A nuclear reactor is a system that controls nuclear chain reactions. It is an arrangement in which fission chain reaction can be carried on in a perfectly controlled manner so that the energy released during the reaction may be used for peaceful purposes. The most common use of nuclear reactor is the production of electricity in nuclear power plants. It is also used for producing radio nuclides, in industry and medicines.
A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. The principles for using nuclear power to produce electricity are the same for most types of reactor. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity.
When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons, collectively known as fission products. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is called nuclear chain reaction.
The reaction can be controlled by using neutron poisons, which absorb excess neutrons, and neutron moderators which reduces the velocity of fast neutrons, hence turning them into thermal neutrons, which are more likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reactor.
1. The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms.
2. Some of the gamma rays produced during fission are absorbed by the reactor, their energy being converted to heat.
3. Heat produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shutdown.
A nuclear reactor coolant i.e. usually water but sometimes a gas or a liquid metal or molten salt is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for e.g the boiling water reactor.
The power output of the reactor is controlled by controlling how many neutrons are able to create more fissions. Control rods that are made of a nuclear poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.
In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.
In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to insert large amounts of poison into the reactor to shut the fission reaction down if unsafe conditions are detected.
1. The core
2. The coolant
3. The turbine
4. The containment
5. Cooling towers
1. The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium U-235 control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
2. The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, or helium.
3. The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
4. The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete.
5. Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapour.
TYPES OF NUCLEAR REACTOR
1. Pressurized water reactor
2. Boiling water reactor
3. High temperature gas cooled reactor
4. Liquid fluoride cooled thorium reactor
5. Sodium cooled fast reactor
1. Pressurized water reactor
The most common type of reactor is the pressurized water reactor that uses regular old water as a coolant. The primary cooling water is kept at very high pressure so it does not boil. It goes through a heat exchanger, transferring heat to a secondary coolant loop, which then spins the turbine. These use oxide fuel pellets stacked in zirconium tubes. They could possibly burn thorium or plutonium fuel as well.
1. Strong negative void coefficient i.e. reactor cools down if water starts bubbling.
2. Secondary loop keeps radioactive stuff away from turbines, making maintenance easy.
Pressurized coolant escapes rapidly if a pipe breaks, necessitating lots of back-up cooling systems.
2. Boiling water reactor
It is similar to the pressurized water reactor. They only have one coolant loop. The hot nuclear fuel boils water as it goes out the top of the reactor, where the steam heads over to the turbine to spin it.
1. Simple plumbing reduces costs.
2. Power levels can be increased by speeding up the pumps , giving less boiled water and more moderation.
1. Primary coolant is in direct contact with turbines, so if a fuel rod had a leak, radioactive material could be placed on the turbine. This complicates maintenance as the staff must be dressed for radioactive environments.
2. It is susceptible to uranium shortage.
3. High Temperature Gas Cooled Reactor
They use little pellets of fuel backed into either hexagonal compacts or into larger pebbles. Gas such as helium or carbon dioxide is passed through the reactor rapidly to cool it.
1. Can operate at very high temperatures, leading to great thermal efficiency and the ability to create process heat for things like oil refineries, water desalination plants, hydrogen fuel cell production, and much more.
2. Each little pebble of fuel has its own containment structure, adding yet another barrier between radioactive material and the environment.
1. High temperature has a bad side too. Materials that can stay structurally sound in high temperatures and with many neutrons flying through them are hard to come by.
2. If the gas stops flowing, the reactor heats up very quickly. Backup cooling systems are necessary.
4. Liquid fluoride cooled fast reactor
They have gotten a lot of attention lately in the media. They are unique so far in that they use molten fuel. So there is no worry of meltdown because they are already melted.
1. It can constantly breed new fuel, eliminating concerns over energy resources
can be maintained online with chemical fission product removal, eliminating the need to shut down during refuelling.
2. No cladding means less neutron-absorbing material in the core, which leads to better neutron efficiency and thus higher fuel utilization.
1. Radioactive gaseous fission products are everywhere, ready to escape at the first breach of containment. This violates the common practice of defence-in-depth where there are multiple levels of protection. All liquid fuel reactors have this problem.
2. The presence of an online reprocessing facility with incoming pre-melted fuel is a concern. The operator could easily divert Pa-233 to provide a stream of nearly pure weapons-grade U-233. Thus, anyone who operates this kind of reactor will have easy access to bomb material.
5. Sodium cooled fast reactor
The first electricity-producing nuclear reactor in the world was SFR (the EBR-1 in Arco, Idaho). As the name implies, these reactors are cooled by liquid sodium metal. Sodium is heavier than hydrogen, a fact that leads to the neutrons moving around at higher speeds. These can use metal or oxide fuel, and burn anything you throw at them.
1. It can breed its own fuel, effectively eliminating any concerns about uranium shortages.
2. It can burn its own waste.
3. Metallic fuel and excellent thermal properties of sodium allow for passively safe operation, the reactor will shut itself down without any backup-systems working, only relying on physics.
1. Sodium coolant is explosively reactive with air, water. Thus, leaks in the pipes results in sodium fires. These can be engineered around but are a major setback for these nice reactors.
2. To fully burn waste, these require reprocessing facilities which can also be used for nuclear proliferation.
3. Positive void coefficients are inherent to all fast reactors. This is a safety concern.
Classification by type of nuclear reaction
1. Nuclear fission: All commercial power reactors are based on nuclear fission. They generally use uranium and its product plutonium as nuclear fuel, though a thorium fuel cycle is also possible
2. Thermal reactors: They use slowed or thermal neutrons. Almost all current reactors are of this type. These contain neutron moderator materials that slow neutrons until their neutron temperature is thermalized, that is, until their kinetic energy approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher cross section of fissioning the fissile nuclei uranium-235, plutonium-239, and plutonium-241, and a relatively lower probability of neutron capture by uranium-238 compared to the faster neutrons that originally result from fission, allowing use of low-enriched uranium or even natural uranium fuel. The moderator is often also the coolant, usually water under high pressure to increase the boiling point. These are surrounded by reactor vessel, instrumentation to monitor and control the reactor, radiation shielding, and a containment building.
3. Fast neutron reactors: They use fast neutrons to cause fission in their fuel. They do not have a neutron moderator, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched in fissile material due to the relatively lower probability of fission versus capture by U-238. Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing Nuclear fusion. Fusion power is an experimental technology, generally with hydrogen as fuel.
4. Radioactive decay: Examples include radioisotope thermoelectric generators as well as other types of atomic batteries, which generate heat and power by exploiting passive radioactive decay.
Classification by moderator material
1. Graphite moderated reactors.
2. Water moderated reactors.
a. Heavy water reactors.
b. Light water moderated reactors: Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperature, if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised than light water reactors. Due to the extra thermalization, these types can use natural uranium/unenriched fuel.
3. Light element moderated reactors: These reactors are moderated by lithium or beryllium.
a. Molten salt reactors are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.
b. Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator.
4. Organically moderated reactors use biphenyl and terphenyl as moderator and coolant.
Classification by generation
1. Generation I reactor.
2. Generation II reactor.
3. Generation III reactor.
4. Generation IV reactor.
The Gen IV was dubbed by the United States Department of Energy for developing new plant types in 2000. In 2003, the French Commissariat a l'Energie Atomique was the first to refer to Gen II types in Nucleonic Week. First mentioning of Gen III was also in 2000 in conjunction with the launch of the Generation IV International Forum plans.
Classification by phase of fuel
1. Solid fueled.
2. Fluid fueled.
a. Aqueous homogeneous reactor.
b. Molten salt reactor.
3. Gas fueled.
Classification by use
a. Nuclear power plants.
a. Nuclear marine propulsion.
b. Various proposed forms of rocket propulsion.
3. Other uses of heat
b. Heat for domestic and industrial heating
c. Hydrogen production for use in a hydrogen economy
4. Production reactors for transmutation of elements
a. Breeder reactors are capable of producing more fissile materials than they consume during the fission chain reaction which allows an operational fast reactor to generate more fissile material than it consumes. Thus, a breeder reactor, once running, can be re-fueled with natural or even depleted uranium.
b. Creating various radioactive isotopes, such as americium for use in smoke detectors, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment.
c. Production of materials for nuclear weapons such as weapons-grade plutonium.
5. Research reactor: Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.
Generation IV reactors
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. These designs are generally not expected to be available for commercial construction before 2030. Current reactors in operation around the world are generally considered second- or third-generation systems, with the first-generation systems having been retired some time ago. Research into these reactor types was officially started by the Generation IV International Forum based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants.
1. Gas cooled fast reactor
2. Lead cooled fast reactor
3. Molten salt reactor
4. Sodium-cooled fast reactor
5. Supercritical water reactor
6. Very high temperature reactor.
Generation V+ reactors
Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present. Though such reactors could be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety.
1. Liquid Core reactor: A closed loop liquid core nuclear reactor, where the fissile material is molten uranium cooled by a working gas pumped in through holes in the base of the containment vessel.
2. Gas core reactor: A closed loop version of the nuclear light bulb rocket, where the fissile material is gaseous uranium-hexafluoride contained in a fused silica vessel. A working gas would flow around this vessel and absorb the UV light produced by the reaction. In theory, using UF6 as a working fuel directly would mean lower processing costs, and very small reactors. In practice, running a reactor at such high power densities would probably produce unmanageable neutron flux.
3. Gas core EM reactor: As in the Gas Core reactor, but with photovoltaic arrays converting the UV light directly to electricity.
4. Fission fragment reactor.
Controlled nuclear fusion could in principle be used in fusion power plants to produce power without the complexities of handling actinides, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as yet none has produced more thermal energy than electrical energy consumed.