Experimental Determination of Resuspension from Surface Contaminated Objects in Severe Mechanical Accident Conditions and Conclusions Regarding Transport Safety

Solid objects that are not radioactive by themselves but contaminated by radioactive material on their surface may be classified for transport as SCO. The contamination limits for SCO-II objects are derived from accident considerations requiring that the potential radiation exposure from inhalation of airborne released contamination of a person close to an accident location would remain below 50 mSv effective dose. Of central importance are accordingly quantitative data on the amount of respirable particulates that becomes airborne by resuspension from contaminated surfaces caused by accident impacts. Contamination limits distinguish between non-fixed and fixed contamination, a distinction which is not based on well-defined criteria. For practical purposes it is assumed that non-fixed contamination is removable by applying a wipe or smears with e.g. a dry cloth or filter paper. An experimental set-up has been applied to measure release fractions from surfaces with defined contamination induced by impact forces simulating the drop of a package from e.g. 9 m onto a hard target. Resuspension of particulates from contaminated surfaces depends on the distribution of adhesive forces to the surface and induced acceleration forces from surface vibrations caused by mechanical impact. Main features of the small scale apparatus are: The surface contamination of metallic plates is prepared in a defined way by deposited particulates, considering both fixed and non-fixed contamination. The induced surface vibrations and acceleration amplitudes from a variable fall hammer impact acting onto the contaminated plate are monitored by a small dynamic acceleration sensor (g-sensor) as used in drop tests of packages. The applied detection and size classification method for single particles released by vibration forces exceeding adhesions forces is very sensitive down to resuspension fractions in the range of 10-7. Conservatively adopting impact forces leading to peak vibration amplitudes of 2000 g potential radiation exposure of a person close to an accident location has been analysed. Even under very cautious assumptions of an IP 2 package filled with 100 m² of metal sheets contaminated at the limits for SCO-II objects the exposure of an individual would remain substantially below the 50 mSv effective dose criterion of the IAEA Transport Regulations.