Theoretical Study On Anti-disproportionation Mechanism And Doping Modification Of ZrCo Tritium Storage Alloy

In pursuit of low-carbon society,as one of the fourth generation of nuclear energy systems,molten salt reactor was intensively studied to meet the future energy needs.In the Thorium Molten Salt Reactor?TMSR?system,Li,Be in the coolant salt and B in the control system will generate a certain amount of tritium under neutron irradiation.In order to reduce the radioactive pollution caused by tritium emission to the surrounding environment,it is particularly important to control tritium generated in molten salt reactor.ZrCo alloy is considered to be the most likely substitute for uranium as the tritium?and hydrogen?storage material due to its low hydrogen absorption/desorption platform pressure at room temperature,excellent hydrogen absorption/desorption kinetics and strong ability of retaining helium,it was also used as a candidate material for tritium storage and transport for ITER project.However,the degradation of hydrogen storage capacity of ZrCo alloy will be induced by hydrogen-induced disproportionation during the process of hydrogen desorption,which is one of the important issues limiting its application,and the micro-mechanism of this problem still needs to be studied.As a naturally radioactive hydrogen isotope,tritium is expensive and difficult to restore,so it is difficult to study the tritium storage properties of ZrCo alloy directly.Owing to the same electrons and similar chemical properties of hydrogen,deuterium and tritium,most experiments on tritium storage materials are primarily carried out using hydrogen and deuterium.Meanwhile,with the help of the supercomputer,first-principles calculations have been widely applied in materials simulation and design.In this project,aiming at the disproportionation phenomenon of ZrCo alloy,first-principles calculations were explored to study the electronic and atomic structures of ZrCoH₃ before and after element substitution systematically,and our results revealed the internal mechanism of disproportionation of ZrCo alloy,offering theoretical prediction to search the alloying elements with good anti-disproportionation properties,and provides sufficient theoretic basis for the experimental study.The effect of isotope effect on hydrogen storage performance of ZrCo alloy was also studied experimentally.The VASP package based on density functional theory was used to systematically investigate the binding energies of H atoms at different sites of ZrCoH₃and the bonding characteristics in ZrCoH₃ before and after element substitution.The results showed that in the perfect unit cell structure of ZrCoH₃,the binding energy of hydrogen atom at the 4c₂ site surrounded by 3Zr and 1Co atoms at?0 y 1/4?or 8f₂ site surrounded by 2Zr and 2Co atoms at?0 y z?is lower,while the binding energy of hydrogen atom at the 4c₂ site surrounded by 2Zr and 2Co atoms at?x 0 0?is higher,which means that the hydrogrn-induced disproportionation is caused by only part of H atoms in 8e site where is less stable in ZrCo alloy.In undoped and doped ZrCoH₃,the binding between H and Zr?or its substitute elements?shows strong ionic and weak covalent feature,and the binding between H and Co?or its substitute elements?displays weak ionic and strong covalent characteristic.In addition,researches were carried out to investigate how the volume of the tetrahedron where H?8e?is located in ZrCoH₃,the corresponding Zr-H?8e?bond length and diffusion barrier influence the disproportionation of ZrCo.The calculated results showed that partial substitution of Ti,Hf,Ni and Fe in ZrCoH₃,Ti and Ni Co-doping ZrCoH₃ can affect the bond length of Zr-H?8e?,the size of 8e tetrahedron and the energy barrier for the diffusion of hydrogen atom from 8e to the nearest neighbor to suppress or accelerate the disproportionation.Therefore,we believe that the influence of element substitution on the anti-disproportionation of ZrCo alloy can be predicted from the size of the 8e tetrahedron and the corresponding bond length of Zr-H?8e?.Based on the above conclusions,we investigated the atomic and electronic structures of Zr or Co substituted by V,Nb,Ta and Ru in ZrCoH₃.The results showed that the substitute elements of Zr?i.e.Nb,Ta and especially V?are predicted to be helpful to improve the ZrCo alloy against the hydrogen-induced disproportionation?which have relatively long Zr-H bond length and relatively small 8e tetrahedron volume?,and the substitution of elements such as Ru for Co can possibly accelerate disproportionation.The formation of heat and cohesive energy indicated that the compound can exist when V or Nb is doped into ZrCo in a certain ratio.In Nb-doped ZrCoH₃,the activation energy of H diffusion pathways from the 8e site to the 4c₂ site is 0.03eV,lower than that in undoped ZrCoH₃.Hence Nb substitution in ZrCo may improve the anti-disproportionation ability of the alloy.Moreover,the electronic structures of ZrCoH₃,ZrCoD₃,ZrCoT₃ and the binding energies of hydrogen,deuterium,tritium atom with different occupancies in the unit cell were investigated,and the effects of hydrogen and its isotopes?deuterium?on the hydrogen storage properties of ZrCo alloys were also studied experimentally.The results showed that the chemical environment between hydrogen isotopes and their surrounding atoms are almost same.The stability of tritide and deuteride are stronger than that of hydride.For atos at 8e site,the stability of hydrogen atom is also lower than that of deuterium and tritium.Hydrogen isotopes only affect the activation properties of ZrCo alloy,but have little effect on hydrogen storage capacity.Under completely activated condition,the total amount of hydrogen/deuterium absorption are almost the same.The plateau pressure of deuterium absorption/desorption for ZrCo alloy are slightly higher than that of hydrogen absorption/desorption,showing a positive isotope effect.The disproportionation effect of hydrogen isotopes on ZrCo alloys is that the extent of deuterium-induced disproportionation is lower than that of hydrogen-induced disproportionation,thus the extent of tritium-induced disproportionation should be the lowest.