Evaluation on migration and transformation of trace nuclides in thermal degradation for low-level radioactive waste

Abstract

The migration and transformation of trace nuclides (cobalt, cesium and strontium) during pyrolysis and gasification of low-level radioactive waste (plastic, glove, coverall, paper towel, sawdust and mixture) were evaluated based on two lab-scale fixed-bed reactors and chemical equilibrium calculation. Co and Cs mobilities in plastic were both higher than that of glove at the same pyrolysis temperature. From the test of cesium migration, the mobilities of cesium in plastic and glove due to the addition of ZrO2 (wt.1%) during thermal treatment almost reduced by one half or above, indicating that ZrO2 was a favorable agent for immobilization of cesium. Phosphorus and sulphur in low-level radioactive waste could resist the migration of Co. The pyrolysis temperature for the mixture should be kept below 509 ºC to prevent the cesium migration. Chlorine in the samples could increase the migration of Cs. The gasification temperature should be kept below 1200 ºC to prevent cobalt and strontium migration. Negative micro-pressure operation did not impact greatly the migration of the trace nuclides. CsOH (g) in plastic and CsCl (g) in mixture decreased both by about 91% by increasing the pressure from 0.1 MPa to 1.0 MPa, indicating that lifting the pressure could effectively resist the migration of the trace nuclides.