Study On The Application Of Magnesium Potassium Phosphate Cement In Emergency Solidification Of High Level Liquid Waste

As the corrosive and highly acidic effluent discharged from the reprocessing process of spent fuel in a nuclear reactor,high-level radioactive liquid waste(HLW)contains the most adioactivity of spent nuclear fuel and has the characteristics of strong radioactivity and great toxicity,is of the great concern.Existing technologies for treatment and disposal of HLW are all aimed at the ultimate geological disposal after solidification;however,the emergency solidification of HLW in the scenario of nuclear accident has not been paid enough attention and studied systematically.Magnesium phosphate cement(MPC)is a novel cementing material that can set and harden rapidly through acid-base reaction between magnesium oxide and phosphate.In this dissertation,magnesium potassium phosphate cement(MKPC)prepared with potassium dihydrogen phosphate is used to directly solidify HLW in nuclear emergency.The feasibility of emergency solidification process is verified.Centeral composite response surface method was adopted to design the experiment scheme,studies have shown that MKPC can realize effective emergency immobilization of HLW while the pH of HLW is of 3?7 and M/P of the MKPC raw material is of 2?5,solidification body can initial set within 10 min and final set within 40 minutes,condensation and setting time is short,comprehensive strength of solidification body reach 7 MPa at age of 3h,50 MPa at age of 28d.The solidified body has good immobilization effect on nuclide Cs⁺and Sr²⁺,the immobilization rate of Cs⁺at 3h can reach 80%,and it of Sr²⁺at 3h can reach over90%.pH and M/P values have significant effects on setting time.The early hydration reaction rate can be adjusted to control the setting time.The M/P has a significant influence on the formation of the early strength of the solidification body.As the age increases,the pore structure affected by pH becomes the main factor affecting strength.The mechanism of MKPC emergency solidifying HLW was investigated Studies have shown that the phosphate generated by the hydration reaction of the net slurry of MKPC at room temperature mainly consists of K-struvite Mg KPO₄·6H₂O and Mg(H₂PO₄)₂·4H₂O.After the mixing water of MKPC was replaced by HLW,the toxic nuclide Cs and Ce in the liquid waste can replace the K ion in K-struvite to generate Mg Cs PO₄·6H₂O and Mg_0.5Ce₂PO₄.The pH of HLW has a great influence on the phase composition of the solidified body in the early stage,but has no significant influence in the late stage while the pH mainly affects the crystallinity of the solidified body in late stage.When M/P=3?5 the influence of HLW pH on the Cs⁺solidification rate is slight.MKPC has a good immobilizing ability on Cs,while Sr and Ce have an adverse effect on the compressive strength of the solidified body,the effect of Ce is stronger than that of Sr.Sr and Ce also have adverse effects on the solidification of Cs⁺.After the heat treatment at 500?,the surface of the solidified samples showed no changes in morphology,such as cracking,and the volume of the solidified samples decreased slightly while the compressive strength of most of the solidified samples increased.In order to improve the volume stability of MKPC at high temperature and realize sintering ceramicization,metakaolin and iron oxide are used to modify.The modification study shows that the incorporation of metakaolin and iron oxide can improve the high-temperature volume stability of the MKPC solidified body,and within the range of the dosage,the larger the dosage,the better the volume stability.The modified solidification body cured at room temperature for 1 hour has good immobilization effect.The Cs⁺leaching rate of 7d of metakaolin-doped solidified body cured at room temperature for 1h is lower than 10×10^-5g/(cm²·d),and the Cs⁺leaching rate of 7d of iron-doped solidified body cured at room temperature for 1h is about 20×10^-5g/(cm²·d).The leaching rate of solidified bodies tended to decrease with increasing of heat treatment temperature,and reached the minimum at 1000?The leaching rate of blank solidified bodies decreased with the increase of heat treatment temperature,reaching the minimum at 1100?.The solidified MKPC solidification body still has the characteristics of early strength.The compressive strength of solidified MKPC body doped with metakaolin or iron oxide are higher than that of the blank control group.Monte Carlo software SRIM was used to simulate the radiation resistance of MKPC under?ray and Kr⁺irradiation.The results showed that the trend of the electronic and nuclear blocking ability of MKPC substrate was similar to that of artificial rock when He²⁺of 10ke V?10Me V and Kr⁺ion of 10ke V?10Me V were irradiated,but its blocking ability was lower than that of pyrochlore and perovskite zircon,while the projected range of incident particles in MKPC is all higher than in pyrochlore and perovskite zircon,indicating that MKPC has lower ability to prevent and shield radiation than artificial rock.In the case of 5.0Me V He²⁺ion incident vertically,the energy loss in MKPC is weighted by ionization loss of incident ions.In the case of 1.0Me V Kr⁺ion incident vertically in MKPC,the energy loss of Kr⁺ion in the MKPC is mainly caused because of ionization by ions,ionization by recoils and phonons by recoils.On average,each He²⁺ion of5.0Me V collision under vertical irradiation produced 204 atom displacement,among which 201 vacancies were formed and 3 replacements collisions occurred.Each Kr⁺ion of 1.0Me V collision under vertical irradiation produced 8431 atom displacement,among which 8302 vacancies were formed and 129 replacements collisions occurred.Under the Kr⁺ion of 1.0Me V irradiation,the average atomic dispersion(dpa)value of MKPC was greater than that of perovskite zircon under the same conditions,but it was still of same order of magnitude,and the dpa value of sintered MKPC substrate decreased somewhat.The atomic dislocation,vacancy and substitution collisions caused by the same ion incidence in the sintered MKPC are slightly lower than the original MKPC,while the distribution law is the same,indicating that the radiation resistance of the sintered MKPC body is slightly improved after heat treatment.Overall,the radiation resistance of MKPC substrate is lower than that of artificial rock,but the dpa value is of the same magnitude.The MKPC solidification body can maintain stable macro performance for a certain period of time under the inner irradiation of HLW,which meets the requirements of emergency solidification.Research has shown that nuclide in the HLW react with other reactants to form the refractory phosphate mineral,at the same time package effect of MKPC also contribute to the solidification,thus proposed process can quickly immobilize nuclide in the solidified body with a large capacity.Emergency solidification of HLW with MKPC is technically feasible,and the immobilization reaction is quite fast.The performance of solidification body meets the needs of emergency solidification in terms of chemical stability,mechanical strength,radionuclide immobilization rate and thermal stability.The volume stability of MKPC solidification body during heating can be further improved by adding metakaolin and iron oxide modification The radiation resistance of MKPC is lower than that of artificial rocks,but considering just obtaining intermediate solidification body as an emergency plan,its radiation resistance meets demands.