| Environment tritium monitoring is widely applied to groundwater tracing,background investigation of civil construction,radioactivity measurement of nuclear facilities etc.Tritium in any form can be transformed into tritiated water(HTO,DTO,T2O etc)after migration and transformation.Since the signing of the nuclear ban order in the 1960 s and the self-decay of tritium,the tritium concentration has been decreasing year by year.At present,the average tritium concentration in precipitation and surface water in the Northern hemisphere is 1~2Bq/L,and the concentration of tritium in seawater and groundwater is often lower.Liquid scintillation measurement is a common way to determine tritium concentration in water.However,its detection limit is 1Bq/L,obviously,it is difficult to ensure the accuracy of data by direct liquid scintillation measurement.Therefore,it is necessary to adopt the method of electrolysis combined with liquid scintillation measurement,which can greatly increase the detection limit of liquid scintillation to 0.1Bq/L.There are three kinds of electrolysis methods,among which solid polymer electrolytic enrichment method has the advantages of small electrolyzer volume,high current density,high safety,easy operation and so on.It is the mainstream method of tritium enrichment in low-level water at present.Based on the principle of SPE electrolysis system,we setted up a SPE tritium electrolysis enrichment system for experiment.,and determined and mastered the preparation method of membrane electrode assembly.In this paper,The separation coefficient was adopted as an index to optimize the electrolysis conditions and the membrane electrode assembly,the system performance is improved.The specific conclusions are listed as follow:Experiments about the influence of electrolytic conditions such as volume reduction factor,current and temperature has been studied,whose results indicate:When the water volume changes from 400 m L to 25 m L,tritium recovery factor at25℃ is 55 %;Separation coefficient β increased first and then decreased withelectrolytic current,When the current is 15 A,the separation coefficient reaches the maximum value of 5.4.The separation coefficient β showed a decreasing trend as temperature increased.According to the fitting function relationship between separation coefficient and temperature,β can be increased by 0.8 for every 1 ℃decrease in the water temperature.Through the optimization of the above electrolytic conditions,the separation coefficient of the self-made SPE tritium electrolysis enrichment system can reach 6.5,and the tritium recovery factor ≥ 65%.A series of optimization experiments were carried out on membrane electrode assembly of SPE system: According to the bonding ability with reactant of catalyst,and the corresponding catalytic activity,the electrode catalyst was screened.Based on the comparison of electrolytic performance and tritium enrichment factor,the material of anode and cathode catalyst was determined.Compared with the electrolysis performance and tritium enrichment factor of anode catalytic,the better catalyst loading was determined.The experimental results show that the optimum loading of anode catalyst is 2.5 mg/cm2.In addition,the experimental results show that the multistage current is beneficial to the tritium recovery factor of system,the tritium recovery factor can be increased to 63% at 25 ℃.Through the above optimization of electrolysis conditions and membrane electrode on the SPE tritium enrichment system,the optimal operation performance of SPE tritium enrichment system was obtained as follows: when the initial water volume ≥ 400 m L,with multistage electrolytic current and low water temperature,electrolyze to residual water ≤ 25 m L,the separation coefficient is more than 7.5,tritium recovery factor is about 70%,which is up to the international level. |
