Non-Linear Design Evaluation of Class 1-3 Nuclear Power Piping

Abstract

A nuclear piping system which is found to be disqualified, i.e. overstressed, in design evaluation using linear analysis software in accordance with ASME Boiler & Pressure Vessel Code, Section III (ASME, 2009a), denoted as ASME III below for convenience, can still be qualified if further design requirements can be satisfied in refined nonlinear finite element analyses in which material plasticity and other non-linear conditions are taken into account. For clarity, a design evaluation using such linear analysis software will throughout this chapter be called a linear design evaluation, and a design evaluation involving a non-linear finite element analysis a non-linear design evaluation. The linear design evaluation according to ASME III is purely based on stress limits. Stresses in piping components are first divided into membrane, bending and localized stresses for formulation consistency with beam and/or shell structures. Thereafter, stresses are further categorized into primary, secondary and peak stresses. The primary stresses are the “not self-limiting” part of responses typically resulted from external forces such as dead-weight, internal pressure, earthquake and so on, and they are important to avoid catastrophic failure and to control plastic deformation. The secondary stresses refer to the “self-limiting” part of responses resulted typically from thermal effects and gross structural/material discontinuities, and they are responsible for eventually progressive/incremental deformation. The peak stresses are the combined “peak” responses which are used to control fatigue failure. In ASME III, design criteria are defined in terms of stress intensity or principal stresses. For Class 1 piping systems, the criteria are defined by the stress intensity which is the largest absolute value of the principal stress difference, or equivalently twice of the maximum shear stress, and for Class 2 and 3 piping systems by the largest absolute value of the principal stresses. In connection with the design-by-analysis approach, the linear design evaluation is performed through comparing stress intensities of above-mentioned stress categories with their allowable limits. Among software commercially available for performing such a linear design evaluation, PIPESTRESS from DST Computer Services S.A. (DST, 2005) is widely used in Sweden. Furthermore, the linear design evaluation is conducted for each of the following load sets: Design Condition and 4 so-called Service Limits of Level A, B, C and D. For different load sets, different design criteria and requirements are used. Through defining various loads