Environmental Engineering Reference
In-Depth Information
method, and the unrepaired tube was plugged. The tube of the OTSG made
by B&W was sensitized, heat treated and carbides were created at the grain
boundary. The tube was SCC-sensitive if it was exposed to acid because of
the tensile stress in the material from the manufacturing process.
In France, cracks have been found in the divider plate of a steam gener-
ator. As of the end of 2007, defects were found in the divider plates of ten
steam generators. Various inspections have highlighted the fact that these
defects are located in the stub of the hot branch, with no signs of signifi cant
evolution, either by fatigue or corrosion.
In Japan in 1976, leakage occurred from the U-bend of the steam genera-
tor row 1 of the Takahama Unit 1. It was assumed that the crack was caused
by the PWSCC due to plastic deformation of the tube. The deformation,
which was located between the U-bend and the tube, had been created by
passing a ball mandrel through the tube, or had developed by the curving
process during tube manufacture. This area usually has high residual stress.
PWSCC of the U-bend also occurred in Ohi Unit 1 and Mihama Unit 2.
Further, another leakage occurred in a small radius U-bend in Ohi Unit 2 in
1994. In this area, the ovality was larger, relatively speaking.
Since 1982, PWSCC of the tubesheet zone has been detected in many
power plants. In Mihama Unit 3 and Ohi Units 1 and 2, PWSCC was found
in both hard rolled areas and expansion transitions (made using full depth
expansion) in tubesheet. PWSCC was detected at expansion transitions
with part depth rolling in tubesheet at Takahama Unit 1 and Mihama Unit 2.
PWSCC in the expansion transition occurs due to high residual stress in the
zone where materials are mechanically rolled. This is caused by insuffi cient
expansion during mechanical rolling over by uneven tubesheet holes. Such
PWSCC of 600 MA pipe can be removed by replacing the steam generator
with one with 690 TT tubes.
In a case of PWSCC of 600 TT tubes in the Japanese Kansai power plant,
some cracks were found by ECT on Alloy 600 TT in tubesheet region of
three power plants since 1999. Inspection was carried out on the damaged
tubes and PWSCC was proven as a result. In three power plants (Sendai
Unit 1, Takahama Units 3 and 4) the depth was expanded by full depth
mechanical rolling after full depth hydraulic expansion. Cracks were found
in the upper part of transitions to the hydraulic expansion area, indicat-
ing that cracks are located near the area expanded by the mechanical roll.
But cracks did not occur where there was hydraulic expansion transition.
Inspection of Takahama Unit 4 found the diameter of the tube hole to be
sectionally large. It is considered that the oval shape was made by polishing
the eccentricity of the tube hole during manufacturing. Cracks occurred in
the zone where oval shaped holes were present. From a mock-up experi-
ment, high residual stress was observed at the zone where there were tube
holes that were irregular in shape, and mechanical rolling had been carried
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