Study On Dissolution Behavious Of Corrosive Impurities And Oxides In Molten Fluorides

Molten salt reactor(MSR)is one of the six internationally recognized Generation IV reactor concepts,which has many advantages such as online fuel cycle,high thermal power density,and inherent safety and so on.Molten fluorides are suitable for use as coolants and fuel salts of molten salt reactors because of their low neutron cross section,excellent heat transfer properties and fairly good chemical stability.Besides,fluoride salts have strong hygroscopic nature and pyrohydrolysis of the absorbed water left in raw materials commonly occurs upon melting,with the formation of HF and O2-:H2O + 2F-? 2HF + O2-.Given its high solubility in molten fluorides,HF(g)can cause the corrosion of structural materials and the component of Cr in alloys would be selectively attacked in molten fluorides through the reaction of Cr + xHF ?(x/2)H2 + CrFx.Meanwhile,the O2-produced by pyrohydrolysis tends to react with the fuel salts UF4 and the UO2 precipitate will form through the reaction of UF4 + 2O2-? UO2 ? + 4F-.Despite the loss of dissolved uranium,these oxides may cause criticality concerns,because the large accumulation of precipitates would generate a superheated area and futher affect the safe operation of MSR.The exsitance of corrosion product and their corresponding concentrations,and the detection of uranium fuel precipitate in molten salt reactor are usually conducted by the periodic sampling and off-line analysis.Through the monitoring of the chromium,oxide and uranium concentration,the material corrosion and uranium precipitation can be predicted.These procedures are time-consuming and complicated.Hence,developing a fast method for online monitoring of chromium,oxide and uranium concentration in molten fluorides is necessary.In this thesis,we mainly focused on the following studies:The electrochemical behavior of Cr(?)in FLINAK melts at 600 ? was studied through cyclic voltammetry(CV)and square wave voltammetry(SWV).Cr(?)reduction proceeded in two steps:initial Cr(?)reduction to Cr(?)followed by subsequent reduction to Cr.The peak corresponding to the reduction of Cr(?)to Cr(?)was also observed in SWVs and CVs even though only CrF2 was added to the FLINAK melts.Moreover,scanning electron microscope-energy dispersive spectrometry(SEM-EDS)and X-ray diffraction(XRD)analyses confirmed that Cr(?)was converted to the more stable Cr(?)via the disproportion:3Cr(?)=2Cr(?)+Cr.Such a conversion could be attributed to the formation of the stable species[CrF6]3-,as determined through Raman spectroscopy.Meanwhile,the compatibility of Cr(?)and Cr(?)with material was also determined by static corrosion test.The results showed that Cr(?)would accelerate material corrosion,while Cr(?)would inhibit corrosion.The oxides in fluoride melts were in the form of O2-and oxygenated anions(SO42-,NO3-,PO43-).The total concentrations of oxide and oxygenated anions in FLINAK melts were detected by using LECO oxygen detector and ion chromatography,respectively.The practical O2-concentration was thus measured by subtracting the concentration of oxide in oxygenated anions from the total concentration of oxide.Square wave voltammeter(SWV)was then applied to determine the electrochemical behavior of O2-in FLINAK melts at 600 ?,which revealed that the process of oxidation from O2-to oxygen was controlled by diffusion.The quantitative relation between O2-concentration and peak current density was also established.Through this quantitative relation,the O2-concentration in molten fluorides can be on-line determined.The solubility product(Ksp)of ZrO2 in FLINAK melts was measured as(5.532-6.327)�10-7 mol3/kg3 through SWV.The electrochemical behavior of U(?)in FLINAK melts at 600 ? was studied by cyclic voltammetry(CV)and square wave voltammetry(SWV)analyses.The reduction of U(?)proceeded in two steps:reduction of initial U(?)to U(?),followed by three-electron reduction of U(?)to U metal.Both reactions were reversible and controlled by ion diffusion at a scan rate of 0.02-0.3 V s-1.A linear relationship between the reduction current density of U(?)/U(0)and the concentration of U(IV)was also established within 0.01-0.084 mol kg-1.When different concentrations of Li2O were added to the FLINAK-UF4 system,the peak current density of U(?)/U(0)detected by CV accordingly decreased because of the formation of UO2 precipitate.The apparent solubility product(Ksp)of UO2 in FLINAK-UF4-Li2O melts was approximately 4.75�10-6 mol3/kg3,which can be used to evaluate the allowable amount of dissolved oxide ions in uranium-based fluorides.The solubility of UO2 in molten FLINAK with ZrF4 additive was studied by chemical analysis.The ICP-OES result showed that the solubility of UO2 was lower to 0.247 wt%,which increased to a maximum value of 1.422 wt%as the added ZrF4 reached to 2.91 wt%.Then,the dissolution behavior of UO2 in FLINAK-ZrF4 system was studied through electrochemical methods(including CV,SWV,and galvanostatic electrolysis)and XRD technique,the results confirmed that UO2 dissolved as the form of uranium oxyfluoride.In order to further explore the mechanism how ZrF4 prevents the formation of UO2 precipitates,the chemical interaction between zirconium and free oxide in FLINAK melts at different zirconium to oxide(nZr/nO)molar ratios was studied by means of LECO oxygen detector and Raman spectroscopy.ZrO2 precipitates were formed with nZr/nO?0.5,while ZrO2 was converted to[Zr2OF10]4-complex with nZr/nO>0.5.The maximum Zr2OFx6-x complexes in FLINAK melts was found to be 0.020 mol/kg.With an initial oxide concentration lower than 0.020 mol/kg and necessary amount of ZrF4,the free oxide O2-in FLINAK melts could be completely converted to Zr2OFx6-x complexes,which would further prevent the formation of UO2 precipitates.