A nuclear power plant (NPP) is a thermal power station in which the heat source is one or more nuclear reactors.
Nuclear power plants are base load stations, which work best when the power output is constant (although boiling water reactors (BWRs) can come down to half power at night).
History
Electricity was generated for the first time ever by a nuclear reactor on December 20, 1951 at the EBR-I experimental station near Arco, Idaho in the United States. On June 27, 1954, the world's first nuclear power plant to generate electricity for a power grid started operations at Obninsk, USSR . The world's first commercial scale power station, Calder Hall in England opened in October 17, 1956.
Systems
The conversion to electrical energy takes place indirectly, as in conventional thermal power plants: The heat is produced by fission in a nuclear reactor (in a coal power plant it would correspond to the boiler) and given to a heat transfer fluid - usually water (for a standard type light water reactor). Directly or indirectly water vapor-steam is produced. The pressurized steam is then usually fed to a multi-stage steam turbine. Steam turbines in Western nuclear power plants are among the largest steam turbines ever. After the steam turbine has expanded and partially condensed the steam, the remaining vapor is condensed in a condenser. The condenser is a heat exchanger which is connected to secondary side such as a river or a cooling tower. The water then pumped back into the nuclear reactor and the cycle begins again. The water-steam cycle corresponds to the Rankine cycle.
Nuclear reactors
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. The most common use of nuclear reactors is for the generation of electric energy and for the propulsion of ships.
The nuclear reactor is the heart of the plant. In its central part, the reactor core's heat is generated by controlled nuclear fission. With this heat, a coolant is heated as it is pumped through the reactor and thereby removes the energy from the reactor. Heat from nuclear fission is used to raise steam, which runs through turbines, which in turn powers either ship's propellers or electrical generators.
Since nuclear fission creates radioactivity, the reactor core is surrounded by a protective shield. This containment absorbs radiation and prevents radioactive material from being released into the environment. In addition, many reactors are equipped with a dome of concrete to protect the reactor against external impacts.
In nuclear power plants, different types of reactors, nuclear fuels, and cooling circuits and moderators are sometimes used.
Steam turbine
The object of the steam turbine is to convert the heat contained in steam into rotational energy. To the turbine shaft, the shaft of the generator is coupled. In nuclear power plants are mostly Saturated steam turbine Application. The turbine has a high-pressure part, and usually two or three low pressure stages. Due to the high moisture vapor after the high pressure part of the steam is dried before entering the low pressure part of means of steam heating and high-speed deposition. At the end of the last blade row of the low pressure part of the steam has a moisture content of about 15%. The expansion into the wet steam region leads to a high working efficiency, but with the disadvantages associated with wet steam.
If the generator to hand over by a disturbance generated electrical energy can, he takes little analogy to mechanical energy. In response to this Load drop would be the Speed the turbine to increase the allowable operating limit by the threat of self-destruction too high Centrifugal. To avoid this process, are close to the turbine inlet valves in the steam pipe installed. If this quick-closing valves activated, they direct the steam bypassing the turbine directly into the Capacitor. In parallel, the reactor is shut down because of the full reactor power capacitor can absorb only a limited time.
The engine house with the steam turbine is usually structurally separated from the main reactor building. It is oriented to fly from the destruction of a turbine in operation as no debris in the direction of the reactor.
In the case of a pressurized water reactor, the steam turbine hermetically separated from the nuclear system. To detect a leak in the steam generator and thus the passage of radioactive water at an early stage is the outlet steam of the steam generator mounted an activity meter. In contrast, boiling water reactors and the steam turbine with radioactive water applied and therefore part of the control area of the nuclear power plant.
Generator
The generator converts kinetic energy supplied by the turbine into electrical energy. Low-pole AC synchronous generators of high rated power are used. The Olkiluoto nuclear power plant was the largest synchronous generator (as of 2010) made. It has a rated power 1992 MW.
Main coolant pump (PWR) and forced circulation pump (BWR)
The reactor coolant pump in the case of the DWR has the task to circulate the coolant between the reactor and steam generators. In western nuclear power plants, the nuclear reactor is fed with four redundant pumps (loops), each separated by Redundancy structurally accommodated in the reactor building. The design of the pump corresponds to a Centrifugal with a one-piece forged body. The throughput is up to 10,000 l / s at a pressure of 175 bar and a maximum allowable temperature of 350 ° C. The increase in pressure through the main coolant pump when DWR indicates pressure loss in the reactor, steam generators and piping system. Even after the failure of the main coolant pumps (RESA is the result of) the circulation and thus the heat dissipation is by so-called Natural circulation guaranteed.
In the case of boiling water reactor are the reactor pressure vessel forced circulation pumps to avoid core wings attached to their interpretation is approximately equal to those in a PWR. You are responsible for the safety of the plant is not absolutely necessary.
Besides these main coolant pump of a nuclear power plant has usually still have several emergency supplies at different pressure levels, the case of disturbances (see Design basis accident) Maintain the cooling of the reactor core.
Safety valves
The pressure in the reactor pressure vessel at an incident, to limit upward, two independent safety valves are available. The pressure relief prevents bursting of pipes or reactor. The valves are in their capacity designed so that they can derive all of the supplied flow rates with little increase in pressure. In the case of the BWR, the steam is directed into the condensate chamber and condenses there. The chambers are on heat exchanger connected to the intermediate cooling circuit.
Should not close the safety valves, are very close again safety shut any, should, if necessary, prevent coolant accident. The non-closing of a safety valve led to a serious accident at Three Mile Island.
Feedwater pump
The Feedwater pump have the task of the water from the feedwater tank to the vapor pressure in the reactor and the steam generator to bring and promote a water with approximately 2200 kg / s. The power required amounts here to about 20 MW per pump. About the feed water system, the water level in the steam generator and nuclear reactor is controlled. Emergency power supply
The Emergency power supply a nuclear power plant is several times redundant built up by diesel generators and battery buffers. The battery backup provides uninterrupted coupling of the diesel units in the network secure. If necessary, the emergency power supply allows the safe descent down the nuclear reactor. Less important auxiliary systems such as, for example, heat tracing of pipelines are not receiving it. The majority of the required power is used to supply the feed pumps and Notspeisepumpen order to shut down the nuclear reactor, the Decay heat even with a Failure of the power system, A Blackout permanently dissipate.
People in a nuclear power plant
Nuclear power plants typically employ just under a thousand people per reactor (including security guards and engineers associated with the plant but possibly working elsewhere).
- Nuclear engineers
- Reactor operators
- Health physicists
- Emergency response team personnel
- Nuclear Regulatory Commission Resident Inspectors
In the United States and Canada, workers except for management, professional (such as engineers) and security personnel are likely to be members of either the International Brotherhood of Electrical Workers (IBEW) or the Utility Workers Union of America (UWUA).
Economics
The economics of new nuclear power plants is a controversial subject, since there are diverging views on this topic, and multi-billion dollar investments ride on the choice of an energy source. Nuclear power plants typically have high capital costs for building the plant, but low direct fuel costs (with much of the costs of fuel extraction, processing, use and long term storage externalized). Therefore, comparison with other power generation methods is strongly dependent on assumptions about construction timescales and capital financing for nuclear plants. Cost estimates also need to take into account plant decommissioning and nuclear waste storage costs. On the other hand measures to mitigate global warming, such as a carbon tax or carbon emissions trading, may favor the economics of nuclear power.
Failure modes of nuclear power plants
There are concerns that a combination of human and mechanical error at a nuclear facility could result in significant harm to people and the environment:
"Operating nuclear reactors contain large amounts of radioactive fission products which, if dispersed, can pose a direct radiation hazard, contaminate soil and vegetation, and be ingested by humans and animals. Human exposure at high enough levels can cause both short-term illness and death and longer-term death by cancer and other diseases."
Nuclear reactors can fail in a variety of ways. Should the instability of the nuclear material generate unexpected behavior, it may result in an uncontrolled power excursion. Normally, the cooling system in a reactor is designed to be able to handle the excess heat this causes; however, should the reactor also experience a loss-of-coolant accident, then the fuel may melt or cause the vessel it is contained in to overheat and melt. This event is called a nuclear meltdown.
After shutting down, for some time the reactor still needs external energy to power its cooling systems. Normally this energy is provided by the power grid to that the plant is connected, or by emergency diesel generators. Failure to provide power for the cooling systems, as happened in Fukushima I, can cause serious incidents.
Because the heat generated can be tremendous, immense pressure can build up in the reactor vessel, resulting in a steam explosion, which happened at Chernobyl. However, the reactor design used at Chernobyl was unique in many ways. For example, it had a large positive void coefficient, meaning a cooling failure caused reactor power to rapidly escalate. Typical reactor designs have negative void coefficients, a passively safe design. However this design may not protect from the meltdown if the cooling system is damaged.
More importantly though, the Chernobyl plant lacked a containment structure. Western reactors have this structure, which acts to contain radiation in the event of a failure. Containment structures are, by design, some of the strongest structures built by mankind. However during the serious incidents engineers may need to vent the containment intentionally as otherwise it might crack due to excess pressure.
All currently operational nuclear power plants lack the containment structure for the spent fuel while it is being cooled in the spent fuel pool. Therefore any accident (eg. loss of coolant) involving the spent fuel and resulting in fuel meltdown or recriticality will not be contained.
Intentional cause of such failures may be the result of nuclear terrorism.
Vulnerability of nuclear plants to attack
Nuclear power plants are generally (although not always) considered "hard" targets. In the U.S., plants are surrounded by a double row of tall fences which are electronically monitored. The plant grounds are patrolled by a sizeable force of armed guards. The NRC's "Design Basis Threat" criteria for plants is a secret, and so what size of attacking force the plants are able to protect against is unknown. However, to scram (make an emergency shutdown) a plant takes fewer than 5 seconds while unimpeded restart takes hours, severely hampering a terrorist force in a goal to release radioactivity.
Attack from the air is an issue that has been highlighted since the September 11 attacks in the U.S. However, it was in 1972 when three hijackers took control of a domestic passenger flight along the east coast of the U.S. and threatened to crash the plane into a U.S. nuclear weapons plant in Oak Ridge, Tennessee. The plane got as close as 8,000 feet above the site before the hijackers’ demands were met.
The most important barrier against the release of radioactivity in the event of an aircraft strike on a nuclear power plant is the containment building and its missile shield. Current NRC Chairman Dale Klein has said "Nuclear power plants are inherently robust structures that our studies show provide adequate protection in a hypothetical attack by an airplane. The NRC has also taken actions that require nuclear power plant operators to be able to manage large fires or explosions—no matter what has caused them."
In addition, supporters point to large studies carried out by the U.S. Electric Power Research Institute that tested the robustness of both reactor and waste fuel storage and found that they should be able to sustain a terrorist attack comparable to the September 11 terrorist attacks in the U.S. Spent fuel is usually housed inside the plant's "protected zone" or a spent nuclear fuel shipping cask; stealing it for use in a "dirty bomb" is extremely difficult. Exposure to the intense radiation would almost certainly quickly incapacitate or kill anyone who attempts to do so.
In September 2010, analysis of the Stuxnet computer worm suggested that it was designed to sabotage a nuclear power plant. Such a cyber attack would bypass the physical safeguards in place and so the exploit demonstrates an important new vulnerability.
Plant location
In many countries, plants are often located on the coast, in order to provide a ready source of cooling water for the essential service water system. As a consequence the design needs to take the risk of flooding and tsunamis into account. Failure to calculate the risk of flooding correctly lead to a Level 2 event on the International Nuclear Event Scale during the 1999 Blayais Nuclear Power Plant flood, while flooding caused by the 2011 Tōhoku earthquake and tsunami lead to the Fukushima I nuclear accidents.
The design of plants located in seismically active zones also requires the risk of earthquakes and tsunamis to be taken into account. Japan, India, China and the USA are among the countries to have plants in earthquake-prone regions. Damage caused to Japan's Kashiwazaki-Kariwa Nuclear Power Plant during the 2007 Chūetsu offshore earthquake underlined concerns expressed by experts in Japan prior to the Fukushima accidents, who have warned of a genpatsu-shinsai (domino-effect nuclear power plant earthquake disaster).
Nuclear safety systems
The three primary objectives of nuclear safety systems as defined by the Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition, and prevent the release of radioactive material during events and accidents. These objectives are accomplished using a variety of equipment, which is part of different systems, of which each performs specific functions.
Reprocessing
Reprocessing of spent nuclear fuel can extend the usefulness of mined uranium. However, it is generally conceded that reprocessed fuel is more expensive than fuel from mined uranium (providing that adequate disposal space is available). Such processing of civilian fuel has long been employed in Europe (at the COGEMA La Hague site) and briefly at the West Valley Reprocessing Plant in the U.S.
Reprocessing of spent fuel to obtain plutonium for nuclear weapons has been done in a number of countries: however these programs are typically separate from civilian activities, and usually classified.
Use of breeder reactors combined with reprocessing could extend the usefulness of mined uranium by more than 60 times. However, breeder reactors, not yet well developed, are currently significantly more difficult to operate.
Accident indemnification
The Vienna Convention on Civil Liability for Nuclear Damage puts in place an international framework for nuclear liability . However states with a majority of the world's nuclear power plants, including the U.S., Russia, China and Japan, are not party to international nuclear liability conventions.
In the U.S., insurance for nuclear or radiological incidents is covered (for facilities licensed through 2025) by the Price-Anderson Nuclear Industries Indemnity Act.
Under the Energy policy of the United Kingdom through its Nuclear Installations Act of 1965, liability is governed for nuclear damage for which a UK nuclear licensee is responsible. The Act requires compensation to be paid for damage up to a limit of £150 million by the liable operator for ten years after the incident. Between ten and thirty years afterwards, the Government meets this obligation. The Government is also liable for additional limited cross-border liability (about £300 million) under international conventions (Paris Convention on Third Party Liability in the Field of Nuclear Energy and Brussels Convention supplementary to the Paris Convention).
Decommissioning
Nuclear decommissioning is the dismantling of a nuclear power plant and decontamination of the site to a state no longer requiring protection from radiation for the general public. The main difference from the dismantling of other power plants is the presence of radioactive material that requires special precautions.
Generally speaking, nuclear plants were designed for a life of about 30 years. Newer plants are designed for a 40 to 60-year operating life.
Decommissioning involves many administrative and technical actions. It includes all clean-up of radioactivity and progressive demolition of the plant. Once a facility is decommissioned, there should no longer be any danger of a radioactive accident or to any persons visiting it. After a facility has been completely decommissioned it is released from regulatory control, and the licensee of the plant no longer has responsibility for its nuclear safety.
Future power plants
The 1600 MWe European Pressurized Reactor reactor is being built in Olkiluoto, Finland. A joint effort of French AREVA and German Siemens AG, it will be the largest reactor in the world. In December 2006 construction was about 18 months behind schedule so completion was expected 2010-2011.
As of March, 2007, there are seven nuclear power plants under construction in India, and five in China.
Russia has begun building the world’s first floating nuclear power plant. The £100 million vessel, the Lomonosov, is the first of seven plants that Moscow says will bring vital energy resources to remote Russian regions.(source:wikipedia.org)
