Solidification Simulated High Level Liquid Waste By Magnesium Phosphate Cement

Plentiful radioactive waste was produced and accumulated with the rapid development of nuclear energy utilization,in which a large amount of nuclear waste liquid was stored and exceed relevant time limit.The safe disposal of high level radioactive waste?HLW?has always been the focus and difficulty of nuclear safety concern.It is a key factor of nuclear safety to carry out rapid disposal of high level radioactive waste liquid to prevent radioactivity leaking and spreading.In this paper,the solidification process of the simulated high level radioactive waste liquid by using magnesium potassium phosphate cement?MPC?was studied,and the sintering ceramics of cement solidified body was studied preliminarily.The short-term performance requirements and long-term stability of the solidified body were achieved through two-step process of cement solidification and sintering.Central composite design was used to optimize the mixture ratio of magnesium phosphate cement solidifying system.By comparing the apparent morphology and chemical stability of the solidified bodies,the appropriate formula of magnesium phosphate cement applied to solidify HLW was determined.Experimental results shows that while adjusting pH value of the high level radioactive waste liquid by using K₃PO₄ as a buffer agent,good solidification can be achieved without denitration under the optimized formula,e.g.mass ratio of MgO and KH₂PO₄ is of 4,pH value of the HLW liquid is between 3 and 7,mixing amount of borax is between10%and 16%?MgO mass?,40%?MgO mass?HLW liquid was solidified effectively.The powder sample of the solidified body keeps more than 85%of Cs⁺after 7 days soaking by deionized water at 90?.Secondly,in view of the radiative heating of radionuclides and the strong acid corrosion of the HLW,paper probes into the influence of the temperature and pH value of HLW on the performance of the solidified body.it is shown that the structure of the solidified body is dense-loose-dense with the increase of temperature in the range of 0?to 1000?.Study found that the solidified body showed good ceramic structure,after 900?sintering,the Cs⁺leaching rate of the ceramic is of7.21×10^-6g/?cm²·d?,just close to the MPC body,below the range of leaching rate of phosphate glass solidified body..When the pH of HLW was not adjusted,the strength of the solidified body developed slowly,7d strength is only about 2Mpa.Using K₃PO₄ to regulate the range of pH to 3 to 7 of the HLW,the 6h strength is about 5MPa,and the 1d strength can reach 15Mpa.After sintering at 900?,the ceramic structure is formed inside the solidified body,and the leaching rate of Cs⁺decreases with the increase of pH value of the HLW.MPC hydration reaction release heat intensively in very short time,this results mass MPC structure cracks frequently,ultimately harms the stability of the solidified body.In this paper,the effect of adding Fe₂O₃ on the properties of MPC solidified body containing HLW liquid was also studied.Result shows that the adding of Fe ₂O₃reducing hydration heat release rate significantly thus lessens the concentrated heat release.Furthermore,Fe₂O₃ is used as an inert material in the cement system at room temperature,which increases the compactness of the structure to a certain extent and reduces Cs⁺leaching.Microscopic analysis shows that sintering at 900?melts more liquid phase and makes the system more dense.The sintered system contains not only mgcspo4 but also ferrophosphate phase e.g.CsFe?PO₄?₂,CsFe?PO₄?₂ brings additional solidification and reduces the leaching rate of Cs⁺.When sintering temperature keeps increase,structure of the solidified body was destroyed and leaching rate of Cs⁺increased significantly.Therefore,the addition of Fe₂O₃ can reduce the concentration of heat in the solidified body of MPC and reduce the sintering temperature of the solidified body.The formation of ferric phosphate is beneficial to reduce the leaching of Cs⁺.Under suitable mixing ratio,MPC can effectively solidify the HLW in range of pH=3-7 to meet the short-term emergency curing performance requirements of the block.The ceramic structure is formed after sintering at 900?,improve the solidified body better in long-term stability.