Element immersed in coolant of nuclear reactor

The method is described of processing an element of a nuclear reactor, the element including a stack of pellets enclosed in a metallic cladding, the method including: positioning the element in a vessel, reducing the pressure of the air in the vessel to a lower magnitude at which there is appreciable oxygen in the atmosphere of the vessel, conducting current through the cladding of the element to elevate the temperature of the cladding, while the cladding is at the temperature introducing gas into the vessel to elevate the pressure in the vessel. The temperature of the cladding reduces the yield strength of the cladding to a magnitude such that the yield strength is exceeded by the elevated pressure and the cladding is collapsed on the pellets of the stack whereby the formation, by exposure to neutron flux, of an unsupported gap in the cladding which would collapse under the pressure and/or at the temperature of the coolant of the reactor is prevented