| Treatment and disposal of high-level liquid radioactive wastes (HLRW) is the focus and difficulty of nuclear safety. Disposal of HLRW without leakage and diffusion is the most important for wastes solidification, which play an important role to meet the requirement of long-term stability of solidify forms. In this paper, the pretreatment of the simulated HLRW was studied at first, and then the magnesium potassium phosphate (MKP) cement was used to immobilize that prepared liquid wastes, finally, the ceramic process of magnesium phosphate cement waste form was studied. The aim of this study is to utilize the two step treatment process of cement solidification and ceramic solidification to realize the stabilization and solidification treatment of HLRW.In this paper, the simulated HLRW has been denitrated, neutralized and concentrated at first. Then the simulated HLRW after treatment was directly added into the magnesium phosphate cement to prepare the cement wastes form. The permanent stability and physical properties of waste forms were investigated. Using high temperature sintering, ceramic solidify forms were prepared and their performances were studied, the phase composition, microstructure and micro element distribution of samples were tested by XRD, SEM and EDX.The results show that most nitrate in the simulated HLRW was removed by adding a certain amount of formic acid (The molar ratio of formic acid to nitrate was 1.9). The addition of NaOH and phosphate adjust the pH of the solution and reduce the content of soluble ions and simulated radionuclides in solution. The compressive strength of the waste forms meet the requirement of state standard, and the forms show good impact resistance, freezing thawing resistance and anti-shocking property. The lowest leaching rates of Sr2+ and Cs+ can reach 0.24×10-5 cm/d and 0.89×10-3 cm/d after the solidify forms immersed in water at 25? for 42 days, which meet the requirement of national standard.The main phase of MKP waste form is struvite (MgKPO4·6H2O), and there are some residual MgO and nitrate being found. Higher content of simulated HLRW lead to the waste forms having poor crystallinity of struvite crystals with more porous structure. The simulated nuclide of Sr and Cs can be found in well crystallinity struvite. There is a great change in the phase composition of the wastes forms after sintering at high temperature. The process of sintering and the ratio of raw materials have great influence on the phase composition of the waste form. EDS results show that the element of Sr and Cs can be detected in the poor crystallinity KMgP04, and the Cs can be detected on the good crystallinity KMgP04. Due to the uneven distribution of the elements, there is no significant change in the content of micro elements of waste forms with different sintering systems and the proportion of raw materials.The leaching rate of elements in the waste forms was tested by the product consistency test (PCT) leaching method. The results show that the element 7d normalized leaching rate of Fe, Ni, Ti, Sr are relatively lower than that of Mo and Cs. High temperature sintering has a great influence on the leaching rate of elements in waste forms. Compared with the leaching rate of Fe and Cs in the waste forms, the leaching rate of Fe and Cs in the waste forms after sintering obviously decreased. The results show that high temperature sintering can improve the solidify performance of Cs, and that of Sr also has a certain degree of improvement.This study indicates that cement solidify process and ceramic process of HLRW can combined together by using magnesium phosphate cement to meet the short and long stability performance of radioactive wastes solidification. |
PDF Full Text Request