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Sizing Vessels Using Limit Analysis

Sizing Vessels Using Limit Analysis. WRC Bulletin 464 describes sizing of vessels using modern limit analysis. The new Section VIII Division 2 (2007 Edition) contains even more updated information on application of limit load analysis.

The ASME Boiler and Pressure Vessel Code provides several, and the sizing by limit analysis is one of them. With limit analysis, the sizing can be achieved by closed-form formulas, equilibrium relations of free bodies, and finite element lower bound analyses. When coupled with a finite element computer code, a lower bound analysis is an effective tool for the sizing of any vessel or its components. Limit analysis addresses the design objective of preventing gross plastic deformation with an agreed-upon design margin. The procedure is simple and straightforward. If a lower bound to the limit load is established that is equal to or greater than the design loading, the objective is achieved. A design based on limit analysis also protects the vessel against burst by tensile plastic instability (ductile rupture), but with a design margin that depends on geometry, loading, and material. The sizing of vessels by limit analysis is applicable to vessels made of any material that is permitted for Section III-Class 1 and Section VIII-Division 2 construction.

NRC Endorses New Fatigue Curves

In Regulatory Guide 1.207 the NRC endorses new, more conservative, fatigue design curves published in 2007 for the design of systems and components in new reactors. The fatigue curves for existing reactors remain unchanged.

The new fatigue curves, as explained in NUREG/CR-6909, introduce two important changes: (1) A new set of fatigue curves in air, and (2) Reduction factors to account for fatigue in a reactor environment.

New Fatigue Curves in Air. The new stress-cycle (S-N) fatigue curves for common steels are lower than the current fatigue curves in ASME III, and therefore yield a shorter, more conservative, design life. For example, in the case of austenitic stainless steel components, this reduction in fatigue life is illustrated in Figure 1. The new fatigue curves include a correction for mean stress and adjustment factors of 2 on stress and 12 on cycles to account for statistical scatter of the experimental data, and the differences in size and surface finish of the laboratory specimen compared to actual components.