Migration Behavior Of Key Nuclides In Repository Materials

With the continuous development of nuclear energy,more and more nuclear waste will inevitably be produced.For high-level radioactive waste(HLW),the generally accepted method is deep geological disposal.Beishan area in Gansu Province is one of the pre-selected sites for geological HLW repository of China.The main component of the surrounding rock is granite,and the proposed backfill material is Gaomiaozi bentonite.It is of great significance to study the migration behavior of key nuclides in the environment of repository.First,the continuous column method was used to study the migration of uranyl ions and strontium ion on Beishan granite column.The influences of flow rate,pH,temperature and speciation on migration were investigated.The results show that flow velocity has a significant effect on the migration behavior of uranyl and strontium ions on Beishan granite.There is an obvious linear relationship between flow velocity and dispersion coefficient,and the slope is the dispersity of the granite column.The distribution coefficient Kd obtained from the continuous column method(the Kd for uranyl ions:0.25 mL/g,and the Kd for strontium ions:0.3mL/g)is much smaller than that from the batch adsorption method.The speciation of uranium during migration was analysed by CHEMSPEC simulation,XPS and SEM.The results indicate that the speciation of uranium didn’t change during the migration experiment.According to the migration parameters obtained in this work,the break-through time of uranium to penetrate 500 meters of granite was predicted.The fastest break-through time is about 23 days,and the slowest is no more than a year.Second,Gaomiaozi bentonite was modified by bismuth nitrate(Bi(NO3)3)to improve its adsorption capacity for I-and IO3-without affecting its initial adsorption ability for cations.The results show that the Kd of modified bentonite for uranyl ions is still higher than 3414.66mL/g,while the KdS for IO3-and I-are more than 800mL/g and 250mL/g,respectively.The adsorption processes of modified bentonite for IO3-and I-are both spontaneous exothermic reactions,and their adsorption rates are very fast,which fulfills the quasi-second-order adsorption kinetic model.The results of this work can provide useful information for the safety assessment of HLW repository.