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The determination of isotope specific activities, and above all, the identification of radioisotopes is an important issue in the waste management of nuclear wastes whatever their origin. Systems based on the detection of the gamma rays emitted by the radionuclides allow the non-destructive assay (NDA) of relatively large volumes of waste. Different NDA systems are commercially available for this purpose. NDA of waste items is subject to the influence of many different physico-chemical characteristics of the sample, most of which are generally not known. In many cases, however, NDA is the only approach that allows measuring these large samples. The unconditional free release of wastes originating from nuclear activities for a great deal relies on NDA systems and procedures for decision-making on free release. The assay of waste of very low activity and even the decision if a radionuclide has been detected or not, is very often difficult. The poor measurement precision, caused by the bad counting statistics, combined with a generally unknown bias of the measurement complicate the interpretation of the results, and accurate measurements are seldom obtained in these cases. The aim of the present paper is to give an overview of the efforts made at SCK-CEN to come to a consistent system that allows performing free release measurements, involving a high throughput of large amount of materials, as expected from a dismantling project.

Detection efficiency calibration of gamma spectroscopy for waste assay systems is a difficult and time-consuming task. Commonly, reference waste packages are used for efficiency calibration but these are sometimes difficult to build, are expensive and have limited use since there is large variability of parameters which influence the assay. Numerical calibrations, however, are an interesting alternative and have begun to find more and more application in waste assay. A dedicated computer code, Solidang, has been developed to compute detection efficiencies for gamma waste assay. Solidang uses the effective solid angle approach to derive detection efficiency and aims at improving and facilitating the calibration of gamma waste assay systems and at serving as a tool to investigate the influence of the many system and sample parameters involved.

The non-destructive quantification of transuranic elements in nuclear waste management or in safeguards verifications is commonly performed by passive neutron assay techniques. To minimise the number of unknown sample-dependent parameters, Neutron Multiplicity Counting (NMC) is applied. We developed a new NMC-technique, called Time Interval Correlation Spectroscopy (TICS), which is based on the measurement of Rossi-alpha time interval distributions. Compared to other NMC-techniques, TICS offers several advantages.

Thorium is used in many different industrial technologies and is widely found in nature. Internal contamination with thorium is considered as highly hazardous because of its radiological and chemical toxicities, which depend on the chemical form in which thorium appears. The assessment of the thorium body burden is then of primary importance in detecting the risks of personal contamination, and for appropriate counteractions when contamination is detected. The in vivo assessment of the thorium lung burden is commonly achieved by gamma ray spectrometry of its progeny. Three methods for the assessment of thorium in the lungs are compared. In the first method, the radionuclide examined is 208Tl, measured with a NaI(Tl) detector. This simple method can be affected by systematic errors due to 220Rn exhalation and because of the assumption of equilibrium between 232Th and 228Th. The second method, based on the measurement of the gamma rays emitted by 228Ac, requires the use of high-resolution gamma spectrometry (HPGe detectors). The accuracy of the thorium quantification is better with this technique. The third method is based on the measurement of exhaled 230Rn.