In-vessel tritium retention and removal in ITER

Tritium retention inside the vacuum vessel has emerged as a potentially ser ious constraint in the operation of the International Thermonuclear Experim ental Reactor (ITER), In this paper we review recent tokamak and laboratory data on hydrogen, deuterium and tritium retention for materials and condit ions which are of direct relevance to the design of ITER. These data, toget her with significant advances in understanding the underlying physics, prov ide the basis for modelling predictions of the tritium inventory in ITER. W e present the derivation, and discuss the results, of current predictions b oth in terms of implantation and codeposition rates, and critically discuss their uncertainties and sensitivity to important design and operation para meters such as the plasma edge conditions, the surface temperature, the pre sence of mixed-materials, etc. These analyses are consistent with recent to kamak findings and show that codeposition of tritium occurs on the divertor surfaces primarily with carbon eroded from a limited area of the divertor near the strike zones. This issue remains an area of serious concern for IT ER. The calculated codeposition rates for ITER are relatively high and the in-vessel tritium inventory limit could be reached, under worst assumptions , in approximately a week of continuous operation. We discuss the implicati ons of these estimates on the design, operation and safety of ITER and pres ent a strategy for resolving the issues. We conclude that as long as carbon is used in ITER and more generically in any other next-step experimental f usion facility fuelled with tritium - the efficient control and removal of the codeposited tritium is essential. There is a critical need to develop a nd test in situ cleaning techniques and procedures that are beyond the curr ent experience of present-day tokamaks. We review some of the principal met hods that are being investigated and tested, in conjunction with the R&D wo rk still required to extrapolate their applicability to ITER. Finally, unre solved issues are identified and recommendations are made on potential R&D avenues for their resolution. (C) 1999 Elsevier Science B.V, All rights res erved.