When a loss of vacuum accident (LOVA) would occur in a nuclear fusion experimental reactor, a buoyancy-driven exchange flow takes place through breaches of a vacuum vessel. To concern the safety analysis, experiments were carried out to examine the effects of the enclosure configuration and the breach arrangement on the exchange flow rate at the vertical and/or horizontal breaches in an annular enclosure. Comparing the experimental results with the existing correlations of the exchange flow rate, the correlation based on the Bernoulli equation was found to provide the conservative results for small breaches in aspect of the release of radioactive materials from the vacuum vessel.