- Organically moderated and cooled reactor
The organic moderated and cooled reactor (OCR) was an early power reactor concept studied in the formative years of nuclear power by the United States Atomic Energy Commission and others around the world. The concept reactor was very similar to light water reactors (LWRs) in fuel element and reactor core design, but used hydrocarbon as a coolant and neutron moderator in lieu of water. The hot organic coolant was pumped through secondary heat exchangers to boil water and generate steam to run a turbogenerator. As in LWRs, the fuel could be slightly- enriched uranium dioxide, though other fuel types were considered.
Though this reactor type has been the subject of extensive study, no large power plants using organic coolant have been built. The Canadians operated a 60 [MWe] heavy water organic cooled reactor from 1965 to 1985. An 45.5 [MWe] demonstration reactor was operated for a brief period in Piqua, Ohio and was the only power reactor of its kind ever constructed in the United States.
The Piqua Nuclear Generating Station was shut down in 1966 due to an instrument scram. In the process of resuming operation, it was discovered that two control rods did not move freely within their guide tubes and four nuclear fuel elements required abnormally high force to remove. Subsequently, these fuel elements would not reseat. A complete unloading of the core showed that large carbonaceous deposits were present on the fuel, control rods, and support structures throughout the core region. These deposits interfered with the movement of the control rods, and altered the heat transfer characteristics of the fuel. As a consequence, the Atomic Energy Commission decided to end the Piqua experiment and the reactor was dismantled.
In terms of safety and economics, the organic moderated and cooled reactor has several inherent advantages. However, these are offset by several key disadvantages that ultimately led to the design being abandoned in the United States.
Advantages Disadvantages Better Moderation Reactor control issues High negative temperature coefficient Decomposition Low system pressure Poor heat transfer
The organic fluids such as biphenyl (which was tested as a moderator in the Piqua OCR) contain considerable hydrocarbon, which has excellent neutron moderating properties. This superior moderation resulted in compact core sizes with respect to common light water reactors, resulting in lower costs for structural materials and reduced shielding weight.
High Negative Temperature Coefficient
A high negative temperature coefficient acted as an automatic stabilizer, causing the reactor to shut down by itself upon a rapid increase in power. This property also allows for complete xenon override, meaning that a reactor of this type could be restarted any time after shut down, without the requisite wait period typical to LWRs (see: Xenon-135).
Low System Pressure
One noteworthy advantage of this moderator type is that temperatures of approximately 700°F can be obtained at low system pressure – in the range of 35 pounds per square inch. Low system pressure reduces sealing and gasket problems and allows the use of thin walls on pressure vessels and piping, significantly reducing manufacturing costs. Also, at low operating pressure the reactor contains less potential energy. This means that a ruptured pipe cannot cause extensive damage (i.e. pipe whip) or cause the release of appreciable quantities of radioactivity.
Reactor Control Issues
The advantage of the high negative temperature coefficient is offset by the fact that it also increases reactor control difficulty. For example, since the coolant and moderator are one-and-the-same, relatively cold, dense coolant entering the core will increase moderation, slowing down more neutrons, and cause reactivity to increase. The resulting power increase would rapidly be quelled by the effect of the negative temperature coefficient but may cause the reactor to shut down prematurely.
At high temperature, organic fluids decompose into lighter and heavier fractions in a process called cracking. This process is accelerated in the presence of high levels of radiation, causing radiolysis. Coolant decomposition increases fouling of the heat transfer surfaces – the problems of necessary continuous cleaning and chemical recombination are difficult to solve. Also, the decomposition rate increases rapidly above 700°F, so the coolant outlet temperature is limited.
Poor Heat Transfer
Organic coolant has poor heat transfer qualities with respect to light water or liquid metal, also used as a coolant in some reactors. The heat transfer coefficient can be improved by nucleate boiling (as in boiling water reactors); however, this further increases fouling due to the decomposition of the fluid. Finned cladding on the fuel elements would help improve heat transfer but would also considerably increase the cost of the fuel.
Engineers in India are showing renewed interest in the organic moderated and cooled reactor. Currently, India’s reactors are almost exclusively pressurized heavy water reactors similar to Canada Deuterium-Uranium CANDU reactors. While the CANDU design has the distinct advantage of being able to be refueled online, it has several drawbacks because of increased system complexity. Due to the significant possibility for cost reduction using a low-pressure design, the Heavy Water Organic Cooled Reactor is again being studied as an alternative. It is believed that an organic coolant purification system can be developed to handle the decomposition of the organic coolant, and research has begun to this effect.
- “Ayres, J.A. and C.A. Trilling. Heavy Water and Organic Fluids as Neutron Moderator and Reflector Materials. Nuclear Engineering and Design 14, February 1971, Pages 363-389.”
- “Nijsing R. and W. Eifler. Heat Transfer Calculations of Organic Cooled Seven-Rod Cluster Fuel Elements. Nuclear Engineering and Design 4, August 1966, Pages 253-275.”
- “Romero, Jacob B. Evaluation of Organic Moderator/Coolants for Fusion Breeder Blankets Argonne National Laboratory, 1980. Retrieved 01/27/09.”
- “Plant/Coolant Types for Nuclear Reactors”
- “The Long-Forgotten Organic Cooled Reactor”
Wikimedia Foundation. 2010.
Look at other dictionaries:
Nuclear reactor technology — This article is a subarticle of Nuclear power .A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a… … Wikipedia
Nuclear reactor — Core of CROCUS, a small nuclear reactor used for research at the EPFL in Switzerland This article is a subarticle of Nuclear power. A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are… … Wikipedia
Obrigheim Nuclear Power Plant — The Obrigheim Nuclear Power Plant … Wikipedia
OCR — may refer to: Optical character recognition, conversion of images of text into characters The OCR A font, designed to simplify character recognition The similar OCR B font Transvaginal oocyte retrieval, a technique used in in vitro fertilization… … Wikipedia
OMR — can stand for: Galería OMR, a gallery in Mexico City Old Mahabalipuram Road, a transport corridor in Tamil Nadu Omani rial, the ISO 4217 code for the currency of Oman Optical Mark Recognition, a data capture technology IATA code for Oradea… … Wikipedia
List of nuclear reactors — Map of all coordinates from Google Map of all coordinates from Bing Export all coordinates as KML … Wikipedia
Scientific research in Canada — This article outlines the history of natural scientific research in Canada including, mathematics, physics, astronomy, space science, geology, oceanography, chemistry, biology, medical research and psychology. The social sciences are not treated… … Wikipedia
Natural scientific research in Canada — This article outlines the history of natural scientific research in Canada, including mathematics, physics, astronomy, space science, geology, oceanography, chemistry, biology, medical research and psychology. The social sciences are not treated… … Wikipedia
Nuclear decommissioning — Example of decommissioning work underway. The reactor pressure ves … Wikipedia
Piqua Nuclear Generating Station — The Piqua Nuclear Power Facility was a nuclear power plant which operated in Piqua, Ohio in the United States. The plant contained a 45.5 megawatt (thermal) organically cooled and moderated nuclear reactor. The Piqua facility was built and… … Wikipedia