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

Example text

The cause of cracking in the CRDRL nozzle blend area observed at several plants was attributed to mixing of cold CRD return flow with hot vessel water. 8] established actions to deal with this problem as well as that of feedwater nozzle blend area cracking caused by seal leakage. Most plants except BWR/2 plants have capped or plan to cap the CRD return lines. Capping reduces fatigue duty on the nozzle to the point where fatigue will not be a significant degradation mechanism. This conclusion applies to all capped CRDRL nozzles.

The procedure is based on the principles of linear elastic fracture mechanics (LEFM). Appendix G provides a reference critical stress intensity factor (KI) curve as a function of temperature, a postulated flaw and a KI expression. The basic premise of LEFM is that unstable propagation of an existing flaw will occur when the value of KI attains a critical value for the material designated as KIC. KIC is called the linear elastic fracture toughness of the material. In the case of ferritic materials, it has been found that the fracture toughness properties are dependent on temperature and on the loading rates imposed.

Vessel shell Thermal and mechanical fatigue cycling of the shell is minimal based upon conservative evaluations documented in vessel design stress reports. Therefore, fatigue crack initiation is not expected during the RPV service life. Thus, fatigue is not a significant ageing degradation mechanism for the vessel shell. The conclusion applies to all shell design variations. Top head Thermal and mechanical fatigue cycling of the top head (excluding nozzles) is minimal. 1 for 40 years. Therefore, fatigue crack initiation is not expected during the RPV service life.

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