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 272

with 281 000 HET). Prior to 2002, 1400 HET of the SGs made of this steel
were plugged in USA (up to 2005 more than 1700, by mid-2006 1884) -
most of them due to wear in the zone of bending grids. Stress corrosion
cracking (SCC) in the HET made of this alloy was detected in the USA
only in 2002 117 .
In France and South Korea, corrosion defects in HET made of this alloy
were observed previously 118 . In 2002, a HET fractured in one of the SGs of
Ulchin-4 NPP in Korea 119 . Since 1988 (Cook NPP) most of the SGs taken
out of operation were replaced by new ones made of 690TT alloy. Corrosion
problems were noted, most of the plugging up took place as a result of
vibrational wear on a very small scale. At the end of 2005, 395 HETs (data
for 78 SGs and 26 units) were plugged in U.S. SGs with HET made of
the alloy. At the end of 2005, 395 heat exchanger tubes (data for 78 steam
generators and 26 units) worked in the steam generators of the USA with
the heat exchanger tubes produced from the same alloy. However, in the
middle of 2005, the first inspection of the steam generators at the Osonee-1
nuclear power plant (14 months after replacement) showed further problems.
Vibrational wear took place in almost all spacing zones in the 3200 heat
exchanger tubes 120 . 48 heat exchanger tubes were plugged. The reasons
have not as yet been established. However, as shown in the presentation
material of the nuclear power plant (at the meeting in NRC), 200 tubes
are being prepared for pluggage by the unit No. 2 shutdown in October
2005. According to the most recent data, only five heat exchanger tubes
were plugged up in the unit No. 2. Three leakages were also detected in
the heat exchanger tube of the steam generator made from this alloy. The
selection of the material for spacing plates was also not very efficient. The
initial selection of carbon steel and of the structure with drilled holes for
these elements in the first models was regarded as the reason of denting
which is a phenomenon typical only of the vertical steam generators. At
the moment, the elements of the vertical steam generators are produced
from stainless steel.
It is well-known that in the horizontal steam generators the stainless
steel was used for spacing elements from the very first steam generator.
The important design solution was the selection of the diameter and wall
thickness of the heat exchanger tube. The vertical steam generators used
several standard dimensions of the heat exchanger tube. There are two
main standard dimensions (the vertical steam generators manufactured by
Westinghouse): 19.05 × 1.05 mm ( k = 0.055) and 21.43 × 1.23 mm k = 0.0
57). In the most recent models (F, D75, etc.), the dimensions of the heat
exchanger tubes are 16.86 × 1.02 mm ( k = 0.060) and 19.05 × 1.09 ( k =
0.0 57). The vertical steam generators produced by the Babcock and Wilcox
company use the heat exchanger tubes with the size of 15.80 × 0.86 mm
( k = 0.054). The vertical steam generators manufactured by CE company
use the heat exchanger tubes with the size of 19.05 × 1.22 mm ( k = 0.0

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