Environmental Engineering Reference
In-Depth Information
problem of HET plugging as a result of the formation of very large numbers
of defects.
Comparison of special features of the formation of defects on differently
oriented surfaces of heat exchangers with different flow pattern may
affect the mechanisms and reasons for the formation and development of
defects. Different materials of HET, their diameters and wall thickness
are also important for various aspects of the HET integrity, but not a
fundamental obstacle for comparative analysis.
Mill annealed alloy 600 (600 MA alloy) was used in the U.S. and
most other countries for the manufacture of CPS and I&C nozzles for
reactor covers and some other primary circuit nozzles. All these nodes
have problems. Covers of PWR reactors are replaced at nearly the same
rate as SG. Moreover, if the U.S. carried out the replacement after the
discovery of defects (to 2004, 10 covers were replaced, and 23 planned to
be replaced), in France covers are being replaced continuously (44 of 54
were replaced by 2005).
Replacement of VSG because of problems with alloy 600MA is
now a routine procedure. There are around 350 SG of this type in the
world. According to the data from US NRC and METI (Japan) websites by
mid-2006 there were around 70 SGs of this type.
The initial approach to choosing material for SG HET for nuclear power
plants in the USSR and the United States was the same. However, the use
of stainless steel type SS304 in the first commercial nuclear power plant in
the USA (Shippingport) gave negative results (leakage through two HETs
of the steam generator 150 hours after start) in early 1957
116
.
Negative experience with stainless steel for HET of steam generators
was confirmed in other nuclear power plants such as Yankee Row and
Indian Point-1, as well as non-commercial reactors Savannah River, Hanford
and Nautilus
116
. Many of the problems with the HET in vertical steam
generators were associated with the wrong choice of a new material for
HET (high-nickel alloy 600MA) in the U.S. and all other countries. In
these countries, nuclear power plant equipment (or SGs for them) were
manufactured by US firms or under US license. The only country which
quickly and independently stopped using Alloy 600MA was Germany. After
the experience gained at the Obrigheim NPP, where HETs of SGs were also
made of alloy 600MA, the new structures used 800NG alloy. Because of
the special features of design and efficient choice of material there were
only a small number of problems with the vertical HETs with pipes made
of 800NG alloy (related to corrosion). Although, if we consider not only
the corrosion causes of breakage of HET, the comparison of the extent of
HET plugging in all German SGs and Russian PGV-1000 shows the practical
equality in this parameter (about 0.6%)!
USA and all other countries were forced to start from 1980 (Surry-2
NPP) the replacement of SG with HET made of the alloy type 600mA by
SG with HET made of alloy 600TT (in the U.S. 17 nuclear power plants,
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