Assessment of Vessel Minimum Wall Thickness Requirements for Ellipsoidal and Torispherical Heads in Section III NB and NCD Using Limit Analysis

Abstract Closed-form relationships to determine the required wall thickness of ellipsoidal and torispherical heads of nuclear vessel are contained in NB-3324.3 for Class 1 and in NCD-3324.6 (ellipsoidal) and NCD-3324.8 (torispherical) for Class 2 and 3 construction. The relationships that are used in NB and in NCD are however different and have different dependencies on the shape parameters of the head. Also, it was reported from the user community that the thickness of a torispherical head sized with the formula in NB-3324.3 was later found insufficient in a plastic limit analysis of the vessel. The objective of the present paper is to investigate the trends of the minimum thickness formulas in NB-3324.3 and in NCD-3324.6, NCD-3324.8 by comparing with the results of a plastic limit analysis. The limit analysis was performed using a commercial finite element package and applied pressure loading. The geometry parameters of the heads were varied widely, covering the likely range of interest and of applicability of the closed-form formulas. Boundary conditions at the head juncture were varied to establish the sensitivity of the limit pressure. The results show that there are indeed some head geometry parameters where limit analysis requires a greater wall thickness than the tentative wall thickness requirement of NB-3324.3.