Thermal Transients Due to Plasma Sweeping on the Monoblock Divertor Plate for ITER

Abstract

In the framework of the feasibility studies of the International Tokamak Experimental Reactor (ITER), the thermal behavior of the monoblock divertor plate has been investigated at the Joint Research Centre of the Commission of the European Communities. The design consists of cooling tubes embedded in a protective armor of graphite, a material that has given good results in plasma physics experiments. Previous parametric studies, based on a thermal flux peak of 15 MW/m² and different materials, led to the choice of a Mo-Re alloy for the tubes and a high-conductivity carbon-fiber composite called SEP for the graphite armor. To comply with a design temperature of 1273 K, an allowable protective layer only 5 mm thick was indicated; however, because of the high erosion rate due to sputtering, the lifetime of such a plate would be unacceptable from an engineering stand-point. To overcome this difficulty, it has been proposed that the separatrix be swept to lower the flux peak during the transient. The nominal working condition then becomes a sweeping of the separatrix moving around the null point with a radius of 30 mm and a frequency of 0.3 Hz; this generates a thermal load varying in time on the divertor plates. The results lead to the conclusion that plasma sweeping can reduce the surface temperature peak of the divertor, allowing a 16-mm-thick protective layer of the armor. A preliminary accident analysis shows the following:

1. A loss-of-sweeping accident has no immediate major consequences, but the plasma must be shut down to avoid significant erosion due to the high temperature reached during the postaccident steady state.

2. A change in frequency from 0.3 to 0.1 Hz does not produce very important peak temperature changes, but the amplitude of the thermal cycle becomes much higher with possible consequences for the structural stability due to fatigue.

3. A change in peak flux from 15 to 30 MW/m² raises the maximum surface temperature to ∼2400 K and the plasma must be shut down to avoid major hazards.

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