Nuclear Reactors

Nuclear Reactors

INDEX

  1. Nuclear Reactor 
  2. Heat Generation
  3. Cooling
  4. Reactivity Control
  5. Xenon Poisoning
  6. Electrical Power Generation
  7. Type of Nuclear Reactors
  8. Nuclear Fuel Cycle
  9. Refueling 
  10. Nuclear Safety
  11. Nuclear Accidents
  12.  Advantages and Disadvantages 


01. Nuclear Reactor  

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in marine nuclear propulsion. 
The heat from nuclear fission is passed to a working fluid (water or gas), which runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear-generated steam in principle can be used for industrial process heat or for district heating


02. Heat Generation

A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of it burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal).


03. Cooling

A nuclear reactor coolant(A substance circulated through a nuclear reactor to remove or transfer heat.) – usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) 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.



04. Reactivity Control

The rate of fission reactions within a reactor core can be adjusted by controlling the number of neutrons that are able to induce further fission events. 

The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via the movement of the control rods.(control the rate of fission of the nuclear fuel) Control rods are made of neutron poisons(a substance with a large capacity for absorbing neutrons in the vicinity of the reactor core) and absorb neutrons. 
This action results in fewer neutrons available to cause fission and reduces the reactor's power output.

  • Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power output.
  • In some reactors, the coolant also acts as a neutron moderator. (A moderator is a substance that slows neutrons down and increases the power output of the reactor by causing the fast neutrons that are released from fission to lose energy and increase the chances of hitting another atom). These thermal neutrons are more likely than fast neutrons to cause fission. 
  • In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. 
  • Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shutdown. These systems insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.
  • Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. 

05. Xenon Poisoning

  • Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. 
  • The common fission product Xenon-135 produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. 
  • Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced.
  • Failure to properly follow a proper procedure for with dealing xenon poisoning was a key step in the Chernobyl disaster.

06. Electrical Power Generation

The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that turns an alternator and generates electricity.

All commercial power reactors are based on nuclear fission. They generally use uranium and its product plutonium as nuclear fuel


07. Types of Nuclear Reactors 

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. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission chain reaction:

Thermal-neutron reactors
Fast-neutron reactors
By moderator material
Graphite-moderated reactors
Water moderated reactors

  • Heavy-water reactors
  • Light-water-moderated reactors
Light-element-moderated reactors
  • Molten-salt reactors
  • Liquid metal cooled reactors
  • Organically moderated reactors

By coolant
  • Pressurized water reactor
  • Boiling water reactor
  • Supercritical water reactor

By generation (of reactor) By phase of fuel By shape of the core By use

09. Nuclear Fuel Cycle

Thermal reactors generally depend on refined and enriched uranium. The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed, and disposed of is known as the nuclear fuel cycle. Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. 


10. Refueling 

  • At the end of the operating cycle, the fuel in some of the assemblies is "spent", having spent four to six years in the reactor producing power.
  • This spent fuel is discharged and replaced with new (fresh) fuel assemblies. Though considered "spent," these fuel assemblies contain a large quantity of fuel. In practice, it is economics that determines the lifetime of nuclear fuel in a reactor.
  •  Long before all possible fission has taken place, the reactor is unable to maintain 100%, full output power, and therefore, income for the utility lowers as plant output power lowers. 
  • The fraction of the reactor's fuel core replaced during refueling is typically one-third but depends on how long the plant operates between refueling.
  • This means that one refueling, replacing only one-third of the fuel, can keep a nuclear reactor at full power for nearly two years. 
  • The disposition and storage of this spent fuel is one of the most challenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years. After being discharged from the reactor, spent nuclear fuel is transferred to the on-site spent fuel pool. 
  • The spent fuel pool is a large pool of water that provides cooling and shielding of the spent nuclear fuel.
  •  Once the energy has decayed somewhat (approximately five years), the fuel can be transferred from the fuel pool to dry-shielded casks, that can be safely stored for thousands of years.

11. Nuclear Safety

Nuclear safety covers the actions taken to prevent nuclear and radiation accidents and incidents or to limit their consequences. The nuclear power industry has improved the safety and performance of reactors, and has proposed new, safer (but generally untested) reactor designs but there is no guarantee that the reactors will be designed, built and operated correctly.


12. Nuclear Accidents

Serious, though rare, nuclear and radiation accidents have occurred. These include the 

  • Windscale fire (October 1957)
  • SL-1 accident (1961) 
  • Three Mile Island accident (1979)
  • Chernobyl disaster (April 1986)
  • Fukushima Daiichi nuclear disaster (March 2011)

Nuclear-powered submarine mishaps include the 
  • K-19 reactor accident (1961), 
  • K-27 reactor accident (1968),
  • K-431 reactor accident (1985).

Nuclear reactors have been launched into Earth orbit at least 34 times. A number of incidents connected with the unmanned nuclear-reactor-powered Soviet RORSAT especially Kosmos 954 radar satellite which resulted in nuclear fuel reentering the Earth's atmosphere from orbit and being dispersed in northern Canada (January 1978).

13. Advantages and Disadvantages of Nuclear Reactors 

Advantages

  • Low cost
  • Reliable
  • Zero carbon emissions
  • Promising future
  • High energy density
Disadvantages
  • Environmental impact
  • Water intensive
  • Risk of accidents
  • Radioactive waste
  • Non-renewable

 

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