Waste acceptance criteria testings of a phosphate-based geopolymer waste form to immobilize radioactive borate waste

Abstract

Immobilization of radioactive borate waste (BW) is an urgent issue in South Korea because of the lack of appropriate waste forms. This study aimed to investigate whether phosphate-based geopolymers could satisfy the waste acceptance criteria established by Korean government. Geopolymers with 40 wt% BW did not exceed the criterion of compressive strength (3.445 MPa) after the water immersion and thermal cycling tests, whereas geopolymers containing 30 wt% BW still showed higher than 3.445 MPa after waste acceptance criteria tests. In particular, the compressive strength increased and the total porosity significantly decreased after the thermal cycling and gamma irradiation tests. The leachability index of boron calculated according to the ANSI/ANS 16.1 test was found to be higher than the criterion value (>6.0), and the controlling leaching mechanism was diffusion. The characteristic analyses indicated that water-soluble borate phases were leached from the geopolymer during the water immersion and leaching tests. However, the structurally incorporated 4-coordinated boron was also present as a sparingly soluble phase. Consequently, phosphate-based geopolymers can be used as stable binder materials to immobilize radioactive BW.