The ITER vacuum vessel heat transfer system - design and safety aspects

ITER is equipped with several heat transfer systems to remove the heat generated in the Tokamak. The vacuum vessel heat transfer system (VV HTS), one of the heat transfer systems, has been classified as a safety related system as its function includes the removal of residual heat, stored heat in the VV and decay heat, from in-vessel components when their respective cooling loops are not available. A fully passive decay heat removal capability has been included in its design to provide inherently safe rejection of residual heat to the ultimate heat sink. The VV HTS consists of two independent loops, each including a circulation pump, an ebullience pipe, a steam separator, and an air cooled heat exchanger which dissipates the heat load into the environment (i.e. the outside air) as the ultimate heat sink. The heat load on the vacuum vessel during pulsed operation is 1.5 MW for each of two VV HTS loops, and for normal operation forced circulation is used. The residual heat, including the decay heat, in the in-vessel components is transferred to the vacuum vessel by radiant heat transfer during off-normal operation. It peaks at about 3.0 MW at 20 hours after a postulated loss of in-vessel cooling initiation. Only natural convection in the VV HTS is credited under off-normal conditions from the safety point of view. The VV HTS has been designed to meet the safety requirements, and the thermal hydraulic behavior during decay heat removal under off-normal conditions has been confirmed analytically. (authors)