Environmental Engineering Reference
In-Depth Information
found that Alloy 750 was a material sensitive to PWSCC, and that sensitivity
increased largely depending on heat treatments. Since it was replaced with
a new cold worked material (CW316 SS), no additional cracks have been
found.
Regarding baffl e former bolts, some cracks were found in an old French
power plant (Fessenheim and Bugey) in 1988. Some bolts had 10-25 dpa
fl uence after being in operation for 10-20 years. They were made of a 316
cold worked stainless steel and showed intergranular stress corrosion crack-
ing (IGSCC). The ageing mechanism was assumed to be IASCC. The cracks
had spread from the shank zone of the head to the lower part of the head.
The material was found to be hardened and radiation-induced segregation
was found in the grain boundary. According to the hardness profi le mea-
surement, it was approximately 5-10 dpa, and there was no evidence of
swelling. The bolts with cracks were detected at rows 2 and 3 from the lower
part of the nuclear reactor where a considerable amount of neutron radi-
ation had accumulated. According to the report, until that point, cracks in
the baffl e former bolts had been found in the 'down-fl ow' design in which
inlet coolant fl ows downward (Gérard, 2009). From 1989 to 1993, the fl ow
of the coolant in the nuclear reactor of the CP0 (name of French PWR 900
MW pre-series units) plant was changed to up-fl ow and, between 2000 and
2003, one third of the bolts were replaced. The cracking rate of baffl e bolts
increases slowly depending on dose. Based on the information on all the
baffl e bolts researched during in-service inspection (ISI) for all CPO units,
the dose threshold is estimated to be approximately 3-4 dpa. The number of
cracked bolts increases slightly at higher doses.
In Japan, two kinds of approach have been applied to the PWR plants
since 1998 in light of the baffl e former bolt issues experienced in France
and also in the United States. The fi rst approach was to replace the baffl e
former bolts. From 2001 to 2002, type 347 stainless steel bolts were replaced
with cold worked 316CW stainless steel bolts in Mihama Unit 1 and 2. The
second approach was to replace the internal structure of the nuclear reac-
tor. In this case, the lower zone including the baffl e former bolts and the
upper zone were replaced. Since 2004, the internals of the nuclear reactors
in three PWR plants have been replaced entirely. The measures described
above were applied to the baffl e former bolts in a 2-loop PWR plant which
was built in the early 1970s.
A research programme aimed at preventing defects of the internals
of nuclear reactors has been ongoing since 2000. For example, there has
been research on IASCC of austenitic stainless steel used in PWR inter-
nals such as in baffl e former bolts. Valuable data was collected through this
research, which showed that IASCC initiation is closely related to stress and
exposed neutron fl uence. In other words, the threshold stress which deter-
mines IASCC occurrence is dependent on the amount of neutron fl uence.
The threshold stress value tends to decrease as neutron fl uence increases.
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