Environmental Engineering Reference
In-Depth Information
to the J-groove weld. 1 Both Alloy 600 and 182 weld metal failed by pri-
mary water stress corrosion cracking (PWSCC). There was no conclusive
evidence that hot cracking contributed to the J-groove weld cracking. The
Davis-Besse event illustrates the severe consequence of in-service crack-
ing of RVH-penetration components fabricated from Ni-base Alloy 600
and 182 weld metal in which PWR water leaking from the cracked nozzles
severely corroded the RPV head low-alloy steel material down to the 308
stainless steel cladding material. 2
In the vessel, internals are exposed to irradiation. Under neutron fl ux
the microstructure of the material can evolve: segregation at the grain
boundaries associated with dechromization and hardening induced by the
recombination of point defects. The fi rst cracked baffl e-former bolts were
observed in 1988 in Bugey Unit 2 (PWR, France), during ultrasonic testing
(UT) controls. Several bolts were examined 3 - 6 and the failure was attributed
to a particular case of SCC: irradiation assisted stress corrosion cracking
(IASCC). Periodic inspections and a replacement program were set up in
the affected reactor types. The assessment of the damage affecting the bolts
revealed that signifi cant differences in cracking behaviors exist between the
various reactors. For instance, taking into account the number of cracked
bolts, Bugey Unit 2 (100 cracked bolts in 140 000 h) and Fessenheim Unit 2
(46 cracked bolts in 140 000 h) are the most affected reactors (the remaining
reactors were mostly less than 30 cracked bolts). Additionally, their bolts
were made from the same heat, suggesting the infl uence of initial composi-
tion and microstructure.
2.3.2 Steam generators (SGs)
In 2004, a failure occurred at Mihama 3, in the pipe of a loop condensate
system between the fourth feedwater heater and the deaerator, on the sec-
ondary side of the PWR. 7 The accident resulted in fi ve deaths among the
workers preparing for periodic inspections at the time of the piping rupture.
The rupture opening in the carbon steel pipe measured as follows: 51.5 cm
(axial direction) by 93.0 cm (circumferential direction). At the time of the
initial plant service, the nominal wall thickness of the pipe was 10 mm, with
the thinnest section only 0.4 mm. Designed with a maximum service tem-
perature of 195°C and a maximum service pressure of 1.27 MPa, the pipe
ruptured when the temperature was only 140°C with a pressure of 0.93 MPa;
the fl ow rate through the pipe was 1700 m 3 h −1 . There were no precursor indi-
cators before the accident or special operations shown on the review of the
plant parameters which could have caused the pipe to rupture. An investi-
gation concluded that water quality had been maintained since the commis-
sioning of the plant. A microscopic inspection was then conducted, which
revealed that a fi sh-like pattern covered almost the entire inner surface of
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