A Fracture Mechanics Evaluation of Observed Cracking at a BWR-2 Reactor Pressure Vessel Weld

The reactor pressure vessel (RPV) to shroud support weld (termed as H9 weld) of a Boiling Water Reactor-2 (BWR-2) was examined during a refueling outage. The ultrasonic (UT) examination was performed from the upper conical support plate surface for all accessible areas of the H9 weld. A fracture mechanics evaluation was conducted to justify continued operation of the RPV in the presence of cracking detected during this examination. The evaluation considered crack growth during future operation and was based on the procedures outlined in Paragraph IWB-3600 of ASME Section XI. The structural margin at the shroud support structure was based on the limit load approach. Projected crack growth based on 10 years (approximately 80,000 hours) of operation was included in the evaluation. The results showed that the safety factor is 21.74 compared to the minimum required safety factor of 2.77. Assessment of the potential for cracking progressing into the low alloy steel material of the RPV was also conducted. The evaluation showed that crack depth in the reactor wall is predicted to be less than the allowable value even after an operating period in excess of 100,000 hours. This result, in conjunction with the evaluation result for the shroud support structure, provided technical justification for continued operation of the RPV with the observed H9 weld cracks for at least 5 additional operating cycles, equivalent to 10 years operation.