4.23.
The generic clearance levels specified in table I.2 of GSR Part 3 [1] for artificial radionuclides were calculated using scenarios involving exposure to a large quantity of homogeneous material. For example, the transport scenario considers a truck containing 10 tonnes of material, and the landfill scenario considers even larger quantities [5]. When applying the clearance levels, it should be recognized that they were derived for these large amounts and that the averaging should be done accordingly, with due consideration of the exposure scenarios. Hence, very small averaging masses are not appropriate for large amounts of material.
4.24.
The regulatory body should determine or approve appropriate averaging masses to be used in the clearance process, in particular for compliance measurements; the averaging procedures used by the operating organization should take these averaging masses into account. Examples of appropriate averaging masses are from a few hundred kilograms to the order of a tonne. The regulatory body should confirm that the averaging procedure is not used solely to allow material that contains radioactivity above the clearance levels to be cleared. The operating organization should ensure that averaging procedures, selected in accordance with the type of material, are an integral part of the clearance process. For small objects with a mass less than the specified averaging mass, a minimum default averaging mass (e.g. 1 kg) could be defined by the regulatory body, which in turn allows the specification of a maximum total activity for these objects for a specified radionuclide (e.g. 100 Bq for 60Co) or radionuclide composition. It should be ensured that this does not result in large objects being divided into smaller ones solely as a means of achieving clearance. In the case of several small objects, an alternative method would be to monitor them together to replicate the averaging mass.
4.25.
In the case of surface contamination clearance levels, the regulatory body should consider, depending on the type of material, and the nature and homogeneity of the contamination, averaging areas from several hundred square centimetres up to 1 m2. In some cases, averaging areas for specific clearance could be higher, for example up to 10 m2 for the clearance of buildings for demolition.
4.26.
For non‑accessible surfaces that might be expected to be contaminated to some degree, the operating organization should make a conservative assessment of the surface activity for comparison with the clearance levels. Similarly to the clearance of small and light objects on the basis of activity concentration clearance levels, the regulatory body could define a default minimum averaging surface area (e.g. 100 cm2) for objects with surfaces below the default averaging area. If there are several of these small objects, an alternative method would be to combine measurements of the surfaces of individual objects to compare against clearance levels that are to be applied to a larger average surface area.
4.27.
Averaging masses and areas for decision making on compliance with clearance levels should be distinguished from masses and areas used for actual measurements. For example, multiple samples of 100 g of soil could be used to determine whether a mass of a few tonnes complies with the clearance levels. In any case, the masses or areas used in measurements should not exceed the averaging masses and areas for decision making on clearance.
4.28.
In deciding on a measurement strategy, the operating organization should batch the material so that it is as homogeneous as possible in relation to both material and origin, and thus in relation to the radionuclide composition and activity level. Variations in the results of individual measurements within one averaging mass or area should be expected. For example, variations in individual results of up to a factor of ten (compared with the average value) are generally considered acceptable, whereas a greater variation would be acceptable if the overall average concentration or surface contamination level was a very small fraction of the clearance level.
4.29.
The operating organization should make use of the maximum permitted averaging areas or masses when designing the monitoring programme for clearance, as this improves the efficiency of the clearance process. The monitoring programme may be constrained by the form and nature of the contamination; for example, the choice of equipment available for monitoring for beta activity inside a small pipe is likely to be limited. Nevertheless, using appropriate time integration in dynamic measurements (e.g. recording counts over a minute, rather than over a second) or numerically averaging over a number of single static measurements will enable a greater averaging area to be achieved.
4.30.
Measurements to determine whether a material or object is in compliance with clearance levels will be based on a measurement unit defined by the monitoring method and the instrumentation used (e.g. contamination monitor, drum monitor, bulk monitor). The size of this measurement unit should be based on practical considerations that reflect the size of an object or sample of material (e.g. a drum of waste, an excavator bucket) and how the measurement will be made (e.g. the geometry of the measurement system).
4.31.
The operating organization should select measurement units and should propose averaging masses and areas that are sufficiently representative of the material, taking into account homogeneity and the necessary detection levels and confidence intervals for the results of clearance measurements. In general, larger measurement units and averaging masses and areas are acceptable where the contamination is reasonably homogeneous. The averaging masses and areas should be agreed with the regulatory body and formally recorded by the operating organization as part of the clearance process. The regulatory body should also provide guidance and quantitative criteria on how the clearance process should address issues associated with inhomogeneity.
4.32.
The averaging masses and areas will indirectly limit the activity of hotspots. The regulatory body should establish limiting criteria for such hotspots when defining the size of averaging masses and areas (see also paras 4.34–4.37).
4.33.
If the results of samples taken from a material are subject to considerable variability, as described in appendix A to Ref. [30], then averaging over the whole material mass is unlikely to be acceptable without a properly documented consideration of the following:
The practicability of separation and segregation of parts of the material;
Suitable revision of the sampling and monitoring plan, including numbers of samples;
Suitable reduction of the size of each measurement unit (mass or volume);
The practicability of making further measurements to identify each volume or area containing significantly elevated levels of radioactivity;
The practicability of removing or segregating small areas or volumes containing significantly elevated levels of radioactivity;
The potential radiological significance of inhomogeneity.
The aspects listed should be considered when planning activities that give rise to the materials, such as decommissioning.