The Divertor Remote Maintenance Project

Remote replacement of the ITER divertor will be required several times during the life of ITER. To facilitate its regular exchange, the divertor is assembled in the ITER vacuum vessel from 60 cassettes. Radial movers transport each cassette along radial rails through the handling ports and into the vessel where a toroidal mover lifts and transports the cassette around a pair of toroidal rails. Once at its final position the cassette is locked to the toroidal rails and is accurately aligned in both poloidal and toroidal directions. A further requirement on the divertor is to minimise the amount of activated waste to be sent to a repository. To this end the cassettes have been designed to allow the remote replacement, in a hot cell, of their plasma facing components. The paper describes the two facilities built at ENEA Brasimone, Italy, whose aim is to demonstrate the reliable remote maintenance of the divertor cassettes. 1.EDIVERTOR MAINTENANCE Remote replacement of the ITER divertor is estimated to be required up to eight times during the life of ITER because of failure, erosion of the armour or to install alternative divertor configurations. To facilitate its regular exchange, the divertor is assembled in the ITER vacuum vessel from 60 cassettes [1], each of them being introduced into the vessel through one of four equispaced handling ports. Of the 15 cassettes installed through each port, 12 are 'standard' cassettes and two are 'second' cassettes, positioned on either side of a 'central' cassette located directly in front of the handling port. Radial movers transport each standard cassette along radial rails through a port and into the vessel where a toroidal mover lifts and transports the cassette around a pair of toroidal rails (fig. 1) to its designated position. To do so, the toroidal mover inserts, between the toroidal rail and the cassette bottom, two forks which include a set of wheels and jacks to lift the cassette by a few millimetres. The jacks are driven by pressurised water whilst the toroidal mover is equipped with pinions, driven by electric motors, operating against racks on the rails to index the cassette around the vessel. The second cassette is transported radially by the second cassette mover until it reaches the position of the central cassette, at which point it is translated toroidally by 61⁄4 along the toroidal rails. The central cassette is transported radially by an ad-hoc mover to its final position. Once at its final position each cassette is locked to the toroidal rails such that it can sustain the off-normal electro-magnetic loads and is accurately aligned in both poloidal and toroidal directions. The transport between the vessel and the hot-cell for refurbishment is performed using a transfer cask with a double door which ensures the reactor and the cask remain closed during removal of the cask to the hot cell. FIG. 1 ITER Divertor Assembly FIG. 2 Exploded View of a Divertor Cassette A further requirement on the divertor is to minimise the amount of activated waste to be sent to a repository. To this ends a second element of this project addresses the remote replacement of plasma facing components (fig. 2), thus allowing the cassette bodies (~ 1000 tons of steel) to be used for the entire lifetime of ITER [2]. Each cassette comprises 10 plasma facing components (PFC's) attached to the body, and the refurbishment of a complete set of cassettes is scheduled to last 1 year. 2.EOBJECTIVES OF THE ITER DIVERTOR MAINTENANCE PROJECT The objectives of the ITER divertor maintenance project were to demonstrate the feasibility of divertor maintenance [3] and to set up two facilities for the future optimisation of the handling equipment, of the handling features incorporated into the components being handled, and of the maintenance procedures. The Divertor Test Platform (DTP) allows full scale simulation of all handling operations inside the vacuum vessel. The Divertor Refurbishment Platform (DRP) allows simulation, also at full scale, of the most critical operations to be realised in the hot cell. Both facilities have been set up at the ENEA Research Centre of Brasimone, Italy. FIG. 3 Overview of the Divertor Test Platform 2.1EObjectives of the Divertor Test Platform The procedures for divertor cassette replacement and the related remote handling equipment have been devised taking into consideration the initial assembly, positioning and manufacturing tolerances of the ITER components ("nominal" conditions). Deviations from the initial tolerances ("limit" conditions), resulting from deformations of the components to be handled or of their supports, will be unavoidable after several years of machine operation. The feasibility of the remote maintainability of the divertor cassettes will be demonstrated after checking the performance of the handling equipment under "nominal" conditions and after assessing their operational limits. The tests foreseen and executed to date include the commissioning of all models, mock-ups and prototypes, the validation of mover operations in nominal conditions, the validation of auxiliary remote handling systems in nominal conditions and a preliminary assessment of the operational limits of the movers [4]. Further tests, e.g. the validation of rescue procedures, will be undertaken before finalising the handling equipment and the component design. 2.2EObjectives of the Divertor Refurbishment Platform The objectives of the DRP are to demonstrate the feasibility of the cassette refurbishment and to assist in the definition of hot cell procedures. The experimental validation includes the manufacture of prototypical attachments between cassette body and PFC's, the manufacture and testing of remote handling tools for handling the PFC's and their attachments and for the cutting and welding of the PFC's hydraulic connections, and a system for dimensional survey of the cassettes before, during and after the refurbishment process. FIG. 4 Assembly Test with an Attachment Key in the DRP