| A large amount of radioactive tritium?T?will be produced during the operation of the molten salt reactor.Compared with the pressurized water reactor?PWR?of the same power,the production rate of tritium in molten salt reactor is much higher than that of the PWR.Tritium will be first dissolved in the molten salt in the form of tritium fluoride?TF?,tritium ion?T+?or gaseous tritium?HT,T2?.As flowing along with the molten salt,it will be distributed in all parts of the molten salt reactor.A large amount of tritium can escape from the molten salt surface and enter the cover gas,then flows into the off-gas system.Some tritium can even permeate through the reactor vessel,pipe or heat exchanger pipe and goes into the environment.As a harmful radionuclide,tritium will affect the safe operation of the molten salt reactor,such as the generated TF can corrode the pipeline,affecting the working life of the pipeline.HT will permeate into the environment,where it is converted into tritiated water?HTO?.It can be easily absorbed through the skin and produce internal radiation to the human body,causing 25,000 times more radioactive damage than gaseous tritium.Therefore,in order to ensure the working life of molten salt pipelines,reduce the permeation of tritium,and reduce the harm to human and the environment,it is necessary to remove tritium produced by molten salt reactors.In nuclear power plants,tritium is generally oxidized to tritiated water?HTO/T2O/DTO?,and then adsorbed for collection.This method has resulted in a large amount of tritiated water in the global,and the safe storage of tritiated water has gradually become a difficult problem in the future.In addition,tritiated water is more hazardous and has lower application value than gaseous tritium,which is not conducive to the follow-up utilization.In order to improve the process of tritium,pressure swing adsorption,molecular sieve adsorption,membrane separation and other methods have been proposed and studied.Among these methods,the inorganic metal membrane represented by palladium and its alloy membranes have been paid more and more attention due to their good selective permeability of hydrogen isotopes,low energy consumption and simple operation.Since the palladium silver alloy membrane?Pd-Ag?has nearly 2 times higher hydrogen permeability than pure palladium,and better thermal stability and anti-toxicity,the Pd-Ag alloy membrane is gradually used for the tritium separation and removing in fusion reactors and various tritium facilities.Based on the above reasons and the features of tritium distribution in the molten salt reactor,in this dissertation,the Pd-Ag membrane was proposed to separate tritium from the molten salt reactor.Since tritium is radioactive and the stable isotopes,hydrogen?H2?and deuterium?D2?,have the similar properties to tritium,the permeability and separation properties of Pd-Ag membrane with H2 and D2 have been researched to simulate the separation of tritium.In this research,an experimental platform to study the hydrogen permeability properties was built up.Based on the experiment,the permeability coefficient of the Pd-Ag membrane was determined,?=5.70×10-8×exp?-4.55/?RT??(mol·m-1·s-1·Pa-0.5).The pressure exponent of Pd-Ag membrane was 0.5,which means the H atoms diffusion in the internal body phase of the Pd-Ag membrane was the rate controlling step of H2 permeation.The working temperature,hydrogen partial pressure,and inlet velocity was changed to study the influence of the hydrogen permeation of the Pd-Ag membrane.The results showed that as the working temperature and the partial pressure difference of hydrogen rise,the flux of hydrogen permeation would increase.While the flux of hydrogen permeation would decrease as the concentration of Ar went up in the gas mixture.At the same temperature,the permeation flux of H2 is 1.311.39 times larger than that of D2,which is close to 1.414,the value of their relationship of diffusion coefficient based on diffusion theory.The permeation flux of T2 could also be deduced according to the results.According to the experiment of the static corrosion in the high temperature molten salt and compared with the Hastelloy N alloy,the Pd-Ag membrane has a good corrosion resistance.The results show that the depth of the Pd-Ag membrane affected by molten salt corrosion was 11?m,which was less than that of Hastelloy N.The content of Cr,Fe in the molten salt were significantly increased while the amount of Ag,Pd were almost unchanged.It means the compatibility of Pd-Ag alloy with molten salt is good.At the temperature of 500?,with a flowrate of 100 mL/min of Ar-H2?H2102 ppm?flowing in the molten salt,the H2 separation efficiency of Pd-Ag could reach3.5%,which indicated that the Pd-Ag membrane could be used in the molten salt to separate tritium.At last,the process of hydrogen permeation and diffusion in the Pd-Ag membrane separator was simulated.It was determined that under the condition of Ar gas,high membrane permeability coefficient and low inlet velocity,serious concentration polarization effect would occur,which could affect the permeability of the Pd-Ag membrane separator.In order to minimize the concentration polarization effect,the volume fraction of H2 in the mixture and the flow rate of the mixture can be increased.However,reducing the concentration polarization effect will affect the separation efficiency of H2,and in order to separate H2 as much as possible,the length of the separator needs to be increased. |
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