Using probabilistic methods for solving the problem of ensuring leak tightness of heat exchanger tubes of nuclear power plant steam generators - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 273

64) and 19.05 × 1.03 ( k = 0.054) mm. In Japan, there is only one standard
size 22.23 × 1.27 mm ( k = 0.057), and in Germany 22.0 × 1.23 ( k = 0.056).
There are only two standard dimensions for the horizontal steam
generators: 16.0 × 1.5 mm ( k = 0.0 94) and 16.0 × 1.4 ( k = 0.088). At this
selection greatly increase the resistance of the heat exchanger tubes of the
horizontal steam generators to corrosion and fracture.
One reason for the heavy damage to the HET of the SG was not the
best choice of a design solution the spacing zone. According to Ref. 117 in
2002 of the 237 nuclear power plants (worldwide) 42% had SGs with spacer
plate with drilled holes. In about 60% of nuclear power plants SGs were
replaced, and new SGs use different spacing structures, 27% grid/eggcrate,
73% broached - with three or four contact zones). All the replaced SGs use
the drilled hole-type spacers. It should be noted that simultaneously with the
change in grid design the material of the elements of the spacer was also
changed. Carbon steel was replaced with stainless steel. The change of the
configuration of this zone of the HET should have significantly changed
the situation with the effect of corrosion. And so it happened. After nearly
20 years of operatione no denting was detected in the VSGs with HET
made of 600TT alloy with new design of the spacing zone and no stress
corrosion cracking with intercrystalline corrosion as well as SCC (stress
corrosion cracking) with intergranular corrosion. The first defects in the
spacing zones appeared in the SG in the US plants only in 2002.
Here, apparently, it is worth mentioning that such a development of
corrosion processes in the HET made of the new alloy was predicted in
many studies by one of the leading U.S. specialists on corrosion - R.W.
Staehle. Critically evaluating the methodology for selecting new materials
(assuming that a mistake was made by choice of the alloy 690TT), he stated
that it takes time (for a variety of factors - 10-20 years) to create suitable
conditions for intensive corrosion processes. This position is described
in detail in, for example, Ref. 121. According to this author, the lack of
corrosion in the HET of the new alloys is largely the result of structural
changes in the spacing zones and fixing areas, and not merely the result
of selecting a new material.
In the HSGs the spacing between HET using spacing strips (option to
which the U.S. came through trial and error - grid/eggcrate - is very close to
the traditional method for HSG). This is one of the most dangerous places of
localisation of all types of corrosion in VSG. Again, it should be noted that
one of the main reasons was the wrong design solution: in the first models
rolling (mechanical) was carried out only on a small part of the sealing
area, so that an annular gap almost half-meter high remained. In the SGs
of different models with HET made of the 600MA alloy this problem has
not been solved. In models 51 manufactured by Westinghouse rolling of the
HET in the tube plates was carried out at full length, first by an explosive
method (to eliminate narrow gaps 122 ), then mechanically, and, finally, with
the model D5 by the hydraulic method. A number of other measures were
taken to avoid re-rolling, reducing the stress state in the transition zone, etc.

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