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 electrical power ( Nuclear power) and for the power in some ships (Nuclear   marine propulsion). This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines. There are also          other  less common uses of nuclear        reactors, to be discussed later.

The first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago by a team led by Enrico Fermi in 1942. It achieved criticality on December 2, 1942. The reactor support structure was made of wood, which supported a pile of graphite blocks, embedded in which was natural Uranium-oxide 'pseudo spheres'  or 'briquettes'. Shortly after the discovery of fission, Hitler's Germany invaded Poland in 1939, starting World War II in Europe, and all such research became militarily classified. 

Reactor Generations

http://www.whitehouse.gov/

World Nuclear Power• 443 Nuclear Reactors 

in 30 Countries in Operation, January 2006

• Provided ~16% World Production of Energy in 2003

• 24 Nuclear Power Plants under Construction

http://www.insc.anl.gov

Classification by use

1.Electricity  Nuclear power plants

2.Nuclear propulsion in marine  and Various proposed forms of rocket propulsion

3.Other uses of heati.Desalination.ii.Heat for domestic and industrial heating.iii.Hydrogen production for use in a hydrogen economy.

4.Production type reactors for transmutation of elementsi.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.ii.Production of materials for nuclear      weapons such as weapons-grade plutonium

5.Providing a source of neutron radiation (for example with the pulsed Godiva device) and positron radiation (e.g. neutron activation analysis and potassium-argon dating).

6.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.

Clean air                       One of the greatest benefits of nuclear plants is that they have no smoke stacks! The big towers many people associate with nuclear plants are actually for cooling water used to make steam. (Some other kinds of plants have these towers, too.) The towers spread the water out so as much air as possible can reach it and cool it down. Most water is then recycled into the plant. The puffs you see coming out of a cooling tower are just clouds of water vapor.

Since early 1990s, Russia has been a major source of nuclear fuel to India. Due to dwindling domestic uranium reserves, electricity generation from nuclear power in India declined by 12.83% from 2006 to 2008.Following a waiver from the Nuclear Suppliers Group in September 2008 which allowed it to commence international nuclear trade, India has signed nuclear deals with several other countries including France, United States, United Kingdom,Canada, Namibia, Mongolia, Argentina,Kazakhstan.In February 2009, India also signed a $700 million deal with Russia for the supply of 2000 tons nuclear fuel.

Nuclear power is the fourth-largest source of electricity in India after thermal, hydro and renewable sources of electricity. As of 2010, India has 19 nuclear power plants in operation generating 4,560 MW while 4 other are under construction and are expected to generate an additional 2,720 MW.India is also involved in the development of fusion reactors through its participation in the ITER project.

Currently, nineteen nuclear power reactors produce 4,560.00 MW (2.9% of total installed base).

In a fission reactor, large fissile atomic nucleuses such as uranium-235 or plutonium-239 undergo nuclear fission when they absorb a neutron. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission products.  A kilogram of uranium-235 (U-235) converted via nuclear processes contains approximately three million times the energy of a kilogram of coal burned conventionally (7.2 × 1013 Joules per kilogram of uranium-235 versus 2.4 × 107 Joules per kilogram of coal).

To turn nuclear fission into electrical energy, the first step for nuclear power plant operators is to be able to control the energy given off by the enriched uranium and allow it to heat water into steam. Enriched uranium is typically formed into inch-long (2.5-cm-long) pellets,the pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are submerged in water inside a pressure vessel. The water acts as a coolant. For the reactor to work, the submerged bundles must be slightly supercritical. Left to its own devices, the uranium would eventually overheat and melt.

To prevent overheating, control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower the control rods. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the control rods are raised out of the uranium bundle (thus absorbing fewer neutrons). To create less heat, they are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.

The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spins a generator to produce powerIn some nuclear power plants, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures.

1.Boiling Water ReactorIn the boiling water reactor (BWR), the water which passes over the reactor core to act as moderator and coolant is also the steam source for the turbine. The disadvantage of this is that any fuel leak might make the water radioactive and that radioactivity would reach the turbine and the rest of the loop.A typical operating pressure for such reactors is about 70 atmospheres at which pressure the water boils at about 285 C. This operating temperature gives a efficiency of only 42% with a practical operating efficiency of around 32%, somewhat less than the Pressurized Water Reactor

2.Pressurized Water Reactors     In the pressurized water reactor (PWR), the water which passes 

over the reactor core to act as moderator and coolant does not flow to the turbine, but is contained in a pressurized primary loop. The primary loop water produces steam in the secondary loop which drives the turbine. The obvious advantage to this is that a fuel leak in the core would not pass any radioactive contaminants to the turbine and condenser.

      Another advantage is that the PWR can operate at higher pressure and temperature, about 160 atmospheres and about 315 C. This provides a higher efficiency than the boiling water reactor , but the reactor is more complicated and more costly to construct. Most of the U.S. reactors are pressurized water reactor.

This is a reactor design that is cooled by liquid metals like sodium, NaK, lead, lead-bismuth eutectic are included  totally unmoderated, and produces more fuel than it consumes. They are said to "breed" fuel, because they produce fissionable fuel during operation because of neutron capture. These reactors can function much like a PWR in terms of efficiency, and do not require much high pressure containment, as the liquid metal does not need to be kept at high pressure, even at very high temperatures. BN-350 and BN-600 in USSR and Superphénix in France were this type of reactors.

3.Liquid-Metal Fast-Breeder Reactor

Pressurized Heavy Water Reactor (PHWR) is a Canadian design (known as CANDU), these reactors are heavy-water-cooled and -moderated Pressurized-Water reactors. Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fueled with natural uranium and are thermal neutron reactor designs. PHWRs can be refueled while at full power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWRs have been built in Canada, Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea.

The CANDU Qinshan Nuclear Power Plant

4.Pressurized Heavy Water Reactor (PHWR)

These are generally graphite moderated and CO2 cooled. They can have a high thermal

efficiency compared with PWRs due to higher operating temperatures. There are a number of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. Older designs (i.e. Magnox stations) are either shut down or will be in the near future. However, the AGCRs have an anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design. Decommissioning costs can be high due to large volume of reactor core.

The Torness nuclear power station— an AGR

5.Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)

Current nuclear reactors use nuclear fission to generate power. In nuclear fission, you get energy from splitting one atom into two atoms. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy, radiation and radioactive wastes that last for long periods of time. In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It's the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.

There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium:Proton-proton chain - This sequence is the predominant fusion reaction scheme used by stars such as the sun.

 1.Two pairs of protons form to make two deuterium atoms.Each deuterium atom combines with a proton to form a helium-3 atom.

 2 .Two helium-3 atoms combine to form beryllium-6, which is unstable.Beryllium-6 decays into two helium-4 atoms. These reactions produce high energy particles (protons, electrons, neutrinos, positrons) and radiation (light, gamma rays).

         3.Deuterium-deuterium reactions - Two deuterium atoms combine to form a helium-3 atom and a neutron.   

         4.Deuterium-tritium reactions - One atom of deuterium and one atom of tritium combine to form a helium-4 atom and a neutron. Most of the energy released is in the form of the high-energy neutron.

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. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER project is currently leading the effort to commercialize fusion power.

Radioactive waste is a waste product containing radioactive  material. It is usually the product of a nuclear process such as nuclear fission,etc.

High level radioactive waste is generally material from the core of the nuclear reactor or nuclear weapon. This waste includes uranium, plutonium, and other highly radioactive elements made during fission. Most of the radioactive isotopes in high level waste emit large amounts of radiation and have extremely long half-lives (some longer than 100,000 years) creating long time periods before the waste will settle to safe levels of radioactivity. Some of the methods being under consideration for dealing with this high level waste include short term storage , long term storage, and transmutation.

Improper waste disposal