Study On Immobilization Of Simulated Actinide Nuclide By Basaltic Glass And Glass Ceramic

The treatment of high-level radioactive waste（HLW）has not been properly solved,among which the solidification treatment of HLW has attracted extensive attention of researchers.Stable natural basalt contains a small amount of radioactive elements,and has a good ability to form glass and glass ceramics,so it is speculated that the high level waste glass and glass ceramic solidified body prepared with basalt as the base material has a good stability.In this paper,based on basalt glass as raw material,Nd₂O₃,CeO₂ and La₂O₃ were used to simulate three and four valence actinide series nuclide and nuclide U respectively,and a Chinese dynamic waste as the object,according to the proportion of the simulated waste elements,the basalt glass and glass ceramics ceramic solidified by high temperature melting-heat treatment process.The the basalt glass and glass ceramics powders were measured by the X’Pert PRO diffractometer（XRD）,scanning electron microscopy（SEM）,Raman spectrometer（Raman）,differential scanning calorimetry（DSC）,X-ray photoelectron spectra（XPS）and inductively coupled plasma emission spectroscopy（ICP-OES）to collect their structure and performance data.The main contents are as follows:（1）The inclusion capacity and crystallization tendency of Nd₂O₃,CeO₂ and La₂O₃respectively and co-doped in basalt glass,the valence distribution of Ce in basalt glass,and the effects of different contents of Nd₂O₃,CeO₂ and La₂O₃ on the structure,thermal stability and leaching resistance of cured glass were studied.（2）Taking a power reactor as the object,five different heat treatment methods were used to prepare basalt glass ceramics.The heat treatment method with the best microstructure of the sample--"high temperature melting-cooling nucleation and crystallization"was selected as the standard to prepare Nd-Ce-La basalt glass ceramics solidified with oxygen apatite crystal as the main crystalline phase.Its structure and stability are analyzed.（3）Taking a power reactor as the object,the simulated waste element was added according to the proportion of the components of the high-level waste,and the whole component of the simulated high-level waste was solidified with basalt glass,to explore the effects of the basalt glass on the solid solution amount of the simulated waste and the influence of the content of the simulated waste on the glass network structure,thermal stability and leaching characteristics of the solidified body.Here are main conclusions were summarized as follows:（1）The inclusion capacity of Nd₂O₃,CeO₂ and La₂O₃ in basalt glass is not less than 30 wt%,18 wt%and 36wt%,but when the content exceeds the inclusion capacity,Ca₂Nd₈（Si O₄）₆O₂,CeO₂ and Ca La₄（Si O₄）₃O crystals will be precipitated,respectively;The thermal stability of glass cured body is proportional to the content of Nd₂O₃,and CeO₂;With the increase of the content of La₂O₃,the thermal stability of glass cured body increases first and then decreases;About half of the Ce⁴⁺ions in the cured basalt glass with 8 wt%～18 wt%CeO₂ were reduced to Ce³⁺ions;All the samples have good chemical stability and meet the requirements of China nuclear industry standard(<1 g m^-2 d^-1).（2）The inclusion fraction of basalt glass to the simulated actinide nuclide mixture is not less than 36 wt%;With the addition of the simulated actinide nuclide mixture,some Si-O-Si bonds in the glass network were destroyed,resulting in an increase of non-bridging oxygen in the glass network;About half of the Ce⁴⁺ions were reduced to Ce³⁺ions in the simulated20 wt%～36 wt%mixture of actinide nuclide in the basalt glass cured body,and the mean binding energy difference between Ce3d_3/2 and Ce3d_5/2 in the sample was about 92 ev;The crystallization kinetics calculation of the samples containing the simulated actinide nuclide mixture of 10 wt%～36 wt%shows that the crystallization tendency of the basalt glass is the highest in the sample containing 10 wt%and the lowest in the sample containing 32 wt%,and the crystallization mode of the basalt glass is mainly volume crystallization;The thermal stability of the cured basalt glass first increases with the content of the simulated actinide nuclide mixture increasing,and then decreases when the content exceeds 25 wt%;The glass with high simulated actinide nuclide content has better leaching characteristics than the glass with low simulated actinide nuclide content.The normalized leaching rate of rare earth ions（simulated actinide nuclide）is 4 orders of magnitude lower than that of Si and Ca ions.The normalized leaching rate of all samples after 56 days is lower than 9.53×10^-3 g m^-2 d^-1,so all samples have good chemical stability and meet the requirements of the Chinese nuclear industry standard.（3）Among the basalt glass-ceramic solidified bodies prepared by five heat treatment methods,the solidified bodies prepared by high temperature melting,cooling nucleation and crystallization show the best microstructure and the highest purity of main crystal phase;When the simulated actinide nuclide content is lower than 20 wt%,the main crystalline phases in the glass ceramics are diopside and plagioclase;when the simulated actinide nuclide content is more than 20 wt%,the main crystalline phase in the glass ceramics sample is oxyapatite,accompanied by a small amount of enlandite;About half of the Ce⁴⁺ions in the simulated actinide nuclide mixture content of 20 wt%～40 wt%basalt glass cured were reduced to Ce³⁺ions.The normalized leaching rate of all samples was lower than 6.96×10^-3g m^-2 d^-1 after 28 days,so all samples had good chemical stability.（4）The inclusion capacity of the whole component simulated high-level radioactive waste in basalt glass is not less than 25 wt%.When the whole component simulated high-level radioactive waste reaches 40 wt%,Yellow phases containing Na_0.5Nd_0.5Mo O₄,Na₂Mo O₄（H₂O）₂ and La₄Mo O₉ were precipitated on the surface of the sample;The static leaching test of MCC-1 showed that the total weight loss per unit surface area of the simulated high-level waste basalt glass solidified body after static soaking for 28 days Respectively are 7.85 g/m²,7.05 g/m²and 8.63 g/m²,which met the requirements of the Chinese nuclear industry standard（<15 g/m²）.