Further Development in Characterization of Radioactive Waste Drums by Non Destructive Gamma Spectrometry at GRR-1

A non destructive technique based on gamma spectrometry and application of the Monte Carlo method for detector efficiency calibration, was used to assay radioactive content waste drums. Exploranium GR-130 miniSPEC portable gamma ray spectrometer was used to externally monitor radioactive waste drums containing ion exchange resin waste from the water demineralization system of GRR-1 open pool-type research reactor facility at NCSR “Demokritos”. Detection efficiency was predicted by Monte Carlo simulation performed with the MCNP-4C2 code and cross-section data from the ENDF-VI-b library. The code was used to perform numerical simulations taking into consideration the energy of the gamma ray emitter, the matrix material, the detector characteristics, the geometric configuration employed, the size of the drum, and the wall material and thickness of the drum. Efficiency curves in the energy range 60 to 1500 keV were predicted assuming a homogeneous distribution of the source activity within the matrix material. A relative error of less than 5% was achieved in all simulated cases. Satisfactory agreement was observed by comparing the results of the non destructive method against analytical results of samples obtained from 25drums. Moreover the MCNP model was used to examine the effect of inhomogeneities, on the accuracy of the technique and estimate the measurement bias. A technique to predict such inhomogeneities by measurements at different source to detector positions was examined.