Three years after Korea Hydro & Nuclear Power (KHNP) found pieces of control rod guide tube support pins in steam generators at Yonggwang 2, the utility completed an upgrading programme to replace at all its PWRs' existing pins with new ones made of advanced materials, and improved loose parts monitoring in all PWR control rooms.
When KHNP found the pin pieces in the Yonggwang 2 steam generators in late 1998, the discovery was the second case of pin failure documented in improved X-750 pins at Westinghouse-supplied PWRs. The improved X-750 pins were used worldwide in Westinghouse units after pin failures were reported in France, Japan and the USA during the late 1970s and early 1980s. The original pins in those units were made of X-750. The improved X-750 pins, installed early on at Yonggwang 2, were heat treated to develop greater resistance to primary water stress corrosion cracking.
The first problems showed up in late 1997. With the unit in startup mode at 50% power, control room personnel noted metal impact signals from the primary channel heads of steam generator A and B. The staff decided that it was necessary to shut the plant to take the assumed loose parts out, but the signals disappeared before the unit was shut, and the reactor was allowed to keep operating. The staff then assumed that the foreign material had either been broken up or else permanently lodged in the steam generator loop, posing no further threat to the integrity of equipment.
A year later, loose parts in the two steam generators were positively identified as broken split pins, and an inspection of the tubesheets of both steam generators showed that welds on the primary side had been significantly impacted. Remote inspections showed that there were four cracked support pins, four pins with missing nuts, and two pins with tilted nuts. Most of the damaged pins were located near the outlet nozzle of the vessel.
Procurement delays meant that in 1998, only 30 of 104 suspect pins were replaced. With only a portion of the pins replaced, in March 1999, Yonggwang 2 again registered warning alarms of metal impact in a steam generator during startup after a reactor trip. The unit was shut down, and nuts and bolts from a support pin assembly were found in the channel heads of steam generators B and C. KHNP determined that these parts were from pins that had not been replaced the year before.By mid-1999, all the pins at Yonggwang 2 could be replaced. By then, the root cause was determined to be primary water stress corrosion cracking, with cracks consistently found at the first thread and the shank-to-flange corner of the pins.
