Development of flow visualization technique for measurement of exchange flow rates under the LOVA condition and results of the preliminary flow visualization experiment

When a loss of vacuum event (LOVA) would occur in a fusion reactor, a buoyancy-driven exchange flow takes place through breaches on a vacuum vessel due to a temperature difference between the inside and outside of the vacuum vessel. The exchange flow through the breaches may occur not only from the outside of the vacuum vessel into the inside but also from the inside to the outside conveying tritium and radioactivated materials. Therefore, it is very important to estimate quantitatively the exchange flow behavior under the LOVA condition from the viewpoint of the fusion reactor safety. In order to develop a new technique for quantitative estimation on the exchange flow rates through the breaches, a flow visualization procedure was investigated. It was concluded from the investigation that the correlation method was the most effective procedure under the LOVA condition. In addition, the flow visualization experiments were carried out using a small vacuum vessel to estimate the exchange flow rates through the breaches. As the result, the exchange flow behavior was observed and the local velocity distribution was obtained from the image analysis results by the correlation method. Furthermore, an average exchange flow rate was calculated using the local velocities. The exchange flow rates estimated by the flow visualization experiments agreed well with the measured exchange flow rates by the electric balance. It was clarified that the exchange flow rates through the breaches under the LOVA condition can be determined quantitatively by means of the flow visualization measurements. (author)