Where There’s Steam, There’s … a Steam Generator
by Moderator
Kenneth Karwoski
Senior Advisor for Steam Generators
News
articles recently brought the phrase “steam generators” into the
national conversation, but we’re not talking about teakettles. ![pwr[1]]()
Steam generators provide vital technical and safety functions at many U.S. nuclear power plants.
In
the United States, steam generators are only found in 65
pressurized-water reactors, one of the two types of U.S. reactors. There
can be two to four steam generators for each reactor unit. The
generators mark the spot where two closed loops of piping meet. The
first loop sends water past the reactor core to carry away heat, and
this loop is at such high pressure that the water never boils. The
second loop is at a lower pressure, so the water in this loop turns to
steam and runs the plant’s turbine to generate electricity.
The
steam generator’s main technical job is to let the first loop pass its
heat to the second loop as easily as possible. To do this, a steam
generator packs thousands of small tubes closely together, allowing the
maximum area for heat to pass through the tubes and into the second
loop’s water.
At
the same time, the steam generators provide an important safety barrier
– the first loop can contain radioactive material, so the tubes must
keep the two loops of water separate. NRC rules require plants to
closely monitor the second loop and immediately shut the reactor down if
a tube leak exceeds very strict limits.
The
NRC’s rules for inspections, maintenance and repair of steam generator
tubes help ensure the tubes continue providing the safety barrier. If an
inspection shows a tube is starting to get too thin, the plant will
repair or even plug a tube to maintain safety.
Steam
generator tube material has improved over time. The first steam
generators had tubes made from a type of stainless steel that experience
showed could be corroded by the chemicals, temperatures and pressures
in the first and second loop. Over time, plants have replaced those
steam generators with ones using more advanced alloys that are less
likely to corrode. Steam generator replacement only happens when the
reactor is shut down for refueling, and plant owners bring in hundreds
of specialized workers to safely remove the old generators and install
the new ones. The old generators have to be safely disposed of as
low-level radioactive waste.
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