Study On Crystallization Behavior And Chemical Stability Of Zirconolite-based Borosilicate Glass-ceramics Solid Used In High Level Radioactive Waste

High level liquid waste produced by nuclear power reactor is destructive to the safety of the environment and health of human beings;the engineering development of the safe disposal of high level liquid waste is far less than the progress of the nuclear industry pushed by our nation;the growing amount of high level liquid waste has restricted the sustainable development of the nuclear industry.The history of us introducing,taking in and exploiting other experience has shown that this kind of using other experience and equipment would impose huge pressure on the economic development,and even worse,it would cause the mismatch between the methods of the solidification formulation of waste of China and the equipment and processing technology of foreign countries.Therefore,creating our own way that fits the solidification characteristics of high level liquid waste is imperative.In this paper,we aimed at the China's characteristics of high level liquid waste of power reactor,based on the former experience on glass solidification,borosilicate glass system was chosen as the basic glass,in which some oxides like CaO?TiO₂?ZrO₂ were added.The glass-ceramics waste solid with higher amount of waste capacity than glass solidification was produced by certain heat treatment processes such as coring and crystallization.Overall,there were three major aspects being carried out.First,the influences that the nuclear agents such as Cr₂O₃?P₂O₅?Fe₂O₃?CaF₂ have on the crystallization behavior of Zirconolite-based borosilicate glass-ceramics solidified body.Second,the effects that the amounts of the major ingredients like Na₂O/SiO₂?B₂O₃?Al₂O₃?CaO and Na₂O/SiO₂ have on the chemical stability.Finally,the influences of leaching time and melting atmosphere have on the chemical stability of solid.During the experiment,detection and analysis methods like differential thermal analysis?DSC?DTA??Microstructure analysis?SEM-EDX?,X-Ray Diffraction?XRD??Analysis of Trace Elements?ICP-OES?were used to represent the material used and the mechanism shown in the process.The results are as follows:?1?The introduction of Cr₂O₃ could effectively reduce crystallization activation energy,decreasing from 218 kJ/mol?without any nucleation agent?to 128kJ/mol.It can also contribute to the crystallization process by means of contributing to glass transition,getting simulated nuclide Ce which was originally in the glass into zirconolite crystal,and getting low valence ion like Al³⁺into the stable Crystalline structure with Ti⁴⁺at its compensation price??2?With the decrease of the ratio Na/Si,the chemical stability of simulated waste solid would decrease,then rise and finally decrease.The chemical stability of the sample would reach its peak point at S4?the ratio of Na/Si is 6:1?due to the instability of B;The chemical stability would rise with the increase of B₂O₃,when B₂O₃ was below 8.47 wt%,the founding of the material got harder,yet when it was beyond 11.47 wt%,B₂O₃ volatilized too much,there even appeared flapping.Therefore,the appropriate amount of B₂O₃ added would be between 8.47 wt%and11.47 wt%;When the molar ratio of CaO:ZrO₂:TiO₂ was 2:1:2,and CaO in the formula was 5.56 wt%,due to the precipitation of zirconolite crystal,it is the best time to solidify Ce;When the amount of Al₂O₃ added was 5.1 wt%,the leaching rate was the lowest and the chemical stability was the best.When the total amount of alkali metals was 8.98 wt%and the addition of Li₂O replacing Na₂O was 2 wt%,the“Mixed alkali”arose so that the chemical stability reached its lowest point.?3?The leaching rate of glass-ceramics was lower than the glass,and the difference became larger overtime.It was shown in the experiment that long-term chemical stability of glass-ceramics was superior in comparison with glass.Specifically,the drop of the leaching rate was relatively high in the first 7 days and grew lower during 7 to 14 days;after 14 days,the leaching rate of Ce slightly reduced,yet the overall status was rather stable.By comparing different kinds of reducing agents and by adding different amounts of reducing agents,it was found that the reducing atmosphere would serve the best to reinforce the solidification effect of simulated nuclide Ce by zirconolite.