| Cement solidification process has significant advantage when applied in thedisposal of medium and low level radioactive wastes. Compared with othersolidification process, technology and equipment for cement solidification process isvery simple and low cost, solidified form also has good durability. So, cementsolidification process has been widely applied in storage and transportation of mediumand low level radioactive wastes. Traditional cement solidification technology hasmany disadvantages, such as high water to cement ratio, high nuclei leaching rate, lowsolidified form strength and etc. Traditional cement must cure under high pH valueenvironment，wastes need treatment previously. The setting time and strength ofcement may affected by PO43-, BO33-, Zn, Sn. Due to the disadvantages above,traditional cement solidification process has low wastes loading, which means morespace must remain to the storage, and the transportation of wastes is also a big problem.Therefore, the selection of new type cement for medium and low level radioactivewastes is necessary，the target of selection is improve the performance of the solidifiedform, improve the loading capacity and speed the solidification process, and finally getsolidified form with better durability, promote cement solidification process in the fieldof treatment and disposal of nuclear waste. Considering the characteristics ofmagnesium phosphate cement （MPC）, magnesium phosphate cement was selected asmatrix material in this study for solidifying medium and low level radioactive waste,and solification of medium and low level radioactive incineration ash was also studied.First of all, the related performances of magnesium phosphate cement which infavor of medium and low level radioactive wastes solidification were studied. In orderto select suitable matrix material, three kinds of phosphate were chose to preparationmagnesium phosphate cement; the effects of mix ration to the strength were alsostudied. Considering the preparation of solidified form of magnesium phosphatecement may be affected by the very low temperature, the strength development ofMPC was studied by curing under-20℃.The properties of MPC after various hightemperature treatments were also studied. Durability is also an important factor ofsolidified form, freezing and thawing test and soaking test were used to investigate thedurability of samples. Results show MPC prepared by ammonium dihydrogenphosphate has highest strength, the latter is MPC prepared by potassium dihydrogen phosphate, diammonium hydrogen phosphate is the worst. M/P plays a key role to thestrength of MPC, followed by effects of borax, particle size of magnesium oxide hasminimal effection. Strength of magnesium phosphate cement has been able to achievegrowth under low temperature conditions. Magnesium phosphate cement-basedmaterials have superior high-temperature performance, the structure of cement stillmaintain integrity even at up to1400℃, volume of samples will reduce under differenttemperature treatment, structural collapse did not occur whthin the full temperaturerange. Durability studies show that magnesium phosphate cement has good resistanceto freeze-thaw and soaking test. The results indicate magnesium phosphate cement isan ideal matrix material.Secondly, because Cs and Sr are two of the most common and dangerest nuclei ofmedium and low level radioactive wastes, the solidification performance of MPC byadding Cs and Sr was studied. The affections of Sr and Cs addition to the strength ofMPC were studied, as well the hydration of MPC. The adsorption performance ofmagnesium phosphate cement to Cs, Sr was also studied. The durability and theexistence state of Cs and Sr in MPC were also investigated. Adding Cs and Sr causethe decline in the strength of MPC. Magnesium phosphate cement have betterperformance on adsorption Sr, absorption rate for Sr is up to97.72%. The42d leachingrates and cumulative leaching factor of solidified form with Cs are5.47x10-5cm·d-1and2.81x10-3cm respectively, and those of Sr are2.85x10-5cm·d-1and5.92x10-3cmrespectively. Sr addition reduces the hydration heat of hydration of magnesiumphosphate cement, slow hydration process, but those of Cs is not so significant.Cs andSr are mainly as MgCsPO4·6H2O and SrHPO4existed in the solidified form, so theleaching rates are significantly lower than other cement system,especially at the earlyleaching test.Immobilization of radioactive incineration ash is a difficult in wastes diaposal.Magnesium phosphate cement was chosed to immbilizaton for simulated radioactiveincineration ash. The loading capacity of solidified form was studied, and theperformance of solidified form was also investigated. Through analysis of compressivestrength, maximum loading capacity of solidified form can reach60wt%. Hydrationheat analysis indicates with higher simulated radioactive incineration ash, the lowerhydration heat of solidified form can be found, caused the hydration process delay. Theleaching rate and cumulative leaching rate of solidified form can reach1.7×10-4cm·d-1and0.15cm for Cs,1.1×10-4cm·d-1and0.015cm for Sr. fluidity of pastes, impact resistance, freezing and thawing resistance and anti soaking are fully met performancerequirements of solidified form.This study showed that magnesium phosphate cement has good performance forsolidification of medium and low level radioactive waste. With the formation ofinsoluble phosphate mineral, and encapsulating with the hydration products of MPC,radioactive ions can be quickly fixed in the solidified form, and the final form has gooddurability. MPC is an ideal matrix material for medium and lowlevel radioactivewastes solidification. |
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