Basic propositions of damage mechanics as a basis for justifying the strength and service life of nuclear power systems

The paper addresses the problem of providing a long-term (up to 60 years) strength reliability of engineering facilities operated under developing damage accumulation. Basic propositions of damage mechanics for derivation of coupled equations descriptive of elastoplastic deformation and fatigue damage accumulation in structural elements of nuclear power systems are given as a tool to assess their expended and residual life from actual operation history. The mathematical model is based on macroscopic variables that integrally characterize the mesoscale structural changes of material under deformation and damage accumulation. The degree of bulk isotropic fatigue damaging is estimated using a macroscopic parameter (0 which is an internal state parameter interpreted as the relative volume fraction of defects in a given material zone. The assessment of fatigue damage accumulation in danger zones of nuclear power systems from actual history of their thermal force loading is based on simulation of the main physical stages of damage accumulation: nucleation and development of micro defects and their coalescence into a fatigue macrocrack. The energy approach used to simulate the damage accumulation allows one to take into account the main peculiarities of elastoplastic deformation in danger zones of structural elements of nuclear power systems under complex nonstationary thermomechanical loading: pronounced localization of damage accumulation due to a high load gradient and design- and process-dependent stress concentration; multiaxial stress-strain states; rotation of principal planes of stress and strain tensors; and nonlinear damage accumulation under changes of loading modes.