Creep buckling of columns with lateral restraint

Abstract

This problem arises from the use of vertical fuel elements in graphite-moderated types of nuclear reactor. Previous theoretical work based on the tangent rate modulus approach has been extended to cover all phases of creep and elastic deformation for both symmetric and asymmetric single-plane bowing. By introducing a ‘solution time’ parameter, a generalized solution can be derived for any given material and initial shape, which is valid for an arbitrary range of sizes of rod, temperatures, and loads (neglecting elasticity). For ‘Magnox’ at 295°C the theory indicates that elastic deformation is negligible.It was found from experiments on round ‘Magnox’ rods which were allowed to ‘lay’ against a simulated channel wall (this is the condition described as continuous lateral restraint) that creep bowing seldom occurred in a single plane. Comparison with the theoretical analysis for such cases was therefore difficult, but it was found that the theoretical predictions were generally very conservative, and that by splitting the measured deflections into major and minor planes some correlation could be obtained. By using a specimen of rectangular crosssection having an aspect ratio of 2:1, bowing was confined to a single plane and very good agreement with the theory was obtained, both with respect to the deflected shape itself and to the time required to achieve it.