The influence of thermomechanical loading history on the stress level in WWER NPP reactor pressure vessels under thermal shock

Abstract

We present the results of calculations of the kinetics of stress-strain state and stress intensity factors for surface and under-the-cladding circumferential cracks in modeling the emergency core cooldown conditions for the WWER-1000 reactor. The calculation procedure is based on a mixed finite-element method statement which provides stability of numerical solution and a high accuracy of results for both the displacements as well as stresses and strains. The authors analyze the influence of the density of the finite-element discretization of the crack-tip area for the surface and under-the-cladding circumferential cracks on the accuracy and convergence of computation of fracture-mechanics parameters in the modeling of thermal shock conditions. The results of calculation of kinetics of stress intensity factors allowing for the thermomechanical loading history and residual process-induced stress fields are reported. It is demonstrated that if the elastoplastic deformation history and residual process-induced stress fields are disregarded in the calculations of stress intensity factors for under-the-cladding cracks the reactor pressure vessel strength and lifetime may turn out to be overestimated. Introduction.To ensure safe operation conditions nuclear power plant (NPP) reactor facilities requires further improvement of the computational models and procedures for calculating the stress-strain states (SSS) in reactor pressure vessels (RPV) allowing for the flaws they may contain and their thermomechanical loading history under various operating conditions.Mathematical modeling of kinetics of thermomechanical state of reactor pressure vessel requires solving rather complex nonlinear thermomechanical transient boundary-value problems. The calculation of SSS kinetics for a reactor pressure vessel can be reduced to a successive solution of transient thermal conduction problems and a nonlinear thermomechanical boundary-value problem. To provide an adequate description of variations of a stress level in RPV under real operating conditions including emergency core cooldown (thermal shock), one should take into account the combined influence of various factors such as loading history, nonuniformity of heating and cooling, plastic deformation, inhomogeneity of physical-mechanical properties.The relevant publications [1][2][3][4][5][6][7][8][9][10][11][12][13][14] present numerous data that reflect the accumulated experience and various aspects of computational substantiation of strength of WWER and PWR reactor vessels. They also discuss the standard approaches, the acting service loads and loads occurring during the reactor core emergency core cooldown and design accidents. Given are the data on physical-mechanical properties of the base metal and cladding of RPV; some materials-science aspects of strength assurance are addressed. The above-mentioned publications describe in