Understanding uranium behaviour at the Askola uranium mineralization

Abstract

Uranium / Rock alteration processes / Uranium migrationSummary. Understanding the behaviour of uranium is essential when assessing the safety of a spent nuclear fuel repository. The geochemical behaviour of uranium, including its reactive transport chemistry, is also a matter of concern when assessing the environmental impact of uranium mining. Subsurface uranium mobility is believed to be primarily controlled by dissolution and (co)-precipitation of uranium mineral solids and adsorption to mineral surfaces. This paper describes a modelling exercise based on characterisation of samples taken from drilled cores at the uranium mineralization at Askola, Southern Finland. In the modelling exercise, current conditions are assumed to be oxidizing and saturated with groundwater. PHREEQC was used for modelling in conjunction with the Lawrence Livermore National Laboratory database, chosen for its extensive coverage of uranium species and mineral phases.It is postulated that weathering processes near the surface have led to uranium dissolution from the primary ore, leaching out from the matrix and migrating along water-conducting fractures with subsequent re-diffusion into the rock matrix. Electron microscopy studies show that precipitated uranium occupies intra-granular fractures in feldspars and quartz. In addition, secondary uranium was found to be distributed within goethite nodules as well as around the margins of iron-containing minerals in the form of silicate and phosphate precipitates. Equilibrium modelling calculations predict that uranium would be precipitated as uranyl silicates, most likely soddyite and uranophane, in the prevailing chemical conditions beneath Lakeakallio hill.

Uranyl Minerals from Abandoned Podgórze Mine (Sudetes Mountains, SW Poland) and Their REE Content

“…As a result of field observations, several characteristic assemblages of secondary uranium minerals could be distinguished in the study area. Based on the literature data, it can be assumed that the decomposition of uraninite begins at a slightly acidic pH of ≈5.7 and partial pressure of CO 2 of 3.87 × 10 −6 atm [1,28,29]. At the beginning, uraninite decomposes into schoepite and metaschoepite.…”

“…However, uranyl oxide-hydroxides are unstable at pH < 6 and begin to slowly dissolve, releasing uranyl ions into the solution. At this point, uranyl-carbonate complexes such as uranyl mono-, di-, and tricarbonate [28][29][30] take on the most important role. In the first assemblage of uranyl minerals in the Podgórze mine, the most common speciation of uranyl carbonate is probably uranyl tricarbonate.…”

Natural Analogue Study on the Disposal of Radioactive Waste Using Uranium Deposits and Geochemical Behaviors of Uranium

“…우라늄은 화산암과 심성암의 주요 구성광 물(석영, 장석, 운모 등)에 미량(<1-10 ppm)으로 함유되 어 있으며, 저어콘(zircon), 인회석(apatite), 설석(sphene) 등의 부수광물에 약 1-10 ppm 정도로 농축되어 존재할 수 있다 (Gascoyne, 1982). 우라늄은 우라늄광상 뿐만 아 니라 다양한 암석에 존재하며, 지화학적환경에 따라 매 우 다양한 거동 특성을 가지기 때문에 방사성폐기물 지 중처분을 위한 핵종 장기거동을 연구하기 위한 이상적인 유사물로 간주되어 왔다 (Baik and Cho, 2022;Baik and Lee, 2020;Cui and Eriksen, 2000;Horie and Hidaka, 2004;Jokelainen et al, 2010;Noseck et al, 2004;Pérez del Villar et al, 1997;Quejido et al, 2005 (Baik et al, 2013). (Miller et al, 2000;NEA, 2004).…”