This is an article about nuclear power
plant equipment. For other uses, see
steam generator.
Steam generators are heat exchangers
used to convert water into steam from
heat produced in a nuclear reactor core.
They are used in pressurized water
reactors between the primary and
secondary coolant loops.
In other types of reactors, such as the
pressurised heavy water reactors of the
CANDU design, the primary fluid is heavy
water. Liquid metal cooled reactors such
as the Russian BN-600 reactor also use
heat exchangers between primary metal
coolant and at the secondary water
coolant.
Boiling water reactors do not use steam
generators, as steam is produced in the
pressure vessel.
Description 
In commercial power plants, there are
two to four steam generators per
reactor; each steam generator can
measure up to 70 feet in height and
weigh as much as 800 tons. Each steam
generator can contain anywhere from
3,000 to 16,000 tubes, each about .75
inches in diameter. The coolant, which
is maintained at high pressure to
prevent boiling, is pumped through the
nuclear reactor core. Heat transfer
takes place between the reactor core and
the circulating water and the coolant is
then pumped through the primary tube
side of the steam generator by coolant
pumps before returning to the reactor
core. This is referred to as the primary
loop.
That water flowing through the steam
generator boils water on the shell side
to produce steam. This is referred to as
the secondary loop. The secondary-side
steam is delivered to the turbines to
make electricity. The steam is
subsequently condensed via cooled water
from a tertiary loop and returned to the
steam generator to be heated once again.
The tertiary cooling water may be
recirculated to cooling towers where it
sheds waste heat before returning to
condense more steam. Once-through
tertiary cooling may otherwise be
provided by a river, lake, or ocean.
This primary, secondary, tertiary
cooling scheme is the basis of the
pressurized water reactor, which is the
most common way to extract usable energy
from a controlled nuclear reaction.
These loops also have an important
safety role because they constitute one
of the primary barriers between the
radioactive and non-radioactive sides of
the plant as the primary coolant becomes
radioactive from its exposure to the
core. For this reason, the integrity of
the tubing is essential in minimizing
the leakage of water between the primary
and secondary sides of the plant. Steam
generator tubes often degrade over time.
There is the potential that, if a tube
bursts while a plant is operating,
contaminated steam could escape directly
to the secondary cooling loop. Thus
during scheduled maintenance outages or
shutdowns, some or all of the steam
generator tubes are inspected by
eddy-current testing, and individual
tubes can be plugged to remove them from
operation.
Entire steam generators are often
replaced in plant mid-life, which is a
major undertaking. Most U.S. PWR plants
have had steam generators replaced.
= History =
The nuclear powered steam generator
started as a power plant for the first
nuclear submarine, the U.S.S. Nautilus.
It was designed and built by the
Westinghouse power company for the
submarine from there the company started
its development and research of nuclear
powered steam generators. Once peaceful
nuclear reactors were legalized for use
as power plants, power corporations
jumped at the opportunity to utilize the
growing development of nuclear powered
steam generators. Westinghouse built one
of the first nuclear power plants, the
Yankee Rowe Nuclear Power Station, which
also used a nuclear powered steam
generator, in 1960. This power plant had
a one hundred MWe output. By comparison,
some modern plants have over 1100 MWe
output. Eventually, other international
companies such as Babcock & Wilcox and
Combustion Engineering began their own
programs for research and development of
the nuclear power steam generator. Since
the 1960s, the US has fallen behind on
some European nations in embracing this
new power source. France and the UK have
been more actively pursuing the benefits
that come with nuclear energy, while the
US is more concerned about the risk.
Finally, it seems China is planning a
massive increase to their nuclear power
supply and ordering many new plants to
be built.
= Three Mile Island =
In the Three Mile Island disaster, a
main feed water pump shut down, although
the cause is not known. Without that
pump, the steam generator wasn't able to
remove heat from the reactor, so
pressure in the reactor began to rise.
The system automatically began to dump
water from the reactor to reduce
pressure, but the relief valve got stuck
open when the automation told it to
close. The control room indicated that
the valve was closed. The staff,
therefore, had no idea that they were
dumping radioactive water out of one of
their reactors. With the water being
pumped out, there wasn't enough
emergency cooling water and the staff
was unaware that they were losing more
by the minute. Without adequate cooling,
one of the reactors began to melt. The
pipes burst and about half the core
melted during the accident. Unlike
Chernobyl and other nuclear disasters,
the containment house around the reactor
held and the damage to the outside world
was very minimal. Since the containment
housing held, no radioactive particles
were released into the atmosphere, like
what happened in Fukushima. The release
of radioactive water did damage and
contaminate the local area, but it did
not spread from there.
Types 
Westinghouse and Combustion Engineering
designs have vertical U-tubes with
inverted tubes for the primary water.
Canadian, Japanese, French, and German
PWR suppliers use the vertical
configuration as well. Russian VVER
reactor designs use horizontal steam
generators, which have the tubes mounted
horizontally. Babcock and Wilcox plants
have smaller steam generators that force
water through the top of the OTSGs and
out the bottom to be recirculated by the
reactor coolant pumps. The horizontal
design has proven to be less susceptible
to degradation than the vertical U-tube
design.
= Materials, Construction =
The materials that make up the turbine
and pipes of a nuclear powered steam
generator are specially made and
specifically designed to withstand the
heat and radiation of the reactor. The
water tubes also have to be able to
resist corrosion from water for an
extended period of time. The pipes that
are used in American reactors are made
of Inconel, either Alloy 600 or Alloy
690. Alloy 690 is made with extra
chromium and most facilities heat treat
the metal to make it better able to
resist heat and corrosion. The high
nickel content in Alloy 600 and Alloy
690 make them well suited for resisting
acids and high degrees of stress and
temperature.
= Degradation =
The annealed, or heat treated, Alloy 600
was prone to tube denting and thinning
due to water chemistry. Plants that used
the Alloy 600 in their water tubes
therefore had to install new water
chemistry controllers and change the
chemicals they put in the water. Due to
this, pipe thinning has been taken care
of, but on rare occasions, tube denting
still occurs, causing leaks and
ruptures. The only way to prevent this
is regular maintenance and check-ups,
but this forces the reactor to shut
down. In some cases, plants replaced
their Alloy 600 tubes with Alloy 690
tubes and a few plants were shut down.
To prevent future problems,
manufacturers of steam turbines for
nuclear power plants have improved their
fabrication techniques and used other
materials, such as stainless steel, to
prevent tube denting.
= Safety Devices =
As the disaster at Three Mile Island
could have been avoided with prior
planning and a water level indicator,
the Nuclear Regulatory Commission has
started to push for water level
controllers. The controller would
regulate water to the reactor using a
combination of sensors such as feedback
controllers and feed-forward
controllers. Yet this is only one of
many such devices that ensure the safe
and efficient production of power by
nuclear reaction. Other tools include
the control rods, relief valves, and
even back up cooling systems. The
control rods work by reducing the amount
of radiation produced. They are built of
a material that absorbs neutrons, and
therefore reduces the number of fission
reactions that take place inside the
reactor. Relief valves function to vent
pressure, sometimes into the atmosphere,
in order to protect the system as a
whole. And if the worst were to happen
and a meltdown was occurring, a
reservoir of cooling water waiting in
standby could mean the difference
between a disaster and a minor incident.
Typical operating conditions 
Steam generators in a "typical" PWR in
the USA have the following operating
conditions:
Tube material 
Various high-performance alloys and
superalloys have been used for steam
generator tubing, including type 316
stainless steel, Alloy 400, Alloy 600MA,
Alloy 600TT, Alloy 690TT, and Alloy
800Mod.
See also 
Nuclear power plant
Power station
Steam shower
Steam turbine
References 
External links 
John M. Dyke and Wm. J. Garland,
"Evolution of CANDU Steam Generators – a
Historical View" pdf
(2007). Pressurized Water Reactor Power
Plant. [4]
World Nuclear Association.(2014).
Outline History of Nuclear Energy. [5]
Jovica R. Riznic.(May 2011). Nuclear
Engineering and Design. Water Level
Controller for a Nuclear Steam
Generator. [6]
Nuclear Regulatory Committee.(March 21,
2014). Backgrounder on Steam Turbine
Issues. [7]
Nuclear Regulatory Committee.( February
11, 2013). Backgrounder on the Three
Mile Island Accident.
[8]
University of Melbourne.(2014).
Nuclearinfo.net. Everything you want to
know about Nuclear Power. Retrieved
from: [9]
