Heat transfer conditions in water-cooling of a fusion reactor divertor

The diverters of planned fusion reactors such as ITER require removal of heat fluxes that are large in magnitude and are spatially peaked. Single-side energy deposition on the plasma-facing surface is transferred by conduction and provides an axially and circumferentially non-uniform heat flux on the wall of the cooling channel. Promising candidate cooling systems are based on the use of highly subcooled water (subcooling more than 100°C) flowing at very high velocities (more than 5 m/s). These conditions are quite different from those used to obtain existing correlations for single-phase convection and subcooled nucleate boiling. Through experimental assessments this paper suggests that existing correlations can be applied for single-phase convection. However, available data in the range of interest for the subcooled nucleate boiling show that extrapolations of existing correlations cannot be used. This is due to the suppression of nuclear boiling, resulting in high wall temperatures, sometimes far above expected and very close to the temperature of homogeneous nucleation (THN). Based on experimental data, a new correlation is proposed for subcooled nucleate boiling region. The range of application for the new correlation is defined as follows pressure 2-3 MPa, bulk water temperatures 19-25°C, heat fluxes up to 25 MW/m², flow velocities from 3 to 15 m/s