Method validation for the characterization of γ-emitting radionuclides in irradiated graphite using the efficiency transfer software EFFTRAN

A large quantity of irradiated nuclear graphite has been produced over the years and has to be treated potentially as radioactive waste. Several challenges arise when considering final disposal of such graphite in Germany, due to the lack of information on the radionuclide inventory as well as the strict guidelines for certain radionuclides. Therefore, experimental characterization becomes essential, not only to identify the specific radionuclides present in graphite samples but also to determine their activities. Gamma emitters are of particular concern during decommissioning activities, as they are the primary contributors to the dose rate to which workers may be exposed. One commonly used technique for this purpose is gamma-ray spectrometry, which relies on an accurate efficiency curve to quantify the sample’s activity. This can be achieved by using the experimental efficiency curve of a standard that has the same geometry as the sample. However, this is not always possible due to the variation in shape, size and matrix of the sample. In order to overcome this challenge, simulation software can be used, such as the Monte Carlo-based efficiency transfer code EFFTRAN. It allows the transfer of the efficiency curve from cylindrical reference standards to any other cylindrical sample. This transfer method has been validated in this work using certified standards as well as irradiated nuclear graphite samples coming from the FRJ-2 (JEN) research reactor.