Immobilization Of Radioactive Fluoride Waste In Phosphate-Based Glass: A Molecular Dynamics Simulation Study

Thorium molten salt reactor energy system?TMSR?is the project named“the future advanced nuclear fission power”which was initiated by the Chinese Academy of Sciences.Fluorides are designed as a fuel carrier and coolant for molten salt reactors because of its good performance in heat transfer,low vapor pressure,and stability to irradiation.Molten salt reactor operation and its nuclear fuel cycle will produce a wide variety of forms of fluorine-containing radioactive waste,most of which exist in solid form.Studies on these fluorine-containing radioactive wastes have progressed slowly,due to the current lack of experimental conditions.Most of the studies focused on the glass-cured fluoride wastes.On the other hand,there is little information on the solidified body obtained from the experiment,especially on the structural mechanism,such as the existence state of fluoride in the glass,the impact on the glass network structure,molecular dynamics simulation can effectively solve these problems.The advantage of molecular dynamics?MD?method has the incomparable advantages over experiments for studying the microstructure of materials,and the feasibility of this method has been confirmed in practice in different research fields.In this paper,we use molecular dynamics method to study the solidification of fluorine-containing radioactive waste from a microscopic point of view.It promotes the safety treatment and disposal for fluorine-containing radioactive waste.This paper studied phosphate glass system immobilizing fluoride waste by using molecular dynamics simulation method.We chose P₂O₅-Na₂O-CaO-Al₂O₃ as glass formula,combined with the composition of fluoridated waste in molten salt reactor,based on the previous experimental.NaF-SrF₂-CeF₃-ZrF₄ groups were selected as simulated fluorine-containing radioactive waste.Molecular dynamics simulation was used to get a series of structure information,including bonding length,bonding angle,coordination,bridge oxygen,etc.We got to know the loading of fluoride and stability in phosphate glass through the analysis of those data.Finally,compared with other experimental results to evaluate the performance of phosphate glass solidified body.Structure and main contents of this article are divided into six chapters,as follows:The first chapter is Introduction.The background of the molten salt reactor?MSR?,the application of fluoride in MSR,the source of radioactive fluoride waste produced from fuel cycle,and the characteristics of fluorine-containing waste were briefly introduced.The existing problems in the treatment methods for fluorine-containing waste are presented,and new solutions are proposed.In the light of the limitation of experimental conditions,this paper put forward the molecular dynamics simulation method in the study on immobilization of fluorine-containing waste.The second chapter is Molecular Dynamics Simulation Method.This chapter introduced basic theory of molecular dynamics simulation and its important concepts.The basic steps of dynamics simulation ware expounded and the Material Studio,carried out this subject,was given a brief introduction.Finally,this chapter discussed four analysis methods of the simulation results:coordination analysis,bonding length distribution,bonding angle distribution,bridge oxygen analysis and mean square displacement.The third chapter is Selection of Key parameters to Build Molecular Dynamics Model and its Simulation Process.This chapter described the initial model and the parameters of potential function in glass system.Combined with previous experimental research,we chose P₂O₅-Na₂O-CaO-Al₂O₃ as glass formula,and Na F-SrF₂-CeF₃-ZrF₄groups were selected as simulated fluorine-containing radioactive waste.The temperature was set of 300 K and the number of amorphous structure was 10.Density of initial model was close to actual phase,and the initial model of glass curing system was constructed.The simulation process chose the form of Buckingham potential function with the analysis of bonding angles and coordination of atoms.The subsection cooling mechanism was selected to simulate the cooling process of glass.The selection of ensemble was NVT ensemble conditions and the integral step length is set to 1 fs.The determination of these parameters for simulation calculations will pave the way for the subsequent two chapters to develop molecular dynamics simulations of phosphate glass-solidified fluorine-containing wastes.The fourth chapter is Molecular Dynamics Simulation of P₂O₅-Na₂O-CaO-Al₂O₃-SrF₂ System.The Na,Ca,Al,P molar ratio was 4:1:3:6.This glass system was with SrF₂ mole ratio for variables.The various ion activity was Na⁺>F^->Sr²⁺>O^2->Ca²⁺>P⁵⁺>Al³⁺.The ions P⁵⁺,Al³⁺reacted with O^2-to form the tetrahedrons,witch connected by the bridge oxygen atoms to make up glass network structure.Along with the increase of the content of SrF₂,[AlO₄]and[PO₄]tetrahedrons could react with F^-by taking place of O^2-to generate 5-coordination polyhedron structures.[PO₄]tetrahedron was more stable than[AlO₄]tetrahedron.The number of bridge oxygen reduced along with the increase of SrF₂ content in the glass system and the decline rate accelerated when its concentration was high,resulting in decline of structural connection degree and more loose structure.Sr²⁺formed a distorted octahedral structure fixed in glass system.Along with the increasing content of SrF₂,octahedral structural instability aggravated.For more than 20 mol percentage of SrF₂,octahedral structure was destroyed.The glass was separated into Na/Ca/Sr/F-rich regions,leading to crystallization easily.The fifth chapter is Molecular Dynamics Simulation of Complex Fluoride in P₂O₅-Na₂O-CaO-Al₂O₃ System.The simulated curable object was changed from a single component of fluoride waste to multiple components.The fluoride waste was Na F-SrF₂-CeF₃-ZrF₄ system with a molar ratio of 1:1:4:4.The results showed that the maximum capacity of fluoride in phosphate glass does not exceed 20 mol%.And the simulation results was similar with the results of experiment.The various ion activity was Na⁺>F^->Ca²⁺>Sr²⁺>O²>Ce³⁺>P⁵⁺>Al³⁺>Zr⁴⁺.The curing effect of high valence ion was better than low valence ion.The sixth chapter is summary and outlook.This chapter summarized the research contents and results of this paper,putting forward the future research direction.