Traceability for measurements of radioactivity in waste materials arising from nuclear site decommissioning

Abstract

Site decommissioning is now a major aspect of the work of the nuclear industry worldwide. One of its many technical challenges is the need to measure levels of radioactivity in a range of materials (e.g. concrete, brick and steel) in order that radioactive waste may be identified, sentenced and consigned to the appropriate waste stream in accordance with national regulations. This is done using any of a number of measurement techniques, falling under three categories: (i) bulk monitoring (for γ and neutron emitters), (ii) surface monitoring (predominantly for α and β emitters) and (iii) radiochemical analysis. The last is often used to determine a ‘radionuclide fingerprint’ for a particular operational area for use in conjunction with data from in situ monitoring. Traceability to national standards can be difficult to demonstrate for measurements of this type. Only a limited number of standards and reference materials are available, and their chemical and physical forms do not match those of the very wide range of samples being measured. Traceability for surface measurements is further complicated by the subjective nature of monitoring using hand-held detectors.This paper describes some of the detector types used for γ non-destructive assay (NDA) and for surface measurements, gives examples of currently available standards and calibration procedures and provides some guidance in how to achieve traceability. A generic analysis regime for an operational area is presented which demonstrates points where traceability can, in principle, be attained. A new methodology for developing ‘realistic’ large-volume standard sources, traceable to national standards, has been developed by the National Physical Laboratory (NPL), and this is described.