Study On Desorption Behavior Of Tritium In Decommissioned Nuclear Graphite By Neutron Irradiation Defects

Nuclear graphite is widely used in various types of reactors.With the decommissioning of reactors,a large amount of long-lived waste nuclear graphite containing radionuclides will be left.Among them,tritium is one of the most important sources of radioactivity,so the removal of tritium is an important prerequisite for the reasonable disposal of decommissioned nuclear graphite.Under high neutron flux irradiation,a large number of irradiation defects,such as interstitial atoms and vacancies,will inevitably occur on the graphite surface or between the layers.The interaction,migration,binding between interstitial atoms,and their adsorption behavior of tritium will affect the form of desorption,rate and products of tritium in nuclear graphite.Therefore,it is important to clarify the evolution law of the migration and combination of irradiation defects and their influence on the tritium adsorption and desorption behavior,which has important guiding significance for the detritium removal of decommissioned nuclear graphite.Based on the first principles,this paper simulates and calculates the interaction behavior between interstitial atoms in graphite,explores the migration and combination of interstitial atoms and vacancy defects in graphite.Hydrogen was used to replace tritium to simulate the influence mechanism of interstitial carbon atoms,vacancies and other radiation defects on the desorption behavior of tritium in nuclear graphite,and to explore the possible desorption products of tritium.On the one hand,the interstitial interaction,migration and binding evolution are studied in this paper.It is found that there is gravitation between atoms in the interstitial space on the same graphite sheet,and the interaction energy can be ignored when the space is larger than 5?.It is found that the interstitial atoms tend to migrate and bond along the armchair direction.The most stable cluster structure in the graphite is chain structure,followed by the structure of the closely adjacent spiro interstitial atoms.While the number of interstitial atoms is more than 6,a cluster,containing six-membered carbon rings,with its bottom will be formed stacking in AB.If more interstitial atoms are combined,new graphite sheet will eventually be formed.On the other hand,the influence mechanism of radiation defects such as interstitial atoms and vacancies on the desorption behavior of tritium was explored.The simulation results show that the interstitial atoms have a strong adsorption capacity for hydrogen atoms on the graphite sheet,the interstitial atoms can gradually adsorb 3 hydrogen atoms until the sp³hybrid structure is formed.The CH₃structure without dangling bonds tends to desorb as ionic CH₃,and finally combines with free H or other CH₃to form CH₄or C₂H₆,instead of directly desorbing in the form of CH₄;Unsaturated CH and CH₂structures will migrate in the graphite sheet.Among them,the energy barrier of CH₂structure migration reaction is slightly higher than interstitial atoms,and CH structure is lower than interstitial atoms.CH(or CH₂)structure tends to be compatible with other CH(or CH₂)structure polymerization,desorb in the form of C₂H₂(or C₂H₄),and its reaction energy barrier is slightly lower than the desorption reaction energy barrier of CH₃structure.Although the migration energy of the interstitial atoms with H adsorbed on the graphite sheet is not much different from that of the interstitial atoms without H,the reaction energy barrier of their recombination with single vacancies is significantly greater than that of the interstitial atoms without H.Therefore,this article predicts that a large number of interstitial atoms in decommissioned nuclear graphite will be one of the important sites for chemisorption of tritium,because of the larger adsorption energy,the reaction energy barrier caused by tritium gas desorption is significantly higher than that of carbon-containing tritium desorption products(including C₂T₂,C₂T₄,CT₄or C₂T₆).The development of this work will provide technical guidance and support for the heat treatment and tritium desorption products process of decommissioned nuclear graphite,which has great application value.