Steam generator (nuclear power)
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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 (PWR) 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.
In commercial power plants, there are two to four steam generators per reactor; each steam generator can measure up to 70 feet (21 m) 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 (19 mm) in diameter. The coolant (treated water), 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 (which is kept at a lower pressure than the primary 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.
The nuclear powered steam generator started as a power plant for the first nuclear submarine, the USS Nautilus (SSN-571). 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 (NPS), which also used a nuclear powered steam generator, in 1960. This power plant had a one hundred MWe (mega watt electric) 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.
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 & Wilcox plants (e.g., Three Mile Island) have smaller steam generators that force water through the top of the OTSGs (once-through steam generators; counter-flow to the feedwater) 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 and 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.
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.
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:
|Side||Pressure, MPa (absolute)||Inlet temperature, °C||Outlet temperature, °C|
|Primary side (tube side)||15.5||315 (liquid water)||275 (liquid water)|
|Secondary side (shell side)||6.2||220 (liquid water)||275 (saturated steam)|
Various high-performance alloys and superalloys have been used for steam generator tubing, including type 316 stainless steel, Alloy 400, Alloy 600MA (mill annealed), Alloy 600TT (thermally treated), Alloy 690TT, and Alloy 800Mod.
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