Vitrification Of Radioactive Wastes Containing Fluorides From MSRs By Phosphates And The Properties Of Waste Forms

The major distinguishing characteristic of molten salt reactors(MSRs) from water reactors is that they simultaneously utilize liquid molten fluoride salts as fuel and coolant. MSR’ fuels are dissolved in the fluoride salts which are of the high-boiling, low-viscosity, low-steam pressure and low irradiance response. So radioactive fluoride salts are generated during the MSRs operation and the reprocessing of spent fuel, which are in the form of solid fluorides. Up to now, we do not how to dispose the residual salts. However, accordding to the requirement of radioactive waste management, there is a strong incentive to develop or identify a feasible alternative technology for ultimate disposition of the MSR wastes.The geological disposal is regarded as the most reasonable and effective way to safely dispose solid radioactive wastes in the world. It is needed to immobilize radioactive waste into a solid, which is compact, high mechanical strength, chemical inertness before disposal. However, borosilicate glasses used to immobilize radioactive oxides in industry, are not feasible for their low solubilities of wastes containing fluorides, and their facility for immobilization is also based on oxide wastes system. Therefore, alternative candidates thus have been an active research topic. Phosphate glasses are particularly attractive for relatively high solubilities for some wastes containing Pu, Mo, Fe, Cr, S, and actinide elements, and they are studied to immobilize these waste. There are no reports about directly vitrification of radioactive fluoride salts into phosphate glasses.In this study, mixed metal fluorides were employed as the simulated MSRs waste and incorporated into iron phosphate(IP) glasses and sodium aluminophosphate(NaAlP) glasses to obtain phosphate-based waste forms. The properties of the waste forms were characterized by several technologies and leaching tests were then performed in deionized water using the product consistency test A(PCT-A method), which simulate extreme conditions(i.e. moist air and high temperature) of underground repositories. The feasibility employing IP and Na AlP glasses as immobilization hosts for simulated MSR wastes was evaluated, the formulation and potential waste loading was investigated, and the influence of additives(e.g. Fe2O3, B2O3, CaO) on properties of wasteforms were also characterized.This paper divides into six chapters.Chapter 1: Introduction. The following considerations, including the waste sources, compositions and characteristics containing-fluorides from MSRs were introduced. The treatment methods and the situation about radioactive fluoride wastes were investigated, and it is concluded that the immobilization of this waste into glasses will be the main approach. The current solidification technology and the application were then summarized. Restricted solubilities of fluorides and actinides in silicate glasses have encouraged wasteform designers once again to judge the potentiality of P2O5 based glass systems as host matrices. We also selected phosphate glasses as base materials to immobilize fluorides and further investigated the historical background, basic features and their application in vitrifying the waste containing halogen. Finally the content and significance of this research were introduced.Chapter 2: The introduction of the experimental method. Firstly, MSRs wastes containing-fluorides could be divided into the low-valent waste containing-fluorides and the high-valent waste containing-fluorides according to the sources and waste characteristics. Secondly, the solidified material and formulation were scheme. Lastly, the experimental devices used in the thesis were introduced.Chapter 3: The preparation and the structural test of waste forms. Through the comparison of the melting times, the melting temperatures and the adding of glass additives, we concluded the best of preparation process, and on this basis we prepared Iron phosphate(Fe2O3-P2O5, IP) glass samples and sodium aluminum phosphate(Na2O-Al2O3-P2O5, NaAlP) glass samples. We also studied the structure of wasteforms.Chapter 4: The leaching resistance performance study. The chemical stability is the primary task in performance requirements of waste forms. The PCT was used to study the leaching rate of IP and NaAlP waste forms in deionized water, and a contrast was made with that of commercial waste forms. The influences of CaO, B2O3 and Fe2O3 on above waste forms were also studied.Chapter 5: Since waste loading is a mirror of base material economy, we then studied the potential waste loading, and the influence of additives(e.g. fluorides, Fe2O3, B2O3, CaO) on the structure of wasteforms.Chapter 6: Summary and prospect. The achievements and the insufficient of the present work were summarized, and the possible improvements were prospected.