Application of Fracture Assessment Method Considering Constraint Effect to Ductile-Brittle Transition Temperature Region

Abstract

With the long-term operation of nuclear power plants, evaluating the integrity of reactor pressure vessels (RPVs) against neutron irradiation has become increasingly important. In the context of pressurized thermal shock (PTS) evaluation, the flaw stability of the reactor vessel has been assessed using fracture mechanics for a postulated flaw. Neutron irradiation may reduce the safety margins of certain plants, potentially raising concerns regarding nuclear safety. For this countermeasure, the applicability of the Beremin model, which is a statistical procedure considering the stress multi-axiality, has been investigated to mitigate excessive conservatism in the conventional fracture mechanics and to perform a realistic fracture evaluation using a physical model for cleavage fracture. In this paper, the applicability of a model coupled with the Beremin model with the Gurson–Tvergaard–Needleman (GTN) models was examined to establish a more precise fracture evaluation method for realistic structures in which cleavage fracture occurs after a small ductile crack growth in the ductile-brittle transition temperature (DBTT) region. After determining the parameters of the Beremin model to characterize cleavage fracture and the GTN model parameters to characterize ductile fracture with the C(T) and SE(B) specimens, these parameter values were used in the coupled model to predict the 5% and 95% confidence limits of critical cleavage fracture of a surface-flawed plate specimen with a thickness of 50 mm under bending or tensile load with nearly the same constraint as a reactor vessel. When the fracture tests using a flat plate with a surface flaw of depth/thickness 0.1 under bending or tensile load were performed at temperatures –80 °C and –120 °C, most of all the critical Ks of the specimens were within the upper and lower bounds of the predicted critical K values. At the temperature –80 °C which caused a small ductile crack, the predicted critical K values by the coupled model were better than those by the Beremin model comparing with the test data. As a result, it was confirmed that the coupled model was a proper procedure for the cleavage fracture associated with small ductile crack growth.