Statistical Model of Micro Crack Growth for the Evaluation of Accumulated Fatigue in NPPs

Abstract

In order to establish sophisticated management of aging degradation and to achieve high reliability of components in nuclear power plants (NPPs), it is required to reveal the mechanism of aging degradation and to quantify its deterioration. Present design code requires the assessment of CUF (cumulated usage factor) using design fatigue curve to prevent the occurrence of low-cycle fatigue failure in class 1 components of NPPs. This assessment of CUF prevents the low-cycle fatigue failure effectively up to the present date. However, the margins of structural factor (safety factor) and environmental fatigue reduction factor Fen need to be clarified for the quantitative assessment of aging degradation based on the mechanism of fatigue accumulation in NPPs,. Structural factor and environmental factor are defined in NUREG/CR6909, etc, but they do not clearly explain the technical basis of these factors according to the mechanism of fatigue and environmental effect. In this study, we quantify the dispersions of crack initiation and crack growth in fatigue test in a certain condition aiming at more sophisticated management of aging degradation based on the mechanism of crack growth. Applying these dispersions, we establish a statistical model of micro crack growth to predict fatigue life. We will propose the application of this prediction model of fatigue crack growth to the management of actual components degradation and establish proper maintenance program in order to achieve high reliability of components in NPPs in future.