COVER STORY | STEAM GENERATORS
Above: Pacific Gas and Electric Company (PG&E) has submitted an operating licence renewal application to add 20 years to the Diablo Canyon lifetime
Operating experience
The licence renewal application details operating experience at several reactors. In 2012 indications related to cracking were found at
dents/dings and in the freespan region in A600TT tube at Seabrook 1. In 2012 bobbin coil inspections and a further inspection using a rotating probe with a +PointTM
coil
confirmed the presence of axially oriented outside diameter stress corrosion cracking (ODSCC) in several areas around dents and dings. Seabrook revised its inspection scope to include 100% of dings and dents with bobbin voltage amplitudes greater than 5 volts and 20% of those with bobbin amplitudes of greater than 2 volts. Tubes with axially oriented ODSCC were removed from service by plugging both tube ends. In 2012 at Braidwood 2 a tube in the Westinghouse Model
D5 steam generators was found to have three indications of axially oriented ODSCC, two at tube support plate elevations
and one in the freespan area between two support plates at a ding in the tubing. The tube was plugged at both ends. Reanalysing bobbin coil data found no additional crack-like indications. In 2009 at Surry 1, an indication of axial primary water
stress corrosion cracking (PWSCC) was found at the hot-leg expansion transition (top-of tubesheet) in one steam generator with A600TT tubing. Further examination at all three steam generators at Surry 1 found no other indications of PWSCC. After an in-situ pressure test the tube was removed from service. A review of Diablo Canyon operating experience revealed
no instances of degradation similar to those at Seabrook, Braidwood and Surry. At the plant Bobbin coil inspections are conducted tube end to tube end, with +PointTM
probe
inspections completed for areas of interest (as determined during bobbin coil inspection). In 2012, the San Onofre Nuclear Generating Station (SONGS) identified significant anti-vibration bar and tube- to-tube wear in replacement steam generators. This is an industry anomaly for units operating with A690TT tubing. The wear discovered at SONGS 2 and 3 was caused by in- plane fluid elastic instability. It is not considered relevant for Diablo Canyon’s replacement steam generators as they have already operated for multiple fuel cycles without similar degradation and the steam generators have a different design. A review of operating experience over the 10 years to
2023 has not revealed any instances of severe degradation at Diablo Canyon. Eddy current testing during refuelling outages revealed a few instances of AVB wear and a small number of tubes have been plugged, but in no instance were the condition monitoring limits exceeded, so performance criteria was satisfied. Given this evidence, the condition of steam generators at
Above: PG&E’s Chief Nuclear Officer Paula Gerfen signs license renewal application 16 | January 2024 |
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Diablo Canyon are not expected to impede an decision on life extension to Unit 1’s operating licence out to November 2044, and Unit 2 through to August 2045. ■
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