Characterization of Radioactive Waste Drums by Non Destructive Gamma Spectrometry

Assay of the radioactive content of waste drums is important for inventory and waste disposal purposes. The sampling of the drum content is a difficult, time and labor consuming procedure, involving special radiation protection considerations. On the other hand the characterization of waste drums containing radioactivity by measuring the drum as a whole (without opening) offers overall simplicity while minimizes labor and radiation protection requirements. In the present work a non destructive technique based on gamma spectrometry was used to assay radioactive waste in drums. For this purpose a portable NaI detector was utilized. The gamma ray detector efficiency for the volume source was derived by Monte Carlo simulations. The MCNP code was used to perform numerical simulations taking into consideration the energy of the gamma ray emitter, the matrix material, the detector efficiency, the geometric configuration employed, the size of the drum, and the wall material and thickness of the drum. The technique was verified by estimating radioactivity levels in drums containing ion exchange resin waste from the water demineralization system of GRR-1 open pool-type research reactor facility. Satisfactory agreement was observed by comparing the results of the non destructive method against analytical results of samples obtained from each drum. The simulations of the present study can easily be extended to model other materials and container types and sizes as well. Preliminary calculations for drums containing solidified radioactive sludge are presented. The technique can also be applied in other radiation protection applications such as biological radioactive waste from hospitals or other physical forms materials that can be met in radioactive waste.