Recommended Revisions to Nuclear Piping Thermal Expansion Stress Limits

Piping thermal expansion stress limits for ASME Section III Class 2 and 3 piping are directly based on the original thermal expansion rules for B31.1 piping published in 1955. Temperature limits for B31 piping are much higher than for Class 2/3 nuclear piping. The fundamental concern in 1955 for piping design was to prevent cyclic creep strains, and the thermal expansion rules were based on a strain range criterion. Fatigue was also a concern, and a stress-range reduction factor for cyclic conditions was incorporated into the strain-based rule. The history and development of the B31.1 thermal expansion rules are summarized. After review of the technical basis, it becomes evident that the nuclear Class 2/3 rules can be simplified and improved by changing to an explicit fatigue-based rule rather than the strain-based rule of B31.1. Recommended Section III code revisions are provided with discussion of related technical issues.