Study On Dissolution Characteristics Of Lead Sulfate And Strontium Sulfate In Borosilicate Glass Melt

High-level radioactive liquid waste（HLW）will be generated during the development and utilization process of nuclear energy.It is a hot and difficult topic for the safe immobilization and disposal of the HLW.Borosilicate glass is the preferred matrix for the immobilization of HLW in the world at present.However,a layer of"yellow phase"（mainly Na₂SO₄）usually forms in the glass surface due to its low solubility for sulfur."Yellow phase"can easily adsorb a certain amount of radionuclides(e.g.,¹³⁷Cs,⁹⁰Sr).In addition,the"yellow phase"is easy to dissolve in water and decompose and volatilize at high temperatures,which will affect the structure and physicochemical properties of the glass waste form.In this thesis,it is proposed that the SO₄^2-ions in the HLW are sequestrated as the form of Pb SO₄or SrSO₄by addition of Pb（NO₃）₂or Sr（NO₃）₂solution into the simulated HLW,and then borosilicate glass waste forms are prepared via melting method.Two base glasses with different compositions,namely barium-containing borosilicate glass（referred to as barium borosilicate glass）and barium-free borosilicate glass（CIAE-5）were chosen.The effects of Pb SO₄or SrSO₄content（0～8 wt%,calculated by SO₃）on the phase composition,microstructure,and leaching resistance of borosilicate glass were systematically studied.The dissolution characteristics and decomposition behaviour of Pb SO₄or SrSO₄in borosilicate glass melts were investigated at different temperatures（800～1150℃）.The Kissinger method and the Doyle-Ozawa method were used to calculate the decomposition activation energy of Pb SO₄and SrSO₄in the glass melt.The results are as follows:（1）Pb（NO₃）₂or Sr（NO₃）₂was used to pretreat the simulated HLW,and the SO₄^2-in the simulated HLW can be sequestrated as Pb SO₄or SrSO₄crystals.For Pb SO₄doped-barium borosilicate glass,the samples are amorphous when the content of Pb SO₄is less than or equal to 4 wt%;when the content of Pb SO₄is greater than or equal to 6 wt%,Ba SO₄crystal appears in the lower layer and a separate phase consisting of Ba SO₄,Li Na SO₄and Pb O exists in the upper layer.The product consistency test（PCT method）shows that the leaching rates of B,Si and SO₄^2-in the samples increase slightly with the increase of Pb SO₄content.For the sample with 6 wt%Pb SO₄,it is a uniform glass-ceramic at both 800℃and 900℃.It mainly contains Si O₂phase and a small amount of Ba SO₄and Pb SO₄phases at 800℃.When the temperature increases to 900℃,the amount of Si O₂and Ba SO₄phases increases,bulk-shape Ca Mg Si₂O₆crystals appear,and the Pb SO₄phase disappears.A white separated phase begins to appear on the glass surface at 1000℃.The erosion characteristics of Pb SO₄in glass melt were observed in situ by high temperature laser confocal microscopy.It was found that round Pb O crystals gradually grew up at 1000～1100℃,and shape of Ba SO₄crystals changed from blocks to strips.X-ray fluorescence spectroscopy（XRF）analysis shows that the sulfur content in the vitreous body of the samples remains basically unchanged at 800～1000℃,and the sulfur content decreases rapidly with the further increase of temperature.（2）For SrSO₄doped-barium borosilicate glass,the samples are amorphous when the content of SrSO₄is greater than or equal to 6 wt%,and a separate phase consisting of（Ba,Sr）SO₄with a small amount of Li Na SO₄exists in the upper layer.The product consistency test（PCT method）shows that the leaching rates of B,Si and SO₄^2-in the samples increase slightly with the increase of SrSO₄content.For the sample with 6 wt%SrSO₄,the glass-ceramic is white at 800～1000℃.When the melting temperature is 800℃,the sample mainly contains SrSO₄with a small amount of Si O₂phase.Si O₂phase disappears at 900℃,and part of SrSO₄transforms into（Ba,Sr）SO₄crystals while bulk shape Ca Mg Si₂O₆crystals appear.Only（Ba,Sr）SO₄phase exists in the samples at 1000℃,and white seperated phase appears in the samples at 1050℃.XRF analysis shows that the SO₃content retained in the barium borosilicate glass samples doped with 6 wt%Na SO₄,Pb SO₄and SrSO₄is about 0.96 wt%,0.86 wt%and 3.21 wt%,respectively at 1150℃.（3）For SrSO₄doped CIAE-5 glass,the samples are amorphous and have a uniform structure when the content of SrSO₄is less than or equal to 4 wt%.When the content of SrSO₄is greater than or equal to 5 wt%,SrSO₄crystal appears in the lower layer and a separate phase consisting of SrSO₄with a small amount of Li Na SO₄exists in the upper layer.The PCT static leaching experiment results shows that when the SrSO₄content increases from 0 to 8 wt%,the 28d normalized leaching rateof B element（LR_B）in the sample increases from 7.05×10^-3to 1.26×10^-2g·m^-2·d^-1,the 28d normalized leaching rate of SO₄^2-(LR_SO42-)increases from 1.63×10^-3to 2.41×10^-2g·m^-2·d^-1,the normalized leaching rate of Si element(LR_Si)does not change significantly(～10^-3g·m^-2·d^-1).For the sample with 6 wt%SrSO₄,a small amount of Si O₂crystals exists at 800～900℃,and they disappear at 1000℃.SrSO₄crystals appear at 1050℃and the white separate phases consisting of SrSO₄and Li Na SO₄crystals appear.With the increase of temperature,the upper granular SrSO₄crystals grow into bulk crystals,and the lower layer is an amorphous phase.The sulfur content retained in the samples decreases rapidly when temperature is about 1100℃,and the sulfur content retained in the glass at 1150℃is about 3.83 wt%.（4）The DSC curves at different heating rates show that for the barium borosilicate glass doped with 6 wt%Pb SO₄,the Pb SO₄decomposes at 691～723℃,and the SrSO₄decomposes at about 846～873℃to form lapis lazuli（Ba,Sr）SO₄crystals.With the increase of the heating rate,the position of the endothermic peak shifts to the higher temperatures.The Kissinger method and the Doyle-Ozawa method are used to calculate the Pb SO₄,SrSO₄and the reaction generate（Ba,Sr）SO₄crystal in barium.The decomposition activation energies in the borosilicate glass melt are 205.29 k J/mol,89.61 k J/mol and 833.38KJ/mol,respectively.The decomposition temperature of the SrSO₄crystal doped in the（CIAE-5）borosilicate glass melt is about 1006～1048℃.The activation energy of the SrSO₄crystal decomposition calculated by the Kissinger method and the Doyle-Ozawa method is 336.24 k J/mol.It is shown that the glass composition have a significant effect on the decomposition behavior of sulfates.