Decommissioning of a Caesium-137 Sealed Source Production Facility

This paper describes the techniques and approaches used in the decommissioning of a Caesium-137 sealed source production facility belonging to Amersham plc. The facility become redundant in 1991. The facility comprised of nine interconnected containment boxes operated from behind a seven-inch lead wall using tongs. Viewing was through lead glass windows. After operations had ceased the facility had deteriorated due to radiation damage. The plastic ventilation system and transfer tunnels between the boxes had collapsed. Some of the viewing windows had also become crazed preventing direct viewing into the boxes. Man entry into the facility was not possible due to the high doserates. The facility contained in the order of 150TBq (4000Ci) of activity with initial ambient doserates in the facility circa 5S v/h γ. There was no acceptable route for transfer of waste out of the facility. The facility was expected to continue to deteriorate further. The project started with an options study in 1993 and was followed on with detailed planning, design and build of bespoke equipment and installing it, followed by the decommissioning operations beginning in 1997. The facility was adjacent to other fully operational facilities less than 1/2 metre away. The equipment designed and built included: • Telerobotic System; • Remote Waste Posting Facility; • New Primary Ventilation System; • Secondary Containment, with its own ventilation system; • Maintenance Facility for repairs to the Telerobotic System. The equipment was successfully deployed and used to size reduce the total contents of the facility and the fibreglass containment boxes themselves. 2300kg of ILW was packed remotely into 14litre waste cans and then remotely posted out for safe storage. The doserates on the waste cans was up to 30S v/h γ. The remaining structure was then decontaminated remotely. When this work was completed in 2001, approximately 150TBq of activity was removed, which was 99.9% of the total activity in the facility. The ambient doserate in the facility was then reduced to 1–3mSv/h allowing man entry into the facility to complete the decommissioning. This is a significant achievement for robotic decommissioning. After a year of manual decommissioning and dismantling the ambient doserate in the facility was reduced to less than 10μSv/h and loose activity levels to less than 0.1Bq/cm². The dose received by the team during remote decommissioning was 30man mSv over a four year period. This included 12man mSv for maintenance of the Telerobotic System. The team received a similar figure of 30man mSv during manual decommissioning operations but over a one year period.