| 239Pu is one of the key nuclides in the high-level radioactive waste, which caused anon-ignorable harm to biosphere. It releases particles with a24100years half-life. Ithas an biological toxicity, and will lead to inner-radiation once being took into bodies,and even hard to be excreted out. So, it is important to study the retardation capabilityof Bentonites on migration of239Pu.The adsorption and desorption abilities of natural bleaching earth,GMZ-bentonite,Na-bentonite, Ca-bentonite, acid-Ca-bentonite and alk-Ca-bentonite were tested. Thewater permeability of natural bleaching earth, GMZ-bentonite, Na-bentonite andCa-bentonite were also tested. The retardation capability of natural bleaching earth,GMZ-bentonite, Na-bentonite and Ca-bentonite on migration of239Pu were studied byusing dynamic leaching experiments under different water levels and pH values.The results of these experiments show that:（1）All the samples reach the adsorption equilibrium after10days. At thismoment, Kdof239Pu is8.3386×104mL/g in acid-Ca-bentonite, Kdis8.0048×104mL/g in alk-Ca-bentonite, Kdis7.8082×104mL/g in Ca-bentonite, Kdis5.5570×104mL/g in GMZ-bentonite, Kdis5.0173×104mL/g in natural bleaching earth, Kdis2.8548×104mL/g in Na-bentonite. The adsorption abilities of the six bentonites fromstronger to weaker are: aci-Ca-bentonite> alk-Ca-bentonite> Ca-bentonite>GMZ-bentonite> natural bleaching earth> Na-bentonite.（2）The four samples reach the desorption equilibrium after10days. At thismoment, the desorption ratio of Na-bentonite is53.5%, the desorption ratio ofCa-bentonite is11.4%, the desorption ratio of GMZ-bentonite is9.6%, the desorptionratio of natural bleaching earth is8.9%. The desorption abilities of the four bentonitesfrom stronger to weaker are: Na-bentonite> Ca-bentonite> GMZ-bentonite> naturalbleaching earth.（3）The water permeability of four bentonitesunderdifferentwaterlevelsshowsthe same features: Ca-bentonite> natural bleaching earth> GMZ-bentonite>Na-bentonite.（4）The total radioactivity in leachates of all samples are very low after leaching100days, less than10Bq. The maximum leaching rate is less than7‰, the minimum is only0.2097‰. All bentomites has blocked the migration of239Pu with the flow.According to the leaching results, the total radioactivity in leachates is proportional tothe water velocity in column. The retardation capabilities of four bentonites fromstronger to weaker are: Na-bentonite> GMZ-bentonite> natural bleaching earth>Ca-bentonite.（5）In five samples, The centroid of230Pu source does not moved, and it shows atiny speed in other five samples. The maximum speed does not exceed0.3mm/d. Allbentomites show a strong retardation capability.Taking all results into consideration: GMZ-bentonite has a strong adsorptionability, a weak desorption ability and water permeability, and it shows a strongretardation capability. Thus, GMZ-bentonite is the best choice for backfill materials inthe four bentonites, with its strong comprehensive properties. |
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