The ITER vacuum vessel is designed to be a double-walled structure made of 316L+N stainless steel with a D-shaped cross section approximately 9 m wide and 15 m high. Its overall thickness is formed by inner and outer shells, 40 to 60 mm in thickness, joined by welded stiffening ribs. The inner shell of the vessel provides the first safety barrier of the reactor. Approximately 65 % of the volume between the shells is filled with plate inserts to provide the required nuclear shielding. These inserts are attached to the ribs to insure the total toroidal electrical resistance with blanket system of over 4.0 micro ohm. The cooling water at 100 centigrade degrees, 2.0MPa, flows in the space around the inserts to remove the 3 MW of nuclear heat deposition. The water introduced into the supply-manifold at the bottom of the vessel, branches off into several passages separated by stiffening ribs, and then joins again in the return-manifold located at the top of the vessel. The required total flow rate is estimated to be 65kg/sec in the normal operating condition. In off-normal operating condition such as a loss of flow accident (LOFA) when the coolant pump would stop, the vessel is expected to be cooled by the natural convection.

A 1/6 sub-scale partial model corresponding to the outboard section of 9 degrees sector has been fabricated to examine the flow distribution and the possibility of flow stagnation in the cooling passages under the forced convection flow condition. Flow velocity in each cooling passage was measured by the tracer method and total coolant head loss was measured by pressure gauge. Flow network analyses for the sub-scale model were also performed to confirm a validity of the calculation model employed for the vessel design. The result indicates that the mass flow rate distribution has a large difference among the coolant passages, with the smallest flow rate in the passage around divertor port and horizontal port.

Natural convection analysis was also conducted for thermal-hydraulic assessment of the vessel cooling system in the off-normal operating condition, using FDM thermal-hydraulic analysis code. Analytical results show that natural convection flow occurs at the coolant with the exit temperature rise of 37 degrees, and the total flow rate resulting from natural convection is to be about 40% of the normal flow. The vacuum vessel temperatures are found to be sufficiently below the design limit of 250 degrees.