Tritium depth profiles in graphite and carbon fibre composite material exposed to tokamak plasmas

Tritium inventories in the plasma-facing surface and the bulk of tiles were investigated with a highly sensitive, accurate full-combustion technique and a PIN-diode method. Examined were (i) a tokamak fusion test reactor (TFTR) graphite tile (D–D plasmas), (ii) a JET graphite tile (low-tritium D–T plasmas), and (iii) several JET carbon fibre composite (CFC) divertor tiles as well as a graphite limiter tile (all high-tritium D–T plasmas). Whilst the bulk tritium concentration in graphite tiles appears to remain at very low levels (about 0.3% of the total tritium) the tritium bulk concentrations in CFC divertor tiles can be as high as three times that in the surface layer. The latter is attributed to plasma-induced trapping of tritium between the fibre planes of CFC in the hot divertor zone. In addition to carbon/hydrogen co-deposition, this contribution constitutes another important source of tritium inventory in the torus that so far had not been recognised.