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Nuclear Chemistry Basics
- 1. NUCLEAR CHEMISTRY Nuclear chemistry is the subfield of chemistry dealing with radioactivity, nuclear processes, and transformations in the nuclei of atoms. 1
- 2. Briefly.. β’ Radioactive decay β’ Nuclear reactions β’ Nuclear reactors β’ Management of radioactive wastes β’ Radiation counting techniques β’ Application of radioisotopes 2
- 3. Nuclear chemistry Radioactive decay Ξ±, Ξ² and Ξ³ decay Nuclear reactions Q value, cross section Nuclear reactions Fission and fusion Reactors Terms in reactor technology Breeder reactor Reactor safety precaution Radioactive waste LLW, ILW, HLW Counters Based on ionzation Ionization chambers GM counter Proportional counter Light sensing Scintillation counter Applications 3
- 5. Binding energy β’ the energy that holds a nucleus together. This is equal to the mass defect of the nucleus. β’ πΈ = Ξmc2 β’ πΈ = βπ Γ 931 MeV β’ 1 amu = 931 MeV 5
- 6. Significance of BE β’ Nuclear binding energy is the energy required to split a nucleus of an atom into its components. β’ Nuclear binding energy is used to determine whether a process will be favorable. 6
- 7. n/p ratio 7
- 8. Radioactive Decay β’ Radioactive decay β’ the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation β’ such as β’ alpha particle β’ beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion 8
- 9. Kinetics Radioactive decay law β’ If N radioactive nuclei are present at time t and if no new nuclei are introduced into the sample, then the number dN decaying in a time dt is proportional to N and so π = β ( ππ ππ‘ ) π Integrating, we get the exponential law of radioactive decay. π = π0 πβππ‘ β’ Where N0 is the number of nuclei present at time t=0. 9
- 10. RADIOACTIVE DECAY β’ Decay series β’ Alpha decay β’ Beta decay β’ Gamma decay 10
- 11. Radioactive Decay β’ Energetics β’ Range β’ Theory of decay: how it happens 11
- 12. Energy and Range: ionization power β’ Can be found by measurement of their absorption by medium β’ Knowledge of interaction with absorption medium tells us idea about the range of the emitted radiation 12
- 13. Alpha decay β’ Parent nucleus emit Ξ± particle producing a daughter nucleus of four mass units less and charge two units less β’ π π΄ π β πβ2 π΄β4 π + 2 4 π»π β’ Reason : nucleus with 210 or more nucleons is so large that the nuclear force holding them together barely counterpart the electromagnetic repulsion between protons. β’ Why alpha decay? β’ Average BE of last 4 nucleon= 7MeV β’ so net of 28 MeV required to remove them separately. β’ BE released upon formation of 1 alpha particle= 28.3 MeV β’ Hence favorable process. 13
- 14. Alpha decay β’ Energy β’ Range β’ Spectra β’ Theory of alpha decay 14
- 15. Energy of alpha particle β’ The maximum energy available for alpha particle is equivalent to the mass defect πΈ = [ππ β (ππ + πΞ±)] 931 MeV β’ π π- mass of parent nucleus β’ π π- mass of daughter (recoil) nucleus β’ πΞ±β mass of alpha particle β’ E= Er +EΞ± 15
- 16. Range of alpha particles: β’ it is the distance in air from source to a point within which the emitted alpha particles produce ionization and beyond which their ionization power ceases. β’ The value of ranges are measured at 150C in air at 1 atm pressure. β’ As the distance of the alpha particles from the source increases the specific ionization increases at first slowly and then more rapidly, reaches a maximum and then falls rapidly and finally decreases somewhat more slowly to zero. 16
- 17. Straggling effect: β’ alpha particle emitted from a given source lose a large fraction of their energy by causing ionization of air along their path. β’ Extend of ionization depends on the number of particles they encounter on their path and the way they encounter. β’ All alpha particles will not lose their energy in an equal way. β’ Some may not convert to neutral atom. Some may exist as He+. β’ Cause a slight variation in the range and hence a point of inflation is observed in the curve. β’ This variation is referred to as straggling. Because of straggling, the actual range of alpha particles is not definite and hence extrapolated range is considered. 17
- 18. Energy β from range of alpha particle β’ Range β energy relationship: β’ The mean range in air is related to the energy E of the particle by the relation β’ π = 0.318πΈ3/2 18
- 19. Geiger β Nuttal rule: β’ Energy β disintegration constant relation β’ it states that short-lived isotopes emit more energetic alpha particles than long- lived ones β’ When the logarithm of the disintegration constant Ξ» (sec-1) is plotted against the logarithm of range R0 (cm) of the emitted alpha particles, a linear plot will be obtained for a given radioactive series. β’ πππ π = π πππ πΉ π + π β’ Considering the range- energy relationship of the alpha particle, Geiger- Nuttal rule may be written in the form β’ πππ π = πβ² πππ π¬ π + π β² β’ aβ and bβ are constants for a radioactive series. 19
- 20. Alpha decay spectra β’ According to law of conservation of momentum β’ Ermr = EΞ±mΞ± β’ When observed with spectrograms of high resolution, alpha decay spectra shows few sharp monoergic lines BE β‘ KE available for alpha particle 20
- 21. 21
- 22. 22
- 23. Alpha decay theory β’ The wave mechanical theory β’ preformation of alpha particles in the nucleus β’ Once the alpha particle is formed, it moves towards the boundary of the nucleus. β’ This alpha particle will be repelled at the nuclear potential boundary. β’ It will oscillate inside the potential well if the KE of the alpha particle is less than zero the nucleus will be stable and there will not be an alpha emission. β’ If the energy is greater than zero, the particle will move towards the boundary of the potential well, collides with it and tends to escape from the nuclear potential boundary (potential barrier). β’ alpha particles will not necessarily cross over the barrier but may leak through and proceed in its forward direction. This barrier penetration of alpha particles is called tunnel effect 23
- 25. Beta decay β’ Types β’ Energy β’ Range β’ Spectra β’ Theory of decay 25
- 26. TYPES OF b DECAY Type Energy released Process Example b- Decay E = Ξmc2 π β π + Ξ²β + πππ‘ππππ’π‘ππππ 11 24 ππ β 12 24 ππ + Ξ²β + πππ‘πππ’π‘ππππ b+ Decay E = Ξmc2 -2me π β π + Ξ²+ + πππ’π‘ππππ 11 24 ππ β 10 22 ππ + Ξ²+ + πππ’π‘ππππ Electron capture E = Ξmc2 βBEcaptured electron π + π β π + πππ’π‘ππππ 26 55 πΉπ β 25 55 ππ + πππ’π‘ππππ 26
- 27. Energetics of b decay 27
- 28. Absorption of b particles and their range through matter β’ Emax found out by absorption measurements β’ The maximum range for a beta particle spectrum may be obtained from an experimental absorption curve β’ By visual inspection β’ Feather plot (absorption curve is compared with absorption curve of a standard beta emitter) Range energy relation: Empirical relations have been proposed π = 0.543πΈ β 0.160 β’ E- maximum energy β’ Valid for 0.6MeV β 15MeV 28
- 29. Explanation for b+ decay β’ Dirac theory β’ n/p ratio 29
- 30. 1. Dirac theory β b+ decay β’ Diracβs relativistic theory: both positive and negative energies are possible for electrons πΈ = Β± π2 π2 + π π 2 π4 β’ For each positive energy state there will be a negative state 30
- 31. 1. Dirac theory β b+ decay 2mec2 +mec2 -mec2 1.02 MeV β’ All negative states are filled : there is no vacancy β’ Vacancy can be created in the negative state if a high energy photon interact with matter and left an electron out to one of the vacant positive energy state. β’ This electron behave as an ordinary electron and the vacancy will behave as a positron (electron with positive charge) β’ For electron hole pair to be produced, the photon must have an energy of 1.02 MeV 31
- 32. Condition for b+ decay β’ Energy needed = 1.02 MeV β‘ 2π π π2 (2 Γ 9.10938356 Γ 10-31 Γ (3 Γ 108)2) β’ Mass defect Ξmβ₯ 2π π π π§ β π π§β1 β₯ 2π π π π§ β (π π§β1+2π π) > 0 Energy (π π§β(π π§β1+2π π))931 = πΈπ + πΈ π£ 32
- 33. Electron-positron annihilation β’ Low energy: (0.51 MeV) electron and positron collide to form Ξ³ ray photon πβ + π+ β πΎ β’ High energy: other particles such as mesons, bosons etc. will be formed. 33
- 34. Electron capture π π΄ π β πβ1 π΄ π + πππ’π‘ππππ Ξmπ2 β₯ πΈ π Eb= πππππππ ππππππ¦ ππ πππππ‘πππ ππ π‘βπ π βπππ Ξm = (Mx- My) 34
- 35. Energy condition for b decay Type Energy released b- Decay 0β€ Ξm b+ Decay 0 β€1.02β€Ξm 931 Electron capture 0 β€BE captured electronβ€ E = Ξmc2 931 35
- 36. Double b decay β’ Process in which an initial nuclei π π΄ π πππππ¦π π‘π π+2 π΄ π emitting 2 electrons and 2 antineutrinos β’ π π΄ π β π+2 π΄ π + 2πβ + 2 πππ‘ππππ’π‘πππππ β’ Typical double b decay is an even even nucleus π π΄ π whose BE is more than its neighbor but less than the π+2 π΄ π β’ 32 76 πΊπ β 34 76 ππ + 2πβ + 2 πππ‘ππππ’π‘ππππ (double b decay) 32 76 πΊπ β 33 76 π΄π + πβ + πππ‘ππππ’π‘ππππ (b decay) b+ decay will also occur in the similar way 36
- 37. β’ After any radioactive process, there is a chance to find daughter nucleus in exited state β’ Deexcitation can occur as β’ Photon emission/ Ξ³ decay β’ Ejection of electrons from outer orbitals 37
- 38. Gamma decay β’ Usually spin and parity of parent and daughter nuclei indicated β’ Probability of Ξ³ transition governed by reduced transition probability β’ reduced transition probability depends on the wavefunctions of initial and final states i.e., the spin and parity of parent and daughter 38
- 39. Based on t1/2 value gamma transitions are termed as 1. instantaneous transitions: Ο< 10-7s 2. isomeric transitions: Ο > 10-7s (occur between metastable and ground state) Every permitted atomic nucleus is a quantum state characterized by definite energy (E), spin (I) and parity (Ο) 39
- 40. parent daughter Change in energy= Ei-Ef =hΚ Change in spin=L= Ii-If Parity may or may not change (Ο) hΚ Photons carry angular momentum (min =h/2Ο) 40
- 41. Nucleus- photon interaction β’ Results in β’ To and fro motion of charges gives rise to electric multipole radiation (EL) β’ Fluctuation of the electric current flowing in closed loops: magnetic multipole radiation (ML) β’ Here L indicate the difference between initial and final spin states πΌπ β πΌπ 41
- 42. Whether ML or EL? β’ ππ + π π + πΏ (Even parity is counted as +1 and odd parity as 0) Sum π π + π π + π³ evenoddML EL Process Parity Spin (L) Sum π π + π π + π³ Type of decayinitia l Final 1 1 1 Even Even EL 2 0 0 Odd Odd ML 3 0 1 Even Odd ML 4 1 0 5 0 1 Odd Even EL 6 1 0 42
- 43. Isomeric transitions β’ Decay life time > 10-7s β’ Nuclear isomers: nuclides with same Z and A but differ in their energy state, spin in some cases parity β’ Parent isomer: metastable state ( due to large difference spin between initial and final state Lβ₯ Β±3) 43
- 44. Internal conversion β’ Alternative to Ξ³ emission β’ Transition of an excited nucleus to ground state by ejection of orbital electrons β’ Energy and angular momentum changes are transferred to orbital electrons β’ Transferred energy must be higher than binding energy of electron β’ KE of electron Ee= EΞ³-Eb β’ EΞ³ β energy difference between initial and final states β’ Eb β binding energy of electron 44
- 45. β’ These conversion electrons display an energy spectrum consisting of discrete lines E1 = EΞ³-EK E2 = EΞ³-EL1 E3 = EΞ³-EL2 E4 = EΞ³-EL3 β’ Total decay constant is Ξ»=Ξ» Ξ³+ Ξ» K+ Ξ» L+β¦ β’ Conversion coefficient Ξ± = ππ’ππππ ππ ππππ£πππ πππ πππππ‘ππππ πππ π’πππ‘ π‘πππ ππ’ππππ ππ πβππ‘πππ πππ π’πππ‘ π‘πππ Representing electrons ejected from K(1s1/2), L1(2s1/2), L2(2p1/2), L3(2p3/2) 45
- 46. β’ During internal conversion and electron capture, vacancies are created in the inner orbitals β’ The process of vacancy filling can be through β’ Radiation emission β’ Electron emission (auger effect) 46
- 47. Radiation emission β’ An electron jump to an inner vacancy β’ Radiation will be emitted β’ Energy of radiation πΈ = πΈ π΅1 β πΈ π΅2 β’ Vacancy is pushed outward β’ Leads the atom to +1 state M 1+ 47
- 48. Auger effect Non-radiative energy transfer β’ Inner vacancy is filled by electrons in higher orbitals β’ Excess energy is non radiatively transferred to electrons in higher levels. β’ Electrons get ejected out β’ The kinetic energy of ejected electron= πΈ = πΈ π΅1 β (πΈ π΅2 + πΈ π΅3) EB1 β binding energy of innermost electron EB2 β binding energy of electron which jumped into the inner orbital EB3 β binding energy of ejected electron The process of ejection of electron can go on as long as the E is +ve Auger effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. 48
- 49. Nuclear reactions β’ Process in which a nucleus reacts with another nucleus, an elementary particle or a photon to produce in a time of order of 10-12 sec or less one or more other nuclei. β’ π1 π΄1 π + π2 π΄2 π β π3 π΄3 π + π4 π΄4 π Target projectile recoil ejectile β’ Representation π1 π΄1 π π, π π3 π΄3 π 49
- 50. Types β’ Elastic scattering β’ Inelastic scattering β’ Photonuclear reactions β’ Radiative capture β’ Other types β’ Special nuclear reactions 50
- 51. Energetics of nuclear reactions β’ Accompanied by release or absorption of energy β’ Expressed by term βQ valueβ β’ π = βπ Γ 931 πππ β’ Barriers β’ Columbic interaction (columbic barrier) β’ Charged particle β’ Provide the minimum energy requirement β’ Angular momentum interaction (centrifugal barrier) β’ Due to interaction of angular moments of projectile and target β’ Keeps any projectile at π 2π de brouglie wavelength from target 51
- 52. β’ Conservation of protons and neutrons β’ π πππππ‘πππ‘ = π πππππ’ππ‘π β’ ππππππ‘πππ‘ = ππππππ’ππ‘π β’ Conservation of momentum Linear momentum β’ ππ£ = π + π π β’ Angular momentum is also conserved β’ Conservation of energy β’ π π + π π β π π β π π 931 = πΈ π + πΈ π β πΈ π = π β’ Q value +ve =exoergic reaction β’ Q value -ve =endoergic reaction X Compound nucleus (M) π π + π πππ π‘ππππ πππ‘πππ π π + π πππ πππππ‘πππ Energy Ea 52
- 53. Threshold energy β’ Exergonic reaction: depends on barrier (a minimum threshold exist) β’ Endergonic reaction: a minimum of Q amount of energy is needed β obtained from the kinetic energy of the projectile β’ π1 π΄1 π + π2 π΄2 π β π3 π΄3 π + π4 π΄4 π For reaction to be possible πΈ π β₯ 1 + π π π π β€ 1 + π π πΈ π Ea KE of projectile m- mass of projectile M- mass of compound nucleus 53
- 54. How much energy needed for a projectile?? β’ π14 (πΌ, π)π17 : β’ Ξ± is charged particle: there is columbic barrier β’ Q value is -1.20MeV. Reaction will not occur if Ξ± particle of KE 1.20 MeV is used When KE =1.20 MeV; Ea= π/(π + π) π =14/18 = 0.77 MeV will be available for reaction. reaction will not occur. For reaction to occur Ea = ((M+m)/M )Q = 18/14= 1.54 MeV is the required KE of the projectile. 54
- 55. Nuclear reaction Cross section (Ο) β’ Probability of a nuclear reaction β’ Expressed in barns (b) β’ 1 b= 10-28 m = 100 fm β’ Οi β i represent the type of nuclear reaction β’ It represents the number of reactions of a type taking place per cm2 per second under conditions of unit density of target nuclei and unit flux of incident particle. 55
- 56. Target with N0 spherical nuclei per unit area n particles per cm31 cm3 Velocity of projectile=v Projectile flux= nv Rate of reaction= ΟN0nv Reaction cross section (Ο) represents the number of reactions of a type taking place per cm2 per second under conditions of unit density of target nuclei and unit flux of incident particle Lower energy particles have higher wavelength and hence they can surround large number of nuclei and hence large reaction cross section 56
- 57. Reaction cross section β’ Energy of projectile β’ Nature of target nuclei β’ Flux of projectile * β’ If energy of projectile is very low, it may not be possible to overcome the barriers β’ Too high energy of projectile can decrease the cross section 57
- 58. Variation of neutron capture cross section with energy (1/v law) β’ Neutron capture: no columbic barrier β’ Slow neutron capture cross section varies inversely with the neutron velocity : 1/v law β’ Neutron capture cross section (Ο) varies as the square root of neutron energy Since v= 2ππΈ Log(Ο)barns Log (neutron energy) eV 58
- 59. Resonance capture β’ According to 1/v law, slow neutron capture cross section area should decrease with velocity of neutron. But β’ In many cases, beyond a limit, the curve for the 1/v law gets superimposed by sharp peaks at definite values of neutron energy. These peaks are known as resonance captures Ο(barns) neutron energy (eV) 59
- 60. Resonance capture β’ Compound nucleus has several excitation levels β’ If capture of a neutron of a particular energy excites the compound nucleus precisely to one of its excited levels then the capture of neutrons of that particular energy will be highly favored. This is termed as resonance capture β’ if neutron energy is slightly greater than or less than this value, then X Compound nucleus (M) products 60
- 61. Photonuclear reactions β’ Interaction of a high energy photon with a nucleus β’ Types: (Ξ³, Ξ³β) reaction (excitation of nucleus followed by de- excitation by radiation emission. (Ξ³, n), (Ξ³, p) and (Ξ³, 2n) reaction High energy is concentrated on a proton or a neutron. When the value exceeds the separation energy that nucleon will be emitted (Ξ³, Ξ±) reaction (Ξ³, m) reaction Photon capture and multiple evaporation fragmentation Photon capture and then fragmentation 61
- 62. Features β’ Careful measurement of photon energy for (n, Ξ³) reaction gives the binding energy of the last neutron in a nucleus β’ (n, Ξ³) reactions give monoergic neutrons 62
- 63. Nuclear Fusion: Thermonuclear reactions β’ Two light nuclei fuse into a single heavy nucleus by a collision of the two interacting particles at extremely high temperatures, with the consequent release of a large amount of energy β’ 41 1 π» β 2 4 π»π + 2π½+ + 2Κ + 26.7πππ β’ Naturally occurs in stars β’ Mechanism: β’ proton-proton chain β’ Carbon-nitrogen cycle 63
- 64. Fusion reactors β’ For commercial production of electricity β’ Hydrogen fusion reactors β’ Fuel: 1 1 π»- to bring about reactions as those occur in stars deuterium, tritium practically used due to the larger reaction cross section and much lower threshold temperature 1 2 π» + 1 3 π» β 2 4 π»π + π + πΎ (energy= 17.6 MeV) β’ hurdles on developing a fusion reactor: 1. attain high temperatures as 20-100 MK (as those in stars) 2. Confine the reaction in suitable structure 64
- 66. Tokomak 66
- 68. NUCLEAR FISSION REACTION Nuclear fission is a process in which the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by- product particles. Thermal neutron 68
- 69. Theory and process β’ Explained using liquid drop model β’ Nucleus : incompressible liquid whose size is proportional to number of nucleons (A) β’ Shape is determined by coulomb force and surface tension β’ When excited by neutron capture or other process, β’ if excitation energy is not high enough, the nucleus revert back to initial state by radiative decay or by emission of particles such as Ξ±. β’ If excitation energy is high enough it may undergo fission RADIATIVE CAPTURE 69
- 70. Fission process U-235 Thermal Neutrons E=0.025 eV U-326 Th - 232 Alpha particle Radiative emission Ξ³ ray fission 70
- 71. 71
- 72. Fission fragments β’ Fission fragments F1 and F2 are not unique pairs β’ About 30 possibilities β’ F1- A= 85-105 β’ F2- A= 150-130 β’ After fission fragments will undergo Ξ² emission since they have excess neutron (high n/p ratio) β’ Asymmetry is favored β’ Similar masses β’ Least probable β’ -Only when the neutron energy is high 117-117 100-134 95-139 90-144 72
- 73. Cumulative chain fission yield β’ Used to describe the distribution of fragments β’ Fission mass yield as curve as a function of mass number A β’ Majority fission events: asymmetric β’ Max yield: A1- 90-100 A2-144-134 70 90 110 130 150 200 Fissionyield Mass number 73
- 74. Fission energy β’ Fission is accompanied by mass loss and liberation of corresponding energy β’ Fission energy= Ξm Γ 931 MeV β’ According to law of conservation of energy, β’ High mass fragment- low KE β’ Low mass fragment- high KE 70 90 110 130 150 200 Fissionyield Energy (MeV) Distribution of energy 74
- 75. Fission Cross Section and Threshold β’ Possibilities β’ (n,n) type reaction (scattering) β’ Radiative capture β’ Fission Fission cross section is the measure of the probability of fission reaction. It depends on β’ Critical size of material β’ Threshold energy of neutrons required 75
- 76. Fission neutrons β’ During fission the fissioning nucleus ejects 2-3 neutrons directly at the moment of fission β’ For U-235 the fission reaction may be written as π235 + π π‘βπππππ β π236 β π + π + π£π( πππ π‘ πππ’π‘ππππ ) V- average number of neutrons in each process Net number of neutrons emitted per neutron used up= Ξ·= π£Ο π Ο π+Ο π Ο π β ππππ π π πππ‘πππ πππ πππ π πππ Ο π β ππππ π π πππ‘πππ πππ ππππππ‘ππ£π ππππ‘π’ππ 76
- 77. Prompt and delayed neutrons β’ Delayed neutrons make 1% of total fission neutrons β’ Emitted after varying time lags β’ Number of neutron ejected decrease exponentially with time β’ Attributed to fission fragments whose Ξ² decay energy is more than the BE of last neutron in daughter product Prompt neutrons emitted during fission Within 10-14 s Energy distribution N(Ξ΅)=e-E sin h(2E)1/2 Neutrons are spread over a energy range 0.05 to 17 MeV Neutron energy intensity 77
- 78. Fission by high energy neutrons β’ In some nucleus fission is possible only by fast neutrons (1-10 MeV) β’ Nuclei Threshold energy Np-237 0.25 U-238 0.92 U-234 0.28 Pa-231 0.45 Th-232 1.05 Th-230 1 Bi-207 15 78
- 79. A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. Nuclear chain reactions are series of nuclear fissions (splitting of atomic nuclei), each initiated by a neutron produced in a preceding fission. For example, 2.5 neutrons on the average are released by the fission of each uranium-235 nucleus that absorbs a low-energy neutron. nuclear chain reaction 79
- 80. Critical size The critical size of a system containing a fissile material is defined as the minimum size in which at least one neutron is available for further fission reaction. Fission chain reaction is not possible if the size is less than critical size. The mass of fissile material at the critical size is called critical mass. 80
- 81. Fertile and fissile materials β’ Fissionable: one capable of undergoing fission U-235, U-238, Pu-239, Th-232 β’ Fissile material: one that can sustain fission chain reaction by thermal neutrons U-235, Pu-239 β’ Fertile material: not itself undergo Sustained chain reaction but can be converted to fissile material U-238, Th-232 Fertile U-238 Th-232 Fissile U-235 Pu-239 81
- 82. NUCLEAR REACTOR A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research. 82
- 83. The reactor core generates heat in a number of ways: ο· The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. ο· The reactor absorbs some of the gamma rays produced during fission and converts their energy into heat. ο· Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time even after the reactor is shut down. 83
- 84. COMPONENTS OF A NUCLEAR REACTOR AND REACTOR CONTROL 1. Core: Part of reactor containing fuel elements (fissile U-235, U-233, Pu239 etc.). Fuel elements are made of plates, or rods of Uranium metal or ceramic, which are usually clad in a thin sheath of SS, Zirconium or aluminium to provide corrosion resistance, retention of radioactivity and structural support. 2. Core moderator: commonly water (H2O or D2O) or graphite is dispersed between the fuel assemblies to moderate or slow down fast neutrons produced during fission. 3. Control rods: Made of neutron absorbing materials (B, Cd, Ag and In)upon movement in or out of the core, vary the number of neutrons available for chain reactions. 4. Reflector: surround the core to reduce the loss of the neutrons. Graphite, light water and beryllium is used as reflector. 5. Coolant: the heat generated as a result of fission is removed by circulation of coolant through the core. Coolant used should have high BP, low MP and should be resistant to corrosion. CO2, H2O, molten Na, mercury etc are used as coolant. 6. Radiation shielding / biological shielding: for preventing the radiation hazard. 84
- 85. 1. Each fission reaction releases large amount of energy 2. Each fission results in an average of 2.5 neutrons (in case of U-235) Mainly 2 characteristics of fission reaction are of great importance: β’ A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction. β’ In nuclear reactors, this chain reaction will proceed in a constant rate by controlling the chain propagation Sustained chain reaction: the key 85
- 86. Its all controlling the number of neutrons!! β’ In a nuclear reactor, self-sustaining chain reaction takes place. β’ The neutrons produced per fission event must be large enough to sustain the chain reaction, despite the loss of neutrons due to leakage and by absorption in non-fissile materials. β’ The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. 86
- 87. 87
- 88. ο· If a system is critical, the multiplication factor is 1. Signifies a steady state Number of neutrons and fissions is constant from cycle to cycle Amount of energy output in unit time is constant ο· Supercritical systems have multiplication factors greater than 1 Signifies a divergent state Number of neutrons, fissions and energy output rapidly increase from cycle to cycle Ends up in explosion ο· subcritical systems have multiplication factors lesser than 1. Signifies a convergent state Number of neutrons, fissions and energy output diminish from cycle to cycle Reactor soon gets quenched 88
- 89. Breeder Reactor β’ A Breeder Reactor is a nuclear reactor that "breeds" fuel. β’ Types β’ Fast breeder reactor β’ Thermal breeder reactor 89
- 90. Fast breeder reactor β’ convert uranium-238 into the fissionable isotope plutonium-239 90
- 91. Thermal breeder reactor β’ converts this isotope into fissionable uranium-233 91
- 92. Fast Breeder Test Reactor β’ breeder reactor located at Kalpakkam, India. β’ The Indira Gandhi Center for Atomic Research (IGCAR) and Bhabha Atomic Research Centre (BARC) jointly designed, constructed, and operate the reactor. β’ The reactor uses a plutonium-uranium mixed carbide fuel and liquid sodium as a coolant. 92
- 93. Counting techniques β’ Ionization chambers β’ Geiger muller counters β’ Proportionality counters β’ Scintillation counters 93
- 94. Ionization chambers β’ the simplest of all gas-filled radiation detectors β’ detection and measurement of certain types of ionizing radiation; X-rays, gamma rays, and beta particles β’ only uses the discrete charges created by each interaction between the incident radiation and the gas β’ good uniform response to radiation over a wide range of energies β’ Cannot distinguish between radiations 94
- 95. β’ β’ A voltage potential is applied between the electrodes to create an electric field in the fill gas. When gas between the electrodes is ionized by incident ionizing radiation, ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. This generates an ionization current which is measured by an electrometer circuit. The electrometer must be capable of measuring the very small output current which is in the region of femtoamperes to picoamperes, depending on the chamber design, radiation dose and applied voltage 95
- 96. GM counter β’ Works on the principle of indirect ionization β’ Can detect alpha, beta or gamma radiation β’ Requires very high voltage for working β’ Cannot detect uncharged particles β’ Independent of energy of incident particle 96
- 97. Geiger Muller counters Voltage - - - -- -- - - 97
- 98. Proportional counters β’ Works on the principle of gas multiplication β’ Can detect low energy X-rays, fast neutron, slow neutrons or gamma rays β’ Requires low voltage for working than GM counter but higher than ionozation chamber β’ Can detect uncharged particles β’ Pulse height depends on energy of incident radiation β’ High resolution β’ Useful to study Mossbauer effect also 98
- 99. Proportional counters Voltage - - - -- -- - - 99
- 100. Scintillation counter β’ A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses. 100
- 103. Chemical investigations Reaction mechanism β’Structure determinations β’Isotope exchange reactions β’ Esterification, hydrolysis, oxidation of H2O2, fumaric acid, CO etc., Friedal Craftsβ reactions β’ evidence for intermediates β’ PCl5, thiosulphate ion 103
- 104. Physico- chemical research Solubility of sparingly soluble compound β’Surface area of a powder or precipitate β’Rates of diffusion Solubility of sparingly soluble compound 104
- 106. Age determinations Dating by tritium content β’Dating by carbon Age of earth Age of minerals and rocks 106
- 107. Medical applications Volume of blood Defects in blood circullation Mechanism of bone fracture healing Study neurotransmit ter pathway in brain Other application radioimmunoa ssay Thyroiditis Brain tumor location 107