Research And Preparation Of Al-based Shielding Composites Having Gd And W Elements Used In Spent Fuel Storage

Nuclear power industry has developed rapidly in recent years.However,it also generates large amounts of spent fuels that have high neutron and y-ray radioactivity.The storage of spent fuels has limited the development of nuclear power and become an urgent problem which needs to be solved.The core of spent fuel storage is shielding materials,so the research and preparation of appropriate shielding materials will be the key to solve the problem of spent fuel storage.The novel Al-based Gd2O3-W metal matrix composites were designed according to the calculation of neutron absorbing and y-ray transmission rates and were synthesized by two process including cold isostatic pressing sintering and hot pressing sintering to overcome the shortcomings of existing neutron shielding materials used in spent fuel storage.The evolutions and reaction mechanisms during preparation process were investigated.The effect of different preparation parameters on the structure,thermophysical properties and mechanical properties of materials was analyzed.In addition,to ensure the safety of the materials in service,the irradiation and corrosion behaviors of the samples were studied,and their reaction mechanisms were discussed.During the material preparation process,ball milling made powder particles refine to nanometer scale,and the collision pressure made part of cubic Gd2O3 transform to monoclinic Gd2O3.Meanwhile,Al3Gd and Gd3Al5O12 were formed from the reactions between Al matrix and Gd2O3 during ball milling,which was further accelerated during sintering.Al12W phase was observed in sintered bulks after adding W particles.Added strengthening phases,including Gd containing particles and W containing particles,were uniformly dispersed in the sintered bulks.With the increase of sintering degree,the number of porosities decreased,whereas the size of the particles increased.Furthermore,various properties of the composites were measured,including hardness,compressive strength,thermal expansion coefficient,thermal conductivity and specific heat capacity,and the effect of material microstructure on macroscopic properties was analyzed.The results indicated that the comprehensive properties of the synthesized composites were better than those of the referenced Al-B4C.After preparation,the effects of neutron and y-ray irradiation on samples were simulated by Fe+ion irradiation.The irradiation defects of the new Al-based Gd2O3-W composites after ion irradiation were observed.After Fe+ion irradiation,the Al matrix region of the material was significantly amorphized,and a large number of irradiated cavities below 3 nm could be observed after amplification.The depth of the irradiation defects was almost the same as the simulation result.In addition,the corrosion behaviors of hot-pressed Al-based Gd2O3-W composites in boron lithium water environment at 40℃ and 300℃ were studied.The weight of the samples initially increased and then stabilized with an increase of corrosion time,corresponding to the rapid formation of flocculent oxides and the occurrence of pitting,respectively.The corrosion weight gain in a low temperature environment was two orders of magnitude smaller than that in high temperature environment.After 40℃ corrosion,smaller and more evenly distributed pitting could be observed visually on samples sintered with shorter time and lower temperature,whereas on samples sintered with longer time and higher temperature,the pitting was concentrated in the Al matrix part and their size was limited by the surrounding added particles.After corrosion,the oxide film formed on the Al matrix had an flocculent AlO(OH)outer layer which had a limited protective effect;and an inner layer was amorphous Al2O3.While on the Gd-containing particles,only a loose outer oxide layer was formed,and there was a diffusion layer inside the particle.When the sintering was more adequate,the material tended to undergo pitting.When the sintering was inadequate,the composites were uniform corroded.The Gdcontaining particles could inhibit the corrosion,whose effect varied with the distribution and the size of particles.The polarization curves showed that with the increase of the sintering temperature and time,the corrosion potential of the samples shifted to a negative potential and the corrosion current density decreased.Impedance results showed that the corrosion resistance of the samples first increased and then decreased with the increase of corrosion time;the corrosion resistance of the samples increased with the increase of the sintering temperature and time.Comparing the corrosion behavior of irradiated and un-irradiated samples,because the irradiation defects in the surface layer provided rapid diffusion paths for corrosion ions,the early stage corrosion of irradiated samples was more serious.In scanning micro-reference electrode tests,the potential peaks of the irradiated samples were higher.The electrochemical tests showed that the corrosion potential of the irradiated samples moved to negative potential,the corrosion current density increased and the corrosion resistance decreased.