6.1.2 Tailings management and mine rehabilitation

From open cut mining, there are substantial volumes ofbarren rock and overburden waste. These are placed near the pit and either used in rehabilitation or shaped and revegetated where they are.

Uranium minerals are always associated with other radioactive elements such as radium and radon in the ore, which arise from the radioactive decay of uranium over a few million years. Therefore, although uranium itself is barely radioactive, the ore which is mined, especially if it is very high-grade such as in some Canadian mines, is handled with some care, for occupational health and safety reasons.

Mining methods, tailings and run-off management and land rehabilitation are subject to government regulation and inspection. For instance in Australia the code of practice and safety guide: Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing was published in 2005, updating previous versions.

Solid waste products from the milling operation are tailings, ranging in character from slimes to coarse sands. They comprise most of the original ore and they contain most of the radioactivity in it. In particular they contain all the radium present in the original ore. At an underground mine they may be first cycloned to separate the coarse fraction, which is returned underground and used for underground fill. The balance is pumped as a slurry to a tailings dam, which may be a worked-out pit as at Ranger and McClean Lake, or an engineered structure.

When radium undergoes natural radioactive decay one of the products is radon gas. Because radon and its decay products (daughters) are radioactive and because the ground rock comprising the tailings is now on the surface, measures are taken to minimise the emission of radon gas. During the operational life of a mine the material in the tailings dam is often kept covered by water to reduce surface radioactivity and radon emission (though with lower-grade ores neither pose a hazard at these levels). This water needs to be recycled or evaporated since it contains radium, which is relatively soluble. Most Australian mines and many others adopt a ‘zero discharge’ policy for any pollutants.

On completion of the mining operation, it is normal for the tailings dam to be covered by some two metres of clay and topsoil with enough rock to resist erosion. This is to reduce both gamma radiation levels and radon emanation rates to levels near those normally experienced in the region of the orebody, and for a vegetation cover to be established. At Ranger and Jabiluka in North Australia, tailings will finally be returned to the mine pit or underground, as was done at the now-rehabilitated Nabarlek mine. In Canada, ore treatment is often remote from the mine that the new ore comes from, and tailings are emplaced in mined-out pits wherever possible, and engineered dams otherwise.

Radon occurs in most rocks and traces of it are in the air we all breathe. However, at higher concentrations than are likely to be found at any modern mine it is a health hazard. Underground, especially in high-grade orebodies, it is removed by mechanical ventilation.

After mining is completed at established ISL operations, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of the operation so that any prior uses may be resumed. Usually this is potable water or stock water (usually less than 500 ppm total dissolved solids). Contaminated water drawn from the aquifer is either evaporated or treated before reinjection.

In contrast to the main US operations, the water quality at the Australian sites is very poor to start with, and it is quite unusable. At Beverley the original groundwater in the orebody is fairly saline and orders of magnitude too high in radionuclides for any permitted use. At Honeymoon the original water is even more saline, and high in sulfates and radium. When oxygen input and leaching are discontinued, the water quality reverts to its original condition over time.

Upon decommissioning, ISL wells are sealed or capped, process facilities removed, any evaporation pond revegetated, and the land can readily revert to its previous uses.

Mining is generally considered a temporary land use, and upon completion the area with any waste rock, overburden and covered tailings needs to be left fit for other uses, or its original use. In many parts of the world governments hold bonds to ensure proper rehabilitation in the event of corporate insolvency.