Uranium mining and milling involves several steps, which may result in radioactive waste which will need to be treated appropriately after closure of the mine to ensure long-term protection of population affected by mining and milling activities. When the ore is mined, either in underground or open pit mines, large amounts of waste rocks are generated, as well as low-grade uranium ore with insufficient concentration of uranium to be economically exploited. This waste can be deposited in waste piles and can pose a threat to the environment, as once mined they are far more exposed to weathering and rainwater leaching than before. In addition, radon exhalation is significantly increased.
Uranium ore with a sufficient amount of uranium for extraction is first crushed and then further processed to produce yellowcake, which is a uranium concentrate powder ready to be shipped for the fabrication of nuclear power plant fuel. A

simplified scheme of such processes involving leaching of milled ore with sulfuric acid and uranium extraction using solvent extraction is presented in Figure 1. The waste generated in such processes are called uranium mill tailings, which are residues after sulfuric acid leaching, and red mud, which is raffinate from solvent extraction. These waste streams can also be deposited on waste piles. Both wastes contain radionuclides from the uranium decay series, where the most important ones for radiological risk estimation are the long-lived radionuclides: U-238, U-234, Th-230, Ra-226 and Pb-210. Short-lived Rn-222 is constantly produced from Ra-226 and represents a large issue as being noble gas it can easily escape from the waste in the atmosphere. Po-210 is also of concern due to its high dose conversion coefficient and consequently high effective doses due to ingestion or inhalation.
After closure of uranium mines and mill sites,

several other objects contaminated with radionuclides should be considered, such as mining, crushing and milling equipment, buildings, etc.

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Figure 1 Simplified scheme for production of yellowcake from uranium ore

Uranium mines and mill sites remediation
The initial phase in remediating uranium mines and mill sites is to define the waste inventories. This can be very problematic in some legacy sites where documentation on past activities is not complete or is completely lacking. In such cases, exploratory work to define inventories needs to be carried out. This may involve drilling cores from waste piles, surveys of abandoned equipment and buildings, taking water and biota samples, etc. Radiochemists support these activities by analyzing these samples. Not only is the total concentration of radionuclides important, but also its speciation as this governs how mobile they are in the environment. Although most of the uranium, usually more than 90%, has been extracted from the ore, what remains is usually very mobile as uranium is oxidized and present in a more mobile form. Th-230 and Pb-210 are usually less mobile as they are highly particle reactive (they prefer to be attached to particulate matter in the environment) but, due to high concentrations in waste, even this portion may be significant. Their transfer via the food chain is also important, as

well as their mobility under surrounding environmental conditions. All this analysis supports the risk assessments.
For the risk assessment all possible exposure routes to the population need to be considered. These are usually via inhalation, ingestion of food, water and dust particles, and external radiation. Some radionuclides might be more important for a specific route. Long-term assessments also need to be done as radionuclides are long-lived. This is usually conducted by modelling with simulation tools such as Resrad or Normalysa. This initial assessment shows what the risk to population would be if no action was taken and the site was left as it is.
If the risk is too high compared to the desired level set by the regulator, remedial measures need to be implemented. How a site is remediated is also dependent on planned future use of the site. Therefore, remedial measures are designed for local conditions and have to take into account initial topography, geology, hydrogeology and geophysics of the sites.
The next step is to prepare the project plan to

define the work required to carry out: earth moving, covering with various types of materials, compacting, site drainage water treatment, special civil works, dismantling of the installations and the disposal of products derived from the dismantling operations. During the working phases, any necessary adjustments are carried out. At the end of the remediation work, the final status includes control of monitoring the topography, water circuits, radioactive mapping, verifying that the residual radioactivity is consistent with the local regional radioactivity and revegetation.
The final step is to design the long-term monitoring strategy, which needs to include all possible exposure pathways and the critical radionuclides identified during the risk assessment. This usually comprises of the monitoring of seepage waters, groundwater, surface water, sediments, fish, food and locally produced feed for long-lived radionuclides and of air for Rn-222. The monitoring strategy should ensure that risk assessments should be periodically evaluated to identify any abnormalities, which could trigger further necessary remedial measures.

References