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Residues from deep geothermal energy
When extracting deep water for energy production, natural radionuclides dissolved in water reach the surface. Depending on water composition and plant control, residues may precipitate inside the plants. The radionuclide content (specific activity) of these residues (is clearly higher than the natural background level of soils and rocks.
Residues with an enhanced content of natural radionuclides result primarily from the use of deep water with a high salinity (up to 250 grams salt per litre of water). According to today's knowledge, however, these high concentrations occur in Germany only in some deep waters from North German Basin and from the Upper Rhine Valley. The salinity of the deep water from the alpine upland used for geothermal purposes is less than 1 gram per litre.
When using near-surface geothermal energy (e.g. for the operation of heat pumps in homes), no residues with enhanced radionuclide content are produced.
Type of residue and specific activity
In the area of deep geothermal energy, there are currently three different technologies to make geothermal energy usable. Radionuclide-containing precipitates (scale) can only arise in hydro- and petro-thermal geothermal energy plants:
- In "hydro-thermal" reservoir, sufficient water (fluid) is available in the host rock for extraction to produce thermal energy.
- For "petro-thermal" reservoir, there is not enough deep water available. Therefore, water is pressed into the host rock when using this method. In gaps and fissures, the water flows around the rock and takes up the heat of the rock. Subsequently it is transported to the surface.
- Borehole heat exchangers have their own closed circuit, so that no contact and, thus, no exchange of substances with water/gases occurs.
During the operation of geothermal plants, scale may arise in all plant components that are in contact with deep water. These precipitates are either poorly soluble carbonate and/or sulphate salts or metallic scale of base metals (such as lead). In the case of metallic precipitates, mainly the radionuclide lead-210 is of importance from the radiation protection point of view, whereas in the other types of scale, radium-226 and radium-228 occur most frequently. Typically, the specific activity of the dominating radionuclides is several ten becquerel per gram. However, specific activities of several hundred becquerel per gram may also occur in the scales.
Apart from the scales, other residues accrue in geothermal plants, such as defective plant components, filter material, sludge, and protective clothing, all of which may also contain enhanced concentrations of natural radionuclides.
Disposal of residues
In Germany, geothermal energy production is a very young technology. Generally, no experience is therefore available about standardised recycling or disposal patterns. According to the Recycling and Waste Management Act, recycling is preferable to disposal. Protective clothing and organic filter materials could be recycled thermally, defective plant components could be melted down. Depending on the radionuclide content, only specialised companies can manage these residues.
Currently, no recycling options are known for scale and sludge. According to current information, these residues must therefore be deposited.
Basically, both workers and the general public have to be considered in the evaluation of possible radiation exposure due to residues from deep geothermal energy. In the evaluation it is taken into account how the residues are disposed of or recycled.
According to currently available data, the reference level of 1 millisievert per year might be exceeded for workers in geothermal plants in unfavourable conditions. In such cases it must be determined which arrangements can be introduced with low effort to reduce the dose. These include, for example, carrying personal protective equipment or searching for alternative disposal paths. From today's point of view and based upon conservative assumptions, however, it is not to be expected that the reference level for the population will be exceeded.
State of 2017.07.18