Devices and Category 3 sources

Fixed radioactive gauges are used for measuring the level, thickness, density, moisture content, or the presence of a specific material while it is being mined, manufactured or processed without contacting the material itself (Fig. 26). Depending on the specific application, industrial gauges may contain relatively small quantities of radioactive material, or may contain sources with activities approaching 1 TBq. The larger activity (~100 GBq)

137Cs, ⁶⁰Co and ²⁵²Cf sources, which are used as level, conveyor, dredger, blast furnace or spinning pipe gauges, are Category 3 sources, while most other thickness, moisture/density and fill level gauges fall into Category 4.

FIG. 26. Various functions of sealed sources in process control.

Blast furnaces, employed in steel making often use ⁶⁰Co sources to gauge the wear of the refractory lining of the bottom hearth. Spinning pipe gauges use ¹³⁷Cs sources to measure the wall thickness of pipes as they are passed through the center of the gauge. While pipe gauges are included in the fixed gauges category, they can also be mounted on trucks. However, they can be quite heavy (~100 kg) due to their lead or tungsten shielding (Fig. 27).

FIG. 27. Examples of different types of gauges.

The devices generally are not large, but they may be located at some distance from the radiation detector, which may have associated electrical or electronic components located within its housing. The locations of such devices or sources within a facility may not be recognized, since the devices are frequently connected to process control equipment. This lack of recognition may result in a loss of control if the facility decides to refurbish some plant or terminate operations.

The disused source may end up in the metals recycling industry. If there is no installed monitoring in the metals recycling route, or the system is not working, the source could be melted down, resulting in the contamination of the foundry and radioactive material incorporated into manufactured articles.

4.3.2. well logging gauges

Well logging is the practice of measuring the properties of the geologic strata through which a well has been or is being drilled. A well log is the trace or record of the data from a downhole sensor tool plotted versus well depth. Its most common application is by the oil and gas industries which seek out recoverable hydrocarbon zones.

For oil and gas production, companies would like to have several kinds of information about a geological layer, such as the hydrocarbon content. To measure these properties, sources and sensors loaded into housings called sondes can be lowered into an existing borehole (a technique called wireline logging) or can be mounted in a collar behind the drilling head for taking measurements while the well is being drilled.

A combination of neutron and gamma sources is used for the determination of density, porosity and moisture, or hydrocarbon content, of geological structures. The most common neutron sources employed are ²⁴¹Am–Be of up to 800 GBq, but some use has been made of ²³⁹Pu–Be and ²²⁶Ra–Be sources as well. The gamma sources most frequently employed are 50–100 GBq ¹³⁷Cs sources. Smaller sources, often containing radium, are still in use for reference purposes. The sources are usually contained in long (typically, 1–2 m) but thin (<10 cm in diameter) devices, which also contain detectors along with various electronic components. The devices are heavy, due to the ruggedness needed for the environments in which they are used. Figure 28 shows a schematic illustration of a neutron source and borehole logging equipment. An example of borehole logging equipment and some typical

241Am–Be neutron oil well logging sources are shown in Figs 29 and 30.

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FIG. 28. A schematic illustration of a neutron source and borehole logging equipment.

FIG. 29. A pipeline crawler source head in a shipping container.

FIG. 30. Typical ²⁴¹Am–Be neutron oil well logging sources.

Figure 31 is a photograph of a gamma–gamma density instrument, showing the source handling device used to insert and remove the ¹³⁷Cs source.

FIG. 31. Atlas Densilog, which uses a vitrified ¹³⁷Cs source for wireline density measurements. The rod extending downward is a source handling device. The light coloured circles are windows for the source and detectors.

The housings in which the neutron well logging sources are stored and transported are large and may appear attractive to thieves. The bulk of the shielding will normally be plastic or paraffin wax and may be thrown away as useless by a thief, leading to a potentially hazardous situation. The housings for the gamma sources will normally be shielded with depleted uranium or lead, which could be attractive for its scrap value. The nature of the work using these sources requires that they be easily removed from their housings to be introduced into a borehole. If they were not subject to adequate control, it would be relatively simple for the source to be removed and left in a hazardous state.

4.3.3. Pacemakers

During the 1970s and 1980s, heart pacemakers (Fig. 6) using radioactive material as the energy source (i.e. very small RTGs) were implanted in a number of patients. The most common radionuclide used was ²³⁸Pu (with a small amount of ²⁴¹Am as a source contaminant). One advantage of using ²³⁸Pu was that it was relatively easy to shield, thereby resulting in minimal external dose.

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It is possible that sources may be discarded after autopsy and end up in recycled metals. The fact that ²³⁸Pu sources are easily shielded also means that they are not easily found.

4.4. DEVICES AND CATEGORY 4 SOURCES