Thermal buckling anlaysis of an LMFBR overflow vessel

During a reactor scram, cold sodium flows into the hot overflow vessel. The effect on the vessel is a compressive thermal stress in a zone just above the sodium level. This condition must be sufficiently controlled to preclude thermal buckling. Also, under repeated scrams, the vessel should not suffer thermal stress low cycle fatigue. To evaluate the closeness to buckling and satisfaction of ASME Code limits, a combination of simple approximations, detailed elastic shell buckling analyses, and correlations to results of thermal buckling tests were employed. This paper describes the analysis methods, special considerations, and evaluations accomplished for this FFTF vessel to assure satisfaction of ASME buckling design criteria, rules, and limits. ASME Code Case N-47 (and its predecessor Case 1331) buckling limits, rules, and safety factors required some interpretation for this application. Analytical model difficulties that were overcome include accounting for nonsymmetrical strip thermal compressions, shell imperfection effects, and plasticity effects. An outcome of these analyses was the recognition that buckling of shells due to a narrow strip of compressive membrane strains occurs at a strain considerably higher than that of a uniformly compressed shell. The cause is the prebuckling deformation and ridging. (orig.)