Nuclear Fusion Research and Development

4-8

Manufacturing the Doughnut-Type Vessel for JT-60SA

6.6 m=High-Precision Structure of Double-Wall Vacuum Vessel=

Actual Size
(Dimension +/- tolerance)

RibReinforce
10 m
Inner Wall

One-turn
ResistanceOuter Wall
One-turn
Resistance

Closure Weld
Inner Wall
(18 mm)Inside double wall

Outer Wall
(18 mm)Rib Reinforce(22 mm)Inner Wall Height6244 mm(6245.0 +6/-4)
Inner Wall Width3124 mm(3119.9 +5/-5)

Fig.4-20 Vacuum vessel and double wall structure
Manufacturing the doughnut-shaped vacuum vessel from adouble-wall welded structure. The vessel has a 10-m diameter
divided in 40°units.

Fig.4-21 Product vacuum vessel
The vacuum vessel was manufactured so that its major
dimensions were within the few millimeters of tolerance
required by the design.

Upgrading the JT-60 to the JT-60SA tokamak machinewith fully superconducting coils is the Japan-EU satellite
tokamak program; this project is under both the Broader
Approach program and Japanese domestic program. Japan isto manufacture a doughnut-shaped vacuum vessel (VV) with aheight of 6.6 m and a diameter of 10 m, as shown in Fig.4-20.
The VV is a double-wall welded structure with 18-mm-thickplates. Openings for plasma heating devices and diagnostic
devices complicate the VV structure. Segments of the vessel
are manufactured in a factory in 40°units; these units arethen assembled on-site into the complete 360°structure. Thiswelded vessel must endure enormous electromagnetic forces;
however, the high precision required by the design means thatthe structure must undergo minimal distortion during welding.
The JT-60SA VV must have the following features: (a) ahigh vacuum for the core plasma, (b) a high one-turnresistance (16 µ Ω) for plasma breakdown, (c) must be able
to endure electromagnetic forces of up to 7.5 MN verticallyand 2.5 MN horizontally, and (d) must be shielded against
neutron penetration. Austenitic stainless steel 316L wasselected as the structural material; each of the inner and outerwalls have a thickness of 18 mm and the two walls areseparated by 22 mm of rib reinforcement. Shielding water
circulates between the two walls. The total weight of thevessel, including the water, is 400 tons. The vessel issupported by nine legs. The operational temperature is 323 K;
however, a baking operation at 473 K is used to improve thequality of the vacuum. The baking is done by circulating high-temperature nitrogen gas.
In manufacturing, the double wall must be precisely weldjointed into the 10-m diameter doughnut-shaped VV. To meet
design tolerances, distortion must be minimized during
welding, which in turn requires that heat input must beminimized. Welding inspections are hampered because it isimpossible to watch, from inside the double wall, welding
penetration at closures. Some structural members weredesigned with a partial penetration weld-joint, which isfocused on the rib reinforcements. Partial penetration
welding, which requires less heat input and therefore causes
less distortion, was adopted for rib welds at the inner andouter walls; full penetration welds were not needed for thestructure to endure large electromagnetic forces. A limit of heat
input was determined before manufacturing and a 20°upperhalf mock-up was fabricated to test the manufacturing
procedure. Feasibility of manufacturing and integrity of thestructure were validated. Major dimensions on the productwere within the few millimeters of allowed tolerances; thus,
high precision was achieved in manufacturing the productvessel (Fig.4-21).
Manufacturing of the VV started in late 2009. The first
40°unit (two sectors) was completed in May 2011, as shownin Fig.4-21, and three 40°units were completed by the end ofMarch 2012. Another three sectors will be completed in fiscal
year 2012. Manufacturing of all sector units will be finishedin fiscal year 2013.

Reference
Masaki, K., Shibama, Y. K. et al., Design and Manufacturing of JT-60SA Vacuum Vessel, Fusion Engineering and Design, vol.86, issues 9-11, 2011, p.18721876.
JAEA R&D Review 201261