Immobilization of Radioactive Borate Waste Using Alkali or Acid-Activated Geopolymer Waste Form

Abstract

Immobilization of radioactive borate waste (BW) which is generated from a pressurized water reactor has been challenging because BW contains a high concentration of B and it hinders the hydration reaction of cement. Consequently, BW negatively affects the characteristics including setting and compressive strength of cement waste forms. Herein, alkali or acid-activated geopolymers were fabricated as alternative waste forms and investigated for the immobilization mechanism of BW in this doctoral thesis consisting of four specific goals. The first goal was to develop the metakaolin-based alkali-activated geopolymer (AAG) using simulant BW as part of the precursor material according to the characteristic that B replaces a part of Si. The AAGs using a KOH alkaline activator (K-AAGs) showed 20 times higher 7-day compressive strength than AAGs using a NaOH alkaline activator (Na-AAGs). In addition, the compressive strength increased proportionally to the Si/(Al+B) ratio regardless of the alkaline cation species. These variations in compressive strength might be due to the viscosity of the geopolymer mixture, atomic size of alkaline cations, and the increase in Si content. The characteristic analyses indicated that B was incorporated into the AAG structure. Thus, the K-AAG has a dense and homogeneous microstructure. In a semi-dynamic leaching test, less B was leached (~0.29) from the AAGs compared to the cement waste form (~0.50). The second goal was to investigate the effect of the Si/Al molar ratio and curing temperatures on the immobilization of BW in AAGs. In this study, K-AAG waste form to immobilize simulant BW was fabricated using different Si/Al ratios (1.0–1.4) and curing temperatures (26 and 60 ℃). The 7-day compressive strength test results revealed that a certain amount of Si and an elevated curing temperature are required to achieve high compressive strength (> 3.445 MPa) and waste loading (30 wt%). Following waste acceptance criteria tests, all K-AAGs exhibited compressive strengths higher than 3.445 MPa. The leachability index of B was higher than 6.0, and the leaching mechanism was identified as diffusion. No significant structural changes in the geopolymer were observed after thermal cycling and gamma irradiation tests. The physically bound or unincorporated BW was initially leached out of the geopolymer during water immersion and leaching tests; however, B, which was chemically connected with Si, was present as an inert phase together with a K-AAG binder. Consequently, immobilizing BW using a geopolymer with a low Si/Al ratio (1.4) is beneficial in terms of BW loading and structural durability. The third goal was to investigate the immobilization of BW in phosphate-based geopolymer (PAG) waste form. In addition, the phase change of BW in the PAG waste form was investigated. The PAG waste forms could immobilize BW up to 50 wt% waste loading depending on the water content and heat curing conditions. In PAG waste forms cured at high temperatures (60 and 90 °C), the 7-day compressive strength increased as the BW waste loading increased up to 40 wt% and the maximum 7-daty compressive strength was achieved at PAG with 40 wt% BW. Heat curing promoted geopolymerization reaction and the precipitation of amorphous boron phosphate, leading to an increase in the compressive strength. The formation of a new amorphous boron phosphate phase was confirmed by performing spectroscopic analyses of the PAG waste forms and reaction products. The final goal was to investigate whether PAG waste forms could satisfy the South Korea’s waste acceptance criteria. Herein, PAG waste form containing 50 wt% BW was not tested because it barely met the criterion of compressive strength (3.445 MPa). PAG waste forms with 40 wt% BW did not exceed 3.445 MPa after the water immersion and thermal cycling tests, whereas PAG waste forms containing 30 wt% BW still showed higher than 3.445 MPa after the all 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 B was found to be higher than 6.0, and the controlling leaching mechanism was diffusion. The characteristic analyses indicated that water-soluble borate phases were leached from the PAG waste forms during the water immersion and leaching tests. However, the structurally incorporated 4- coordinated B was also present as a sparingly soluble phase. This study suggests that AAG or PAG waste forms are more suitable and promising binder materials for immobilizing BW. It also provides insights into the mechanism of immobilizing BW within AAG and PAG waste forms.