ITER Tritium Retention Estimates and Uncertainties

Erosion and deposition constitute one of the key topics in preparation of ITER operation, as they influence the in-vessel tritium inventory and the lifetime of divertor tiles and other PFCs. Assessment of erosion is discussed elsewhere [39,41]. 

Accurate estimates of the accumulation of tritium on the surface and in the bulk of the materials of the various PFCs of ITER and the degree of tritium permeation to the coolant are very important for determining the tritium supply requirements, for assessing the radiological hazards from routine operation and from potential accidents, and for decisions regarding the de-tritiation system. 

Tritium co-deposition with eroded carbon, during the D-T phase, is anticipated to be the dominant tritium retention mechanism in ITER even if the use of carbon is limited to the divertor strike plates. Retention by implantation and bulk diffusion at the Be-clad surfaces of the first wall and W-clad surfaces of the divertor are expected to rapidly reach saturation, or to marginally contribute to the in-vessel tritium-uptake [95,96], In addition, adsorption of tritium at the inner surfaces of the porous CFC of the vertical target is expected to be very small (few g-T) and adsorbed tritium is not expected to significantly diffuse into the carbon matrix because the temperature of the majority of the bulk CFC will be significantly below 1200 K (Table 12.4). 

The quantification of the co-deposition rate in ITER is still subject to large uncertainties and analysis carried out to date provide only general trends rather than accurate predictions. This subject is thoroughly discussed in several publications (see for example [97]). The main uncertainties arise primarily from the plasma edge physics parameters, which are anticipated to strongly affect the erosion, deposition and co-deposition patterns and rates, from uncertainties of the chemical erosion yields at high fluxes [33] and from the effect of temperature and H-atomic flux on deposition patterns and rates in areas hidden from the plasma. 

Divertor erosion/co-deposition estimates in ITER are based on the RE-DEP/WBC code package [98-100]. Power fluxes as well as plasma particle fluxes (i.e., D-T ions and atoms, and impurities such as helium and carbon) 

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