Environmental Engineering Reference
In-Depth Information
The pitting of plant A was related to high copper dissolved from condenser
material, chloride and high levels of dissolved oxygen. Transgranular SCC of
plant B seemed to be related to lead compounds. ODSCC and IGA in plant
A were related to a caustic environment in the crevices. PWSCC in plant
A and plant C originated from the inherent characteristics of the materi-
als, which were not properly thermally treated (Hwang, 2003). After failure
analysis, the performance of non-destructive testing was evaluated based on
destructive metallographic examination, and some counter measures, such
as material change, inhibitor injection, molar ratio control and temperature
reduction operation, were suggested.
In a typical case of high cycle fatigue, a complete 360-degree break
occurred in the cold leg side tube in Row 9 of the North Anna Unit 1 plant
in the United States on 15 July 1987. The case was explained as follows
(Shah and MacDonald, 1993):
1 An anti-vibration bar (AVB) was not installed around row 9.
2 A small dent was found in the tube. It opened due to mean stress as the
fatigue strength of the material had dropped.
3 An uneven AVB was installed around the troubled tube, which caused
sectional high speed coolant fl ow.
4 High amplitude and deteriorated fatigue strength caused fatigue
destruction.
After this accident in North Anna Unit 1 plant, the US Nuclear Regulatory
Commission (USNRC) ordered an inspection of the power plants that
showed potential fatigue destruction due to denting around the TSP and
fast sectional fl ow. Except for the U-bend area, well-installed TSP structures
have caused no high cycle fatigue.
It is said that in the mechanical ageing progress, fretting, wear and thin-
ning are caused by the vibration between the tube and tube support struc-
ture (TSP and AVB). But thinning occurs where there is no fl ow-induced
vibration, so it is diffi cult to say that tube vibration is a cause of thinning,
Only in certain cases can we can say that thinning derives from pure corro-
sion wastage. There are many factors infl uencing fretting, wear and thinning
including the distance between the tube and support plate, coolant fl ow,
oxide fi lm formation and corrosion product accumulation. Of these, friction
of the same side causes fretting and a large vibration causes wear. When a
combination of vibration and corrosion predominates, thinning results.
One example of low temperature PWSCC is the stress corrosion cracking
which was detected in the tube of OTSG in the Three Mile Island (TMI-1)
plant in 1981. Most of the cracks were circumferential, and were found mostly
in the HAZ of the weld or Top of Tubesheet (TTS) of the expansion part. The
tube of the lower defective zone was repaired using the explosive expansion
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