Mining

A major contentious issue with respect to nuclear power production is the mining processes necessary for fuel production. There exist a number of types of uranium mines appropriate in different geological conditions:

1. Open-cut
2. Underground
3. In-Situ Leaching

While these techniques differ considerably certain considerations are common to all. In particular, contentious issues due to environmental, health and social considerations of a mining process and mine site are often more related to the mined product and by-products than to the mine itself.

Open Cut Mining

This is the most common type of uranium mining in Australia; the uranium is accessed either by digging or setting off controlled explosions to reveal the uranium ore. The loose rock can then be separated into waste ore and uranium ore. The uranium ore then begins a journey as described in the figure below. This process is used when the ore is relatively close to the surface, if not, underground mining may be necessary. The sheer magnitude of rock removed is a significant strain on the environment, much more so than underground or in situ methods.

The process of obtaining the raw ore is identical to the open cut mining of coal and other materials. The most important difference is that the raw ore is at a higher activity when it is transported to the first stage of treatment.

Underground Mining

Underground mining, also known as closed cut mining, is used when the ore is deep underground. The ore is accessed through tunnels and shafts. Less rock is removed then in open cut mining, which means that there is generally less waste and less environmental impact. Most of the world's uranium comes from closed cut mines.

Once mined, the ore is crushed and the uranium dissolved out using sulfuric acid, which is then separated from the tailings. The tailings retain most of the radioactivity though the radioactivity is not at a high level. Around 85% of the radioactivity of the original ore is left over in the tailings. In particular the tailings contain radium, selenium, uranium and thorium. Radium, however, is the most dangerous as it decays into radon gas which is radioactive and can escape into the air. Precautions are taken against both the emission of gas, the level of radioactivity and to ensure that the tailings do not leak into the ground water. While a mining site is in use the tailings dam is covered with water and when a mine is no longer in use the tailings are generally returned underground or covered with two meters of clay and topsoil which means that the radioactivity is reduced to around the same level as before mining.

In-Situ Leaching

In-Situ Leach (ISL) mining, also known as solution mining, is a process that extracts minerals from sediment while leaving the ore-body where it is (in-situ) rather than removing it as with the more conventional mining techniques. In this respect the surface disturbance is minimal for ISL mining. Estimates place this technique as accountable for between 10 and 20% of world uranium production and it is expected to become much more dominant in the future mostly due to the low costs involved with respect to other types of mining. Currently all Kazakhstan and Uzbekistan production uses ISL, and it is prominent in the US mining techniques.

ISL is essentially a reverse of the precipitation process that places a sedimentary precipitation deposit, and is only applicable to this type of deposit, due to the geological requirements of the technique. An acid or alkali solution is pumped into the uranium-rich aquifer via a system of wells in order to create the appropriate conditions for re-ionisation. The uranium can then be brought to the surface by extraction of the solution.

There are two crucial geological requirements for a deposit to be considered for ISL mining:

• In order for the acid/alkali solution to move to the uranium deposit it must be contained within a permeable sediment (sandstone) porous enough for liquid transfer. This type of sediment is usually termed an aquifer since it is rock-body that contains ground water.
• The aquifer must be confined between impermeable sediments (clays), so that the acid solute does not contaminate external aquifers and ground water away from the ore body (see fig. 2). An aquifer can be artificially confined, or its sealing improved using cementing to block fractures and leakages.

An ISL mine is more of a well field than conventional mine. It usually consists of several injection wells placed 15-30m apart accompanied by production wells to recover the solution. In this way the solute can be cycled through the aquifer - returning groundwater to the aquifer after the uranium has been extracted. Monitor wells are also placed externally to the deposit to test that solute does not escape the confined uranium-rich aquifer.

Once a deposit is considered sufficiently contained, the procedure can begin:

• The first step is to remove ground water from the aquifer to create a pressure gradient in the sediments that encourages water flow into the ore-containing aquifer rather than away from the ore-body.
• The leaching solution is then pumped into the injection wells. This solution is either acid or alkali depending on the host aquifer constituents and an oxidant (dissolved oxygen or hydrogen peroxide). Australian proposals suggest the solution be pumped into the aquifer for periods between 5 and 7 days. Very long periods have resulted in aquifer damage that leads to major environmental impact (2).
• A 'pregnant' solution is one that has in it uranium dissolved from the ore body. This solution is pumped from the production wells to the treatment plant, where it undergoes an ion-exchange and precipitation process followed by dewatering to give a hydrated uranium peroxide (UO4.2H2O) product.
• The leaching solution then has its pH restored (if necessary) and is recycled through the well field. A small amount is withheld, as waste-water to maintain the pressure gradient in the aquifer.
• Once the ore body has had the maximum uranium extracted it is generally a legal requirement to restore the site to original conditions. This requires the removal of all pipes, pumps and chemicals and the return of ground water to its original conditions. Concentrations of uranium and other heavy metals should not be higher than in pre-mine conditions. In practice this can be difficult to achieve, although flushing the aquifer with clean water at the end of the extraction life has been successful in many mines. Usual a complete and successful restoration process takes between 15 and 20 years.

Acid leaching

In Australia and Europe conditions generally require acid leaching. A pH between 2.5 and 3.0 (about the acidity of house-hold vinegar) is required to dissolve the uranium oxides and is maintained with a weak sulphuric acid. Acid leaching is generally faster and more effective than alkaline leaching but can be more expensive do to the requirement of acid resistant well, pipes and pumps.

Alkaline leaching

When the calcium content is high, alkaline solutions are used, because otherwise there is the risk of significant degradation to the strength and structure of the ore-containing sediment. Also, acid solutions tend to dissolve heavy metals so where these are present, it is an inappropriate technique. This is the principal reason that alkaline solution is used in the US. Restoration of aquifers after ISL with the alkaline method is generally quicker than for the acid method.

Only the leaching process to recover the uranium is unique to ISL mining: the remaining processing and enrichment phases are carried out as part of the conventional fuel cycle.

Aspects of the general process are summarised below:

References:

1. http://www.uic.com.au/nip40.htm
2. http://www.aph.gov.au/library/pubs/rp/1997-98/98rp12.htm