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Authors (affiliation): 1. Rita Plukienė (FTMC), 2. Elena Lagzdina (FTMC), 3. Darius Germanas (FTMC), 4. Kristina Mikalauskienė (FTMC), 5. Marina Konstantinova (FTMC), 6. Artūras Plukis (FTMC), 7. Arūnas Gudelis (FTMC), 8. Vidmantas Remeikis (FTMC)
One of important tasks for smooth and successful nuclear power plant (NPP) decommissioning process is optimization of nuclear facility metallic radioactive waste (MRW) management by applying grouping and separation of MRW. In order to reduce the amount of MRW to be disposed of in final repositories the efficient characterization, decontamination and/or melting processes should be applied. Modelling tools (MCNP6, SCALE6.2) are usually used for obtaining activation of materials in the reactor core for characterization and separation of waste streams of high activation, intermediate, low activation metal waste and also non-activated materials for which only surface contamination is relevant. For efficient characterization of very low-level metallic waste the determination of surface contamination part is needed by simple nondestructive γ-spectrometry measurement or combination of dose rate/γ-spectrometry measurement application. The aim of this work is to investigate the γ-spectra of 60Co (activation) and 137Cs (surface contamination) sources in different shielding geometries and from the shape/intensity and peak/Compton ratio of γ-spectra analysis identify surface and volume activity by using different HPGe and CeBr3 detectors.
The measurements of the same known-home-made different geometry metallic waste laboratory samples with 60Co and 137Cs sources have been performed using HPGe semiconductor and CeBr3 scintillation detectors. MCNP6 modelling of both detectors and different sample geometries of the experiments are carried out for comparison reasons. The coincidence-summing effect was taken into account. Detectors were efficiency calibrated using reference materials traceable to NMI. The GammaVision software (v. 6.06) was applied for spectra acquisition and analysis. A good consistency (discrepancies not more than 3%) of experimental and modelled results has been obtained during comparison of measured and modelled γ-spectra of laboratory samples with HPGe, on the contrary, not so good agreement was obtained for CeBr3 detector (discrepancies up to 15%). This was associated, probably, with CeBr3 detector measured γ-spectrum thermal dependence which is not taken into account in the modelling case. The analysis of experimental measurement and modelling of the 60Co and 137Cs source samples has shown, that there is peak/Compton ratio dependence for different thickness iron shield laboratory samples. MCNP modelling of different source cases: planar source shielded with different metal plates and volume source have revealed, that surface contamination 137Cs source can be distinguished if compared with a reference source case by using modelling and measurement techniques from the shape/intensity and peak/Compton ratio of γ-spectra analysis. This allows investigating of γ-spectra of surface contaminated and volume activated different geometry metallic waste samples. Some recommendations for application of different spectrometers for surface activity determination in metallic contaminated samples are proposed.
