Dissolution experiments with surrogate treated organic wastes immobilized by hot isostatic pressing

Abstract

The presence of organic compounds in radioactive wastes might be incompatible with long-term waste management options. Consequently, as part of the H2020-PREDIS project, different thermal treatment routes and conditioning techniques were first used to process surrogates of solid organic wastes. Then, the chemical durability of the conditioned wastes was investigated performing dissolution experiments in a synthetic cementitious water at pH 12.7, 22 °C and in diluted conditions. These tests were conducted with a glass–ceramic, produced by hot isostatic pressing using a mixture of ashes and sodium tetraborate (95/5 wt%). The results showed that glass–ceramic dissolution was incongruent and controlled by diffusion. SEM–EDX analysis showed the presence of an altered zone of about 45  µm after 730  days of alteration, with amorphous and crystalline phases (spinel and chlorapatite), and containing voids due to the dissolution of the glass matrix. The average diffusion coefficients were in the range of (4.26 ± 1.36) × 10⁻¹⁵ – (2.88 ± 0.59) × 10⁻¹³ m²/s, with the lowest and highest values for Zn and B, respectively. This corresponds to leachability indexes in the range of 10.40 ± 0.14 and 8.54 ± 0.07. The maximum dissolution rate of the glass matrix based on the B release between the start of the tests and 28  days was similar to that found when borosilicate glass was corroded by a KOH solution at pH 12.5. Hot isostatic pressing was thus successfully used to immobilize ashes, and the resulting glass–ceramic exhibited good chemical durability under alkaline conditions.