Redistribution of stresses in the header–PGV-1000 steam generator connector weldment under loading after thermal treatment

Abstract

Additional heating of a thickened portion of the steam generator connector during postrepair thermal treatment of the header-connector weldment leads to a decrease in residual tensile stress levels. It is shown that during hydrostatic tests performed after thermal treatment inelastic strains on the inner connector surface are reduced, which inhances the cycle life of the weldment.Introduction. The relief of undesirable residual stresses induced by welding during repair jobs of the header-PGV-1000 steam generator connector weldment of a WWER-1000 reactor is provided by thermal treatment, including heating, holding at elevated temperature for residual welding stress relaxation, and further cooling. The phenomenon of residual stresses arising during thermal treatment of the header-steam generator connector weldment is not completely understood, which determines a current interest in investigation of this problem.Earlier data [1] on the optimization of thermal treatment of the header-steam generator connector weldment after repairment were used to propose its conditions that ensure a decrease in maximum residual stresses. These calculations account for the bending moment effect on the weldment on the side of the main circulation pipe (MCP), but do not take into consideration the rigidity of MCP-reactor pressure vessel (RPV) connection and stress redistribution under further service loadings.The present communication reviews refined calculation results for stress-strain state kinetics in the headersteam generator connector weldment, with simulation of thermal treatment and further service loading, including hydrostatic tests and normal operations.Choice of a Calculation Scheme. Kinematic ties of elements (steam generator, MCP hot branch, and RPV) of "small"-and "large"-series reactors are similar and differ only in their layout. The calculation model of a small-series steam generator is shown in Fig. 1. The loading scheme accounts for the rest of the steam generator and RPV on rollers and a support collar, respectively, as well as the rigid fixing of the MCP end in the RPV wall. The weldment design, its most loaded element being the thinned portion of the steam generator connector, including the weld joint, is depicted in Fig. 2. This connector portion takes up loads, affecting the steam generator on the MCP side, the weight of structural elements, connected with the header, and of a coolant, as well as pressure-induced loads in the primary and secondary steam generator circuits.The calculations were performed with SPACE software [2], providing the solution of the three-dimensional problem of thermoplasticity with the account of loading history.Physicomechanical properties of the steam generator metal were assumed to be dependent on temperature. Inelastic deformation of the material during relaxation by the end of holding was measured by the equation of isotropic strain hardening with the account of the temperature effect [3]

Improvement of Fracture Resistance Calculation Procedure for NPP Primary Equipment Components

“…Результати прикладних досліджень із застосуванням розроблених методів розрахунку відображені в публікаціях [1,4,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Для оцінки міцності і ресурсу корпусів реакторів ВВЕР у процесі експлуатації розроблено галузевий нормативний документ [8].…”