Modelling of Water Ingress into Vacuum Vessel Experiments Using RELAP5 Code

Abstract

An event of water coolant ingress into vacuum vessel is one of the most important events leading to severe consequences in nuclear fusion reactors. The ingress of coolant to the vacuum vessel could appear due to coolant pipe rupture of in-vessel components. Injected water, contacting with hot surfaces of in-vessel components and evaporating that leads to fast increase of pressure into the vacuum vessel. Vacuum vessel is designed for vacuum conditions and could not withstand the high pressure inside, thus, during the accident with pressure increase the safety valves are opened to release the steam. The activated dust could be released at the same time together with the steam. Therefore, it is important to understand thermo hydraulic processes in vacuum vessel during the ingress of coolant event to prevent the over pressurization of vacuum vessel and following discharge of radioactive materials. There are few experimental investigations performed, modelling of ingress of coolant event. In this article the ingress of coolant event in vacuum vessel was modelled for two cases. In the first case, water ingress in the vacuum vessel with no heated in-vessel components and temperature of injected water is the same as temperature of vessel walls. The second case is the ingress of coolant event in vacuum vessel with heated in-vessel components. In this case, the temperature of injected water is lower than the surface of in-vessel component. For the modelling of these two experiments RELAP5 code was used. RELAP5 is a “best estimate” system code suitable for the thermo-hydraulic analysis of all transients and postulated accidents in nuclear fission, light water reactor systems, including both large and small-break loss-of-coolant accidents as well as the full range of operational transients. The use of RELAP5 code for the accident analysis in nuclear fusion reactors allows to perform integral analysis of thermal–hydraulic processes in the cooling system and vacuum vessel. The comparisons of calculation results and experimental data showed that with some limitations the RELAP5 code could be used for the analysis of the thermal–hydraulic processes in the vacuum vessel during ingress of coolant event.