Confirmation of the retention of 226Ra in U-mine tailings by barite

Abstract

Uranium ore from the Athabasca basin in Saskatchewan Canada, particularly from the McClean Lake and Cigar Lake deposits, is extremely rich in uranium (U): 2 to 20–25 % U₃O₈. The ore is also rich in many elements considered to be Constituents of Potential Concern (COPC) for the environment, including arsenic (As), selenium (Se), nickel (Ni), molybdenum (Mo), lead (Pb), cobalt (Co) and radium-226 (²²⁶Ra). Following uranium extraction, residual materials (tailings) containing these elements in partially immobilized forms are placed in the McClean Lake plant Tailings Management Facility (TMF). Uranium tailings are sulfate-rich low-level radioactive waste containing ²²⁶Ra, with activities of up to 3000 Bq/g. Barium chloride (BaCl₂) is added to precipitate ²²⁶Ra in barite as a solid-solution. Following this neutralisation treatment, ²²⁶Ra in the tailings is generally considered as sequestered in barite and sorbed onto ferrihydrite. To assess the subsequent stability of ²²⁶Ra in the tailings, an extensive study of its retention potential was performed. Characterisations incude assessment of tailings pore water and solids chemistry, tailings mineralogy, cation exchange capacity measurement, scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM/EDS), and alpha-autoradiography analyses supplemented with thermodynamic modelling. This work confirms that barite is the main ²²⁶Ra binding-phase, in agreement with the ²²⁶Ra concentration measured in the porewater (6 Bq/L) resulting from a coprecipitation reaction. A (Ba,Ra)SO₄ solid-solution regulates the average ²²⁶Ra concentration now and over the long term, through a distribution coefficient which is subject to a gradual shift towards a recrystallisation equilibrium. Recrystallisation causes an increase in the long-term concentration of ²²⁶Ra in the tailing water to 10 Bq/L. The work, conducted to assess the management strategy of the tailings, allowed us to ensure that the tailings have no significant effects on the downstream environment over the long term.