Abstract
On the monitoring for compliance with clearance level, the concentrations of objective nuclides, such as alpha or lowenergy beta emitters, can be estimated without direct gamma measurement by assuming the existence of objective nuclides with geometric mean concentrations or using previously assessed information on nuclide spectra and measurement results for a key gamma nuclide. To determine whether clearance can be carried out, deviations in the mean concentrations and nuclide ratios to the key gamma nuclide should be appropriately considered, in addition to the measurement error. In this paper, the concept of clearance level has been reconsidered and a new approach has been proposed to establish an appropriate safety factor of the monitoring for compliance with clearance level. To apply the approach to practical use, a probability distribution calculation system has been developed and verified. A simple method has also been proposed to determine whether the safety factor is required. The other option is to adopt initially a conservative value for the mean concentrations or the nuclide ratios. It has also been clarified that the use of arithmetic means for the mean concentrations or the nuclide ratios is more conservative than the present approach.
