Research On Irradiation Of Carbon Materials And Fabrication Of Transmutation Targets

Every year tens of thousands tons of high radioactive nuclear waste has been produced,which will pose a great threat to humanity when they accumulate gradually. Presenting ofPartitioning and Transmutation technique bring dawn and hope to this problem. The technique isthat, first separating MA and LLFP nuclides from high radioactive nuclear waste and thenirradiating them to make them change to stable and short life nuclide. In addition, carbonmaterials have been widely applied in nuclear power and aerospace field. So the structurestability of them in severe radiation environment is quite important.In this paper, we studied the structure and property change of carbon fiber, MWCNT andgraphite under gamma ray, electron and Cu²⁺irradiation systematically. The results revealed thatthe degree of graphitization of the carbon fiber gradually increased and the surface wettabilityenhanced after gamma ray irradiation. Forced resonant peak method and single filament drawingmethod were used to determine the Young’s modulus of the carbon fibers before and aftergamma irradiation, which showed that the Young’s modulus of the fiber increased withincreasing gamma dose and it reached the maximum at a dose of2MGy. In FETEM, focusedelectron beam was performed on the carbon fiber surface. It could create a gap and nano-sizedcarbon onions. Low dose Cu2+irradiation could raise the degree of graphitization of the carbonfiber while high dose ion irradiation damaged the carbon fiber. The carbon fiber surfacewettability was first enhanced and then down with increasing the ion dose and reached themaximum at the dose of1015ions/cm².With increasing gamma irradiation dose, the degree of structural order in MWCNTs wasdecreased first and then increased, the surface wettability of the nanotube was enhanced, and theclear straight fringe of the nanotube was changed to amorphous structure before it changed backto ordered structure. The damage degree to the nanotube after electron irradiation increased withincreasing the electron energy and current density. Electron irradiation could also create carbononions on MWCNTs. In addition, it could make two nanotubes weld together. When carbonnanotubes were irradiation by Cu2+, with the increase of irradiation dose, the structure ordergradually reduced, and the surface wettability presented enhancement trend before weakening like carbon fibers. At the ion dose of1015ions/cm², the carbon nanotubes showed a specialmorphology which contained part of the hollow structure and part of closure structure. However,the carbon nanotubes completely transformed to the amorphous nanowires with the increase ofirradiation dose. Ion irradiation could also make carbon nanotubes welding, which made theyform the amorphous nanowire reticular structure.In contrast to MWCNTs, the degree of graphitization of graphite was increased beforedecreased after gamma irradiation and it reached the maximum at the dose of200kGy. The mainreason for the different evolution of structural order between graphite and MWCNTs was thecurvature-induced strain. Studies have found that only low rate gamma irradiation could raise thestructure order in flaky graphite we used. When graphite with thin graphene layer was underelectron irradiation, the layers curled and showed the trend to sphere structure. The highresolution images became disorderly structure after electron irradiation of graphite with thickgraphene layer. However, the electron diffraction pattern did not change after electron irradiation.Unlike carbon fiber after Cu2+irradiation, there were a lot of holes, pits and some melting traceon the graphite surface, and the high resolution images turned to be disorderly. The amorphouscarbon film can be observed gradually clear high resolution images of fringes after ionirradiation and the fringe distance is about0.36nm. Graphitization of the amorphous carbon filmis caused by rearrangement of atoms and irradiation heating effect.The fabrication and properties of Al₂O₃-CNTs and Si₃N₄-Re transmutation simulationtargets were investigated. The results showed that the relative density, Vickers hardness andfracture toughness of the Al₂O₃-CNTs transmutation target sintered by SPS increased beforedecreased with increasing the content of CNTs, and they reached the maximum when the contentof CNTs was1vol.%. The bending strength of the Si₃N₄-Re transmutation target sintered bypressureless sintering method decreased with the increasing of the Re content. The relativedensity was maintained at about98%. The Si₃N₄matrix would react with Re if the sinteringtemperature exceeding1550oC. The two kinds of target were irradiated for24h and48h inXi’an pulse reactor to perform transmutation experiments. Due to the strong inducedradioactivity, we will investigate mechanical properties and irradiation swelling mechanism ofthe transmutation targets after cooling and moderation of the seal cans.