The ITER cryostat is the vacuum chamber containing the tokamak reactor. Its functions are (1) to provide a high vacuum environment to limit thermal loads to the superconducting magnet system by gas conduction and convection; (2) to be part of the second radioactivity confinement boundary; and (3) provide passive removal of decay heat for beyond design basis accidents. A separate thermal shield along the inside wall limits thermal radiation to the coils. An external concrete shield provides radiological protection. The cryostat consists of 1 cylindrical section bolted to torispherical heads at top and bottom. The top head is split into flanged concentric sections to permit upper poloidal field coil and central solenoid removal without removing the entire head. The vessel is made up of two concentric walls connected by horizontal and vertical ribs. The space between the walls can be filled with helium gas at slightly above one atmosphere for thermal coupling of the two walls, to block inbound air microleaks, and for leak detection. The cryostat has many penetrations, some as large as four meters diameter, providing various types of access from the outside to the tokamak. These include heat transport system cooling pipes, cryogenic feeds, auxiliary heating, diagnostics, and blanket and divertor removal ports. Large bellows are used between the cryostat and the tokamak to accommodate differential thermal expansion.
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