The ITER vacuum vessel is the first line of defence for any leak of radioactive tritium from the ITER plasma. As such its integrity is very important. As the fundament on which the high heat flux resisting in-vessel components are mounted, it must also be assembled with sufficiently high accuracy that the mountings of those components can compensate for any remainder. The in-vessel components if they fail can leak coolant into the vessel, quenching the plasma and requiring considerable machine down time to repair. To demonstrate that the necessary vessel accuracies can be obtained for such a large but thin component, and that the high heat flux components will resist the necessary loads without breaking, three large projects were conceived:

• Vacuum Vessel Sector - L3 - to build 1/18 of the ITER vessel to high accuracy
• Blanket Module - L4 - to test high heat flux blanket components and shield module manufacture
• Divertor Cassette - L5 - to test high heat flux divertor components and cassette manufacture