Hydrogen isotopes retention in the outboard first wall tiles of JT-60U

We have investigated hydrogen isotopes retention in graphite tiles used as outboard first wall armors of JT-60U by means of SEM, TDS and SIMS. The SEM examination confirmed that these tiles are mostly eroded. Different from graphite divertor tiles, in which H retention was dominant owing to isotopic replacement during HH discharges preformed after DD discharges to remove tritium, deuterium is dominated in hydrogen isotopes retention of the outboard first wall tiles showing deeper penetration. This is attributed to the injection of high energy D and less isotopic replacement due to the lower temperature of the outboard first wall tiles. This type of hydrogen retention could not be avoided for the metallic wall. Although the amount of hydrogen isotopes retained per unit area in the outboard first wall tiles was much less than that for the divertor tiles, the integrated retention over the whole first wall surface could be very large.