Decay heat removal in the ITER outline design

Decay heat densities are fairly small in ITER when compared to fission plants. The global decay heat in the water cooled shielding blanket design of ITER (fusion power 1.5 GW) is about 20 MW at shutdown and decreases to 1.5 MW after one day after which it stays fairly constant (1 MW after one month). These numbers are calculated assuming pulsed operation of ITER. With active blanket cooling in operation the removal of this decay heat does not pose any problem. Active vacuum vessel cooling alone can limit in-vessel temperatures to below 500 °C which is required to limit H2 production from long-term Be—steam reactions. Several strategies to deal with decay heat in situations ranging from normal operation to beyond design basis accidents (occurrence rate of event sequence < 10−6 per yr) are analyzed. In beyond design basis states it is important to access the heat sink of the magnets and/or cryostat. To promote passive decay heat removal one has to optimize the radial thermal conductivity through blanket and radiation transport between the blanket and vacuum vessel.