A package for radioactive material which comprises a radiation- absorbing vessel having a plug-type inner cover sealingly engaging this vessel and an outer security cover overlying the massive plug-type cover and defining a clearance therewith in which gas can be monitored to detect failure of the seal, this safety cover being sealed to the vessel so that the latter chamber is sealed from the atmosphere. According to the invention, this chamber is sealed with gas at a superatmospheric pressure which is also above the pressure within the vessel and the pressure of the chamber is monitored, a drop in pressure signaling a breach either of the seal between the plug-type cover and the interior of the vessel or between the safety cover and the vessel.
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