Approximate Theory for the Thermal and Irradiation Creep Buckling of a Uranium Fuel Rod and Its Magnesium Can

Abstract

A simplified theory is developed for calculating the creep buckling of a fuel element, including both thermal and irradiation creep of the uranium, and thermal creep of the magnesium alloy can. This is made possible by introducing creep rate moduli for both uranium and can, analogous to Young's Moduli for simple elastic buckling. The relationship between these rate moduli and the fundamental creep equations is discussed in detail and justification given for the proposal that the rate modulus should be defined as the slope of the stress strain-rate curve at the mean stress. The theoretical inter-relation between the results of sagging bar tests and the fundamental creep equation is also discussed. A typical calculation for a highly rated channel is included, and it is shown that the contribution of the can to resisting buckling is quite significant, more than doubling the buckling time for half the elements, and adding about half the buckling time for the remainder. Suggestions for further work are put forward.