MECHANISMS FOR EXTREME HEAT-TRANSFER CONDITIONS IN WATER-COOLING OF FUSION-REACTOR COMPONENTS

In existing fusion reactors conceptual designs, water-cooled impurity control components employ operating heat fluxes up to 15 MW m(-2), coo lant velocities above 5 m s(-1), subcoolings more than 100 K, and pres sures below 5 MPa. These conditions are quite different from those mos t used to obtain existing correlations for subcooled nucleate boiling and critical heat flux. In addition, some available data in the range of interest for fusion reactor components show that extrapolations of the correlations cannot be used. This paper suggests that two heat tra nsfer mechanisms must be incorporated in developing correlations for f usion reactor high heat flux components. First, boiling can be suppres sed, resulting in observed non-boiling wall temperatures far above tho se expected for nucleate boiling. Second, critical heat flux (CHF) can apparently occur with no prior boiling, when the wall temperature rea ches the temperature of homogeneous nucleation (THN) while a single ph ase liquid-is adjacent to the wall. A limit of this nature can apparen tly occur only under conditions of a very high mass flux and a very hi gh bulk subcooling. These two mechanisms must be incorporated in fusio n thermal-hydraulics analysis: suppression of nucleate boiling and cri tical heat flux caused by homogeneous nucleation.