12 Apr Creative decomposition

For the past six months, a group of workers has been demolishing a storage block that stands close to the old DR 3 reactor. The team has to test its mettle to find the most effective methods.

K The coal bucket descends over the dusty pile of ball concrete, and with a secure grip in the remote control, Thomas Nielsen causes the shovel's open gap to make a inroads into the pile. The robot arm is now transferred to a powerful sack, and as the shovel is carefully placed inside, it empties its cargo into the interior of the bag.

Thomas Nielsen and his colleagues are breaking down a 6 meter high storage block. The block, located right next to test reactor DR 3, was used at the time to store the spent fuel elements while they cooled down. The old drawings show an ingenious structure with storage pipes surrounded by lead blocks and a thick layer of concrete mixed with a myriad of steel balls to protect the best possible radiation from the radiation.

Before decomposition, the team had to clear some test tubes and other items that have been stored in the block since the reactor's closure. In the last six months, the team has been in the process of decomposition itself, alongside other tasks.

Risø's old design drawings do not necessarily reflect the reality that meets the employees as the decomposition progresses. Therefore, it is time and again necessary to adjust the work plans and come up with creative solutions.

Step by step
First, the surface of the block had to be cleaned of old, environmentally hazardous paint filled with heavy metals. Here ice blasting was chosen as the best method (see article "Icy solution to environmental problem"). Then you had to find a machine that was strong enough to mill the concrete – which in addition to steel balls also contains rebar. A five-ton heavy decomposition robot of the type Brokk 400 proved capable of coping with the must.

Now the employees are switched to remotely control the robot's special milling head of a type normally mounted on 25 tons of excavators. When an appropriate amount of concrete is gnawed off the sides of the storage block, the Brokk gets a break. The concrete piles must be cleaned away.

To minimize the dust, the team first chose to vacuum up the concrete with a very powerful vacuum system. However, the steel balls in particular wear a lot of equipment, so the team had to devise an alternative method. A coal shovel that can be mounted on a smaller Brokk was purchased, and that's the one Thomas Nielsen now serves – with finger-pointing so that the dust doesn't swirl up.

Inside the sides of the storage block there are also some tons-heavy lead blocks. They slowly appear one by one as the team gets its way into the concrete. It takes hoisting and machining to get such a block of pry free, and it is only afterwards that one can find out how much it really weighs.

The storage block is located between the test reactor and the wall of the reactor building, and there is not much room to give off. With the machines and methods now in use, you cannot remove the entire block. So when the team gets further, new creative solutions will be needed in the decomposition work.