02 Jul Airborne dust proved stubborn

The mechanical arms, responsible for decontaminating the highly contaminated Hot Cells of radioactivity, have experienced an involuntary pause. The fourth cell slated for decontamination was found to contain significant quantities of fine airborne dust, which almost adheres to the filter.

Decommissioning of Risø's historical facilities is a unique undertaking. Documentation is often incomplete, and as work progresses, unexpected elements repeatedly emerge. This has also been the case during the ongoing preliminary decontamination of the Hot Cells:

The first three of the six Hot Cells were sandblasted using remote-controlled mechanical arms in 2017. These three cells were primarily used for chemical experiments and had also undergone attempted decontamination with water in the 1990s. Sandblasting commenced here to gather experience with the equipment for the final three cells, where measurements indicate higher radioactivity.

At the beginning of 2018, work began on the fourth cell. This cell had been used for numerous mechanical experiments, including grinding, and had never previously been decontaminated, so a significant accumulation of radioactive dust was anticipated. However, it was not foreseen that the airborne dust, during extraction, would adhere so tenaciously to the filter:

“During the initial decontamination, we use a powerful vacuum extractor positioned next to the cells. It features an air filter at the top and a so-called cyclone at the bottom, which collects particles from the sandblasting and directs them into a shielded drum,” explains Project Manager Bjarne Rasmussen, continuing: “If radioactive dust accumulates in the air filter, we have two methods to dislodge it and move it down to the cyclone: compressed air and vibration. However, in the fourth cell, these proved insufficient; the very fine, cesium-laden airborne dust virtually adhered to the filter and its housing.”

Need for More Space
The filter had to be replaced before sandblasting could resume. During the preparation phase, filter changes had been practiced on scaffolding, but with the abruptly elevated radioactivity in the filter housing, scaffolding was no longer suitable.

“To work efficiently and achieve the lowest possible radiation doses, three personnel were required for the task, necessitating more workspace,” states Bjarne Rasmussen.

The past few months have been dedicated to constructing a three-meter-high platform with a workspace on top. The structure was assembled in a nearby building and subsequently positioned next to the vacuum extractor, extending an existing workspace near the filter. The entire workspace maintains a lower air pressure than its surroundings to contain radioactive contamination.

Three employees have now successfully replaced the filter. The successful operation took half an hour, from the loosening of the first screws until a forklift had moved the filter and shielding out the door and into a container.

Further Improvements
The platform is designed to remain in place for future filter changes without impeding other work occurring in the vicinity of the vacuum extractor. However, it is anticipated that the platform will only be required on a few occasions.

Concurrently with the design and construction of the platform, another improvement was implemented: the addition of a smaller filter designed to capture airborne dust before it reaches the vacuum extractor and its internal filter. This new filter is built directly into a shielded drum, which is easy to replace and transport away once a suitable quantity of radioactive contamination has been collected.

With the two new constructions in place, the initial decontamination can proceed. Similar to the fourth cell, airborne dust is also expected to be prevalent in the final two cells slated for sandblasting, but the team is now prepared to address this.