12 Apr Creative Deconstruction

Over the past six months, a team of employees has been engaged in dismantling a storage block situated close to the old DR 3 reactor. The team must experiment to identify the most effective methods.

TThe grab bucket descends over the dusty pile of spherical concrete, and with a steady hand on the remote control, Thomas Nielsen directs the bucket's open jaws to penetrate the pile. The robotic arm then transports the load to a robust bag; once the bucket is meticulously positioned within, it discharges its contents into the bag's interior.

Thomas Nielsen and his colleagues are in the process of dismantling a 6-meter-high storage block. The block, located directly adjacent to test reactor DR 3, was historically used to store spent fuel elements while they cooled down. The old drawings reveal an intricate construction featuring storage tubes encircled by lead blocks and a thick layer of concrete intermixed with numerous steel spheres to provide optimal shielding against radiation.

Prior to demolition, the team had to clear experimental tubes and other items that had been stored in the block since the reactor's shutdown. For the past six months, the team has, concurrently with other tasks, been engaged in the demolition itself.

Risø's old construction drawings do not necessarily reflect the reality encountered by the employees as the dismantling progresses. Therefore, it repeatedly proves necessary to adjust work plans and devise creative solutions.

Step by step
First, the block's surface had to be cleaned of old, environmentally hazardous paint containing heavy metals. Here, ice blasting was chosen as the optimal method (see the article “Ice-cold solution to an environmental problem“). Subsequently, a machine powerful enough to mill the concrete – which, in addition to steel spheres, also contains rebar – had to be identified. A five-ton demolition robot, a Brokk 400, proved capable of handling the task.

Currently, employees take turns remotely controlling the robot's specialized milling head, a type typically mounted on 25-ton excavators. Once a sufficient quantity of concrete has been milled from the sides of the storage block, the Brokk is paused. The concrete debris must then be cleared.

To minimize dust generation, the team initially opted to vacuum the concrete using a very powerful vacuum system. However, the steel spheres, in particular, caused significant wear on the equipment, necessitating the team to devise an alternative method. A coal shovel, adaptable for a smaller Brokk, was procured, and this is what Thomas Nielsen is now operating – with dexterity, to prevent dust from being stirred up.

Within the sides of the storage block, several ton-heavy lead blocks are also concealed. These gradually emerge one by one as the team mills deeper into the concrete. Extracting such a block requires hoisting equipment and mechanical force, and its actual weight can only be determined after its removal.

The storage block is situated between the test reactor and the reactor building wall, with limited available space. Given the current machinery and methods, it is not possible to remove the entire block. Therefore, as the team progresses, new creative solutions will be required for the decommissioning work.