Elastic-plastic behaviour of thin tubes subjected to internal pressure and intermittent high-heat fluxes with application to fast-nuclear-reactor fuel elements

A sealed reactor fuel can when subjected to sufficiently high thermal stresses in the presence of an internal pressure will yield plastically. A simple model of the can is used to show that the plastic strains so produced may cause ratchetting or plastic cycling as the temperature gradient across the can wall cycles because of startup and shutdown of the reactor. On the assumption that creep is negligible, approximate criteria are derived for the onset of ratchetting and plastic cycling, simple expressions are obtained for the plastic strains incurred by each cycle, and failure of the can due to the above mechanisms is discussed both for work-hardening and non-work-hardening material. Consideration is then given to the effect of stress relaxation due to creep when the mean temperature of the can is sufficiently high to cause complete relaxation of the thermal stress while the reactor is at power, creep being ignored while the reactor is shut down. Under these conditions, it is found that the criterion for ratchetting is simply the criterion for plastic yielding during the first temperature cycle. Finally, it is shown in an appendix that the results obtained from the simple model also hold, with minor modifications, for the similar problem of a thin spherical shell subjected to an internal pressure and a temperature gradient across the shell wall which is cycled. Use is made of this to discuss the accuracy of the results obtained from the simple model when applied to a thin can.