Results on the use of tungsten heavy alloys in the divertor of ASDEX Upgrade

Tungsten heavy alloy (97 wt% W, 2 wt% Ni, 1 wt% Fe) was investigated as an alternative for tungsten (W) as plasma facing material. It is produced commercially by several companies and compared to bulk W it is readily machinable and considerably cheaper. In order to qualify the material for use in the divertor of the mid-size tokamak ASDEX Upgrade (AUG) dedicated laboratory investigations as well as high heat flux tests in the neutral beam facility GLADIS were performed. These investigations revealed that the thermal conductivity at high temperature is close to that of W, the magnetisation is small and saturates already at low magnetic field and the hydrogen retention is similarly low as that of W. In high heat flux tests at power densities up to 20 MWm(-2) no failure was observed up to the melting temperature (approximate to 1500 degrees C) of the binder phase. Even at surface temperatures of up to 2200 degrees C the mechanical integrity was sustained. Mechanical tests confirm the ductile behaviour of the W heavy alloy at room temperature and finite element analyses using the aforementioned data suggest a lower tendency for cracking. The increase of the long term dose-rate resulting from the activation of Ni under neutron irradiations appears to be moderate. During the 2017 campaign more than one fifth of the AUG divertor tiles consisted of W heavy alloy. Under nominal operation conditions the tiles showed no macroscopic failure and no increased Fe/Ni influx into the plasma was detected. Even though a few tiles showed strong melting at the edges due to accidental misalignment no failure due to cracking was observed. (c) 2018 Published by Elsevier B.V.