Progress of divertor study on DEMO design

Recent progress of the physics and engineering design study for the 8 m-sized DEMO is reported. Parametric study for the divertor of the compact DEMO (a machine size ∼ 5.5 m) by using the SONIC code shows that the target heat load less than 10 MW/m^2 around the fusion power of ∼1.5 GW and the impurity radiation fraction of more than 80 %. In the 8 m sized DEMO with these parameters, the partial detachment is obtained at the outer divertor, even in the low SOL density, due to the large impurity radiation in the SOL and divertor region. The SONIC simulation shows the peak of the target heat load is 7 MW/m^2. However, the peak of the ion temperature at the target is considerably high, which causes significant erosion of the target. The divertor power handling and decrease in the ion temperature have to be proceeded by the scenario development of the divertor plasma operation as well as the core plasma design and the engineering design. In the engineering study side, the tungsten monoblock target with the water cooling and the Cu-alloy cooling tube is designed. The MCNP-5 neutronics analysis shows applicability of the Cu-alloy cooling tube for the divertor unit on the high heat flux region. Also the divertor cassette with a heat removability of peak heat load of 10 MW/m2 is studied. The heat transport analysis shows the maximum temperature of 1021 °C at the tungsten surface and 331 °C at the Cu-alloy pipe, which are acceptable level for mechanical toughness and thermal fatigue