Vacuum vessel manufacturing | Last phase for Korean sector #6

On 7 September, contractors from Hyundai Heavy Industries in Ulsan, South Korea, initiated the final fabrication activities for vacuum vessel sector #6: the assembly of four completed segments into the final D-shaped component, plus the installation of upper and lower port stub extensions. At the end of this first-of-a-kind undertaking, the first 440-tonne ITER vacuum vessel sector will be ready for shipment to France. On the shop floor in Ulsan, a curtained-off clean space has been created around the assembly platform that supports the building blocks for ITER first vacuum vessel sector: four poloidal segments ranging in weight from 35 to 125 tonnes. When assembled and welded, the final D-shaped component will measure 13.8 metres in height, 6.6 metres in width, and 7.8 metres in length. All nine sectors needed to form ITER's toroidal plasma chamber are currently in fabrication in the factories of two ITER Members—Korea (for four sectors) and Europe (for five sectors). As the first sector programmed for arrival at ITER, sector #6 has the honour, and the challenge, of "proceeding first" through every fabrication stage. The lessons learned on sector #6 will facilitate the way for all other sectors. "We are definitely breaking new ground," says project director Kyungho Park at Hyundai. "The ITER vacuum vessel is an absolutely unique component with very complex geometry, hundreds of interfaces to other components, and very strict nuclear safety compliance requirements. Each step of the process has been challenging, from manufacturing qualification and procedures, machining, and welding, to non-destructive examination and tolerances. We are learning and problem solving as we go, and we are making sure that the knowledge and experience we acquire benefits the other sectors." The vacuum vessel is a double-walled component, with 34 to 75 cm of space between the inner and outer steel shells. "Space" is rather a misnomer, however, because the interspace will be nearly completely filled with in-wall shielding blocks (packs of borated stainless steel plates), cooling water channels, and steel supporting ribs. The interspace of Sector 6, for example, contains 850 blocks of in-wall shielding and 850 supporting ribs. Inside the vessel, 172 flexible support housings are welded for the attachment of the blanket shield blocks while outside, two port stub extensions are attached to the sector's upper and lower port stubs through splice plates. All of this complexity makes the final assembly stage quite challenging. Activities will be carried out in the following order: TIG welding of the inner shell (full penetration welds), followed by welding of structural ribs (so-called "t-ribs") and flexible support housings, keys, the installation of in-wall shielding, and—in the final activity to complete the segments—TIG welding of the outer shell (full penetration welds). One hundred percent volumetric inspection is required as a nuclear safety requirement. Taking all assembly activities on sector #6 into account, the Hyundai team estimates the total length of full penetration welds at 1,380 metres. In order to increase confidence that the stringent tolerance requirements for the final geometry can be achieved, the Hyundai team—in collaboration with the ITER Organization and the Vacuum Vessel Project Team¹—performed  a "virtual fitting" this summer in which all available dimensional measurements of the completed poloidal segments were processed with specialized software (Space Analyzer)  and matched virtually. "This process was key," explains Chang Ho Choi, who leads the ITER Vessel Division. "It allows the demanding tolerances to be achieved by identifying the areas that require reverse engineering to compensate welding deformation and shrinkage." From 7 to 15 September, the team completd the inner shell welding on all poloidal segments. Six Project Team inspectors, two Agreed Notified Body² inspectors, and the Hyundai quality departments are performing continuous quality control—day and night—as the work advances. A total of 45,000 inspection points are planned and 500 manufacturing documents, including reports, have been developed. ¹ The Vacuum Vessel Project Team reunites staff from the European, Korean, Indian and Russian Domestic Agencies with staff from the ITER Organization in regular meetings to improve the overall efficiency of vacuum vessel procurement execution.² An Agreed Notified Body (ANB) is a private company authorized by the French Nuclear Regulator ASN to assess the conformity of components in the pressure equipment category (ESPN).