REFRESH — Where There’s Steam, There’s … a Steam Generator
Kenneth Karwoski
Senior Advisor for Steam Generators
When
the NRC talks about “steam generators,” we’re not talking about
teakettles. 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
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.
![pwr[1]](https://nrcpublicblog.files.wordpress.com/2013/07/pwr1.jpg?w=296&h=321)
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.
REFRESH is an occasional series where we revisit previous posts. This first ran in July 2013.
