Optimization Of Membrane Electrode For Tritium Enrichment In Solid Polymer Electrolyte Electrolysis

Tritium is one of the main radionuclides discharged from nuclear facilities to the environment,mainly in tritiated water,which is relatively easy to absorb by the human body through inhalation,intake,and skin absorption,posing potential radiation hazards to human health.With the continuous growth of the proportion of nuclear energy in China’s energy structure and the rapid development of nuclear science and technology,the monitoring of tritium in water is an important content of radiation environmental quality monitoring and environmental monitoring around nuclear facilities.Since implementing the Comprehensive Nuclear Test Ban Treaty,most of the tritium in the environment has been reduced to the natural background level after several half-lives,which is close to the detection limit（1 Bq/L）of the ultra-low background liquid scintillation spectrometer.Obviously,it is difficult to ensure the accuracy of the data by directly measuring the tritium activity in low-level environmental water through liquid scintillation.At present,various pretreatment methods have been developed to achieve tritium concentration,such as thermal diffusion,gas chromatography,distillation and electrolysis.Among them,electrolysis has the advantages of simple operation,good concentration effect and high safety performance.Thus,the measurement of tritium activity in water by electrolytic concentration combined with liquid scintillation measurement can significantly improve the detection level of liquid scintillation,and its detection level can reach 0.1 Bq/L,satisfying the monitoring of low-background tritiated water samples in the environment.Solid polymer electrolyte（SPE）electrolysis is the simplest and most widely used pretreatment method at present.Still,the hydrogen/tritium separation coefficient(β_H/T)of the domestic SPE electrolytic concentration device（5～6.5）is still relatively lower than that of the devices from the United States,Japan and other foreign countries（4.6～12.6）.In SPE electrolysis,the hydrogen isotope separation mainly occurs in the membrane electrode part of the electrolytic cell,andβ_H/T is limited by factors such as the materials（e.g.,catalyst,proton exchange membrane）,preparation process,and electrolytic conditions of the membrane electrode.Therefore,the optimization of membrane electrode is still challenging for the SPE electrolytic concentration of tritium.In order to improveβ_H/T,the optimization of the membrane electrode is carried out in this thesis through electrolytic experiments and theoretical simulations.This thesis mainly includes the following parts:（1）A SPE electrolysis experimental system for concentrating tritium was built independently.The fabrication method of membrane electrode,i.e.,catalyst fabrication,catalyst slurry configuration,ultrasonic atomization spraying and hot press transfer,was developed,which is the critical component of the SPE system.At the same time,the separation coefficientβ_H/Twas selected as the evaluation index of the system,and its performance was improved by optimizing the electrolysis conditionsA Pt-membrane-Pt Ir Ru membrane electrode was prepared in this thesis to investigate the electrolytic performance of the developed SPE system.And the effects of electrolytic conditions,e.g.,electrolytic current,volume concentration factor and temperature,on theβ_H/Tvalue of the system were explored.The experimental results showed that the best performance of the self-built SPE electrolytic tritium concentration system could be obtained when operating under the following conditions:the initial water volume of electrolysis was 400 ml;the temperature of electrolytic cell and sample water was 5℃;the electrolysis experiment was carried out with three-stage electrolytic current(stage 1_{（400⁹0）}:20 A;stage 2_（90⁶0）:15 A;3_（60^）:10 A);and the volume concentration factor was controlled to be 18.0±0.3 times.（2）Based on the membrane electrode structure of the SPE electrolytic tritium concentration system,graphene（Gra）and hexagonal boron nitride（h-BN）were used to establish the“sandwich”structured membrane model respectively.The separation performance of these two materials for hydrogen isotope ions in proton exchange membrane was studied based on density functional theory,and the separation mechanism was revealed from the perspectives of penetration barrier and zero-point vibration energy difference.Moreover,the effects of penetration path,water environment and defect structure on the penetration barrier were explored.The simulation results showed that the energy barrier of the hydrogen ion adsorption-penetration path is much higher than that of the direct penetration path in the vacuum environment.A similar phenomenon was observed in the water environment,indicating that the hydrogen ion tends to pass through Gra and h-BN preferentially.At the same time,it was found that the presence of water molecules would increase the penetration barrier to a certain extent,and the existence of Stone-Wales defects would significantly reduce the penetration barrier of these two materials to hydrogen ions.At room temperature（298.15 K）,theβ_H/D of intact Gra and h-BN were～11.2 and～9.5,respectively,similar to the experimental result(β_H/D～10).And it was further calculated that theβ_H/T of intact Gra and h-BN molecular models were～35.2and～25.5,respectively,which is better than the best performance of the SPE electrolytic concentration tritium system over the world.Based on the above experimental optimization and theoretical simulation for the membrane electrode,a new set of SPE electrolytic tritium concentration system with highβvalue(β_H/T～6.98)was successfully built,and a novel,efficient graphene SPE electrolytic tritium concentration system was conceptually designed,which would advance the establishment of a highly efficient,economical,practical,safe and reliable hydrogen isotope separation technology.