Evaluation of the solid radioactive waste activity in nuclear medicine: Definition of shape factors to real geometries

This work provides a critical analysis of the point source methodology applied in nuclear medicine radioactivity waste management routine. To evaluate the point source approximation used in the practice to determine the activity and its effect on activity and storage time was used deterministic and Monte Carlo calculations. The first step of this work was to validate the Monte Carlo Geant4 toolkit using measurements with standard calibration sources. After that the deterministic and Monte Carlo calculation was performed. In a realistic geometry the waste is not a point source but a box of dimensions (30.5 x 31.5 x 25.0) cm³. The impact on activity estimation comparing the real source point to the box geometry was observed from 20% up to 800% depending on the distance between the source and the detector and the selected radionuclide. The simple case evaluated in this work considering different radioisotopes proves that uses the point source approximation cause significant influence on the activity estimation. However it does not have significant impact in the storage time. The non significant influence in time storage can be explained by the activity limits recommend to release the radioactive waste that is significant smaller than the initial activity and the exponential behavior of the radioactive decay associated to value of the physical disintegration constant for the radionuclide commonly used on nuclear medicine applications.