An operational radiation safety intervention: Minimizing dose in lab spaces due to photon sources in adjacent storage room

Abstract

The Nuclear Engineering Laboratory of NTUA operates a radiography installation incorporating a 200 kVp -x ray unit in a basement vault. During licensing characterization phase of the control, supervised and general public zone(s) by the Regulator, it was made obvious that there existed potentially harmful dose, greater than the background, within the control zone(s). At a limited number of points, easy to be avoided, dose reached 1.5 μSvh^-1. This dose was due to -γ emitting sources stored in adjacent to the control zone(s) storage room. The situation was further investigated by dose measurements collected using a portable radiation monitor. The collection grid was set to about 0.5´0.7 m for approximately the following elevations: 0.00 m (floor), 1.00 m, 1.70 m and 2.20 m. Maps in 2D were then drawn numerically for these elevations using the kriging method. Results demonstrated that photons contributing to dose emerged at a height of about 1.00 m, from the NW wall of the vault, behind which the source storage room of the Laboratory is located. It was then realized that photon sources were placed behind this wall in drums without any particular shielding. It was consequently decided to move the sources away from this wall to the SW side of the storage room and shield them circumferentially with standard lead bricks. The result of this intervention was verified by remapping the dose rate in the control zone(s) for the worst case scenario of the 1.0 m elevation. New doses were found close to the background.