Feasibility study of a non-destructive assay system for certifying identified isotopes and their activity concentrations for radioactive waste in a large container

Abstract

We have generated a system concept for a non-destructive assay (NDA) system to certify identified isotopes and their activity concentrations for radioactive waste in a large container. Various optimization studies have evaluated the system performance in terms of the MDA (minimum detectable activity) results using the Monte Carlo simulation in conjunction with experimental studies. The proposed system consists of a total of eight HPGe (high-pressure germanium) detectors: four detectors on the top and the other four detectors on the bottom. The top and the bottom detector arrays are aligned and face each other. This detector arrangement has advantages in employing the attenuation correction as proposed by T. Chang [IEEE Trans. Nucl. Sci. NS-25, 638 (1978)]. We also found that the UFOV (useful field of view) of the detector highly influences the system efficiency, which leads to an improvement in the MDA performance. However, while the wider FOV of the detector improves the detection efficiency by allowing incoming radiation from other segmented volumes, it may suffer from non-uniform performance or increased errors in estimating an activity concentration for each segmented volume. To prevent such potential errors in employing the wider FOV, we have proposed an activity estimation algorithm, a so-called ‘fine volume reconstruction’, based on a back-projection method that estimates the activity concentration of each segmented volume. We demonstrate the feasibility of the conceptual system for use as a free release assay system for ISO containers up to a size of 2.4 × 6.0 × 1.3 m³ (W x L x H). The detector efficiency was about four times higher than those of typical commercial systems. Future studies include fine-tuning of the activity reconstruction algorithm and a validation study on various materials and non-uniform activity concentrations.