A device for releasing nuclear energy under continuous controlled conditions is a nuclear reactor. In some wasy a common fission type nuclear reactor is similar to a coal burning furnace in both the cases the principal output energy heat is converted to a more convenient form of energy electricity. The big difference is in the source of heat inside the thick walled usually dome toped building containing the nuclear reactor. This is where the nuclear fuel is stored and burnt. The common nuclear fuel is uranium 235 rods which are clad in aluminum. When U-235 atoms split some fission fragments remains and some matter and energy are mainly heat. This happens in nature too at a slow rate an atom here and an atom there over thousands of thousand of year. But when the atoms are close enough and of sufficient quantity their released neutrons accelerate the fission.
- Moderate accelerating useful amounts of heat for running the steam turbine generated.
- Greater acceleration: more heat is generated than used the excess heat would cause a rise in temperature to meltdown. The highly corrosive, dangerously radioacster never to be recaptured.
- Still greater acceleration a nuclear bomb. The thermal energy generated in the reactor is constantly removed from it by circulating in it liquid which itself is known as a coolant. In a boiling water reactor the reactor the water boils into steam which is piped to the steam turbine. In a pressurized water reactor water is kept under pressure to prevent it from boiling whilst allowing it to reach a very high temperature.
- Its heat is transferred via coiled pipes to other water which turns to steam for the turbine and so on. In gas- cooled reactor the heat of fission is transferred a gas cooled reactor the heat of fission is transferred to a gas such as helium or carbon dioxide. The heated gas is piped through a coil surrounded by water. The water turns to steam for the turbine. Indian reactor use heavy water as they use natural uranium as fuel.
(A) CLASSIFICATION BY MODERATOR MATERIAL
- Water Moderated Reactor:
- Heavy water reactor: They use heavy water (deuterium oxide) as the neutron moderator Light water reactor: 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.
- Organically Moderated Reactors (OMR) Use Biphenyl and Terphenyl as Moderator and Coolant
- Graphite Reactor: The first nuclear reactor, Chicago Pile-1, a graphite-moderated device that produced a microscopic amount of heat, was constructed by a team led by Enrico Fermi in 1942. The construction and testing of this reactor (an “atomic pile”) was part of the Manhattan Project. This work led to the construction of the X-10 Graphite Reactor at Oak Ridge National Laboratory, which was the first nuclear reactor designed and built for continuous operation, and began operation in 1943.The nuclear reactor in the Chernobyl disaster was an RBMK graphite-moderated reactor
(4) Light Element Moderated Reactors: These reactors are moderated by lithium or beryllium.
- Molten salt reactors (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.
- Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator
(B) CLASSIFICATION BY COOLANT
Water Cooled Reactor
Pressurized water reactor (PWR)
- A primary characteristic of PWRs is a pressurizer, a specialized pressure vessel. Most commercial PWRs and naval reactors use pressurizers. During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. During normal operation, the pressurizer is connected to the primary reactor pressure vessel (RPV) and the pressurizer “bubble” provides an expansion space for changes in water volume in the reactor. This arrangement also provides a means of pressure control for the reactor by increasing or decreasing the steam pressure in the pressurizer using the pressurizer heaters.
- Pressurised channels. Channel-type reactors can be refueled under load.
Pressurized Heavy Water Reactor (PHWR)
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 fuelled with natural uranium and are thermal neutron reactor designs. PHWRs can be refuelled while at full power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWR’s have been built in Canada, Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea. India also operates a number of PHWR’s, often termed ‘CANDU-derivatives’, built after the Government of Canada halted nuclear dealings with India following the 1974 Smiling Buddha nuclear weapon test
Boiling Water Reactor (BWR)
- BWRs are characterized by boiling water around the fuel rods in the lower portion of primary reactor pressure vessel. During normal operation, pressure control is accomplished by controlling the amount of steam flowing from the reactor pressure vessel to the turbine.
Liquid Metal Cooled Reactor
Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid metal coolants have included sodium, NaK, lead, lead-bismuth eutectic, and in early reactors, mercury.
- Sodium-cooled fast reactor
- Lead-cooled fast reactor
Gas Cooled Reactors are cooled by a circulating inert gas, usually helium. Nitrogen and carbon dioxide have also been used. Utilization of the heat varies, depending on the reactor. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine.
Molten Salt Reactors (MSRs) are cooled by circulating a molten salt, typically a eutectic mixture of fluoride salts, such as LiF and BeF2. In a typical MSR, the coolant is also used a matrix in which the fissile material is dissolved
(C) CLASSIFICATION BY USE
- Nuclear power plants
- Nuclear marine propulsion
- Various proposed forms of rocket propulsion
Other Uses of Heat
- Heat for domestic and industrial heating
- Hydrogen production for use in a hydrogen economy
- Production reactors for transmutation of elements
- Breeder reactors. Fast breeder reactors are capable of enriching Uranium during the fission chain reaction (by converting fertile U-238 to Pu-239) which allows an operational fast reactor to generate more fissile material than it consumes. Thus, a breeder reactor, once running, can be re-fuelled with natural or even depleted uranium.
- 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.
- Production of materials for nuclear weapons such as weapons-grade plutonium
Thermal Reactor: It is one which makes use of a slow moving neutron to bring about a nuclear chain reaction. For this purpose it necessarily uses a moderator. It uses Plutonium or enriched uranium as a fuel. e.g. Two reactor established at Tarapur with U.S. assistance are thermal reactor. Two reactor which are being established in Kondakulam (T.N) with Russian assistance are thermal reactor.
Fast Reactor: It is a reactor which makes use of fast moving neutrons to bring about nuclear chain reaction. It uses very little or no moderator If D20 is used in such a reactor it is used primary as a coolant. It makes use of Plutonium, Uranium-238( Natural) as a fuel.
All the indigenously established nuclear reactor of India are at-
- Rawat Bhatta in Rajasthan
- Kalpakkan in Tamil Nadu
- Narora in U.P
- Kakrapara in Gujarat
- Kaiga in Karnataka
Breeder Reactor: It is one that produces more fissile material than it burns. Plutonium can also be made to sustain a chain reaction just like U-235. Thus non-fissionable U-238 that predominates in natural uranium ore, by being made to yield plutonium can work a breeder reactor. The plutonium produced in such a reactor by conversion of U- 238 is more than the U-235 is more than the U-235 consumed in the process it is as if fuel is breeding in these reactors neutrons are not slowed down thus they are fast breeder. In these reactors neutrons are not slowed down thus they are fast breeder reactor But they need special heat removal system such as liquid sodium or steam coolants in view of their higher power density.
Fast Breeder Reactor (FBR): It is a reactor that makes use of fast moving neutrons in order to breed more nuclear fuel. India is the only country in the world that is actively engaged in the development of Fast Breeder Reactor. India commissioned the world’s first Fast Breeder Test Reactor (FBTR) at Kalpakkam in 1985 with the successful operation of FBTR. The Indira Gandhi Centre of Atomic Research (IGCAR) is at present engaged in establishing the world’s first Proto Type FBR (PFBR) with a capacity of 500Mw at Kalpakkam. PFBR is expected to be commissioned by 2010. After its successful operation the FBR will be built in India.
India’s Fast Breeder Reactors
- India’s first 40 MWt Fast Breeder Test Reactor (FBTR) attained criticality on 18th October 1985.
- India becomes the sixth nation having the technology to built and operate a FBTR besides USA, UK, France, Japan and the then USSR.
- The unique features of Indian FBTR are :
- Indigenously developed U-Pu carbide fuel rich in Pu
- Design, development and fabrication of all machineries, peripheral units and materials are by the Indian Scientists in close co ordination with industry.
Status: Initial operational problems sorted out and the reactor operates smoothly at a steady power level of 10.5 Mwt- maximum possible power output owing to its small core.
- Future plans based on the Design, setting up and operation of FBTR has provided rich experience and immense information with liquid metal cooled Fast Breeder Reactor Technology and also confidence to embark upon the design of a 500 MWe prototype fast breeder reactor [PFBR], planned to be constructed at Kalpakkam.
- 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)
- 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.
APSARA, a 1MW ‘swimming pool’ type indigenously built research reactor attained criticality in 1956, heralding the nuclear age in Asia. ZERLINA, a zero energy research reactor was built indigenously in 1961. CIRUS, a 40 MWt reactor was commissioned in 1960 for engineering ‘ experimental work with facilities for material testing and radioisotope production. With tit commissioning of PURNIMA, Asia’s first ‘Fast’ reactor, in 1972, India achieved an important milestone in its ‘fast reactor’ programme. DHRUVA an indigenous 100 MWe reactor, was commissioned in 1985 for research in nuclear sciences and technology. The commissioning of KAMINI, a 30 KW reactor, in 1996 marks an important landmark in India’s long cherished endeavour at mastering thorium-U-233 fuel cycle. This reactor will be mainly used to study the highly radioactive fuel elements which are discharged from FBTR at Kalpakkam. This will help in development of high performance plutonium fuel elements for the! prototype FBR to be built in the next century. KAMINI will also serve as neutron source for variety of research applications.
BARC has developed comprehensive technology for industrial operations in fuel; reprocessing and waste management. Reprocessing plants are operational in Trombay and Tarapur. A comprehensive waste management technology for; handling and safe disposal of all types of waste generated in nuclear industries has been perfected by the centre. Advanced mixed carbide fuel for FBTR was fabricated by BARC from indigenously separated plutonium. Two mixed uranium-1 plutonium oxide (MOX) fuel bundles fabricated by BARC as an alternative fuel for Tarapur have been loaded into TAPS for study.
BARC has initiated research in cold fusion. Research is going on for pulse power and particle beam technology and is continuing with a view to developing intense sources of particles and radiation for plasma fusion research. Indigenously built tokamak, ‘Aditya’, set up in 1989 at the Institute of Plasma Research in Gandhinagar, can generate plasma at 5 million °C.