The Study Of Tritium Decontamination From Nuclear Graphite Base On Hydrogen Isotope

Due to the outstanding properties against radiation and high temperature,graphite has been widely used in nuclear reactors.As these reactors come to its lifespan,they have to be decommissioned.Because of the big volume and various radionuclides contained,irradiated nuclear graphite should be taken great care for decommission.Tritium and carbon-14 are the two radionuclides with highest content in nuclear graphite.Currently,thermal treatment is the most viable and effective method for the removal of tritium from nuclear graphite,and the removal efficiency is influenced by the treatment temperature,the type of carrier gas and its flow rate,the type of nuclear graphite.However,the optimum treatment temperature for carbon-14 in nuclear graphite was proved to be 700oC,hence the study of tritium removal under 700oC is crucial.This study combine computational simulation and experiment,investigate the absorption and desorption behavior of hydrogen and deuterium in nuclear graphite,and predict the behavior of tritium according to the results of hydrogen and deuterium experiment.Base on first principle calculation,this study simulate the absorption and desorption behavior of hydrogen in graphite.A 6×6 carbon super cell was constructed to simulate the graphite matrix using Vienna Ab-initio Simulation Package.Using this model,the present study calculated the absorption energy of a hydrogen atom on a graphite surface,a hydrogen molecule on a graphite surface and a graphite surface with point defect.The results showed that the absorption energy of a hydrogen molecule was so low that most of the absorption will take place physically,the existence of point defect help the absorption of hydrogen.In order to investigate the desorption of hydrogen,the desorption of a hydrogen from a point defected graphite surface was simulated.The energy barrier for desorption was calculated to be 2.17eV,corresponding to a desorption temperature around 600oC.Since the simulation results agreed with the experimental results in this study,the removal of hydrogen from graphite can be simulated using first principle calculation.Finally,the zero point energy was calculated to investigate the isotopic effect on the behavior of hydrogen,deuterium and tritium.The results showed that the bond between tritium and carbon was the most stable among others,hence the absorbed tritium would retained firmly in graphite matrix,and the desorption of tritium should be more difficult than hydrogen and deuterium.Due to the high permeability of hydrogen and its isotopes,a set of experimental apparatus was developed for the investigation of the removal of hydrogen and its isotopes from nuclear graphite base on gas chromatography.By modifying the gas chromatograph and improving the measuring method,the separation and measurement of high concentration hydrogen and deuterium was achieved.Furthermore,a new method of measuring the amount of absorption and desorption was developed.Using the apparatus and experiment method set up,the absorption and desorption behavior of hydrogen and deuterium in three types nuclear graphite was investigated experimentally.The total absorption of hydrogen in IG-110,NBG-18 and NG-CT-10 was 9.9×10^-3±3.1,3.4×10^-3±0.9,5.8×10^-3±0.8 mL/g,respectively,however,the fraction of strong absorbed hydrogen was 33%,6%and 49%respectively.Consequently,although the total amount of absorbed hydrogen was the highest in IG-110 nuclear graphite,the amount of strong absorbed hydrogen in NG-CT-10 was the highest,the difference in total absortion was due to the difference in degree of graphitization,while the difference in chemical absorbtion was because of the different average pore size of the samples.As in the desorption experiment,the desorption of hydrogen in all three graphite samples show little difference,when temperature was below 600oC,less than 1%of the absorbed hydrogen was released.While temperature was raised to 700oC,as the energy provided overcome the energy barrier for desorption,big amount of hydrogen was released at700oC,which matched the computational results obtained from desorption simulation.Through dynamic analysis of the desorption data,desorption of hydrogen was a diffusion controlled process,the rate of desorption was mainly determined by the diffusion of hydrogen in graphite lattice.The remaining hydrogen came from the interior of graphite,when the bonds between carbon and hydrogen was broken,their energy was not enough to escape from the graphite matrix.As for the absorption and desorption experiments of deuterium,the amount of absorption was two times higher than that of hydrogen,and because of the difference in diffusivity,longer time required to complete the absorption for deuterium.But the desorption of deuterium showed similar trend as temperature increased from 400oC to 700oC,and similarly,the desorption of deuterium was much slower.According to the computational simulation and experiments,the behavior of tritium was predicted.First of all,the adsorbed tritium should also be classified as strong adsorption and weak adsorption.Furthermore,tritium absorption should be also slower than other two isotopes due to its lowest diffusivity.Moreover,the simulation and experimental results all showed that desorption of hydrogen and deuterium required a temperature above 600oC.Therefore the energy barrier for desorption and the temperature that big amount of tritium desorbed should be similar to hydrogen and deuterium.Lastly,this research prove the possibility of the removal of tritium at 700oC,meaning the removal of tritium and carbon-14 can be joint together.However,the removal efficiency for tritium at 700oC has to be improves through adjusting process parameters,including the type of carrier gas,flow rate.