Large-diameter thin-walled piping such as that used in the Liquid Metal Fast Breeder Reactor (LMFBR) plant is characterized by relatively stiff straight pipes welded to flexible elbows. According to Robinson [1], such a piping system configuration may experience elastic follow-up during elevated temperature operation. Therefore, ASME Code Case N-47 requires that the secondary thermal expansion stress “. . . with large amounts of elastic follow-up . . .” be considered a load-controlled primary stress. A procedure to calculate the extent of potential elastic follow-up and thereby classify the thermal expansion stress as either primary or secondary is presented here. The elastic follow-up effect is investigated in detail by evaluating spatial and temporal redistribution of loads and inelastic strains computed for a typical LMFBR piping system.

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