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=Organically Moderated and Cooled Reactor=

The organic moderated and cooled reactor 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 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 turbine-generator set. As in LWRs, the fuel could be slightly-enriched uranium dioxide, though other fuel types were considered.

History
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 11.4 [MWe] demonstration reactor was operated for a brief period in Piqua, OH and was the only power reactor of its kind ever constructed in the United States. The Piqua Nuclear Generating Station was shut down in 1967 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.

Organic Moderator
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.

Better Moderation
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.

Decomposition
At high temperature, organic fluids decompose into lighter and heavier fractions in a process called cracking (chemistry). This process is accelerated in the presence of high levels of radiation. 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.

Possible Future
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.