| Nuclear energy,a high energy density,clean and low carbon,has received unprecedented attention and orderly development in recent years.The nuclear fuel cycle is the basis of nuclear industry.Advanced nuclear fuel cycle system with purpose of efficient utilization of uranium,puts forward higher requirements for the purity of uranium products obtained by reprocessing.In the process of uranium conversion and spent nuclear fuel reprocessing,uranium is often converted or recovered by fluoride volatility process.The UF6products obtained usually contain high volatile metal fluorides represented by MoF6,which are difficult to be separated and removed.At present,distillation and selective adsorption methods are widely used to remove molybdenum and other impurities from uranium,however,the separation effect between uranium and molybdenum is not satisfactory,because the volatile properties of high-value fluorides of uranium and molybdenum and their adsorption-desorption behaviors on the adsorbents are similar.In recent years,due to the characteristics of liquid fuel,molten salt reactor(MSR)has unique advantage in producing 99Mo by regarding the whole core fuel as irradiation target.However,in order to achieve the effective extraction of 99Mo without affecting the operation of the reactor,the key issue is how to separate and purify 99Mo without changing the chemical form of uranium in the nuclear fuel.Therefore,the separation of uranium and molybdenum is one of the key issues to be solved urgently in industry,whether in the process of uranium conversion and spent fuel reprocessing for purification of uranium products,or in the production process of medical nuclide 99Mo for the removal of uranium from molybdenum.In order to solve the above issues,this paper proposed a new idea to realize the effective separation of uranium and molybdenum only through orderly fluorination reaction of NF3with molybdenum and uranium,by applying the sensitivity of NF3's reactivity to temperature,and accurately controlling the reaction temperature.Centering on the above idea,the on-line fourier transform infrared spectroscopy(FTIR)analysis technology was established,and on this basis,the separation experiments of uranium and molybdenum in gas-solid system,molten salt system and irradiated molten salt system were systematically studied,through the thermodynamics analysis,experimental research and radioactive tracer experiment verification.The main results are as follows:(1)The on-line FTIR analysis technology for NF3reaction with uranium or molybdenum and uranium-molybdenum separation was established.The FTIR analysis system is stable and reliable.The standard deviation of spectrum detection data is 3.9%,the confidence degree is greater than 95%,which lays a foundation for the study of uranium-molybdenum separation by NF3.(2)In the gas-solid reaction system,the temperature has a significant effect on the reaction of NF3with Mo powder and UF4.Near 230?or higher,the reaction between Mo powder and NF3occurred,meanwhile,the average rate increased from1.00 g Mo·h-1to 2.31 g Mo·h-1when the temperature increased from 230?to 430?.However,the reaction between NF3and UF4occurred only when the temperature was above 450?,and the reaction rate of UF4increased from 1.53 to 2.68 g U·h-1in the temperature range of 450?to 600?.The on-line FTIR assisted by XRD analysis indicated that Mo powder was converted to MoF6through an intermediate product MoF3:2NF3(g)+2Mo(s)=2MoF3(s)+N2(g),then 2MoF3(s)+2NF3(g)=2MoF6(g)+N2(g),and UF4was directly converted to UF6with the following reaction mechanism:2/3NF3(g)+UF4(s)=UF6(g)+1/3N2(g).Based on a mathematical model with unreacted shrinking core,the calculated rate constant for the reaction of NF3with Mo power,increased from 1.8×10-2to 3.5×10-2min-1in the temperature range of230?to 430?,and the activation energy was 10.13 k J/mol.For the reaction of NF3with UF4,the rate constant increased from 1.6×10-2to 3.1×10-2min-1in the temperature range of 450?to 600?,and the activation energy of the reaction was22.56 k J/mol.Kinetic study results showed that the reaction between NF3and Mo was easier and faster than UF4.On this basis,the effect of mass ratio of uranium to molybdenum on the separation efficiency of uranium-molybdenum was investigated at 350?,which showed that the separation factors of molybdenum to uranium was1.71×103to 3.23×103.(3)In Li F-Be F2-Zr F4-UF4(64.5-29.5-5.0-1.0 mol.%)molten salt system,the temperature of reaction between NF3and Mo powder was determined to be 600?or above,and the reaction between NF3and UF4occurred when the temperature was above 650?.Mo powder in the molten salt system reacted with NF3directly to produce MoF6through XRD and XPS analysis.Considering that the fission product molybdenum in molten salt reactor exists in the form of fluoride or metal,MoF3was prepared and the effective reaction temperature of NF3and MoF3in molten salt was determined to be 570?or above.In the range of 570-600?,the average fluorination rate of MoF3increased from 0.09 g MoF3/min to 0.16 g MoF3/min and the conversion rate increased from 76%to 97%with the increase of temperature.At a specific temperature of 585?,the conversion rate of MoF3was more than 90%and the collected yield of MoF6by Na OH solution was about 90%,and the separation factor of molybdenum product to uranium was about 100.The XRD assisted by XPS analysis indicated that the reaction mechanism between MoF3and NF3in molten salt system was as follows:2MoF3(d)+2NF3(g)=2MoF6(g)+N2(g).(4)The separation of fission product 99Mo in Li F-Be F2-Zr F4molten salt system was carried out by irradiation of UF4with photo-neutron source.The results showed that in Li F-Be F2-Zr F4-UF4(irradiated,63.5-28.5-5.0-3.0 mol.%)molten salt,the specific activity of 99Mo in molten salt decreased from 7608.2 Bg/g salt to 155.0 Bg/g salt,and the separation rate was over 98%,under experimental condition of 585?,0.2 L/min NF3for 90 min.At the same time,the specific activity of 237U in the molten salt almost did not change during the reaction,which indicated that most uranium still remained in the molten salt.The decontamination factor(DF)of 99Mo to 237U was about 270.Through the research of this work,a new idea of separating molybdenum from a large amount of uranium was proposed by using the sensitivity of NF3's reactivity to temperature.The fully reaction between NF3and Mo can be achieved by precisely controlling the reaction temperature,and when most of Mo is separated,uranium is separated by reacting with NF3at increased temperature.In this way,the separation factor of uranium-molybdenum increased by 1-2 orders of magnitude.The results of this study not only lay the scientific foundation for the separation and purification of uranium in spent nuclear fuel reprocessing,but also provide a new technical route for the production of 99Mo in molten salt reactor. |
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