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i18

Uranium mining

Radioactive decay products

Uranium is a radioactive metal, which decays by alpha and gamma emission into other elements, called the decay products or 'daughters'. The decay products are also radioactive, most of which are potent alpha-emitters. Consequently uranium bearing rock contains a number of radioactive elements. In the natural condition the radionuclides are confined in more or less insoluble minerals in the rock of the uranium ore deposit. Uranium ores are generally very old geologic formations, with ages of 1-2 billion years. In spite of its old age uranium-bearing rock is anything but harmless, for the rock emits gamma radiation and the dust of it contains dangerous alpha emitters. By far the most uranium deposits of the world are located in sparsely inhabitated and often arid regions, for example in Australia, Namibia, Kazakhstan and in the USA.

Mill tailings

To obtain uranium it has be extracted from uranium ore by physical and chemical separation processes. At the uranium mine the ore is mined, then milled (ground to powder) and finally chemically treated to extract the uranium. The other radionuclides in the ore, the decay products of uranium, remain in the tailings (waste stream) of the extraction process. The mill tailings have the appearance of a watery mud and consist of the ore powder, chemicals and large volumes of water. The radioactive mud is stored in large ponds. From then on the radioactivity from the uranium ore is mobile. A part of the water from the mud will evaporate and the other part, including the dissolved radionuclides, drains into the ground. When the mill tailings go dry, the remaining fine powder will be easily spread by the wind. This situation occurs when one pond is filled up and a new one is taken into operation and after the mine has been mined out and is abandoned.

Satellite photographs show dust from the Sahara desert crossing the Atlantic Ocean under certain conditions. An indication how far dust, and so radioactive dust, can be transported by the wind. Hundreds of thousands of square kilometers are contaminated in this way. The dust blown off the mill tailings contains highly radiotoxic elements, such as Ra-226, Pb-210 and Po-210. The lethal dose of polonium-210 is some 50 nanogram. Inhalation of the dust is a dangerous contamination pathway, for most of the radionuclides are potent alpha emitters. The decay products can also enter the body via drinking water, as the groundwater at large distances from the mining area may be contaminated with soluble compounds of the radionuclides, seeping from the mill tailings. The groundwater table in vast areas is also being contaminated with non-radioactive chemicals, such as arsenicum.

Mine reclamation

Mine reclamation (also called rehabilitation) comprises the actions needed to restore the mining area to a habitable one again. The chemically mobile radionuclides in the mill tailings should be immobilized again and put back in the mining pit, as deep as possible. To prevent remobilization by groundwater flows, the mill tailings must be shielded from the groundwater by an effective barrier, for example thick layers of bentonite. Bentonite is a clay mineral with special properties: it swells by the uptake of water, effectively closing fissures and microchannels, and has strong ion-exchange properties, resulting in a very low migration rate of nuclides, other than hydrogen ions and some alkali metal ions.

Nowhere in the world, as far as known, has the impact on the environment by uranium mining been compensated for in a way that can be considered ecologically adequate and safe to the local inhabitants. Uranium mining companies leave the mill tailings unshielded in the mining area. After the last kilogram of uranium has been removed from the site, the lights are turned off and the gate is closed.

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rangersatphoto2

Figure 18-1. Satellite photo of the Ranger uranium mine in Australia

Satellite photo of the Ranger uranium mine in Australia, a medium-sized and one of the cheapest operating uranium mines in the world. The large green geometric pond on the lower left is the mill tailings pond. The light colored areas are the overburden and waste rock dumps. The active mining pit is the one at the upper right, partially flooded. The round dark object at the lower center is a former, mined out, pit which is fully flooded. The extraction plant (mill) is on the far right, barely discernable. Note the scale bar in the lower left corner. Source photo: Google Maps.

mm

Figure 18-2. Uranium mining and mine reclamation

Outline of uranium mining, the first step of the nuclear process chain. The area directly disturbed by the mining operations of a large uranium mine may come to some 100 km2. The indirectly disturbed area, by wind blown dust and contaminated groundwater, may run into hundreds of thousands of square kilometers. When the ore is exhausted, the dangerous mill tailings should be immobilized and the mine and its surrounding area should be restored to the original situation, a process called mine reclamation.

saharasandstorms

Figure 18-3.

A massive sandstorm blowing off the Northwest African desert has blanketed hundreds of thousands square kilometers of the Eastern Atlantic Ocean with a dense cloud of Saharan sand. This photo shows how far dust from arid areas, including radioactive dust from uranium mines, can be transported by the wind. Photo SeaWIFS/NASA.