Organically moderated and cooled reactor
Encyclopedia
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 reactor
s (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 exchanger
s 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.
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 rod
s 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.
(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.
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
).
– 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.
. 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.
, also used as a coolant in some reactors. The heat transfer coefficient can be improved by nucleate boiling
(as in boiling water reactor
s); 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.
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.
United States Atomic Energy Commission
The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...
and others around the world. The concept reactor was very similar to light water reactor
Light water reactor
The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...
s (LWRs) in fuel element and reactor core design, but used hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
as a coolant and neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
in lieu of water. The hot organic coolant was pumped through secondary heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s to boil water and generate steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
to run a turbogenerator. As in LWRs, the fuel could be slightly- enriched
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
uranium dioxide
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
, 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 waterHeavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
organic cooled reactor from 1965 to 1985. An 45.5 [MWe] demonstration reactor was operated for a brief period in Piqua, Ohio
Piqua, Ohio
Piqua is a city in Miami County, Ohio, United States. The population was 20,738 at the 2000 census. It is part of the Dayton Metropolitan Statistical Area.Piqua was one of the cities that experienced severe flooding during the Great Dayton Flood of 1913....
and was the only power reactor of its kind ever constructed in the United States.
The Piqua Nuclear Generating Station
Piqua Nuclear Generating Station
The Piqua Nuclear Power Facility was a nuclear power plant which operated just outside the southern city limits of Piqua, Ohio in the United States. The plant contained a 45.5-megawatt organically cooled and moderated nuclear reactor...
was shut down in 1966 due to an instrument scram
Scram
A scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...
. In the process of resuming operation, it was discovered that two control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s did not move freely within their guide tubes and four nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
elements required abnormally high force to remove. Subsequently, these fuel elements would not reseat. A complete unloading of the core showed that large carbonaceous
Carbonaceous
Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....
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
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change 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.| Advantages | Disadvantages |
|---|---|
| Better Moderation | Reactor control issues |
| High negative temperature coefficient | Decomposition |
| Low system pressure | Poor heat transfer |
Better Moderation
The organic fluids such as biphenylBiphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...
(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 coefficientTemperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
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
Iodine pit
Iodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the core of a nuclear reactor. The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135. Iodine-135 is a weak neutron...
, 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
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...
).
Low System Pressure
One noteworthy advantage of this moderator type is that temperatures of approximately 700°F can be obtained at low system pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
– 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
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
. 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 crackingCracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...
. This process is accelerated in the presence of high levels of radiation, causing radiolysis
Radiolysis
Radiolysis is the dissociation of molecules by nuclear radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux...
. Coolant decomposition increases fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
of the heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change 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 metalLiquid metal cooled reactor
A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...
, also used as a coolant in some reactors. The heat transfer coefficient can be improved by nucleate boiling
Nucleate boiling
Nucleate boiling is a type of boiling that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux. For water, as shown in the graph below, nucleate boiling occurs when the surface...
(as in boiling water reactor
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...
s); 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 IndiaIndia
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
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.