Modelling of tritium retention and target lifetime of the ITER divertor

Erosion, deposition and transport of eroded material in the divertor of ITE R are modelled with the Monte Carlo impurity transport code ERO taking into account c hemical erosion, physical sputtering, enhanced erosion of redeposited carbon and beryllium deposition. With increasing exposure time continuous deposition of beryllium leads to reduced carbon erosion along the divertor plates. The limit of discharges possible from t he estimated long-term tritium retention ranges between 440 using an effective sticking o f unity for redeposited hydrocarbons and 380 for an effective sticking of zero. The lifetime of the divertor plates is less critical than tritium-retention, in the worst case about 7000 I TER discharges. Erosion due to transient heat loads (ELMs or disruptions) are not yet included in the modelling. Introduction In the current design of ITER, carbon fibre composites (CFC) are foreseen for the divertor target plates where the highest heat loads are expected. The advant age of non-melting of graphite materials opposes their strong erosion due to chemical form ation of hydrocarbon molecules even at low plasma temperatures. Extrapolations from c urrent experiments to ITER indicate a critical amount of tritium retention due to co-depositi on. However, experimental data are based on full-carbon cladded devices whereas in ITER bery llium is foreseen for the main wall and tungsten for the baffles and the dome. No experimental data exist for this material choice and extrapolation must therefore base on a reasona ble understanding of the processes determining the fuel retention taking into account the ITER materi al mix. This contribution presents ERO modelling of erosion and deposition along the di vertor target plates taking into account a uniform background beryllium influx of 1% r elative to the incoming deuterium ion flux. Eroded particles are followed through the d ivertor plasma until they are redeposited at the divertor plates or escape the plasma volume cons idered in the code. Particles not locally redeposited are assumed to form carbon-laye rs at remote areas leading to long-term tritium retention. The chemical erosion yield by deut erium impact (ions and atoms) is calculated using the new semi-empirical "Roth" formula dep ending on the incoming deuterium flux density, surface temperature and deuterium impact e nergy (1). The chemical erosion of redeposited carbon species is assumed to be enhanced by a factor of ten compared to the erosion of graphite (2). Physical sputtering is caused by ba ckground deuterium and beryllium ions and also by eroded impurities. Input parameter for the ERO modelling The plasma background parameters for the ERO calculations are taken from B2-EIRENE calculations (3). Figure 1 shows profiles of the deuterium ion and at om flux density, electron density and temperature as function of the distance d along the div ertor plates. The distance