Impact of reactor pressure vessel degradation

This paper discusses the impact of the reactor pressure vessel (RPV) degradation that was discovered at the Davis-Besse Nuclear Power Station in March 2002. On February 16, 2002, the Davis-Besse Nuclear Power Station near Oak Harbor, Ohio, began a refueling outage that included a special inspection of the Alloy 600 nozzles that enter the closure head of the low alloy carbon steel reactor pressure vessel (RPV). The inspection focused on the nozzles associated with the mechanism that drives the control rods, known as the control rod drive mechanism (CRDM). This inspection was consistent with the plant's commitments made in response to United States Nuclear Regulatory Commission Bulletin 2001-01, 'Circumferential Cracking of Reactor Pressure Vessel Head Penetration Nozzles,' which was issued on August 3, 2001. In conducting this inspection, it was discovered that three CRDM nozzles had indications of axial cracking (one with circumferential cracking also), which had resulted in leakage of the RPV's pressure boundary. Specifically, these indications were in CRDM nozzles 1, 2, and 3, which are located near the center of the RPV head. Significant degradation of the RPV's head was found adjacent to CRDM nozzle 3 and the plant entered an extended repair shutdown. (author)