THERMOHYDRAULIC EXPERIMENTS ON A WATER-JET INTO VACUUM DURING INGRESSOF COOLANT EVENT IN A FUSION EXPERIMENTAL REACTOR

An event of a water coolant ingress into a tokamak vacuum is one of th e most important events leading to severe consequences in a nuclear fu sion experimental reactor. The ingress of coolant event (ICE) starts f rom a coolant pipe rupture inside the vacuum vessel, and then water co olant ingress into vacuum happens followed by an impingement of the wa ter onto the plasma-facing wall and water evaporation with chemical re actions on the plasma-facing wall. The ICE might cause enough pressuri zation to break the vacuum vessel and a possible release of tritium an d/or activated materials to the outside of the vacuum vessel. Therefor e, one of the key issues in fusion safety design is to evaluate the pr essurization in the vacuum vessel during the ICE. Thermohydraulic expe riments with a water jet injected into a vacuum enclosure were carried out to examine the pressurization due to the evaporation in vacuum, a nd water jets were impinged onto a hot plate placed in the vacuum encl osure. Water was injected from an injection nozzle to the vacuum enclo sure. Injection nozzles with diameters of 0.5, 1.0, 2.0 and 5.0 mm wer e used to simulate a small break of the coolant pipe. Mass flow rates of the water jet ranged from 1.3 x 10(-3) to 0.50 kg s(-1). Water free zing occurs under the limited conditions of the injection nozzle with 0.5 mm diameter and the mass flow rate less than 2.42 x 10(-3) kg s(-1 ). The pressurization rates due to the evaporation were obtained durin g the impingement of non-freezing water on the hot wall superheated to 300 K. The existing correlations of the boiling heat transfer in wate r jet impingement onto a hot plate cannot predict the present results of the pressurization rates.