AbstractOne of the serious thermofluid safety issues in a fusion experimental reactor is an ingress of coolant event (ICE), when the water in a coolant pipe is ejected into the vacuum vessel (VV) if an accident occurs, such as a runaway electron impact on the coolant pipes. If the VV cannot withstand the high pressurization caused by water evaporation in the VV, and on the high-temperature plasma-facing components, the steam containing activated dusts could be transferred from the VV to the environment. Therefore, it is important to understand the entire thermofluid characteristics during ICE conditions. This paper describes the first set of ICE experimental results, such as pressure transients in VV and transient temperature distributions on a water-jet impingement wall. It was found that pressurization under vacuum conditions, as an initial pressure condition in VV, is very different from that under atmospheric pressure conditions when the VV is filled with air at 0.1 MPa. The reasons for this discrepancy and some heat transfer characteristics on the target wall are discussed.