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.

SynopsisAn expression is derived for the Van der Waals force between two semi-infinite bodies with small surface irregularities. Calculations are given both for the plane-plane and the plane-sphere configuration. The value of the correction from the surface irregularities upon the Van der Waals force is shown to amount easily to lo-50%.

A theoretical investigation is made of incremental growth in a thin pressurized tube when the tube wall is subjected to large cyclic variations of temperature and temperature gradient, the mechanisms of deformation being plastic yielding during application and removal of the temperature gradient and creep during the high temperature part of the cycle. In a previous paper a simple model of the tube was developed and used to determine approximately when the mode of incremental growth known as ratcheting occurs and to estimate the ratchet strain per cycle, both when creep is ignored and when it causes complete relaxation of internal stresses. The above work is extended in the present paper to cases where only partial relaxation of internal stresses occurs and a method of evaluating the ratchet strain per cycle is presented. In addition, the method allows the increased creep strain due to non-linear interaction between internal stresses and the pressure stress to be calculated. This accompanies ratcheting when the relation between creep rate and stress is non-linear.

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