MIGRATION OF SR, ND AND CE IN UNSATURATED CHINESE LOESS UNDER ARTIFICIAL SPRINKLING CONDITIONS: A FIELD MIGRATION TEST

摘要

Double concentration peak phenomenon has been observed in nuclide migration tests in unsaturated Chinese loess during the cooperative research between China Institute for Radiation Protection (CIRP) and Japan Energy Atomic Research Institute (JEARI), and was considered conflicting with traditional theory of solute migration. In order to confirm the existence of this phenomenon and better understand its formation mechanism, we conducted a nuclide migration test that lasted 470 days using Sr, Nd and Ce which are analogues of ~(90) Sr and actinides, with loess and fine arenaceous quartz, respectively, as tracer carriers. In addition, we examined the efficacy of capillary barrier which is constructed by placing fine-grained soil on a layer of course-grained material, according to its influence on nuclide migration. When using loess as tracer carrier, a fraction of Sr migrated downward from the source layer to form a migration peak, and the residual Sr formed another concentration peak which corresponds to the source layer; while Nd and Ce hardly migrated during the 470 day test with only one concentration peak in the source layer. When using fine arenaceous quartz as tracer carrier, double concentration peak phenomenon occurs for all the nuclides examined, with the peaks distributing, respectively, on the upper and lower sides of the source layer. This phenomenon was suggested to result from the very low water containment ability and nuclide retentivity of the source layer arenaceous quartz. Thus, the so called double concentration peak phenomenon is formed by the source and subsequent migration of part of the source. The obviously reduced migration of Sr when taking fine arenaceous quartz as tracer carrier demonstrated significant influence of the capillary barrier formed by the fine arenaceous quartz layer and overlying loess on nuclide migration. Considering that the fine arenaceous quartz layer is very small (7 mm) in thickness and horizontally placed and the small dimension of the test pit, capillary barrier could be an effective way to protect the underlying waste from leaching.