Environmental Engineering Reference
In-Depth Information
out. It is thought that mechanical rolling causes PWSCC where there are
irregularly shaped tube holes.
7.4.4 Degradation of CRDM and pressurizer nozzles
In September 1991, leakage occurred from the Bugey 3 T54 vessel head pen-
etration in France. After 10 years of monitoring the leak was detected using
the acoustic emission method as part of the thermal-hydraulic test and it was
estimated to be approximately 1 L/h. Non-destructive inspection using dye
penetrant testing, eddy-current testing and ultrasonic testing confi rmed a ver-
tical crack penetrating the pipe in the opposite direction to the weld. Through
metallographic analysis, it was concluded that the crack was PWSCC. By
the end of 1992, Non-Destructive Examination (NDE) programmes using
Eddy Current Test (ECT), Ultra sound Test (UT) and Visual Test (VT) had
found penetrating cracks in fi ve 900 MW vessel heads and four 1300 MW
vessel heads (after 30 000-40 000 h of operation). Due to vessel head and
RPV homogeneity, EDF (the operating utility) decided to replace all of the
vessel heads with Alloy 600 penetrations (54 out of 58 vessel heads are Alloy
600, the remaining four are Alloy 690 penetrations). In France, pressure ves-
sel heads at Bugey Unit 5 have been replaced since 1993; in Japan, replace-
ment of the pressure vessel heads started at Takahama-1 in 1996; in Spain at
Almaraz-1 in 1996; in Sweden at Ringhals-2 in 1996; in Belgium at Tihange-1
in 1999; in the United States at North Anna-2; and in China at Guangdong-2
in 2003. As of 2005, the pressure vessel heads of 93 power plants across the
world had been replaced with Alloy 600 penetrations.
The importance of carbide was fi rst established for steam generator
tubes, but this has also been applied to penetration of the upper heads.
Interpretation was based on carbon content, as well as forging, tempera-
ture of rolling and resistance strength upon hot forming. Three classes were
determined in terms of grain boundary carbide decoration: Class 1 - well-
developed intergranular carbide structure; Class 2 - mainly prior grain
boundary carbides with recrystallized grain; Class 3 - mainly intragranular
carbides with recrystallized grain. Modelling for crack initiation probability
took into account the composition of materials considering the infl uence of
other classes, angle of penetration and location (i.e. near or opposite to the
weld zone) and the infl uence of cold working treatment.
In 2004, after over 100 000 operating hours, leakage of the 47
th
CRDM
head penetration was detected in Ohi Unit 3 in Japan through visual inspec-
tion. Ohi Unit 3 is a power plant where RV head reactor coolant system
(RCS) temperature had been revised from 289
°
C to 310
°
C in 1997. In order
to distinguish the leakage ratio, helium leakage testing, eddy-current test-
ing, dye-penetration testing and ultrasonic testing were carried out on the
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