An anticipated design basis accident for ITER is the Loss of Flow Accident (LOFA). A LOFA can result from a multitude of initiating events, ranging from an inadvertently closed valve, coolant pump failure, or coolant line break. The consequences of a LOFA would be most severe were the LOFA to occur in the divertor cassette module since the divertor coolant tubes, which cool the divertor cassette's armor tiles, are subjected to the highest heat flux (5 MW/m² normal, up to 30 MW/m² off-nominal) within the vacuum vessel. Should these coolant tubes rupture because the LOFA has caused their critical heat flux limit to be exceeded, tritium release and vacuum vessel damage are but two of the more critical results.

The authors previously simulated a LOFA using Sandia's 30-kW Electron Beam Test System. The mockup was machined from oxygen-free high-conductivity copper and had a coolant channel diameter of 7.5 mm. Time-to-burnout data was acquired by (1) establishing a steady- state surface heat flux via electron beam rastered heating, (2) tripping off the coolant loop's circulation pump, and (3) continuing electron beam heating until the decreased coolant flow through the mockup caused a high-temperature trip by the K-type thermocouples embedded 0.5 mm below the mockup's heated surface. Time-to-burnout was defined as the number of seconds between the circulation pump's trip and the high-temperature trip.

The aforementioned LOFA experiment was performed with no heat transfer enhancement scheme and at water conditions (1 MPa and 11 C) unlike ITER's current specifications (4 MPa, 150 C, and 10 m/s). The current LOFA experiment remedies these shortcomings by using ITER water conditions, the ITER approved heat transfer enhancement scheme (swirl tape insert with twist ratio of 2), a heat sink width of 16 mm, and a coolant channel diameter of 10 mm. To assist with modeling the heat transfer coefficient's response, as a function of angle around the tube, during the simulated LOFA, a ring of 11 K-type thermocouples were embedded 1.25 mm from the coolant channel's wall. Preliminary analysis of the experimental data suggests that time-to-burnout parabolically increases with increasing inlet velocity and parabolically decreases with increasing surface heat flux.

Keywords: loss of flow accident, critical heat flux, swirl flow, twisted-tape insert, high subcooling, fusion reactors, thermal hydraulics, high heat flux