Solidification of radioactive waste in lignite slag and bismuth oxide filled elastomer matrices: Release mechanism, immobilization efficiency, long term radiation stability and aging
Piotr Szajerski
Abstract
This paper presents results of the experimental studies on immobilization of radioactive waste in composite elastomeric matrices based on the natural rubber and mineral fillers lignite slag and bismuth oxide. For preparation of the matrices a classical mixing procedure using a roll mill was applied using isotopically labeled fillers with adsorbed Cs-137, Co-60, Sr-90 and Am-241 tracers, followed by vulcanization at 160 °C and 20 MPa pressure. The composites were verified towards efficiency of radioactive isotopes immobilization and resistance against high radiation doses up to 1000 kGy. For this purpose such parameters as diffusion coefficients, leachability indexes, leaching rates as well as mechanical and radiation shielding properties for the native and radiation modified composites were determined and analyzed. The results proved very good immobilization efficiency of the waste bearing matrices and moderate radiation resistance. The results obtained from leaching experiments show effective diffusion coefficients of order 10−13–10−11 cm2·s−1, leachability indexes significantly above 9, and the normalized leaching rates after 28 days at most at a level of 2.78·10−6 g·cm−2·day−1. The composites exhibit relatively good radiation resistance against doses up to ca. 100 kGy, after which their mechanical properties (stress at 100% elongation, tensile strength and elongation at break) drop, with a significant increase of the hardness and crosslinking density. The composites exhibit good radiation shielding properties, especially in case of the heavy weight bismuth oxide filler. The proposed methodology has a strong potential to be successfully applied for radioactive waste stabilization in industrial scale.