Index > 4 Environmental remediation of radioactively contaminated sites >

4.5.3 In-sito vitrification (CT)

Contents
4.5.3.1 Traditional in-situ vitrification
4.5.3.2 Planar in-situ vitrification
4.5.3.3 Plasma arc in-situ vitrification

Heat treatment is aimed at in-situ vitrification (ISV) whereby loose sand is fused into a lump containing the contaminants (see Figure 4.13 and Figure 4.14) [IAEA-2006b]. Resistance or inductive heating methods are available. They are best suited to areas with contamination in relatively homogeneous media. Mixed contaminated sites that are very heterogeneous, such as buried waste sites, require careful pre-treatment characterization in order to assess the safety of the process implementation and production of a uniformly high quality product. Characterization is needed to identify waste forms, such as intact containers of liquids, pressurized gas cylinders and residues of explosives, which can cause significant pressure excursions during treatment. Characterization is also needed to ensure that the base chemical constituents are suitable and adequate to form an acceptable vitrified product. If not, addition of glass forming constituents, for example sand, may be necessary. Care is also needed if substantial amounts of metal debris are present.

The vitrification process will either destroy organic compounds or volatilize them in its early stages. It has to be considered, however, that an incomplete combustion process may lead to more toxic degradation products, such as dioxins. Another problem with heat treatment may be the volatilization of 210Po, [^137^Ce], Pb and Hg, where present. This can be overcome, albeit at additional cost, with the installation of abstraction hoods, high efficiency particulate air (HEPA) filtration and exhaust gas scrubbing. Secondary wastes from air emission control may require special treatment and disposal at licensed facilities. The vitrified block may be either left in-situ or removed (Figure 4.15) to an engineered disposal facility.

Figure 4.13 In-situ vitrification [IAEA-2006b]
Figure 4.13 In-situ vitrification [IAEA-2006b]

.

Figure 4.14 Examples of in-situ vitrification (after GeoMeltTM)
Figure 4.14 Examples of in-situ vitrification (after GeoMeltTM)

.

Figure 4.15 In-situ vitrification for removal and disposal (after GeoMeltTM)
Figure 4.15 In-situ vitrification for removal and disposal (after GeoMeltTM)*

.
The evolution of the in-situ vitrification technology resulted in three different configurations of the process discussed below:

  1. Traditional in-situ vitrification;
  2. Planar in-situ vitrification;
  3. Plasma arc (or bottom-up) in-situ vitrification.

4.5.3.1 Traditional in-situ vitrification

The traditional in-situ vitrification process employs an array of electrodes placed vertically into waste or contaminated soil, and an electric current is passed through the soil between the electrodes. The heat generated from the resistance of the soil to the passage of the current is referred to as Joule heating. As the heated soil melts progressively downwards, the electrodes are allowed to sink through the melted soil, enabling melting depths of 7 m or more.

An off-gas hood covers the entire melt and some distance around the outside edge to control release of gases and airborne particles generated within or near the melt. The off-gases are drawn into the hood by the negative pressure created by a fan, then treated in a process train before being discharged to the atmosphere. When the melting has progressed to the desired depth, the power to the electrodes is shut off and the melt is allowed to cool. The electrodes are left in place in the melt and are sawn off at the ground surface. New electrodes are installed at each new melt location. The final melt is smaller in volume than the original waste and associated soil due to:

  • Removal of volatile contaminants;
  • Reduced void space;
  • Higher density of glass relative to waste materials.

Each melting produces a single block shaped monolith of glass. Most vitrification projects require multiple, overlapping melts to cover the area and the volume of the contaminated site.

4.5.3.2 Planar in-situ vitrification

Like traditional in-situ vitrification, planar in-situ vitrification employs the same Joule heating principle but differs in the application of electric current and in the starter path configuration. In planar in-situ vitrification, the current travels between pairs of electrodes, causing two parallel planar melts to form. As the melts grow downwards and spread laterally, they eventually meet in the centre of the electrode array and fuse together into one melt. The final planar melt has the same size and shape as a traditional in-situ vitrification melt.

4.5.3.3 Plasma arc in-situ vitrification

Plasma arc in-situ vitrification is a newer and much less tested technique based on established plasma arc technology. In this process, electrical energy is applied as direct current between two electrodes within a torch, creating a plasma of highly ionized gases at very high temperatures. The resistance to the flow of current between the two electrodes generates the plasma.

The operation involves lowering the torch into a pre-drilled borehole of any depth and heating the wastes and soil as the torch is gradually raised. The organic fraction of the wastes is pyrolysed and the inorganic fraction is vitrified, thus converting a mass of soil and or waste into a highly stable, leach resistant slag column.

Although this ‘bottom-up’ in-situ vitrification process is experimental, it has advantages over the traditional and planar in-situ vitrification applications. A primary advantage is the ability of gases and vapours to escape the subsurface above the melt zone rather than being trapped beneath it. As a result, the likelihood of melt expulsions is reduced.

The in-situ vitrification process can immobilize extremely hazardous materials and radionuclides that may be difficult to treat.