As soon as operation begins using tritium, the vessel internal components - blanket modules, divertor cassettes, distributed in-vessel diagnostics, and port plugs, can only be maintained by remote handling systems. These all operate on the same principles. The items are attached/detached using a remote manipulator, and removed through a port into a transport cask docked at the port. A temporary door is placed over the vessel access port and the cask is closed to prevent contamination spread. The cask, which contains no shielding to save weight, is moved on air bearings to the hot cell via a path from which personnel have been evacuated. A similar docking occurs at the hot cell and the component is removed, to be replaced by a new or refurbished one. The process is then reversed to bring that component to the tokamak.

The need for remote handling depends on the use of the components, and how quickly or easily they become damaged. All the above components are envisaged to need replacement in full or in part during the machine life, and procedures must be fully developed and tested beforehand. In fact, due to the weights involved, such systems will be used in the initial assembly process, to train operators and to put in place automatic procedures, while human access is still possible. It is of course possible that other types of remote maintenance are required during the plant life - for instance replacement of a TF coil or vessel sector - but these are examined from the feasibility standpoint, on the understanding that such a major disruption to the machine would only be considered if it occurred during the first few years of operation in hydrogen, before the tokamak vessel becomes radioactive.

Blanket Remote Handling

Blanket Remote Handling Scheme

Four equatorial ports are reserved for remote handling, approximately equidistant around the torus. 2 transfer ports are shared with limiters, which would be removed to provide access. 2 are occupied by the remote handling deployment system for blanket maintenance, or by diagnostics during operation. The blanket handling system consists of a rail mounted vehicle. The rail is deployed circumferentialy round the torus from the deployment port plug, and braced against the vessel at various intervals. Each rail can in principle reach completely round the circumference, but the more normal deployement will be with two systems each covering half the torus. The principle of blanket replacement is that the rail-mounted vehicle can reach all blanket modules in a given vertical plane.

Estimates have been made of the time needed to replace a defective module or the complete blanket (as might be done during the last 10 years of plasma operation in order to install a tritium-breeding blanket). These have been based on the tests carried out on the blanket module remote handling project. This shows that a single module should be replacable in 25 days, and the complete blanket in 9 months.

Divertor Remote Handling

Divertor Remote handling Scheme

Divertor remote replacement is carried out through up to three lower ports equidistant around the machine. Thus typically 18 cassettes are maintained from a given port. The divertor cassettes are moved radially from the cask docked at the port using the cassette multifunction mover, which positions them on the divertor rails mounted on the vacuum vessel floor. The cassettes are then moved to their correct toroidal position using the cassette toroidal mover.

It has been estimated from experimental tests on the divertor remote handling project that the time to replace a single cassette in the worst location would be 2 months and to replace the complete divertor will take 6 months. This latter operation is expected to be necessary several times during the machine operation, but will fit during normal periods of maintanance on the complete plant.

After used cassettes are removed from the machine, they are taken by cask to the hot cells for refurbishment.

Port Plug Remote Handling

All of the ports around the machine, except those for the neutral beams, contain plugs. These provide diagnostics and heating system elements, pumps, etc., which from time to time need refurbishment or replacement. The plugs are removed in similar fashion to the divertor cassettes, on a remote handling arm cantilevered from within a cask, and the plug is pulled into the cask before sealing and transport to the hot cell.

Further details of remote handling are available in the ITER Technical Basis .