Mobilization and Redeposition of NORM

Mobilization and Redeposition of NORM

The following discussion about mobilization and redeposition is largely taken from the CRCPD E-4 Committee Report on NORM Contamination and D&D (CRCPDa 1994).

Mobilization by human activity can be intentional or unintentional. Two examples are:

Uranium extraction by in situ leaching maximizes solubility of uranium.


Unintentional mobilization occurs when the element is desirable for its non-radiological properties. Vanadium and uranium were originally mined for their non-radiological properties. Usually, however, the TENORM isotope is mobilized along with some other mineral of interest.

Generally, redeposition involves the same factors as mobilization. Changes in any of the parameters of a stream of material may result in reduced mobility and subsequent redeposition. These processes may also take place preferentially; concentrations of specific minerals may occur. Examples include: Chlorination of metallic ores as one step in metal production mobilizes radium, which accompanies uranium in the ore. The high solubility of RaCl₂ relative to other species leads to extraction of radium wherever the parent mineral is exposed to chloride ions. Production of brine or brine-contaminated oil includes dissolved radium as well, since the brine contains chloride ions.

• Low solubility of alkaline earth SO₄^2- (sulfate) species is also a factor in redeposition of NORM. Movement of sulfuric acid solutions through piping in mineral extraction processes is known to cause precipitation of scale containing high concentrations of radium. Production of water containing sulfate-bearing solutions can also cause precipitation of pipe scale containing elevated concentrations of uranium.
  
• Groundwater chemistry may change as the water reaches the surface or as it passes through different strata and the dissolved minerals form at the surface. Changes in pH, oxidation state, or chemical equilibrium may result in precipitation of dissolved minerals. This mechanism accounts for the existence of many ore bodies. Addition of alum and softening chemicals in drinking water treatment plants similarly precipitates radium with the other minerals. Uncontrolled discharges from tailings piles may contain extracted radionuclides (as well as other heavy metals).
  
• Oxidation-reduction potential can affect solubility. Variation in oxidation state affects solubility since complex formation depends strongly on oxidation state. For example, water exposed to sulfur-bearing minerals generally exhibits reducing potential, which may alter the oxidation state of other minerals in contact with the water.
  
• Adsorption depends on the substrate and the specific species in question. Clays are known to adsorb some chemical species preferentially over others; passage of groundwater through a layer of clay may strip out NORM species that would otherwise travel with the water. Adsorption of radon on activated charcoal is an equilibrium process. Desorption can occur if the ambient radon concentration drops; saturation can prevent further radon removal.
  
• Ion exchange is used to control water chemistry, typically to remove contaminants, soften potable water, or remove radium. Ion exchange does not cause mobilization of radionuclides, but once mobilized in water, any subsequent treatment by ion exchange has clear potential for reconcentration.
  
• Temperature-dependent variations in solubility result in increased concentrations of radionuclides, together with other elements in geothermal waters. Thermal processes can mobilize radionuclides. Any high-temperature process such as furnaces, kilns, roasters, calcination, and smelting can volatilize lead and polonium. Combustion of coal or lignite volatilizes some isotopes (thorium, uranium, radium, and bismuth). Subsequent redeposition may occur in process equipment, in pollution control equipment, or in the environment. Minerals dissolved in naturally occurring geothermal waters typically plate out as the temperature drops. Deposits of scale containing substantial concentrations of radionuclides may result.
  
• As the water comes to the surface and the partial vapor pressure drops, radon dissolved in water partitions into the air. In open air, dilution, convection, and diffusion minimize increases in concentration, but in caves or buildings, higher concentrations of radon and its progeny may result. Brines exhibit similar behavior. Oil mixed with brine brings radium with it; as the chemical and physical conditions change in the pipe string and at the well head, the radium precipitates with other minerals and forms scale inside the piping.
  
• Mechanical reduction in particle size enhances the potential for mobilization of material, such as erosion, movement of alluvial or glacial particles, bulldozers, mining, and construction. In addition, radionuclide concentrations can increase when part of the matrix is removed, leaving radionuclides behind in increased concentrations in the residue. Extraction of phosphate from ore leaves uranium behind in filter cake; bauxite ore contains aluminum, which is extracted leaving "red mud" containing uranium. Combustion of coal leaves uranium in the ash and slag.
  
• Suspended materials settle out of material streams as the velocity decreases. Fines settle out where the current is slowest, with successively coarser material settling out in the faster sections. Such gravimetric separation may result in deposits of zircon or monazite sands. Airborne particles exhibit similar behavior, with the coarser material settling closest to the source.

Temporal Variations in Background

Temporal changes in background range occur during short to long time-frames; hours to days, months to years, and centuries or more. Short and medium term changes in background have been measured (Maiello 1997). There are changes in background from terrestrial and cosmic sources that can be summarized as follows (NRC 1994):

Cyclic changes on a daily basis are due to changes in radon concentrations in air, which are dependent on the atmosphere (Fig. 3.). Early mornings are typically calm, so radon escaping from the ground stays near the surface causing radon levels to rise. During the day, the sun warms the ground and air near it rises, generating a mixing effect that sends the radon (and its progeny) to higher levels in the atmosphere, thus lowering the radon level.

Click Here for Figure 3.

• Dose rates rise gradually as soil dries out. Water shields the radiation coming from the NORM in the ground, and dilutes the concentrations of NORM in the soil. After rainfall, the background values drop.
  
• Rainfall scours radon and its progeny from the atmosphere, causing radiation levels to rise at ground level. Some larger storms may double the gamma exposure rate for a short period of time.
  
• The shielding effect of snow is substantial (Fig. 4.). Shielding is dependent on the water equivalent of the snow because a heavy wet snow is more effective at shielding than a dry snow.

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Month-to-month variations in background are due mostly to changes in soil moisture content and snow cover. Winter months trend to lower radiation levels because of higher soil moisture and summer months trend toward higher levels because of lower soil moisture. Seasonal effects actually average out over the course of a year (Fig.5.).


Anthropogenic activities also affect background values. According to NUREG 1501, about two-thirds of the background gamma dose comes from NORM contained in the top 15 cm of soil out to at distance of 6 meters from where a person stands. Therefore, ground coverings, (e.g., asphalt, concrete) may decrease the gamma exposure rate. Conversely, building materials may contribute to dose. The magnitude of the change depends on the amount of material involved, and the relative radionuclide concentration in the old and new situations.

Click Here for Figure 5.

Increases in background have also been noted after accidents and weapons tests.

Cosmic ray variations tend to be small and result from changes in barometric pressure. With a high pressure system, there is a larger mass of air to provide a shielding effect, as compared to a low pressure system, which has less air mass and less shielding.


Cosmogenic radionuclide production varies according to changes in the cosmic ray intensity. A more active sun produces changes in the solar wind and magnetic field, which oppose the cosmic rays coming from outside the solar system.


Seasonal changes also occur in the deposition of cosmogenic radionuclides on the ground. Deposition is greater during the spring months when air in the stratosphere mixes with air in the troposphere, where it gets washed out by precipitation.

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