Reliability Assessment of Welds of Reactor Pressure Vessels in Nuclear Power Plants
Date Issued
2009
Date
2009
Author(s)
Lin, Hsien-Chou
Abstract
The reactor pressure vessel (RPV) is the most important pressure boundary component in the steam supply system of a nuclear power plant. The RPV is too large to be manufactured as a single piece. Instead, it is welded together by several steel plates. Based on their orientations, the welds can be classified into circumferential ones and axial ones. Different orientations may require different weld processes. It results in different material properties and different directions of pre-existed cracks. In general, cracks occur more frequently in welds rather than in base plates of a RPV. Radiation embrittlement is considered the major cause of cracks. To evaluate the degree of radiation embrittlement and ensure the safety of a RPV, the domestic nuclear power plants adopt the standard set by the United States Nuclear Regulatory Commission (USNRC) and carry out aging assessment programs periodically. Aside from it, the present study employs a probabilistic fracture mechanics approach taking into account the radiation embrittlement to find crack failure probabilities of RPV welds. The result shows that, for the particular vessel studied in this thesis, if it is inspected and repaired every ten years, the probabilities of failure due to cracks at circumferential and axial welds are and respectively after forty years of usage. If its usage is extended twenty more years beyond its design life and a ten-year inspection period is considered again, the probabilities of failure of circumferential and axial welds become and respectively at the sixtieth year of usage. They are still considered low. However, the difference is large in comparison of failure probability of circumferential welds and that of axial welds. Based on this observation, axial welds should be inspected more frequently than circumferential welds. The latter can even be exempted from inspection as proposed by USNRC.
Subjects
Reliability
Probabilistic Fracture Mechanics
Reactor Pressure Vessel
Stress Corrosion Cracking
Monte Carlo Simulation
Type
thesis