Patent Number: 046817298
Section: claims

1. A vessel provided internally with a device for enabling external detection of temperature changes within the vessel, said device comprising a permanent magnet, means mounting the magnet within the vessel for movement towards and away from a boundary wall of the vessel, means having sufficient magnetic coupling with the magnet to restrain the magnet against movement away from a predetermined position with espect to said boundary wall of the vessel, and temperature-responsive means for urging the magnet away from said predetermined position such that, in the event of a substantial rise in temperature within the vessel, the latter means becomes effective to move the magnet relative to the boundary wall to vary the magnetic field strength detectable by detector means located on the external side of said boundary wall. 2. A vessel as claimed in claim 1 in which said vessel boundary wall is composed of a magnetically soft material and constitutes said restraining means, and in which the urging means develops sufficient power to move the magnet away from said boundary wall in the event of a substantial rise in internal temperature. 3. A vessel as claimed in claim 1 in which a magnetically soft element determines said predetermined position, said element being fixed against movement with respect to said boundary wall and constituting said restraining means, and in which the urging means develops sufficient power to move the magnet away from said element in the event of a substantial rise in internal temperature. 4. A vessel as claimed in claim 1 in which said urging means includes a thermal link which, while intact, restrains the urging means from exerting its urging force on the magnet but breaks in response to substantial temperature rise and thereby renders the uring means effective to move the magnet away from said predetermined position. 5. A vessel as claimed in claim 1 in which said urging means includes a bimetallic component. 6. A vessel as claimed in claim 1 in which said boundary wall is constituted by a top wall portion of the vessel. 7. A handling and monitoring facility for spent nuclear fuel material comprising at least one vessel as claimed in claim 1, and detector means external to the vessel and responsive to changes in magnetic flux caused by movement of said permanent magnet away from said predetermined position. 8. A facility as claimed in claim 7 in which said detector means comprises a Hall effect transducer. 9. A facility as claimed in claim 7 in which said detector means is operable to induce a time-varying magnetic flux in said vessel wall for superimposition on the steady state magnetic flux of the permanent magnet in said boundary wall or said element and to measure changes resulting from movement of the magnet away from said wall. 10. A facility as claimed in claim 7 in which a plurality of said vessels are provided, said vessels each incorporating a device as aforesaid and being arranged in an array such that said vessel boundary walls are all accessible at one face of the array, and in which means is provided for moving the detector means from one vessel to another whereby all of the vessels can be periodically monitored. 11. A facility as claimed in claim 10 in which said moving means is operable to bring the detector means into close proximity with said boundary wall of each vessel and scan the detector means over said boundary wall.