Nuclear Fission and Fission Reactors Dr. BC Choudhary,

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1 Nuclear Fission and Fission Reactors Dr. BC Choudhary, Professor, NITTTR, Sector-26, Chandigarh

Nuclear Constituents: The Nucleus Protons (positive charge) Neutrons (neutral charge) Nuclear Diameter ~10-15 m Atomic diameter ~10-10 m Density 10 44

2 Nuclear Constituents: The Nucleus Protons (positive charge) Neutrons (neutral charge) Nuclear Diameter ~10-15 m Atomic diameter ~10-10 m Density nucleon/m 3 The term nucleon refers to either a proton or a neutron in the nucleus. The term nuclide refers to a nucleus with a specific number of protons and neutrons.

3 The Nuclear Force Look Deep Into Nature, And Then You Will Understand Everything Better Einstein When you get a clear grasp of the concepts and know how they fit together to explain natural phenomenon, solving problems is fun and meaningful. Nucleon-Nucleon Interaction Repulsive electrostatic force decreases with squared power of distance between the charged particles, becoming relatively stronger than the attractive nuclear binding force.

4 Nucleus and Binding Energy B.E. is the energy required to separate the full nucleus into its individual protons and neutrons A measure of stability of nucleus. BE=M (Zprotons) + M (Nneutrons) M (Nucleus) Mass of a nucleus is slightly less than the total mass of the protons and neutrons separately. The lost mass accounts for binding force in the nuclei. Energy can be released by creating nuclei that are more strongly bound (increasing E b )

5 Mass - Energy Equivalence In 1905, Einstein gave an outstanding important Energymass relation E m. Energy evolved 2 c Mass defect Speed of light In nuclei, the mass of a nucleus is slightly less than the total mass of the protons and neutrons taken separately mass defect ( m) For a small mass of matter (say 1g), the amount of energy produced = 10-3 ( ) 2 = J About 33,000 tons of TNT The idea was revolutionary if put into actual practice. Physicists had made hard efforts and at last the discovery of nuclear fission come out in 1939.

NUCLEAR FISSION Discovery comes from work on artificial radioactivity in an attempt to produce transuranic elements (Z>92), when 92 U 235 was bombarded by slow neutrons and certain anomalies

6 NUCLEAR FISSION Discovery comes from work on artificial radioactivity in an attempt to produce transuranic elements (Z>92), when 92 U 235 was bombarded by slow neutrons and certain anomalies witnessed in the product nuclei. Otto Hahn & Fritiz Strassmann (1939) : Explained that final products yielded two lighter nuclei Barium and Krypton accompanied by three neutrons U 0n 56Ba 36Kr 30 n Q The disintegration process was given the name Nuclear Fission

Nuclear Fission The fission process, accompanied by a huge amount of energy, larger than that encountered previously in any atomic or nuclear processes.

7 Nuclear Fission The fission process, accompanied by a huge amount of energy, larger than that encountered previously in any atomic or nuclear processes. The energy provided is by the difference in mass between the initial and the final products and is in accordance with massenergy relation. In 92 U 235 fission, 200 MeV of energy is released, which is 20 times as large as released in average -particle disintegration and millions of times greater than that released in combustion process.

8 Example of a fission diagram = Nucleus = fission fragment = Neutron = Energy

9 How does fission work? A neutron hits a uranium nucleus. The nucleus becomes unstable and needs to release energy. The nucleus breaks releasing energy, neutrons and 2 smaller atoms. 2 smaller atoms formed are called fission fragments. Fission does not destroy the particles, it just releases the nuclear binding.

10 Energy release in Fission of U-235 n Neutron absorption 235 U U Kr 36 Fission 141 Ba 56 n n n n U 92 _ 141 Ba Kr n Change in mass, dm = kg Energy released, E = (dm)c 2 = ( ) 2 = J In chemical combustion C + O 2 _ CO 2 E = J Energy release from fission 1 uranium nucleus = carbon atoms 1 tonne of 235 U = tonnes of coal

11 Energy Released by Fission Process Instantaneous Release (per fission) MeV Fission products 168 _ heat Neutrons 5 g-rays 7 _ heat Delayed Release (per fission) b-particles g-rays Antineutrinos Neutron Capture by 238 U b b n U 92 _ 239 U 92 _ 239 Np 93 _ 239 Pu m 2.3 d MeV 8 _ heat 7 _ heat 12 lost from reactor In practice many neutrons do not contribute to fission because they are absorbed by 238 U

13 Fission-Nucleon Equation

14 Nuclear Models The Liquid-drop model The Shell model Models tell us various aspects of the structure of nuclei and how they behave during fission.

Liquid- Drop Model Bohr & Wheeler: First to evolve theory for nuclear fission.

A droplet is a cluster of particles held together by short-range forces.

spherical shape of nucleus is due to the forces of surface tension Property of liquids

15 Liquid- Drop Model Bohr & Wheeler: First to evolve theory for nuclear fission. Considered target nucleus similar to liquid drop. A droplet is a cluster of particles held together by short-range forces. Examples: Liquid drop Atomic nucleus Molecular and nuclear forces are short-range The spherical shape of nucleus is due to the forces of surface tension Property of liquids only. These forces try to maintain the shape of nucleus as such while the Coulomb s repulsive forces among the protons in the nucleus try to destroy its shape.

16 When some energy (excitation energy) in imparted to the drop through capture of neutron, oscillations are set up in the drop, which tend to distort the spherical shape of the nucleus, while surface tension forces try to restore it. In case the excitation energy is sufficiently large, the nucleus may undergo distortion till it attains the shape of dumb-bell. Possible steps in the process of nuclear fission The Coulombic repulsive forces then tear the nucleus apart which again exist in the spherical shape and thus cause the nuclear fission.

17 Theory of Nuclear Fission On basis of Bohr s theory; the fission of U 235 takes place in two steps Capture of a neutron by 92 U 235 nucleus results in the formation of Compound nucleus ( 92 U 236 ) Compound nucleus undergoes distortion due to excitation energy to attain the shape of a dumb-bell and finally splits up into fission products (Ba 141 and Kr 92 ) with the release of three neutrons. In case, the excitation energy is not sufficient to cause fission, it is liberated in the form of g-rays through n-g reaction with Uranium. Why neutrons are used as bombarding particles?

18 Thermal Neutrons Although fission is energetically possible in all naturally occurring heavy nuclei, it is a very rare event. Detailed analysis shows that fission with thermal neutrons occurs much more often in nuclei with an odd number of neutrons than in nuclei with even number of neutrons. For example, 92 U 238 nucleus waits for on the average for years before undergoing fission while 92 U 233, 92 U 235 and 94 Pu 239 undergo fission with thermal neutrons readily. U 235 and Pu 239 are most commonly used for obtaining energy by fission. Thermal Neutrons or Slow neutrons: Neutrons at room temperature (0.025 ev) or having energy less than 1eV (Slow neutrons).

19 Neutron Cross-sections on Uranium (Barns) 10 3 s~1/v Fission by 235 U 92 s Capture by 238 U 92 Fission by 238 U (ev) E Low energy neutrons (Thermal) for fission of 235 U. High energy neutrons (Fast) for fission in 238 U.

Nuclear fission usually results in the emission of 2 or 3 free neutrons per fission Fission is a peculiar type of nuclear reaction which has amongst its

20 Nuclear fission usually results in the emission of 2 or 3 free neutrons per fission Fission is a peculiar type of nuclear reaction which has amongst its products the same kind of particles which initiated it. It is the emission of these neutrons which renders the process of nuclear fission a real source of energy.

21 Chain Reaction When U 235 nucleus splits up, it generally releases 3 1 n 0 One of the neutrons may escape without hitting any other nucleus and thus get lost. The other 2 may strike against other fissionable nuclei to produce further fission accompanied by the release of still more neutrons and so on. If on the average, more than one neutron per fission produce further fission, the number of fissions taking place at each successive stage goes on increasing at a rapid rate giving rise to what is called Chain Reaction.

22 Example of a Chain Reaction n U 92 _ 141 Ba Kr n +Q

23 Critical Mass Important to note that a chain reaction can be set up only if the mass of the fissionable material is greater than a certain Critical mass. It is the amount of nuclear fuel necessary to sustain a chain reaction. You must have a critical mass in order to build a nuclear reactor. You must have a critical mass in order to make a nuclear weapon. If the chain reaction is set up in a certain mass, the reaction will accelerate at a tremendous rate. Within small fraction of a second, the entire fissionable material shall get exploded, resulting in the process a tremendous amount of energy Uncontrolled Chain Reaction Such uncontrolled chain reaction, resulting in sudden explosion of a fissionable material is the underlying principle of Atomic Bomb.

24 Critical Size A neutron emitted by fission may either cause another fission or may escape through the surface and be lost. In order to minimize the losses through the surface, the area of the surface S (through which fission neutrons escape) must be made as small as possible in comparison to the volume (in which fission take place). Larger the size of the fissionable material, smaller the surface to volume (S/V) ratio becomes. In a cube of side d : Volume = d 3 ; Surface area = 6d 2 S/V = 6/d ; inversely proportional to size of the material d = 0.2 cm ; S/V = 30 d = 20 cm (100 times larger) ; S/V=0.3 (100 times smaller)

25 Critical Size If U 235 piece is two small, fission neutrons escape through its surface and no chain reaction takes place As the size increases, S/V becomes smaller. Eventually a size (critical size) will reach at which exactly one of the two or more neutrons are retained to carry on the chain reaction. For a size larger than the critical size, the loss of neutrons through the surface will be less so that more than one of the fission neutrons are able to cause further fission and chain reaction grow rapidly releasing tremendous amount of explosive energy.

$Within a fraction of second, the entire material gets vaporized to dissipate tremendous amount of explosive energy into the atmosphere.$

26 A primitive atom bomb contains two pieces, each slightly smaller than the critical size. They are brought together by means of a conventional explosive (Gun powder or TNT). The resulting piece is much larger than the critical size hence, the chain reaction builds up rapidly releasing huge energy. Within a fraction of second, the entire material gets vaporized to dissipate tremendous amount of explosive energy into the atmosphere. Mechanism of Atom bomb Atom bomb Little boy detonated in Hiroshima, Japan during world war-ii (1945) Fat Man, the atomic bomb detonated over Nagasaki, Japan (1945).

27 Applications of Nuclear Fission Controlled Fission: Nuclear reactor/power plant Research Reactors Uncontrolled Fission: Atomic bomb/ Hydrogen bomb Nuclear Weapon

28 Nuclear Reactor: Controlled Fission

29 A Nuclear Sunset : Atom bomb Uncontrolled Fission

Enrico Fermi (1901-1954) Italian Physicist most noted for his work on Beta decay and development of the first nuclear reactor Won the 1938 Nobel Prize in Physics for his work on nuclear reactions.

30 Enrico Fermi ( ) Italian Physicist most noted for his work on Beta decay and development of the first nuclear reactor Won the 1938 Nobel Prize in Physics for his work on nuclear reactions. First nuclear reactor- a massive pile of Graphite bricks and Uranium fuel was built and went critical on 2 December 1942, at University of Chicago. The experiment was a landmark in the quest of energy and it was typical of Fermi s brilliance- Manhattan Project The chain reacting pile was important not only for its help in assessing the properties of fission but it serve as a pilot plant for a massive reactor used to breed the plutonium needed for bombs used for destruction in World War II at Hiroshima and Nagasaki, Japan. After World War II, development of Civilian nuclear program (Atlantic Energy Act of 1946); In 1954: first commercial nuclear power plant commissioned

31 How to control the chain reaction? Whether the chain reaction remains steady, builds up or dies down depends upon the competition between the emission of neutrons through fission and their losses through absorption by various processes and escape from the surface of the system. The chain reaction may be controlled by absorbing a desired number of neutrons, so that on the average, one neutron from each fission is left to excite further fission. The number of fissions occurring per second thus remains constant. and the energy thus liberated does not get out of control. Control Rods Control Rods are made of a material that absorb the neutrons. Generally, made of Cadmium or Boron. As the control rods are removed the chain reaction increases, and if we want to slow the reaction control rods are inserted.

32 Factors affecting Chain Reaction 1) For each thermal neutron absorbed, h:effective fast neutrons emitted, h < n, mean number produced (n 2.42 for 235 U), because not all neutrons absorbed by fuel cause fission. Natural Uranium (0.72% 235 U) h ) Some fast neutrons cause fission before slowing down which increases the number of neutrons by the fast fission factor 3) The probability that a neutron will avoid resonance capture by 238 U - the resonance escape probability p - depends on the moderator 4) The fraction of thermal neutrons that are absorbed by the fuel in the core (fuel, moderator, clad) is called the thermal utilization factor f 5) There are a fraction l f of fast neutrons and a fraction l t of thermal neutrons that leak out of the reactor The neutron multiplication factor k is therefore given by: k h p f (1- l f ) (1- l t ) For chain reaction-self sustained without explosion k = 1

33 Working Schematic Major Parts are: Reactor Core - Fuel Moderator Control rods Coolant & Heat Exchangers

by water, forming core Pressure Tubes Inside a Zr-Nb

34 U 2 O pellets in a steel fuel bundle assembly loaded into a pressure tube Multiple pressure tubes surrounded by water, forming core Pressure Tubes Inside a Zr-Nb pressure tube. Assembly of pressure tube and Calandria tube

35 Enrichment Naturally-occurring uranium consists of Fissile isotope 235 U Stable isotope 238 U }ratio = 1/138 ~ 0.7% In a reactor, neutrons are lost by : Absorption by 238 U _ 239 U _ 239 Pu Absorption by 235 U _ 236 U Absorption by moderator Absorption by reactor structure Escape from reactor core Not enough n to continue chain reaction with H 2 O moderation _ Enrichment necessary to increase ratio 235 U/ 238 U _ 3% b

36 Enrichment is process of increasing proportion of fissionable nuclei in natural uranium (0.7% 235 U) Methods of Enrichment Fuel Enrichment 1. Electromagnetic Separation vacuum chamber accelerating electrodes B mv 2 /r qbv _ r mv/qb heavy isotope light isotope Used for Manhattan project (1g/day) and by Iraq before Gulf War

37 2. Gaseous Diffusion Uranium ore converted to UF 6 gas passed through very thin porous membranes. Light 235 U molecule diffuses faster than the heavier 238 U molecule. ~1400 stages to achieve 3-5% 235 U/ 238 U 3. Ultracentrifuge Gaseous UF 6 is rotated at high angular velocity in a cascade of centrifuges, causing partial separation 4. Laser Separation Tuned lasers selectively ionise the lighter isotope in UF 6 vapour. Positive ion attracted to charged collector plates Testing Technology

38 Enrichment Facilities US Enrichment Corporation(USEC), Maryland - A global energy company, is a leading supplier of enriched uranium fuel and nuclear industry related services for commercial nuclear power plants.

39 Collectively these steps are known as the 'back end' of the fuel cycle.

40 Moderator Its purpose is to slow down the neutrons as a result of the collision the neutrons suffer against the atoms of the material of the moderator. Suitable materials used as moderator are Heavy Water Carbon taken in the form of pure graphite.

41 Moderator is a medium for reducing the kinetic energy of neutrons from MeV to thermal level without losing many in the resonant trap of 238 U m M For 180 o scattering E s = [(M - m)/(m + m)] 2 E i = [(A - 1)/(A + 1)] 2 E i For 0 o scattering E s = E i Neutron Moderation neutron Averaging : E s = ½{1+[(A-1)/(A +1)] 2 }E i = [(A 2 +1)/(A +1) 2 ]E i (Averaging over all angles gives the same result) 1 H 12 C 238 U A E s /E i nucleus

42 How many collisions are required to reduce neutron energy from 2 MeV to 1 ev? (factor of ) Put (E s /E i ) n = 1/( ) = For example; 1 H gives n ~ 21, 12 C gives n ~ 96 Moderating Ratio, MR = (1- E s /E i ) s el /s c Good moderators require large s elastic (s el ) low s capture (s c ) significant loss in KE per collision chemical stability (in hot, radioactive environment)

Control rods Rods of Cadmium are generally used as control rods. Cadmium atoms are very good absorbers of neutrons and thus prevent the chain reaction from going fast.

43 Control rods Rods of Cadmium are generally used as control rods. Cadmium atoms are very good absorbers of neutrons and thus prevent the chain reaction from going fast. The rods are inserted in the reactor and the chain reaction is controlled by careful adjustment of their lengths inside the core. The movement of a single rod through a few centimeter length makes a significant difference in the performance of the reactor.

44 Thermal Power Stations Note: Thermal includes fossil-fuel and nuclear power Heat source is part of Steam Cycle Thermodynamics of cycle independent of nature of heat source Steam Cycle: Main Components Boiler Water Pump Heat in Condenser Turbine (expander) w Electrical power Cooling water Heat out

45 How does a nuclear Reactor generate Electricity? Heat from reaction is carried away from the core by coolant Pressure sufficiently high to increase boiling temperature ( Atm) to >250 o C. Hot coolant either transfers heat to different pool of water and boils it, or boils itself High temperature steam is used to drive turbine, generate electricity.

46 How does a nuclear reactor control the reaction? Fast neutrons from fuel are slowed down by a moderator Fission chain reaction occurs by U 235 absorbing slow neutrons (i.e. low energy) Reaction controlled by Cadmium control rods, which are raised and lowered to control the neutron multiplication factor, k

47 Fission Reactor Dynamics Moral of the story: The more fission reactions there are, the more neutrons are available to make even more fission reactions. For controlled (stable) reaction 1neutron, 1fission reaction (k=1) P(t) P 0 e t (k-1) where P(t) is the time-dependent power, P 0 is the initial power, and k is the neutron multiplication factor K = >1, supercritical state =1, critical state <1, subcritical state Adjusting control rod positions adjusts the core power!

48 Fission Reactor Safety What safety measures are there? Vertical Control Rods Gravity-fed, held by electromagnets Emergency Core Coolant Mechanical system that dumps fresh coolant in case of LOCA (Loss of coolant accident) Gadolinium Nitrate Poisoning Gadolinium is a very strong neutron absorber

49 Nuclear Power : Historical Milestones 1896 Becquerel Fogging of photographic plates near U salts 1905 Einstein Special theory of relativity- E = mc Rutherford Discovery of nucleus - -particle scattering 1913 Bohr Quantum model of H atom 1932 Chadwick Discovery of neutron 1936 Bohr, Frenkel Liquid drop model of nucleus 1938 Hahn, Strassmann Discovery of fission 1939 Joliot, von Halban Discovery of neutrons produced in fission Kowarski reactions _possibility of chain reaction 1939 Szilard, Wigner Advised Roosevelt of feasibility of uranium bomb

50 1939 Booth, Dunning, Start of projects to separate isotopes of Urey 235 U and 238 U 1940 Anderson, Fermi Showed that 12 C would be a good moderator 1940 Joliot, Dautry Transferred D 2 O from Norway to UK 1940 Seaborg Discovered Plutonium 1942 Groves Manhattan Project started 1942 Fermi First nuclear reactor- demonstrated that chain reaction controllable 1943 Bethe, Weisskopf Defined specification of atomic bomb Teller, Feynman (sub-critical sphere surrounded by explosives _ compression _ criticality) May 1945 Experimental Uranium bomb exploded July 1945 Experimental Plutonium bomb exploded Aug 1945 Hiroshima destroyed by U-bomb Nagasaki destroyed by Pu-bomb

51 1945 UKAEA established in UK, CEA established in France 1954 Fast reactor programme started 1956 First prototype power station (Calder Hall) gas cooled 1956 Suez crisis _ oil shortage _ nuclear power stations _ first commercial reactors 1962 (Berkeley, Bradwell) 1957 Pressurised Water Reactor (PWR) developed for nuclear submarines by the USA 1957 Windscale fire (Wigner energy underestimated) 1957 Campaign for Nuclear Disarmament (CND) established 1959 Dounreay fast reactor _ critical 1964 UK decide to build Advanced Gas cooled Reactors (AGR) 1976 First AGRs commissioned (Hinckley B, Hunterston B) 1979 Three Mile Island accident (operator errors) 1986 Chernobyl accident (design faults, operator errors, no regulation) 1991/2 Collapse of communism in Europe _ nuclear cooperation (civil and military) 1995 First PWR in UK (Sizewell B)

52 The End Thank You for the patience We are on the web at Animated Gifs compliments of bellsnwhistles.com

Nobel prizes in nuclear and reactor physics. Szabolcs Czifrus Institute of Nuclear Techniques BME

Nobel prizes in nuclear and reactor physics Szabolcs Czifrus Institute of Nuclear Techniques BME Nuclear physics in everyday life Electricity: production in nuclear power plants Sterilization by the application