Development of an In-Situ Radiological Classification Technique for Material from CERN's Accelerator Facilities

CERN, the European Organization for Nuclear Research, operates high energy accelerators for particle physics research. Because of beam losses and subsequent particle interactions, radioactivity can be induced in certain accelerator components. Material and waste taken out of the accelerators facilities as a result of maintenance repair and upgrade actions as well as in case of decommissioning needs to be radiologically classified for future handling. Depending on the level of residual activity, some of these components are candidates for clearance from regulatory control in Switzerland. The Swiss radiation protection ordinance sets as criteria for clearance of material and waste from regulatory control the compliance with radionuclide specific limits for surface contamination and for specific activity as well as an ambient dose equivalent rate criterion. For objects with a mass below 1 kg a radionuclide specific clearance limit for total activity has to be respected. This work is focused on the specific activity criterion, whereby the Swiss clearance limits for specific activity correspond to the exemption limits for specific activity. The clearance from regulatory control performed at the exit of the facility is special, mainly due to the restricted infrastructural conditions and a limited time budget. In most cases the measurement has to be non-destructive, i.e. without measurements of disassembled subcomponents or sampling works for detailed mechanical, chemical or radiochemical analysis. In addition, some information of the complete activation process might not be available, e.g. the irradiation time prole, the chemical material composition or the object location when it has been in the facility. The present work tackles this challenge by defining enveloping conditions and parameter sets that allow to perform material clearance measurements, even without a detailed knowledge of object characteristics and radiological history. Representative input parameter ranges, e.g. reference material compositions or irradiation conditions, were determined and used for the calculation of the resulting radionuclide inventories. More than 28000 radionuclide inventories were calculated, analysed and visualized. A 600-page report containing these data supplements this work. It was studied whether the detection limit of a total gamma counter, based on six large area scintillation detectors (RADOS RTM 661/440 lnc, produced by Mirion Technologies, Germany), is sufficiently low to reliably detect the radioactivity that corresponds to the clearance limits for the calculated radionuclide inventories. The gamma-ray signal per Swiss clearance limit was determined for the calculated radionuclide inventories of all selected reference materials and for all considered irradiation conditions and compared to the detection limit of the RADOS RTM 661/440 lnc. To include the detector response in the calculations, a photon energy dependent efficiency function was determined for the total gamma counter. The dependence of the signal strength on the various parameters was analysed and the most conservative resulting radionuclide inventories, i.e. the ones with the lowest detectable gamma-ray signal per clearance limit, were identified and categorised by material as well as by irradiation condition. These enveloping radionuclide inventories allow performing clearance measurements, even without a detailed knowledge of object characteristics and radiological history. Furthermore, self-absorption effects as a function of the object geometry and material density were quantified by Monte-Carlo simulations and integrated in the clearance method by specifying self-absorption factors. The gamma-ray signal per Swiss clearance limit of the considered common materials for objects with a mass of 1 kg, the most penalising mass configuration according to the Swiss radiation protection ordinance, is above the detection limit of the RADOS RTM 661/440 lnc, even if the clearance limit is dominated by radionuclides that have no measurable gamma-ray emission. Consequently, by using the developed characterization method, this device can be used for the clearance of material and waste from CERN's accelerator facilities from Swiss regulatory control for the present Swiss clearance limits in force.