I've attached a graphic to show the location and role of the steam
generator divider plate. It is the vertical piece in the lower domed
area of the U-tube steam generator. The U-tube steam generator features
thousands of U-shaped tubes that have the U at the top and the two tube
ends at the bottom. The divider plate functions to separate the flow
coming into the steam generator from the flow leaving it. Thus, the
divider plate forces incoming flow to enter one side of the U-tubes and
leave the other end.
I am at a loss to explain "We do not have data because no one has ever
looked at the issue."
In about a minute, an electronic search of my files obtained from NRC's
ADAMS library returned at least a dozen reports on steam generator
divider plate inspections and results. I've attached just one of these
reports to illustrate the point. In fall 2003, the NRC hosted a
conference on inspections and results. The very first presentation - the
very first - was by Westinghouse. The 13th slide in Westinghouse's
presentation stated that France inspected the hot side (incoming flow
side) of 42 divider plates and the cold side (outgoing flow side) of 26
divider plates with only one plate showing crack indications.
A conference conducted in 2005 reported that France considers steam
generator divider plates as being among the top four priorities for
primary water stress corrosion cracking (largely because the plates are
made from Alloy 600, which is vulnerable, and because the plates
experiences high thermal stress due to hot water on one side and cold
water on the other).
So, I cannot explain and do not understand NRC's apparent unawareness of
all these readily available reports on steam generator divider
inspections and results.
Despite their cluelessness, at least the NRC seems to be on the right
track questioning the aging management of the divider plates.
Westinghouse's 15th slide concludes that Alloy 600 will crack, the base
metal will crack before the weld metal, Alloy 600 base metal will crack
after 6 years, and that Alloy 600 "cracking is increasingly likely."
If the divider plate were only to crack, the consequences would be only
financial. If a hole or crack opened in the divider plate, the higher
pressure incoming flow would flow through to the other side of the
plate, bypassing the journey through the tubes. Thus, the energy used to
heat the water in the reactor would be wasted as that hot water would
not produce any steam for the turbine to use in making electricity.
But the NRC's concern is not about this outcome. They want to know if
divider plate cracking could induce cracking in the tubes and/or
tubesheet. With an intact divider, there is force (and thus stress)
applied by the incoming flow hitting the divider plate. The divider
plate is welded to the tubesheet at the top and the lower dome of the
steam generator at the bottom. The force against the divider plate
"tugs" on these welds. If the divider were to crack, the water flowing
through the crack could the divider plate to oscillate. Just as one can
break a paper clip by folding and unfolding it, the divider plate
oscillations and the cycling tug/no tug on the tubesheet welds could
create cracks in the tubesheets and then propagate those cracks - or at
least, that the concern the NRC wants PSEG to address.