Hoverdust was stubborn

The mechanical arms that clean the heavily contaminated Hot Cells for radioactivity have had an involuntary break. The fourth cell to be cleaned was found to contain large amounts of fine hoverdust that almost stick to the filter.

DDecommissioning of Risø's historic site is a unique task. Documentation is often inadequate and, as the work progresses, unexpected elements reveal themselves time and again. This is also the case with the ongoing rough cleaning of the Hot Cells:

The first three of the six Hot Cells were sandblasted with remote-controlled mechanical arms in 2017. These three cells have been primarily used for chemical experiments and have also been attempted to be cleaned in the 1990s with water. The blast started here to gather experience with the equipment for the last three cells, where measurements show that radioactivity is higher.

At the beginning of 2018, the fourth cell was set in motion. It has been used for a myriad of mechanical experiments, including for grinding, and has never been attempted clean before, so it was expected that there would be larger amounts of radioactive dust. However, it was not prepared that the hover dust for extraction would get so firmly in the filter:

"During rough cleaning, we use a powerful vacuum suction that stands next to the cells. It has an air filter at the top and a so-called cyclone at the bottom that collects the particles from the blast and sends them down into a shielded drum," explains project manager Bjarne Rasmussen and continues: "If radioactive dust settles in the air filter, we have two different methods to loosen it and get it down to the cyclone: compressed air and vibration. But in the fourth cell it turned out not to be sufficient; the very fine and cesium-filled hoverdust almost stuck to the filter and the filter box."

Need more space
The filter had to be changed before the blast could continue. During the preparation phase, filter changes had been practiced on a scaffold, but with the abruptly increased radioactivity in the filter box, a scaffolding was no longer appropriate.

"In order to work efficiently and get the lowest possible radiation doses, three men were needed for the task, so we had to have more jobs," says Bjarne Rasmussen.

Recent months have been spent constructing a three-metre-high platform with an upstairs workspace. The structure is constructed in a nearby building and then placed next to the vacuum cleaner as an extension of an existing working room by the filter. The total working space has a lower air pressure than the surroundings in order to keep radioactive contamination inside.

Now three employees have changed the filter. The successful work operation took half an hour from the first screws being loosened until a truck had lifted the filter and shielding out of the door and into a container.

More improvements
The platform is designed so that it can remain for future filter changes without bothering the other work that occurs in the area around the vacuum suction. But the expectation is that the platform will only be needed a few times.

In addition to the design and construction of the platform, another improvement was initiated: the addition of a smaller filter to pick up the hover dust before it reaches the vacuum sucker and filter therein. This new filter is built directly into a shielded drum that is easy to replace and transport away once an appropriate amount of radioactive contamination is collected.

With the two new structures in place, the rough cleaning can continue. As in the fourth cell, the hoverdust is also expected to house in the last two cells to be sandblasted, but now the team is ready to bond with it.



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