Tritium inventory in the materials of the ITER plasma-facing components

Abstract

Managing the interface between a thermonuclear plasma and the solid material environment is arguably one of the highest technical challenges in the operation of a next-step deuterium-tritium (D-T) tokamak and in the successful development of future fusion power reactors. The most crucial issues (e.g., erosion/redeposition, tritium retention) came in to sharp focus during D-T operation of TFTR and JET and, in particular, in the process of designing ITER. Carbon-based materials have superior thermo-mechanical properties and do not melt (they sublime), however they retain high levels of tritium by co-deposition with eroded carbon that could severely constrain plasma operations in a next-step device with carbon plasma-facing components. Metallic materials avoid the tritium retention issue, but melt layer losses due to transient high energy deposition (e.g., type I ELMs and disruptions) may lead to severe damage and unacceptably short lifetimes and, in addition, maintaining plasma purity with high-Z materials is a concern. A mix of several different plasma facing materials is likely to be used in ITER to meet the requirements of areas with different power and particle flux characteristics. Erosion, and the subsequent transport of impurities, will inevitably lead to a certain amount of material mixing between these materials, whose behavior in a tokamak is uncertain. Despite remarkable advances in the knowledge base from experiments and theory, significant uncertainties associated with tritium retention and erosion lifetime remain in ITER. This area remains one of the grand challenges that must be met for magnetic fusion to achieve its promise as an attractive, environmentally acceptable energy source. This paper surveys factors relevant to material choices in ITER, and reviews some of the most recent experimental findings, which shed some light on the complexity of the problem of predicting in-vessel tritium inventory. Current modeling predictions and the attendant uncertainties are discussed, together with the outstanding issues still remaining, leading to recommendations for dedicated research and development work required to address these topics.