Thermohydraulic experiments on a water jet into vacuum during ingress of coolant event in a fusion experimental reactor

Abstract

An event of a water coolant ingress into a tokamak vacuum is one of the most important events leading to severe consequences in a nuclear fusion experimental reactor. The ingress of coolant event (ICE) starts from a coolant pipe rupture inside the vacuum vessel, and then water coolant ingress into vacuum happens followed by an impingement of the water onto the plasma-facing wall and water evaporation with chemical reactions on the plasma-facing wall. The ICE might cause enough pressurization to break the vacuum vessel and a possible release of tritium and/or activated materials to the outside of the vacuum vessel. Therefore, one of the key issues in fusion safety design is to evaluate the pressurization in the vacuum vessel during the ICE. Thermohydraulic experiments with a water jet injected into a vacuum enclosure were carried out to examine the pressurization due to the evaporation in vacuum, and water jets were impinged onto a hot plate placed in the vacuum enclosure. Water was injected from an injection nozzle to the vacuum enclosure. Injection nozzles with diameters of 0.5, 1.0, 2.0 and 5.0 mm were used to simulate a small break of the coolant pipe. Mass flow rates of the water jet ranged from 1.3 × 10 −3 to 0.50 kg s −1. Water freezing occurs under the limited conditions of the injection nozzle with 0.5 mm diameter and the mass flow rate less than 2.42 × 10 −3 kg s −1. The pressurization rates due to the evaporation were obtained during the impingement of non-freezing water on the hot wall superheated to 300 K. The existing correlations of the boiling heat transfer in water jet impingement onto a hot plate cannot predict the present results of the pressurization rates.