A vessel for the long term storage and monitoring of nuclear fuel materials incorporates a device (12) within its interior for providing an externally detectable signal indicating whether any pressure loss has occurred. The device (12) comprises a magnet (32) attracted towards a boundary wall (11) of the vessel and urged in that direction by the internal pressure within the vessel acting on a diaphragm (26). The magnet (32) is subjected to oppositely directed forces by pressurized fluid in chamber (40) bounded by diaphragm (26) and a spring (30), the arrangement being such that these latter forces collectively or individually are effective to drive the magent (32) away from the wall (11) in the event of a pressure loss within the vessel thereby producing an externally detectable change in magnetic flux.
