Development of non-volatile immobilization process for radioiodine by cold-sintering of ceramic-based waste forms

Abstract

Volatile radionuclides present in the radioactive waste stream of the used nuclear fuel as well as other nuclear materials reprocessing facilities are one of the major causes of environmental hazards. Among these radionuclides radioiodine is most important because of its very long half-life and high environmental mobility. Therefore, effective adsorption and immobilization of radioiodine are required to protect the environment from its harmful effects. Radioiodine has low volatilization temperature of $500 ^\circ C$ and therefore cannot be retained in immobilization matrix treated at high temperature (e.g. vitrification, HIP, etc.).

Cold-sintering is a very low temperature consolidation process. Higher densification can be achieved by the combined effect of sintering temperature and pressure during the cold-sintering process. Cold-sintering is facilitated by the re-arrangement and densification of amorphous phase of the starting material under the mechano-chemical effect of applied pressure and the temperature in the range of room temperature to $300 ^\circ C$. This temperature window is safe for the consolidation of volatile radionuclides such as I-129 and efficient immobilization can be achieved by this process.

Calcium Hydroxyapatite and sodalite have shown loading capacity of iodine up to 8.4 wt% and 19.8 wt%, respectively, in their crystal structures. This loading can be achieved by the substitution of iodine in the form of iodate or iodide with the hydroxyl groups of the apatite and sodalite. However, sintering of the synthesized substituted ceramics is required to increase the density and to produce a compact matrix which has been carried out at high temperatures ($>800 ^\circ C$) using sophisticated sintering techniques (HIP, SPS etc.). In this study, cold-sintering of iodine-hosted calcium hydroxyapatite and sodalite was investigated for the development of a durable matrix for radioiodine immobilization. Single-phase, nano-crystalline, iodate-substituted calcium hydroxyapatite (IO-HAp) was synthesized by a wet precipitation method and sintering of the dried IO-HAp powder containing ~7wt.% of substituted iodine was carried out at $200 ^\circ C$ under a uniaxial pressure of 500 MPa. Meanwhile, the cold-sintering of sodalite (iodosodalite) containing ~ 14.97 wt.% of iodine was carried out at $300 ^\circ C$, 500 MPa for 10 min. It was demonstrated that sintered relative densities of 96.8% & 100% can be achieved without affecting the iodate nature of the substituted iodine. The cold sintered matrices shown good microhardness and compressive strength and the measured values were higher than the defined regulatory requirements. A product consistency test of the sintered samples was also carried out under standard conditions. The normalized leaching rates of Ca, P and I from cold sintered IO-HAp after seven days were 6.9 ($\pm 0.5$) $\times 106{-7}$, 2.6 ($\pm 0.2$) $\times 10^{-7}$ and 2.4 ($\pm 0.4$) $\times 10^{-5} g/m^2/d$, respectively, whereas the normalized leaching rates of iodine from cold sintered iodosodalite were on the order of magnitude $10^{-4} g/m^2/d$. The very low leaching rate of I provides evidence of the durability of the cold sintered matrix and promise of using the cold-sintering process for the conditioning of volatile element-bearing radioactive waste into solid waste forms.