Effect of Structural Weld Overlay on Recommended Leak - Before - Break Margins

摘要

The U.S. Nuclear regulations require that nuclear power plants be designed and built to shut down safely in the event of a sudden pipe break. Accordingly, high energy piping systems are traditionally designed to account for the effects of postulated pipe breaks. The leak-before-break (LBB) methodology provides a technically justifiable approach for eliminating postulation of double-ended guillotine break (DEGB) in high energy piping systems. Therefore, a vast majority of pressurized water reactor plants have adopted the LBB approach as their structural design basis.Several plants in the U.S. have applied structural weld overlay to the pressurizer nozzle dissimilar metal weld locations in order to mitigate Primary Water Stress Corrosion Cracking (PWSCC). Since many of these plants have already adopted LBB as a design basis for the pressurizer surge lines, it is necessary to reconfirm the LBB status of the affected welds after the structural weld overlay is applied. The flaw size margins would be affected since both the leakage flaw size and critical flaw size would increase due to added thickness resulting from the weld overlay. In addition, effects of PWSCC crack morphology on leakage need to be addressed. The impact of these changes on the recommended LBB margins are investigated in this paper for a typical pressurized water reactor (PWR) pressurizer surge nozzle.