Environmental Engineering Reference
In-Depth Information
6) damage to the tube 60-96, SG3:
- no defects were found between the spacing grids 1 and 2;
- traces of pitting corrosion were found between the spacing grids 2
and 3 in the form of clusters of pits with a diameter of 0.3 to 2 mm. One
of them is a through-wall pit; the diameter of the outer surface 3 mm, on
the inned surface an ellipse 0.1
×
8 mm. Dead-end cracks propagated from
both the surface and from pits, one through-wall crack from a pit;
- pits 0.2-0.3 mm deep were found below the second spacer grid;
7) damage to tube 60-102, SG3:
- below the spacer grid 3 - pits and cracks up to continuous, emerging
from both the pits and on the surface;
- below the spacer grid 4 - corrosion pits and stress corrosion cracking
on the outer surface. A crack developed parallel to the surface.
8) damage to tube 61-1, SG1:
- under the spacer grid 4 there were small pits with a diameter of
0.1 mm with a density of 2 pits at 1 mm, and also cracks developing from
both the surface and from the pits. The maximum depth of cracks was 51%
of the thickness of the tube wall;
9) all the tubes contained deposits, including those under the spacer
grids. Sulphur (0.1-1%), copper, traces of chromium, chlorine, sodium and
other elements were found in the cracks;
10) the crack trajectory is wave-shaped which indicates that it is
determined mainly by the influence of corrosive environment and
microheterogeneity of the steel;
11) in general, cracks are oriented along the axis of the tube which
indicates that the tangential stresses exceed the axial stresses
Photographs of different types of defects are shown in Fig. 9.7-9.11.
Causes of damage in the HET of SG1, 3, 4 unit 2 of Balakovo NOO
analyzed by retrospective network modeling.
At the first stage the main characteristics of HET damage of SG-1, 3,
4, unit II Balakovo NPP were determined, and significant information from
other units was collected. One of the main criteria for the correct analysis
of the causes of damage is that these (this) reason(s) should explain all the
features of damage and the process of damage.
At the second stage assumptions about the causes (mechanisms) of
damage were identified. All assumptions suggested at meetings of the
working group and other meetings dedicated to discussion of SGs on unit
2 of Balakovo NPP (of 8 hypotheses) were taken into consideration.
At the third stage facts, data received as a result of reviews, calculations
and experimental analysis as well as other circumstances that could prove
or disprove a hypothesis were subjected to analysis.
As a result of tests carried out at stages 3 and 4, it was found that the
root cause of damage such as pits and pits with cracks is the transfer of
corrosion products from the condensate feed circuit and their accumulation
in the form of deposits and sludge.
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