Uranium Deposits

Resource Classification

In today's nuclear reactors, the basic fuel is uranium. Uranium is NOT a renewable energy source and so one of the important questions is "How much is there?" Some of the following definitions under the McKelvey Classification system can help us to consider a response:

• Resources - refer to all deposits from those that are well known to undiscovered, that are currently economically viable or may become economic in the future.
• Identified Resources - are those deposits whose location, quality and quantity are known, as a result of geological testing and studies.
• Reserves - exploitable resources: the subset of identified resources that are currently economic and legal.
• Measured - reserves for which estimates of the quantity and quality of ore has been computed to within 20%, using sample analysis and measurement from closely spaced and well-known geological sampling sites.
• Indicated - reserves whose quantity and quality of ore can be partly computed from sample analysis and measurements, but also including some degree of reasonable geological projections.
• Inferred - Unexplored extensions of resources for whom estimates on quantity and quality of ore are based on geological projection alone.

These divisions are represented in the following figure:

Clearly considerations such as economic feasibility are dependent on social and political factors as well as geological conditions so the evolution of a deposit within this classification table depends on uranium demand as well as available technology.

Types of Deposits

There is a large range of types of deposits in which economic concentrations of uranium is found. The three main types in order terms of frequency of occurrence are:

1. Unconformity-related deposits
2. Breccia Complex deposits
3. Sedimentary Precipitation Deposits
   

The first two are often although not necessarily related, while the later is an altogether difference emplacement mechanism.

In recent years mining of sedimentary precipitation deposits has been substantially carried out with the In-situ Leaching technique [link to subheading in mining procedures page] due to the low costs involved, although these deposits otherwise generally constitute only low to medium grade and size ore bodies.

Unconformity-related deposits

Unconformity-related deposits occur with the changing conditions related to major unconformities. Faulting, and bending of sediments due to volcanic and seismic activities can create enough heat and pressure mobilise minerals by dissolution in ground water and magmas. Precipitation then occurs in the faulted area, which is usually overlaid by more recent, undeformed sediments.

Breccia Complex deposits

The Olympic Dam Deposit in South Australia is one of the world's largest uranium deposits and is a Breccia Complex type. Uranium and other economic minerals are found in cracks and fractures around brecciaed lithology.

Details of the placement of uranium in these deposits are still unknown. The process for fracturing is thought to be hydraulic and caused by the movement of hot, magma-related water into the region. Minerals such as uranium probably precipitate in the fractures as the water cools.

Sedimentary Precipitation Deposits

On a world-wide scale, a major proportion of uranium deposits occurs in enriched zones in fluvial sandstone sequences - either flat or 'roll-front' (C shaped, see figure 2) in cross section. These zones develop due to a natural weathering process linked to the movement of oxidised ground water through the sediment layer. Uranium (and other minerals) is dissolved in the water that provides a mechanism for ion transport. However, the ability for water to dissolve these ions is critically dependent on the pH and oxygen concentration, and uranium precipitates along oxidation-reduction (redox) interfaces when saturation point for the given conditions is reached. At such an interface the uranium minerals that precipitate are primarily uranium oxides: uraninite, UO2 or 'pitchblende', U2O5.UO3 - better known as U3O8 and coffinite: USiO4.

Fuel Projections

Projections as to the amount of uranium available for exploitation are based on identified resources. These identified recoverable world reserves are estimated at about 3.6 million tonnes, of which Australia has more than 1 million. Currently the annual natural uranium (mined state) consumption of the world's nuclear energy industries is at about 70000 tonnes. That places about 50 years left for the known low-cost uranium reserves. Since uranium is not a particularly scarce mineral, further significant discoveries are considered probable. Also, as the demand increases, lower-grade and more expensive sources of uranium will almost certainly be considered. Extensions to the nuclear fuel supply could arise with the following fuel source considerations.

Sea water extraction
The amount of uranium dissolved in seawater is on average 3 mg/m3 and the total amount in the world's oceans is calculated as 4.5 billion tons. Some research has been directed to the extraction of uranium from seawater (2). Currently this method is not comparable to the production or yellow cake via conventional means (on average only a tenth of the cost), however, the technology is in its infancy and has potential for improvement, and with increased demand the economic feasibility could be vastly increased.

Thorium as a fission source
Strictly speaking this is the conversion of thorium-232 (Th-232) via slow-neutron absorption to produce uranium-233 (U-233), which is fissile. Thorium is about three times more abundant than uranium in the Earth's crust, and potentially all mined thorium could be used in a reactor with no enrichment process necessary, making it a vastly more energy-rich fuel source (3). Yet another benefit is that there are already reactor designs in use, such as the CANDU reactor, that could be used as thorium conversion reactors with little or no modifications.

Fuel recycling and breeder reactors
These are other methods for which estimates suggest the amount of effective uranium could be increased by several orders of magnitude.

References:

1. http://pubs.usgs.gov/bul/b1693/html/bull1ghh.htm
2. http://www.ans.org/pubs/journals/nt/va-144-2-274-278
3. http://www.world-nuclear.org/info/inf62.htm