Speaker
Description
For future burning-plasma fusion devices that will operate with a D-T fuel mix, the scarcity and radioactivity of tritium represent critical constraints on reactor safety and availability. Therefore, it has long been recognized that tritium retention and accumulation in Plasma-Facing Components (PFCs), along with its permeation into structural materials and cooling loops, require continuous monitoring and control. These topics have been addressed by extensive experimental studies, ranging from fundamental laboratory-scale investigations to machine-scale experiments in tokamaks and stellarators, with analysis methods ranging from local in-situ diagnostics to global gas balance measurements and ex-situ post-mortem analysis. In parallel, modelling efforts have provided important insights into hydrogen transport and retention, spanning first-principles calculations, atomistic and continuum approaches, up to whole-device scale analysis. The recent ITER re-baselining with tungsten as the material for both the first wall and the divertor, together with the prospect of next-step burning-plasma devices expected to reach high neutron fluence, call for a comprehensive summary and critical revision of the accumulated knowledge.
This contribution reviews recent advances in the description and understanding of fuel retention mechanisms in metallic PFCs, based on both experimental and modelling studies. This includes the implications of the transition from a beryllium to a tungsten first wall in ITER, with particular attention to boronization and associated fuel co-deposition; the role of self-ion- and neutron-induced displacement damage; isotope effects in retention and permeation; the application of in-situ fuel retention diagnostics; global-scale fuel recovery and accountancy; aspects of tritium breeding; progress in code development and validation; integrated modelling, and associated uncertainty quantification. Notable recent progress includes the development of open-source macroscopic rate-equation codes FESTIM and TMAP8 for advanced tritium transport modelling; the development of the global-scale simulation framework Hydrogen Inventory Simulations for PFCs (HISP) and its application to fuel retention and outgassing analysis in tokamaks; the recent record D–T campaigns at JET and subsequent tritium clean-up experiments, including successful demonstration of in-situ laser-based local fuel retention measurements that are also in planning for ITER; experimental and modelling studies addressing the creation and annealing of displacement damage in tungsten and fusion relevant steels. The remaining knowledge gaps and forthcoming challenges in view of reactor-scale fusion devices will be summarized.