[en] The invention concerns a process for measuring the leak rate of an appreciably leak-tight vessel containing a gas under pressure, by determining the change in the refraction index of the gas in the vessel over a given time interval. The invention also concerns a system for bringing this process into operation. The invention particularly aims at the checking of the leak-tightness of confinement vessels of boiling or pressurized water nuclear power stations. Such vessels are designed to hold the contaminated gases in the event of a burst pipe in the primary circuit. The conditions which would arise in the vessel further to a burst pipe are defined. Such conditions are called reference accident conditions, or 'A.D.R.' conditions and the acceptable maximum leak rate is computed so that the contaminant level in the environment outside the vessel does not exceed the accepted figures