Research On Immobilization Of Radioactive Iodine Waste By Phosphate Binder

In this paper,the research results of radioactive iodine curing at home and abroad are summarized,and the evaluation methods and related research contents of phosphate curing materials at home and abroad are deeply investigated.On this basis,several phosphate binder systems suitable for curing zeolite containing iodine are designed and studied experimentally.Phosphate binder was used as the substrate to cure silver zeolite solidified with iodine by using a phosphate binder system made by high temperature solid phase method.The leaching resistance test of the prepared solidified body for 42 days was carried out by using GBT7023-2011 Standard Leaching Test Method for solidified body of Low and Medium Level Radioactive Waste.The concentration of each element in the leaching solution was determined by plasma emission spectrum-mass spectrometer.Based on the anti-leaching performance of the cured body,the cured body of phosphate binder was screened and the best preparation process was determined.Then,three kinds of solidified bodies containing Pb,Eu and Nd phosphate binders were selected for further study.The optimum selection of each factor and the degree of influence of each factor were obtained through the experiment combined with 42 days of anti-leaching test to determine the amount of phosphate binder added,the solid capacity of iodine and the amount of metal oxide added during the preparation of the curing system of three phosphate binder systems.The normalized leaching rates of the cured phosphate binder system on day 42 were Ag(10^-5 g·m^-2·d^-1),I(10^-4～10^-5g·m^-2·d^-1)and added metal oxide(10^-2～10^-3 g·m^-2·d^-1).The prepared phosphate binder curing zeolite containing iodine has good chemical stability.The phase and structure of the phosphate binder were analyzed by X-ray diffractometer and Fourier infrared spectrometer.XRD results show that most of the Ag I components did not undergo phase change during the curing process and still exist stably in the cured body,but some of the Ag I reacted with phosphate binder or metal oxide additive to generate new iodide.In addition,there is a small amount of zeolite crystal phase,which indicates that during the formation of the solidified body,the zeolite phase will participate in chemical reactions to a certain extent,resulting in the formation of Al₂Si O₅ crystal phase.The addition of metal oxide will not only react with Ag I to produce new iodide,but also promote the polymerization reaction of phosphate binder to produce pyrophosphate and metaphosphate chain structure,so as to improve the chemical stability of solidified body.The results of infrared analysis also show this result.X-ray fluorescence spectrometer was used to analyze and determine the composition of the solidified powder,and the iodine conversion rate of the solidified body was calculated.The iodine conversion rate of the cured body prepared by the selected formulations of the three phosphate binder systems is above 97%.This shows that the phosphate binder system has high curing efficiency for iodized zeolite and less iodine loss.Then,scanning electron microscope was used to analyze the microscopic appearance and element distribution on the surface of the solidified body.The whole sample prepared was relatively uniform,and there were defects in a few areas of the solidified body when the amount of phosphate binder was insufficient.The element distribution of the prepared sample is more uniform.