An experimental investigation of the post-CHF enhancement factor for a prototypical ITER divertor plate with water coolant

In an off-normal event, water-cooled copper divertor plates in the International Thermonuclear Experimental Reactor (ITER) may either experience heat loads beyond their design basis, or the normal heat loads may be accompanied by low coolant pressure and velocity. The purpose of this experiment was to illustrate that during one-sided heating, as in ITER, a copper divertor plate with the proper side wall thickness, at low system pressure and velocity can absorb without failing an incident heat flux, qi, that significantly exceed the value, qiCHF, which is associated with local CHF at the wall of the coolant channel. The experiment was performed using a 30 KW electron beam test system for heating of a square cross-section divertor heat sink with a smooth circular channel of 7.6 mm diameter. The heated width, length, and wall thickness were 16, 40, and 3 mm, respectively. Stable surface temperatures were observed at incident heat fluxes greater than the local CHF point, presumably due to circumferential conduction around the thick tube walls when qiCHF was exceeded. The Post-CHF enhancement factor, η, is defined as the ratio of the incident burnout heat flux, qiBO, to qiCHF. For this experiment with water at inlet conditions of 70 C, 1 m/s, and 1 MPa, qiCHF and qiBO were 600 and 1,100 W/cm2, respectively, which gave an η of 1.8