NUCLEAR POWER REACTORS

     Nuclear reactors function on a principle that from fission reaction large amount of heat is produced and it heats up the coolant. According to European Nuclear Society, there are 438 operable nuclear power plants around the world and there are 67 plants that are under construction [1]. Obninsk Nuclear Power Station is the first grid-connected nuclear power plant in the world[2]. It is located in Obninsk and its construction started in 1951 and finished in 1954.

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     From 1950s, many different commercial and experimental nuclear reactor models are developed and still being developed. The types of nuclear reactors are [3], [4];

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Notes for Table-1 and Table-2 [4]:


• The purpose of the reactor does not depend on the choice of coolant or moderator, but rather on reactor size and on how the reactor is operated, and on what ancillary materials are put into fuel rods besides fuel.

• Not all fuel type necessarily included.

• The enrichment of fuel refers to the percentage of the isotope of uranium-235 compared to uranium-238 present in fuel.

The distribution of reactor;



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     There are also different classifications like thermal or fast reactor which is based on energy of neutron. In the following, commercially using reactors are listed.


Light Water Reactor (LWR)


     Light water reactors use light water for coolant and moderator. It is thermal reactor that fast neutrons come to media and they are moderated by water and lose energy to thermal energies. It is the most common type of reactor in the world. There is different varieties of light water reactors: the pressurized water reactor (PWR), the boiling water reactor (BWR) and the supercritical water reactor.


Pressurized Water Reactor (PWR)


     It is the most common type of reactor in the world. This reactor based on coolant and moderator light water circulates in single phase under high pressure. The coolant is heated up and it goes to steam generator. In steam generator, there is heat transfer from first loops’ coolant which is radiated to second loops’ coolant. By this way, there is no radiation pass to second loop. There are different PWR models in the world.

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Vessel

     “The reactor core, and all associated support and alignment devices, are housed within the reactor vessel. The reactor vessel is a cylindrical vessel with a hemispherical bottom head and a removable hemispherical top head. The top head is removable to allow for the refueling of the reactor. There will be one inlet (or cold leg) nozzle and one outlet (or hot leg) nozzle for each reactor coolant system loop. The reactor vessel is constructed of a manganese molybdenum steel, and all surfaces that come into contact with reactor coolant are clad with stainless steel to increase corrosion resistance.”[7]

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Pressurizer

     “The pressurizer is the component in the reactor which arranges the systems pressure. It operates with a mixture of steam and water in equilibrium. If pressure starts to deviate from the desired value, the various components will actuate to bring pressure back to the normal operating point.”[7]

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Steam Generators

     “The reactor coolant flows from the reactor to the steam generator. Inside of the steam generator, the hot reactor coolant flows inside of the many tubes. The secondary coolant, or feedwater, flows around the outside of the tubes, where it picks up heat from the primary coolant. When the feedwater absorbs sufficient heat, it starts to boil and form steam.”[7]

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Reactor Coolant Pump

     “The purpose of the reactor coolant pump is to provide forced primary coolant flow to remove the amount of heat being generated by the fission process. Even without a pump, there would be natural circulation flow through the reactor. However, this flow is not sufficient to remove the heat being generated when the reactor is at power. Natural circulation flow is sufficient for heat removal when the plant is shut down (not critical).”[7]


Westinghouse Standart



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     Four-loop Westinghouse design has four steam generators, four reactor coolant pumps and a pressurizer. Examples in the USA are Braidwood 1 and 2, Byron 1 and 2, Callaway, Catawba 1 and 2, Comanche Peak 1 and 2, D. C. Cook 1 and 2, Diablo Canyon 1 and 2, Indian Point 2 and 3, McGuire 1 and 2, Millstone 3, Salem 1 and 2, Seabrook, Sequoyah 1 and 2, South Texas Project 1 and 2, Vogtle 1 and 2, Watts Bar 1, and Wolf Creek.


Combustion Engineering


     A Combustion Engineering plant has two steam generators, four reactor coolant pumps, and a pressurizer. The Combustion Engineering units in the United States are Arkansas 2, Calvert Cliffs 1 and 2, Fort Calhoun, Millstone 2, Palisades, Palo Verde 1, 2, and 3, San Onofre 2 and 3, Saint Lucie 1 and 2, and Waterford 3.

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Babcock and Wilcox


     This design has two once through steam generators, four reactor coolant pumps, and a pressurizer. The Babcock & Wilcox units in the United States are Arkansas 1, Crystal River 3, Davis Besse, Oconee 1, 2, and 3, and Three Mile Island 1.

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VVER


     VVER is the Russian design pressurized water reactor. There is different versions of VVER like VVER-440, VVER-1000 and VVER-1200 etc. The main differences from western PWR are horizontal steam generators and hexagonal fuels.

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APWR


     This is the Generation III reactor that includes several addings to typical PWR. This reactor is Mitsubishi It has a neutron reflector and its efficiency is increased. The safety system is improved too and it is combination of passive and active systems.

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AP 600 and AP 1000 (The Westinghouse Advanced Passive)


     AP-600 is a 600 MWe pressurized water reactor (PWR) with advanced passive safety systems and extensive plant simplifications to enhance the construction, operation, and maintenance of the plant. The plant design utilizes proven technology which builds on approximately 40 years of operating PWR experience.

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EPR (European Pressurized Reactor)


     It is the third generation reactor that designed by French AREVA and Électricité de France and German Siemens companies. Rather than enriched uranium, EPR can use MOX and reprocessed uranium as fuel. Different than standart PWR, EPR’s safety systems are increased and also passive safety systems are added.

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Boiling Water Reactor (BWR)


     It is the second most common type of the reactor in the world. Rather than PWR, it allows the core turn water into the steam and this steam goes to the steam turbine. The system is one loop and because of this situation, it is more expensive system due to increased radiation shielding. System has lower pressure than PWR design.

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ABWR


     It is the only Gen III reactor whose designer is GE Hitachi Nuclear Energy that operates and advanced version of BWR. Safety systems are improved and internal pumps are added and attached to bottom of core by this way recirculation pumps are neglected so crack probability on pipes are avoided.

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ESBWR (Economic Simplified Boiling Water Reactor)


     This reactor is Gen III+ reactor with passive systems. This reactor has no pumps so coolant circulates with natural circulation.

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Heavy Water Reactor (HWR)


     This reactor is pressurized nuclear power reactor that uses natural or slightly enriched uranium as fuel and heavy water as moderator. By this way, reactor uses rather expensive moderator than PWR or BWR but it can be operated without expensive uranium enrichment. Also reactor uses online refuelling so there is no need to shut down for fuel exchange. The most typical examples for this reactor are CANDU and ACR.


CANDU


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ACR


     This is the Gen III+ generation reactor that is development of existing CANDU technology. It uses low enriched uranium as fuel and light water as coolant. This reactor has enhanced safety systems than CANDU.

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Graphite Moderated Reactor


     This reactor design uses graphite as moderator. Graphite has better moderating ratio than light water and this concept based on using natural or slightly enriched uranium as fuel and turning this uranium to plutonium. Because of this reason, these reactors can be used for producing plutonium.


RBMK


     RBMK is Russian reactor with unique design. Graphite moderator is used with fuel tubes and water in coolant tube between these fuel tubes flows vertically. This reactor can be refuelled on-line like CANDU. Chernobyl is one of the RBMK reactor.

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GCR (Gas Cooled Reactors) and AGCR (Advanced Gas Cooled Reactors)


     This design uses slightly uranium dicarbide or uranium metal as a fuel which is natural or slightly enriched. Coolant is in gaseous form and mainly carbon dioxide but also nitrogen and helium is used for coolant. Main advantages of using these gases are they have low absorption and scattering crossections and they do not change phase. Magnox is developed by United Kingdom and UNGG is developed by France. AGCR uses slightly enriched uranium dioxide as fuel and stainless steel for cladding.

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Fast Breeder Reactor


     Breeder reactor technology consists on generating fissile material from fertile material. To achieve this, there is no moderator is used for reactor. This design is for high neutron economy with higher conversion ratio. The main design is liquid metal fast breeder reactor. Mainly sodium is used for coolant and three loops are used for this design.

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     There is also a ongoing work for new or advanced designs. 6 designs are chosen by several countries to research. These are called Gen-IV designs.


Gas Cooled Fast Breeder Reactor (GFR)


     This system is a high temperature, helium cooled, fast-spectrum reactor with closed cycle. It has high efficiency, it produces hydrogen and it can give generated heat for other industrial use. Wastes are minimized because of being closed cycle.

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Lead Cooled Fast Reactor (LFR)


     This design is fast spectrum with high temperatures and molten lead or lead-bismuth eutectic is used for coolant. Using liquid lead or lead-bismuth eutectic is advantages because they are non-reactive with good thermodynamic properties. It produces electricity, hydrogen and process heat. This reactor also prevents obtaining plutonium from core. The main difficult of this design is corrosion of lead in components[8].

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Molten Salt Fast Reactor (MSR)


     This is old concept but the research is ongoing. Fuel is dissolved in molten fluoride salt. It has extended fuel utilization and minimazes the wastes. This design choice is motivated by the study of parameters such as feedback coefficient, breeding ratio, graphite lifespan and 233U initial inventory.

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Sodium Cooled Fast Reactor (SFR)


     This design uses liquid sodium as the reactor coolant, allowing high power density with low coolant volume fraction and operation at low pressure[8]. System needs to be oxygen-free. If not,sodium reacts chemically with air and water. The SFR is an attractive energy source for nations that desire to make the best use of limited nuclear fuel resources and manage nuclear waste by closing the fuel cycle.

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Supercritical Water Cooled Reactor (SCWR)


     SCWRs are high temperature, high-pressure, light-water-cooled reactors that operate above the thermodynamic critical point of water (374°C, 22.1 MPa)[8]. These conditions increase thermal efficiency and prevent boiling of coolant.

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Very High Temperature Gas Reactor (VHTR)


     This is the most dedicated design of all 6 design. It is a graphite-moderated, helium-cooled reactor with thermal neutron spectrum. It can supply nuclear heat and electricity over a range of core outlet temperatures between 700 and 950°C, or more than 1 000°C in future. The VHTR has two typical reactor configurations, namely the pebble bed type and the prismatic block type.

 

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REFERENCES


• [1] European Nuclear Society

• [2] World Nuclear Association

• [3] Paul R. Josephson (2005). Red Atom: Russia’s Nuclear Power Program from Stalin to Today. University of Pittsburgh Pre. P. 2. ISNB 978-0-8229-7847-3.

• [4] Lamarsh, John, Introduction to Nuclear Engineering, (Reading, MA: Addison-Wesley publishing Co., 1983), 120-143.

• [5] Institute of Energy and Environmental Research

• [6] Nuclear Engineering International Handbook

• [7] NRC, Reactor Concept Manual, Pressurized Water Reactor

• [8] Gen-IV International Forum