Ageing of Nucl. Powerplant Compnts. - BWR Pressure Vessels

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Extra info for Ageing of Nucl. Powerplant Compnts. - BWR Pressure Vessels (IAEA TECDOC-1470)

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The specification identifies the design conditions and operating conditions (normal conditions, upset conditions, emergency conditions, faulted conditions and testing conditions). Sub-article NB-3200 deals with the stresses and stress limits which must be considered for the analysis of the component. e. the requirement for normal conditions are based on minimizing cumulative damage and distortion whereas limits for very infrequent conditions allow more damage and distortion albeit still within conservative limits.

2. Significance Based on reported in-service cracking incidents, fatigue is a significant degradation mechanism for some BWR pressure vessel components. Most BWR vessels and components have been designed against fatigue crack initiation by using conservative amplitudes and recurrence frequencies for normal and upset loading cycles together with detailed fatigue design procedures of the ASME Code, Section III, and Appendix I fatigue design curves. The remainder either considered designs against fatigue during the design process using simplified fatigue strength reduction procedures, or have addressed fatigue during implementation of the NRC Systematic Evaluation Program (SEP).

The principles of limit design, which were used can be described briefly as follows. The assumption is made of perfect plasticity with no strain-hardening. This means that an idealized stress-strain curve of the type shown in Fig. 3-2 is assumed. Allowable stresses, based on perfect plasticity and limit design theory, may be considered as a floor below which a vessel made of any sufficiently ductile material will be safe. The actual strain-hardening properties of specific materials will give them larger or smaller margins above this floor.

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