The Level of Uncertainty in Materials Clearance

Measurement of surface levels is essential in waste assessment and in clearing materials from nuclear licensed sites. Radiation measurements in general are much less accurate than most forms of engineering measurement, even in relatively simple conditions, such as radiochemical laboratories. Waste assessment during clearance is far more difficult. The areas of uncertainty include: (1) The intrinsic limitations of the equipment employed. For surface activity measurement, the detector is often a large area alpha + beta scintillation probe connected to a ratemeter. Any detector has an effective energy threshold below which it fails to register an event. The detector response is rarely uniform over the window area. The detector is connected to a ratemeter. The design of a ratemeter and the way it is set up will have a large influence on how easy the user finds it to classify correctly materials close to the exempt limit. (2)Calibration of the equipment. There is only a limited set of surface contamination sources available. Prediction of the response to other nuclides can be complicated. (3)Determination of the fingerprint. For many practical situations, the potential contaminant is a mixture of nuclides emitting a mixture of alpha, beta, gamma and X radiation. Any detector will have a response which depends on the radiation type and energy. Frequently, the response to the fingerprint of the most operationally robust and convenient detectors will be dominated by only a small fraction of the total activity present. It is thus vital that that fraction is well established and any area zoned so that the fraction remains reasonably stable. (4)The influence of natural activity in materials and of gamma radiation from elsewhere. Many building materials have levels of natural activity in the Bq/g region. These often vary significantly from sample to sample and area to area, particularly where buildings and equipment have been extended or modified. (5)Surface condition. For short range emissions such as alpha particles and low energy betas, the range for detection in air is a few mm. Any material covering the activity, such as paint or grease, will reduce the emission rate.