Cryostat lower cylinder
Another major assembly achievement
7 Sep 2020
The setting, the atmosphere, the tension, and the sequence of operations were similar, but the challenges were slightly different. The cryostat base, which was inserted into the assembly pit back in May, was an exceptionally heavy, uniform and rather rigid component. The ring-shaped lower cylinder, although much taller (12 metres vs 4), was a considerably lighter load (375 tonnes vs 1,250) and—due to its diameter, thickness and many large openings—more "flexible" and likely to deform. In addition, its mass was unevenly distributed: on one side, the neutral beam port area added significant weight, which required a specific rigging arrangement.
Another image out of a science-fiction movie: the cryostat lower cylinder is slowly lifted prior to its transfer and installation in the assembly pit.
For more than one year, since April 2019, the cryostat lower cylinder had sat in storage, solidly encased in its steel frame and carefully cocooned in airtight material. On 22 August 2020, a 192-wheel self-propelled transport trailer slowly transferred the component into the Assembly Hall, passing through the wide door with very limited clearance.
Preparing for the lifting, transfer and insertion of the cryostat lower cylinder took the better part of one week, including laser measurements, the installation of complex rigging, the connection to the overhead crane, and finally a test lift to determine the importance and range of deflections.
Some 15 hours after the beginning of operations, the cryostat lower cylinder reached its near-final position above the base section. Another week of adjustments, and the welding operations can begin.
On 31 August everything was ready, and the cautious, progressive process of lifting and transferring a steel ring the diameter of the Stonehenge stone circle could begin.
By mid-day, the lower cylinder hovered just about the mouth of the Tokamak pit (see video below). An empty well at the time of cryostat base insertion, the pit has since been equipped with a number of protruding components that reduced already tight clearances to less than 10 centimetres at the narrowest points. Alignment with the cryostat base and the openings in the bioshield wall meant also that stringent rotational tolerances had to be met.
Watching the huge and almost frail component slowly slide down the pit, each opening in the steel ring aligning with the corresponding opening in the concrete bioshield, was an unforgettable experience. As the lower cylinder settled on temporary hydraulic jacks, just a few centimetres above the base, one felt a sense of awe for the precision engineering that had just been demonstrated.
Now, one week later, the lower cylinder is settled in its final position. The gap between the two cryostat sections has been reduced to just 6 millimetres or less and automatic welding operations are ready to start. Expected to last 4 to 5 months, the task will be carried out by specialists from MAN Energy Solutions—the same Indian Domestic Agency contractor that was in charge of assembling and welding the cryostat segments in the Cryostat Workshop.
Watch a video of the operation on 31 August here.
On 22 August, the cryostat lower cylinder is transferred by modular transporter from the Cryostat Workshop, where it had been relieved of its weatherproof cocoon, into the Cleaning Facility.
The 10-metre-tall component on its transporter just fits through the open doors of the Cleaning Facility. As the double doors close behind, the room will function as an airlock between the "clean" Assembly Hall and the dust of the outside environment.
The teams watch as the giant component enters the Assembly Hall. Another large component—the first 40-degree sector of the ITER vacuum vessel—had been moved further into the building to make room for the lower cylinder.
Ten metres tall, 30 metres in diameter, 375 tonnes ... the lower cylinder will be the largest component yet installed in the Tokamak pit. With a large number of openings to be matched exactly with interfacing components at the bottom of the pit, the rotational tolerances are particularly exacting.
Due to the component's cylinder shape, some deformation is to be expected during the lift operation. Ahead of the scheduled lift the teams performed a trial to confirm that the actual deformations were, in effect, in the expected range of the calculated deformations.
On Monday 31 August, the lower cylinder of the ITER cryostat is successfully lifted and transferred to the tokamak pit. The operation takes about 15 hours.
The cryostat base sits at the bottom of the pit; the lower cylinder is positioned for lowering. The rigging arrangements play an important role in keeping the component balanced and steady as it begins its descent.
One of the challenges of the operation is the lack of space between the steel cylinder and the surrounding bioshield wall. At the tightest points, the gap did not exceed 10 centimetres.
The success of the lift operation is dependent on collaboration between a large number of actors—the ITER Organization assembly teams, the CNPE Consortium (machine assembly contractor), MOMENTUM (ITER's construction management-as-agent), the crane operator (REEL), and safety and security (APAVE). Each group plays an essential role and coordination is of paramount importance.
Each opening in the cryostat must match up exactly with corresponding openings in the bioshield wall. To the right in this image are oval opening for ITER's planned neutral beam injectors. The corresponding "ports" in the lower cylinder caused a significant difference in weight on this side of the component that had to be compensated through the rigging arrangements.
About four metres remain before the bottom edge of the lower cylinder touches the cryostat base. In fact, the component will first rest on hydraulic cylinders while alignment is confirmed through metrology.
In the 30-metre-deep Tokamak pit, the role of the cryostat is to contain the vacuum vessel and magnets in a vacuum environment. It has a total volume of 16,000 m³ and a pump volume (once all components are installed) of 8,500 m³. The neutral beam injection ports and their reinforcements are clearly visible on the right side of the component.
After a round of final measurements and verification, the cryostat lower cylinder is lowered to the base and detached from the overhead crane. Indian Domestic Agency contractor MAN Energy Solutions will soon begin welding the two components together, an operation that is expected to take five months including quality tests and final documentation.
