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Nuclear energy

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Atomic structure – Atoms are fundamental subunits of matter. Matter is anything that takes up space and has mass. Air, water, trees, cement, and gold are examples of matter.

Figure 4.2 diagram of oxygen

All atoms have a central region know as the nucleus, which is composed of two kinds of relatively heavy particles: positively charged particles called protons and uncharged particles called neutrons. Surrounding the nucleus is a cloud of relatively light weight, fast moving, negatively charged particles called electrons. The atoms of each element differ in the number of protons, neutrons, and electrons present.

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Isotopes• All atoms of the same element have the same

number of protons and electrons but the number of neutrons may differ.

• Atoms of the same element that differ in the number of neutrons are called isotopes, For eg. Uranium-235, 236, 238 (atomic no. 92)

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Fissile and Fissionable nuclides• fissile nuclides: those nuclides that can be made to

undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting.

• Fissionable: the only nuclides that are fissionable are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction.

• As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. 

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Introduction1. Nuclear power is the fourth-largest source

of electricity in India after thermal, hydro and wind power. 2. As of 2012, India had 20 nuclear reactors in operation in 6 nuclear power plants.3. India's Nuclear plants generate 3.75% of total electricity produced in India.4. 17% of electricity in the world is generated by nuclear energy.Tarapur Atomic Power Station, MaharashtraMadras Atomic Power Station (Kalpakkam), Tamil NaduRajasthan Atomic Power StationKaiga Atomic Power Station, KarnatkaNarora Atomic Power Station, Uttar PradeshKakrapar Atomic Power Station, GujaratKudankulam Nuclear Power Plant, Tamil NaduProposed Nuclear Energy Parks:Jaitapur Nuclear Power Plant, MaharashtraMithi Virdi Nuclear Power Plant, GujaratKovvada Nuclear Power Plant, Andhra PradeshHaripur Nuclear Power Plant, West BengalKumharia Nuclear Power Plant, Haryana

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History of Nuclear Energy Development• The first controlled fission of an atom occurred in 1938 in

Germany• The US was the first to develop an atomic bomb• In 1945, the US military dropped bombs on the Japanese

cities of Hiroshima and Nagasaki

• A legacy of the military research is that a great deal of soil, water, and air are contaminated with radioactive material (Hanford, Savannah River sites).

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History - Continued•The world’s first electricity generating reactor was constructed in the US in 1951 (Arco, Idaho).

•First commercial power plant, England 1956.

•The first commercial nuclear generator to become operational in the United States was the Shipping port Reactor (Pennsylvania, December 1957).•“APSARA” – THE FIRST NUCLEAR RESEARCH RECATOR IN Trombay•“CIRUS” (Canadian Indian reactor, US): supplied by Canada but used heavy water supplied by U.S.A. It was shut down on 31st December 2010, because of INDO-US nuclear accord

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Nuclear fuel reserves India's domestic uranium reserves are small and the country is dependent on uranium imports to fuel its nuclear power industry. Since early 1990s, Russia has been a major supplier of nuclear fuel to India.

Large deposits of natural uranium, which promises to be one of the top 20 of the world's reserves, have been found in the Tummalapalle belt in the southern part of the Kadapa basin in Andhra Pradesh in March 2011.

http://www.ecolo.org/photos/uranium/uranium-black.jpg

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To appreciate the consequences of using nuclear fuels to generate energy it is important to recognize the nuclear fuel cycle. Mining produces low grade uranium ore. The ore contains 0.2 % uranium by weight. After it is mined, the ore goes through a milling process. It is crushed and treated with a solvent to concentrate the uranium. Milling produces yellow-cake, a material containing 70-90% uranium oxide.

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Naturally occurring uranium ore contains about 99.3% nonfissionable U-238 and only 0.7% fissionable U235 (the U235 is the uranium isotope needed in nuclear reactors). This concentration of U-235 is not high enough for most types of reactors, so the amount of U-235 must be increased by enrichment. Since the masses of the isotopes U-235 and U-238 vary only slightly, enrichment is a difficult and expensive process. However, enrichment increases the U-235 content from 0.7% to 3%.

Fuel fabrication converts the enriched material into a powder, which is then compacted into pellets about the size of a pencil eraser. These pellets are sealed in metal rods and transported to the reactor site.

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Nuclear Fission• Nuclear fission is the process

of splitting a nucleus into two nuclei with smaller masses.

• Fission means “to divide”• The process of splitting a

heavy nucleus into a number of fragments of much smaller mass by suitable bombardment with sub-atomic particles is called nuclear fission.

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Fission cont.

• Only large nuclei with atomic numbers above 90 can undergo fission.

• Products of fission reaction usually include two or three individual neutrons, the total mass of the product is somewhat less than the mass of Uranium-235.

• During this process some of the mass of the original atom is converted into energy in accordance with the equation E = mc2

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By appropriate calculations, 1 g of URANIUM-235 releases 8.22x107 kJ of energy which is equal to that released by 2.5 metric tones of good quality coal

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A chain reaction is an ongoing series of fission reactions. Billions of reactions occur each second in a chain reaction. Only certain kinds of atoms are suitable for the development of a nuclear chain reaction. The two materials most commonly used are uranium-235 and plutonium-239.

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Chain Reaction cont.• On earth, nuclear fission

reactions take place in nuclear reactors, which use controlled chain reactions to generate electricity.

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Chain Reaction cont.• Uncontrolled chain

reactions take place during the explosion of an atomic bomb.

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Nuclear Reactors A nuclear reactor is a device that permits a controlled fission chain reaction. In the reactor, neutrons are used to cause a controlled fission of heavy atoms such as Uranium 235 (U-235). U-235 is a uranium isotope used to fuel nuclear fission reactors.

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Components of a nuclear power reactor: a moderator, controlling rods, fuel element and coolant. All these parts are enclosed in a chamber.

How to slow down the speed of secondary neutrons: use of moderators:

The three neutrons that are released in the fission of U-235 are fast neutrons (14,000 km/s) and cannot further cause fission. To sustain a nuclear chain reaction these fast neutrons need to be converted into thermal neutrons (2.2 km/s) which are slower as compared to fast ones. When high speed neutrons collide with moderator they lose some kinetic energy. Examples of moderators are water, heavy water, graphite.

How to decrease the number of secondary neutrons: use of controlling rods

If the number of neutrons are more then it will result in explosion. In order to regulate the excess no. of neutrons controlling rods are used (cadmium or boron rods).

Their length is a crucial aspect. This is adjusted from outside the reactor. If no. of neutrons is to be decreased then they are pushed further.

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Fuel element:U-235 CAN BE FISSIONED. Natural uranium consists of chiefly 99.3 % of U-238 and only 0.7% of U-235. U-238 can be fissioned by fast moving neutrons (breeder reactors).

Enriched uranium uses both isotopes and is used in the form of long rods or plates. The uranium rods are inserted into the graphite core in such a way that cadmium or boron rods lie between the uranium rods.

Coolant (heat transfer agent): the energy produced in the reactor is heat energy. It should be immediately transferred to the heat exchanger. This is done by circulating a coolant which is attached to both reactor and heat exchanger. It must have a high boiling point. For eg. Water, liquid sodium

Protective chamber: thick walls made from cement and concrete

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Various possibilities in a nuclear reactor

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Critical mass: minimum mass required to sustain a chain reactionNeutron multiplication factor (K): the average number of released neutrons which cause further fissionCritical: K=1 (chain reaction is sustained)Subcritical (K<1): (chain reaction is not sustained; safe; maybe some secondary neutrons have escaped into the atmosphere)Supercritical (K>1): (leads to explosion, bomb)

The critical mass for lower-grade uranium depends strongly on the grade: with 20% U-235 it is over 400 kg; with 15% U-235, it is well over 600 kg.

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Nuclear Fission from Slow Neutrons and Water Moderator

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How does a Nuclear Power Plant Work?

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How a Nuclear Reactor works • 235U fissions by absorbing a neutron and producing 2 to 3 neutrons, which

initiate on average one more fission to make a controlled chain reaction• Normal water is used as a moderator to slow the neutrons since slow

neutrons take longer to pass by a U nucleus and have more time to be absorbed

• The protons in the hydrogen in the water have the same mass as the neutron and stop them by a billiard ball effect

• The extra neutrons are taken up by protons to form deuterons• 235U is enriched from its 0.7% in nature to about 3% to produce the

reaction, and is contained in rods in the water• Boron control rods are inserted to absorb neutrons when it is time to shut

down the reactor• The hot water is boiled or sent through a heat exchanger to produce

steam. The steam then powers turbines.

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Types of nuclear reactor: (classified according to the fuel, coolant and moderator)Light water reactor

(water acts as a coolant, moderator, heat exchange liquid)

Boiling water reactor Pressurized water reactorHeat from the fuel rods causes water to boil, producing steam at the top of the reactor

The water is under sufficiently high pressure to prevent boiling even at a temperature above the normal boiling point. The high temperature water still under high pressure leaves the reactor vessel and enters a device called a heat exchanger which contains a separate, secondary water system. The lower pressure in the secondary water system causes the water the boil and this steam is then fed to the steam turbines.

Advantage of pressurized water reactor: As steam is generated outside the reactor; it is radiation free

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Cooling Tower

Emergency corecooling system

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•Pressurized heavy water reactor

•Gas cooled reactors (heat exchange fluid can be a gas)

•Liquid metal cooled breeder reactors (breeder reactors are used to prepare fissile nuclides)

Types of nuclear reactor..Contd…

U-uraniumNp- neptuniumPu- plutonium

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How much energy is produced?

• Nuclear power is an extremely rich energy source.

• One gram of Uranium-235 delivers as much energy as 2.5 metric tons of coal!!!

• One in every 5 houses in the U.S. is supplied with nuclear energy.

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Nuclear Fusion• Nuclear fusion is the combining

of two nuclei with low masses to form one nucleus of larger mass.

• Nuclear fusion reactions are also called thermonuclear reactions (because they require high temperature, generally greater than

4x106 oC, in order to overcome electrostatic repulsion between 2 nuclei when they come together)

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Nuclear Fusion cont.• Fusion reactions exist in stars.• Our sun is a good example of

a thermonuclear (fusion) reaction.

• It is almost impossible to create fusion reactions on earth since they need temperatures above one million degrees Celsius in order to take place.

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Nuclear Fusion cont.• Nuclear fusion produces

less nuclear waste than nuclear fission and the materials are easier to obtain.

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The energy that would be released by combining the deuterium in one cubic meter of ocean water would be greater than that contained in all of the world’s entire fossil fuels. Even though in theory fusion promises to furnish large amounts of energy, technical difficulties appear to prevent its commercial use in the near future. Even the governments of nuclear nations are budgeting only modest amounts of money for fusion research. And, as with nuclear fission and the breeder reactor, economic costs and fear of accidents may continue to delay the development of fusion reactors.

Nuclear Fusion

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U-235 U-236 U-238Half-life (t½) is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period

7.038 ×108 years 2.348 x107 years 4.468×109

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Nuclear Power Countries

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NPPs AROUND THE WORLD

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Pros for Nuclear Power• Rich energy source.• 1 gram of Uranium-235 delivers as much

energy as 2.5 metric tons of coal.• Reactors run for years without refueling or being shut down and

need little maintenance.• No air pollution! (Nuclear energy annually prevents 5.1 million tons

of sulfur 2.4 million tons of nitrogen oxide 164 metric tons of carbon dioxide)

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Cons about Nuclear Power• Produces Radioactive Waste• There is no permanent long-

term disposal site for commercial nuclear waste.

• There is a relatively short supply of 235U

• Nuclear Power Plants are expensive to build.

• Minor maintenance problems can be very expensive to fix.

• Safety concerns!!!• Nuclear Proliferation

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Nuclear Proliferation: Nuclear proliferation is the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information to nations not recognized as "Nuclear Weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the Nuclear Nonproliferation Treaty or NPT.

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Nuclear wastesType Mildly active Moderately active Highly active

Examples residues from filter and purification plants

radioactive residues from purification plants (ion exchange resins)

spent fuel (burnt) which is sent for reprocessing

Way of disposal

“Dilute and Disperse”. Disposed off in oceans or buried in soil.

“Delay and Decay”. Delay disposal by storing at the site in sealed tanks till radioactivity levels falls somewhat. After the period of decay they are packed in double wall containers and sunk in the ocean or stored in disused salt mines (because salt mines will be free from water and thus radioactive contamination of groundwater is unlikely)

“Concentrate and contain”. Reduce volume as much as possible. It is contained above ground in double walled steel and concrete tanks, constantly refrigerated and agitated with air because if they settle down they might corrode the floor due to heat energy released.

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Hazards of above nuclear waste disposal: refrigeration might fail due to power failure, earthquake, flood, human error, chemical corrosion of tanks, etc

Other ways of disposal: under polar ice sheets, buried in the sea bed, interplanetary disposal.

Waste can be minimized by usage of breeder reactors

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DRY STORAGE IN CASKS ON SITE

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YUCCA MOUNTAIN SITE

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Yucca Mountain

The Future of Nuclear Waste Storage

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Problems with Yucca Mountain• The nuclear waste currently sitting around is enough to fill the repository

(commonly refers to a location for storage, often for safety or preservation) • Danger to the public with the transportation of the waste to yucca mountain• Possible health risks to those living near Yucca Mountain

• Eventual corrosion of the metal barrels which the waste is stored in

• Located in an earthquake region and contains many interconnected faults and fractures

• These could move groundwater and any escaping radioactive material through the repository to the aquifer below and then to the outside environment

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Is Nuclear Energy Safe?

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Three Mile Isle

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Chernobyl nuclear accident

• 4 reactors (2 built in 1970’s, 2 in 1980’s, graphite moderated water coolant reactor)

• Combination of design and operator error during electrical power safety check resulted in cascade of events leading to core breach of Reactor 4 with subsequent chemical (not nuclear) explosion

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Chernobyl is a small city in Ukraine near the border with Belarus, north of Kiev.

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The largest lump of graphite shows an intact control rod channel.

A mutated piglet

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One impact of Chernobyl is that it deepened public concern about the safety of nuclear reactors. Even before Chernobyl, between 1980 and 1986, the governments of Australia, Denmark, Greece, Luxembourg, and New Zealand had officially adopted a “no nuclear” policy. Since 1980, 10 countries have cancelled nuclear plant orders or mothballed plants under construction. Argentina canceled 4 plants, Brazil 8, Mexico 18, and the US, 54. There have been no orders for new plants in the US since 1974. Sweden, Austria, Germany, and the Phillipines have decided to phase out and dismantle their nuclear power plants.

Decommissioning Costs

Decommissioning a a fossil fuel plant is relatively easy a wrecking ball is about all that is required. Nuclear power plants are not demolished they are decommissioned. Decommissioning involves removing the fuel, cleaning the surfaces, and permanently preventing people from coming in contact with the contaminated buildings and equipment.

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The Fukushima Daiichi nuclear accident (2011)

59http://www.greenfacts.org/en/chernobyl/, Chernobyl Forum(2006)

Pathways Of Exposure To Man From Release of Radioactive Materials

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Safety of Nuclear Plants

•Steel-reinforced concrete and a dome-shaped containment buildings surround all US reactors (inner wall several feet thick and outer wall at least 15 inches thick)• Designed to withstand hurricanes, earthquakes, high winds• Reactors have detectors to quickly shut down in event of tremor (about 20% are in regions with seismic activity like Pacific Rim)• In considering safety, must address…

• Faults in plant design• Human error• Risks associated with terrorism/political instability

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“The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine."However, within 10 years the world's first nuclear reactor had been built and by the mid-1950s nuclear power stations started supplying electrical power for industrial and domestic use.

Ernest Rutherford (1871-1937)The discoverer of the nucleus of the atom.