Hydrogen recycling and tritiurn retention at plasma facing wall in a fusion reactor

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The recently developed imaging plate (IP) technique gives a detailed map of tritium deposition on plasma facing carbon tiles used in JT-60U D-D discharges and makes it possible for the direct comparison of tritium distribution and carbon deposition pattern. This motivates us to compare the retention behavior of H, D and T in different tokamaks and to investigate from where the differences originate. In JET, most of the T is co-deposited with carbon (D/C〜0.75) in the divertor shadowed region, particularly on louvers in front of the cryogenic pumps, while hydrogen and deuterium retention (in H+D/C ratio) in the deposited layers on JT-60U divertor tiles is much smaller. Both machines are operated at elevated wall temperatures (around 550K). In JT-60U, the divertor tiles are inertially cooled and tile surface temperatures easily reach 600K, sometimes more than 1000K. However, in JET the divertor mounting structure is water cooled, so that although the surface temperature of divertor tiles … More can also exceed 1000K near the strike points during plasma discharges, their bulk temperatures are restrained to a much lower temperature (〜355-420K). This lead us to conclude that the temperature of the divertor and therefore of the co-deposited layer could play an important role on the tritium retention and it may be possible to reduce tritium retention significantly by tailoring the surface temperature of plasma facing components.In TFTR, even in erosion dominated regions of the bumper limiter, we have found large amount of tritium codeposition on the sides of all tiles, which do not directly face the plasma. This indicates that large amount of eroded carbon does not travel long distances but is rather promptly re-deposited, where of course it is immediately re-eroded in erosion dominated areas. Taking the available data on carbon deposition and retention of H, D and T in the plasma facing carbon materials into account, tritium retention in a carbon based D-T reactor, particularly from the aspect of retained tritium inventory reduction is estimated. Less