SWEDEN | COVER STORY


control rod safely. A camera and a dose rate probe were placed in the opening, for monitoring when the control rod was hoisted up. The radiation protection tube with the control rod could then be moved just a few centimetres above the floor to the shaft where it was lowered towards the scissors. Powerful alligator scissors were mounted for vertical clipping (model DTX600-11kW-Xtra Torque with cutting power of 240t). This method was chosen to minimise heat generation and the creation of secondary waste. To keep the spread of contamination in a confined space, but still be able to monitor the cutting via camera, an enclosure of steel frames and polycarbonate was mounted around the scissors. A ventilation exhaust was mounted on the installation and connected via filter box to the general ventilation.


Segments of the absorbers were collected in an 80-litre


barrel placed in a single-hole concrete container. The barrel was reinforced with an additional inner drum for shielding against radiation and damage from the cut segments. The waste packages were tightly docked below the scissors to avoid radioactive contamination on the outside of the concrete mould. A lifting table on a remote rail car was used to manoeuvre waste packages to the docking position below the scissors (Pos 3), to the position for opening and closing the waste packaging (Pos 2), and to the central position (Pos 1), where the concrete mould could be lifted up and down to the reactor hall. It was realised early on that the compression and length


of the cut segments were important considerations for overall volume reduction. Here the use of the scissors had another advantage, as it both pressed together and cut off the segments. Tests of gasketing with different segment lengths were done, using an inactive dummy and the tests indicated that segment lengths of about 125mm would be enough to house two full absorbers in an 80-liter barrel. Locking the lids onto the 80-litre barrels also offered


challenges. The lids have a locking mechanism that is released by pressing a plunger placed in the middle of the lid. For this, a custom-made tool with an electrically controlled ball screw was used that was mounted on a used industrial robot. The robot was programmed with pre- determined movement patterns to be able to mount the


three lids: for the inner barrel; for the 80-litre barrels; and for the outermost concrete cap. Three laser pointers and a camera were used to fine-tune the robot’s position between these steps.


Testing It is important that a design concept which looks suitable in theory or on paper is verified in reality. A key to ensuring the equipment was functional and optimised has been to assemble and test the equipment off-site before transporting it to Ågesta. This was done at a local workshop, which was also commissioned to manufacture and modify parts of the equipment as necessary. After installation at Ågesta, inactive tests of all the


constituent parts were again carried out. Documentation and risk analyses were supplemented, and CE marking was made by moving machines.


Performance On 15 December 2020, the first active control rod was cut. With functional controls completed and the control rod in position above the scissors, the cutting of absorbent could be initiated and 20 minutes later there were 24 segments in the barrel. However, unlike in the inactive testing, a longitudinal bend was created in the rod, which meant that the remaining part of the control rod could not be lifted up again. As one of the least active control rods had been selected as the first active test, the lower part could be cut manually and the remaining control rod returned to its original position. After this, a minor modification of the equipment was made. The narrow section that sat above the scissors was removed prior to cutting and this turned out to be enough to handle the bend. The length of the segments was also slightly reduced to ensure a sufficient degree of packing. It was also found that the lower part of the control rod,


after segmentation, had a higher dose rate than expected — about 10mSv/h. The method and equipment for handling the remaining part had to be developed. Moreover, a fully- extended absorber was a prerequisite for segmentation in the scissors and, pending restart, work was done on releasing the absorber locking mechanism on the control rods. This was done with a simple mechanical tool that U


Above left: Robot mounting a lid Photo credit: John Guthed Above right: Transportation to SVAFO Photo credit: Linda Ekstrand


www.neimagazine.com | February 2022 | 35


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