that the hard embrittlement of thermal aged materials can be significantly alleviated by a continuous applied electropulsing with low parameter.The dislocation density was the lowest in the pulsed tensile sample,because the electron-wind force introduced by electropulsing can promote the dislocation lines to unhook from the nanoclusters,weakening the negative effect of nanoclusters.Molecular Dynamics results also shown that dislocation lines are more likely to detach from nanoclusters under electric field.In summary,pulsed current technology provides a new method and idea for repair of nuclear reactor pressure vessel damage.