substantially pure uranium hexafluoride is shipped in a conventional overpack . a cylinder includes a vessel with a cylindrical sidewall a head permanently affixed to the sidewall closes one end of the vessel . a valve controls the flow of matter into and out of the vessel . a sealing surface connected to the vessel surrounds the valve . a cap covers the valve and a pair of seals is located between the sealing surface and the cap . the volume between the two seals , the cap and the sealing surface defines a test volume . a test port connects the test volume and an exterior surface of the vessel , and fasteners press the cap against the sealing surface . a maximum rate of leakage from within the cap to the atmosphere outside the cap is determined by measuring the leakage rate into the test volume with a leak testing apparatus connected to the test port . thereafter the cylinder is placed in a conventional overpack . the maximum leakage rate is determined using a test apparatus consisting essentially of a vacuum pump , a pressure gauge , a container of known volume , a valve to control the flow of air from the atmosphere 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 .