[en] The reactor primary coolant circuit is filled with water by natural circulation. The pressure in the hermetic compartment is increased to 0.12 MPa and successively reduced. This cycle is repeated until the humidity at reduced pressure decreases to below 70%. The cycle is discontinued and pressure is stabilized. The amount, temperature and pressure of the filling air as well as the pressure, temperature and relative air humidity in the hermetic compartment are then measured for 6 to 12 hours. The primary asset of this leak testing procedure is a shortening of the reactor unit shutdown period. (M.D.). 4 figs