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steam generator (nuclear power)

The nuclear powered steam generator uses a nuclear reaction to rapidly heat water and turn a turbine. The hot, radioactive water then travels down thousands of small pipes under pressure so high that even its extreme temperature can’t turn it to steam. The radioactive water is run through a condenser, where it is cooled by non-radioactive water. In the process the clean water is turned to steam and is put to use turning the turbine^[1]. The turbine is connected via a shaft to the generator, which rapidly spins powerful magnets across coils of wires, creating an electric current. The condensed radioactive water is pumped through the reactor again to start the process over again. The roundabout way that the cooling water is used to create the steam is a safety precaution. The non-radioactive water is condensed in a condenser to temperatures just below the flash point. The flash point is the temperature where water flashes to steam. Once it is turned back into water, the clean water is run back through the system to begin the cycle again.

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^[2]. 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 MW (mega watt) output. By comparison, some modern plants have over 1100 MW output. Eventually, other international companies such as B&W and Consumer Electrics 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^[3].

In the Three Mile Island disaster, a main feed water pump shut down, although the cause is not known. Without that pump, the steam turbine wasn't being cooled and pressure in the reactor as rising ^[4]. The system automatically began to dump the cooling water, 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.

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 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 ^[5].

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 ^[6]. 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.

The nuclear powered steam generator uses a nuclear reaction to rapidly heat water and turn a turbine. The hot, radioactive water then travels down thousands of small pipes under pressure so high that even its extreme temperature can’t turn it to steam. The radioactive water is run through a condenser, where it is cooled by non-radioactive water. In the process the clean water is turned to steam and is put to use turning the turbine. The turbine is connected via a shaft to the generator, which rapidly spins powerful magnets across coils of wires, creating an electric current. The condensed radioactive water is pumped through the reactor again to start the process over again. The roundabout way that the cooling water is used to create the steam is a safety precaution. The non-radioactive water is released after the turbine has been turned.

Of the two types of nuclear reactor, the pressurized water reactor (PWR) uses a steam generator to create the electricity. The other type of generator, the boiling water reactor, doesn't use a steam generator. Of these two designs, the pressurized water reactor seems to have a history plagued with obstacles. Three-Mile Island, the worst nuclear disaster in the United States, used a nuclear powered steam generator to generate its electricity. To ensure that disasters such as these are minimized, the tubes and turbine blades are made out of specially made materials that can withstand the radiation and heat for years. The relatively recent improvement in materials for the components of the steam generator can be attributed to the Nuclear Regulatory Commission’s tightening its regulatory oversight ^[7].

In the Three Mile Island disaster ^[8], a main feed water pump shut down, although the cause is not known. Without that pump, the steam turbine wasn't being cooled and pressure in the reactor as rising. The system automatically began to dump the cooling water, 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 ^[9]. 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.

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 either Alloy 600 or Alloy 690 ^[10]. 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 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 Committee 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 ^[11].

(2007). Pressurized Water Reactor Power Plant. [1]

World Nuclear Association.(2014). Outline History of Nuclear Energy. [2]

Particulars of Radiolysis and Mass Transfer in Two-phase Water Coolant of a Nuclear Power Facility with Steam Generation at an Interphase Surface

Experience in the Operation of Channels with Single-pass Steam Generation in the Reactor at the First Nuclear Power Station

Combining the Nuclear Power Plant Steam Cycle with Gas Turbines

Jovica R. Riznic.(May 2011). Nuclear Engineering and Design. Water Level Controller for a Nuclear Steam Generator. [3]

Nuclear Regulatory Committee.(March 21, 2014). Backgrounder on Steam Turbine Issues. [4]

Nuclear Regulatory Committee.( February 11, 2013). Backgrounder on the Three Mile Island Accident. [5]

University of Melbourne.(2014). Nuclearinfo.net. Everything you want to know about Nuclear Power. Retrieved from: http://nuclearinfo.net/Nuclearpower/WebHomeSafetyOfNuclearPowerPlants

All sources were found using the articles tool on the CSUM library page.