Application Research On Molten Salt Frozen-wall Technology In The Pyroprocessing Of Fuel Salt From Molten Salt Reactor

Pyroprocessing is the best choice for molten salt reactor with the purpose of nuclear fuel recycling,and fluoride volatility method?FVM?is one of the core technologies in pyroprocessing process.Corrosion is the biggest challenge in R&D of FVM due to high-temperature fluoride salt and strong oxidizing gas?F₂?,which not only shorten the service life of the process equipment,but also increase the content of impurities in fuel salt.To a great extent this corrosion problem hinders the development of FVM.Molten salt frozen-wall is considered as a method to solve the corrosion problem in FVM.The frozen-wall is a layer of solidified salt formed on inner-surface of vessel by controlling heat transfer,and plays a role in physical isolation by preventing vessel from direct contact with the corrosion gas and liquid source.The key technical issues of frozen-wall technique include heat transfer controlling,thickness monitoring,formation and maintenance adjustment during application process.In this paper a series of work have been carried out on the molten salt frozen-wall technical problems in application.Special experimental device and method were established.Physical model of heat transfer,thickness monitoring method,frozen-wall formation and maintenance technique,application mode and protection performance were studied by theoretical analysis,numerical simulation,experimental research and test verification.The main results are as follows:?1?The basic heat transfer physical model of molten salt frozen wall was established.The formation and maintaining process of frozen-wall was described by solid-liquid phase interface moving model.The relationship between solidification interface location with time was obtained.The calculation method and the results are of importance for experimental research and practical technological parameters setting.?2?A variety of methods for monitoring of frozen-wall thickness were studied in the status of unsteady heat transfer and steady heat transfer,respectively.An online thickness monitoring technique based on the temperature gradient calculation method for frozen-wall was established.The measurement error could be controlled within 30%and it fully meets the requirements for thickness monitoring of frozen-wall.Such online frozen-wall thickness monitoring method provided a technical foundation and guarantee for follow-up research.?3?The effect of heat flux on the compactness of solidified salt,the formation rate of frozen-wall and the equilibrium thickness of frozen-wall was investigated.The heat transfer law and control technique in the formation and maintenance of frozen wall were obtained.The porosity of the formed frozen-wall was below 10%and the strength was about 12MPa when the average heat flux was 2⁸kW/m².The formation rate of the frozen-wall could be controlled within the range of 0.2⁰.5mm/min,and the frozen-wall could be maintained at any thickness within the range of 5⁵0mm.The molten frozen-wall adhered to the inner metal wall uniformly and firmly.?4?The simulation study of the frozen-wall application was carried out in batch processing mode.The influence of some factors on the thickness fluctuation was investigated during the application process.These factors include the temperature of molten salt,the heating power of molten salt,the temperature of the cooling medium and the initial thickness of the frozen-wall.The flow rate of cooling medium was real-time regulated to control the fluctuation based on thickness feedback of frozen-wall.The process was verified by the tracer element Y in the molten salt.The results show that the thickness variation could be controlled within±3 mm by adjusting flow rate of cooling medium.The distribution of element Y in frozen-wall layer could reflect the thickness fluctuation of frozen-wall during application process.At the same time,the distribution of Y element also indicated that there was no material migration in the solid salt layer of the frozen-wall.?5?The protection performance of molten salt frozen-wall were conducted in molten FLiNaK salt?500-510??with a fluorine gas purge?0.2 L/min 20%F₂/Ar?which was considered as the simulation of FVM process.In the case of direct exposure of SS304,SS316L,Inconel 600 and graphite in molten salt,the corrosion rates were57.94?m/h,25.3?m/h,8.1?m/h and 49.5?m/h,respectively.When protected by the frozen-wall with a thickness of 5¹0 mm,the corrosion rates decreased by at least one order of magnitude to 1.37?m/h,0.43?m/h,0.17?m/h and 0.34?m/h,respectively.This results show that the frozen-wall has a good protection performance.In conclusion,a series of experiments on molten salt frozen-wall technique have been conducted on the basis of theoretical analysis in this paper.An online thickness monitoring method and the process control technique for formation and maintenance of frozen-wall was established.The stability of frozen-wall could be controlled wonderfully by the thickness feedback in the process of application.The real protection performance of frozen-wall for different materials was also obtained.The results of this study can provide good experimental data support for the design of process vessel with frozen-wall,and a technical foundation for the application of the frozen-wall in pyroprocessing of fuel salt from molten salt reactor.