Theoretical Study Of The Adsorption And Diffusion Of Metallic Fission Products On Nuclear Graphite

The Generation IV nuclear reactors have the advantages of high inherent safety,nuclear fuel cycle, physical protection of nuclear nonproliferation and so on. Nucleargrade graphite has been used as neutron moderator, reflection layer and structure materialof reactor core in nuclear reactors, with excellent performances of high moderating ratio,low thermal expansion coefficient, high temperature resistance, thermal shock resistanceetc. Tristructural-isotropic (TRISO) coated fuel particles, which are coated by multilayersilicon carbide and graphite, are used as nuclear fuels in Fluoride salt-cooled Hightemperature Reactor, one of the Generation IV nuclear reactors. When the reactors areunder extreme conditions, radioactive metallic fission products can be released from thenuclear fuel cores. And then they immediately contact with nuclear graphite used asneutron moderator, reflection layer and the barrier of fission product. Defects can beinevitably generated under neutron irradiation in nuclear graphite. So it is of criticalimportance to understand the behaviors of metallic fission products on defective graphitefor evaluating the safety of the Generation IV nuclear reactor. Cesium, Strontium andSilver are the typical radioactive metallic fission products and having very long half-life.Therefore, the interaction mechanism between metallic fission products and the nucleargraphite surface with or without defects is exploratively studied based on thefirst-principles calculations.Firstly, we have investigated the interaction between metallic fission products (Cs,Sr and Ag) and the nuclear graphite surface with or without defects. We found that themost favorable adsorption site on pristine graphite surface for fission products Cs and Sris the hollow site. They are both chemical adsorption while Ag is physical adsorption.Defects enhance the adsorption of Cs, Sr and Ag on defective graphite surface. Thediffusion behaviors of adatoms on the graphite surface with or without defects are studiedby using nudged elastic band method. Cs and Sr, especially Cs, can easily diffuse on thepristine graphite surface. Defects can serve as the trap sites for constraining the metallicfission products on defective graphite surface.Nuclear power is an important means to ensure national energy security and promoteemission reduction and fuel consumption. With the gradually extensive use of nuclearenergy and the serious consequences of nuclear accident, the treatment and disposal of radioactive nuclear waste have become the focus of world attention. Carbon nanotubespossess excellent properties such as high mechanical strength and elasticity, high specificsurface, strong adsorption energy and so on, which are good candidates for radionuclidepreconcentration. At present, the research group of Prof. Xiangke Wang in Institute ofPlasma Physics Chinese Academy of Sciences have used carbon nanotubes in nuclearwaste management and achieved good results. Therefore, the interaction between metallicfission products and armchair single walled carbon nanotubes (SWCNTs) with differentdiameters is also studied. Comparing the adsorption parameters of Cs and Sr on (6,6),(7,7)and (8,8) SWCNTs, we found that Cs and Sr adatoms are more stable on the interiorsurface of SWCNTs than those on the exterior surface. With the increasing of thediameter of SWCNTs, the difference of the adsorption energies between the interior andexterior surfaces of SWCNTs decreases and the adsorption energies eventually approachthose of adatoms on the graphene.