Substantially pure uranium hexafluoride is shipped in a conventional overpack. The cylinder's valve is covered with a cap, and seals are located between the cylinder and the cap. The volume enclosed by the two seals defines a test volume. A test port leads to the test volume. A leakage rate from within the cap to outside the cap is determined by measuring the leakage rate into the test volume. The leakage rate is determined using a vacuum pump, a pressure gauge, a container of known volume, a valve to control the flow of air into the container which are all adapted to be connected to the test port at the same time, a valve controlling the flow of gas from the container into the test volume, and a valve controlling the flow of gas from the test volume to the vacuum pump.