Nuclear Weapons Nuclear Weapons They produce incredible - PDF document

Nuclear Weapons 2 Observations about Nuclear Weapons 1 Observations about Nuclear Weapons Nuclear Weapons � They release enormous amounts of energy They release enormous amounts of energy Nuclear Weapons Nuclear Weapons � They produce incredible temperatures They produce incredible temperatures � They produce radioactive fallout They produce radioactive fallout � They are relatively difficult to make They are relatively difficult to make � They use chain reactions They use chain reactions Nuclear Weapons 3 Nuclear Weapons 4 3 Questions about 3 Questions about Question 1 Question 1 Nuclear Weapons Nuclear Weapons � Where is nuclear energy stored in atoms? Where is nuclear energy stored in atoms? � Where is nuclear energy stored in atoms? Where is nuclear energy stored in atoms? � Why are some atomic nuclei unstable? Why are some atomic nuclei unstable? � How does a nuclear chain reaction work? How does a nuclear chain reaction work? Nuclear Weapons 5 Nuclear Weapons 6 Atomic Nuclei Atomic Nuclei Structure of Nucleus Structure of Nucleus � Atoms are usually electrically neutral Atoms are usually electrically neutral � Nucleus contains two kinds of nucleons Nucleus contains two kinds of nucleons � They must have as many + charges as They must have as many + charges as – – charges charges � Protons are positively charged Protons are positively charged � Each electron must be matched by a + charge Each electron must be matched by a + charge � Neutrons are electrically neutral Neutrons are electrically neutral � At the center of an atom is its nucleus A A At the center of an atom is its nucleus h h f f i i i i l l � Two forces are active in a nucleus T Two forces are active in a nucleus T f f i i i i l l � Extremely small (1/100,000th of atom’s diameter) Extremely small (1/100,000th of atom’s diameter) � Electrostatic repulsion between protons Electrostatic repulsion between protons � Contains most of the atom’s mass Contains most of the atom’s mass � Nuclear force attraction between touching nucleons Nuclear force attraction between touching nucleons � Also contains most of the atom’s potential energy Also contains most of the atom’s potential energy � At short distances, the nuclear force dominates At short distances, the nuclear force dominates � Evidence is related to: E=mc Evidence is related to: E=mc 2 � At long distances, the electric force dominates At long distances, the electric force dominates 1

Nuclear Weapons 7 Nuclear Weapons 8 Question 2 Question 2 Nuclear Stability Nuclear Stability � The nucleons in a nucleus are in equilibrium The nucleons in a nucleus are in equilibrium � Why are some atomic nuclei unstable? Why are some atomic nuclei unstable? � To be classically stable, that equilibrium must be stable To be classically stable, that equilibrium must be stable � To be quantum To be quantum- -mechanically stable, that equilibrium mechanically stable, that equilibrium must also be the overall potential energy minimum must also be the overall potential energy minimum ust a so be t e ove a pote t a e e gy ust a so be t e ove a pote t a e e gy u u � Quantum mechanics and the Heisenberg uncertainty Quantum mechanics and the Heisenberg uncertainty principle allow the nucleons to “try out” arrangements principle allow the nucleons to “try out” arrangements that are quite different from their equilibrium positions that are quite different from their equilibrium positions � If they find a path to a new, lower If they find a path to a new, lower- -potential potential- -energy energy equilibrium, the nucleus may fall apart equilibrium, the nucleus may fall apart Nuclear Weapons 9 Nuclear Weapons 10 Radioactivity Radioactivity Question 3 Question 3 � Large nuclei have two possible problems: Large nuclei have two possible problems: � How does a nuclear chain reaction work? How does a nuclear chain reaction work? � Too many protons: too much electrostatic potential Too many protons: too much electrostatic potential � Too many neutrons: isolated neutrons are unstable Too many neutrons: isolated neutrons are unstable � Balance between protons and neutrons is tricky Balance between protons and neutrons is tricky B l B l b b d d i i i k i k � Large nuclei tend to fall apart spontaneously Large nuclei tend to fall apart spontaneously � These breakups are known as radioactive decay These breakups are known as radioactive decay � and can include a splitting process called fission and can include a splitting process called fission Nuclear Weapons 11 Nuclear Weapons 12 Induced Fission Induced Fission Chain Reaction Chain Reaction � A large nucleus can break when struck hard A large nucleus can break when struck hard � Since neutrons can induce fission Since neutrons can induce fission � Collision knocks its nucleons Collision knocks its nucleons � and induced fission releases neutrons, and induced fission releases neutrons, out of stable equilibrium out of stable equilibrium � this cycle can repeat, this cycle can repeat, � Collision � Collision Collision altered nucleus Collision-altered nucleus altered nucleus altered nucleus a chain reaction! a chain reaction! a chain reaction! a chain reaction! may undergo induced fission may undergo induced fission � Each fission releases energy Each fission releases energy � Since a neutron isn’t repelled Since a neutron isn’t repelled � Many fissions release Many fissions release by nucleus, it makes an ideal by nucleus, it makes an ideal prodigious amounts of energy prodigious amounts of energy projectile for inducing fission projectile for inducing fission � Sudden energy release Sudden energy release produces immense explosion produces immense explosion 2

Nuclear Weapons 13 Nuclear Weapons 14 Requirement for a Bomb Requirement for a Bomb The Gadget & Fat Man The Gadget & Fat Man Each of these fission bombs started as a 239 239 Pu � A fission bomb requires 4 things: A fission bomb requires 4 things: � Each of these fission bombs started as a Pu � sphere below critical mass (6 kg) sphere below critical mass (6 kg) � An initial neutron source An initial neutron source � a fissionable material (undergoes induced fission) a fissionable material (undergoes induced fission) � It was crushed explosively to supercritical mass It was crushed explosively to supercritical mass � each fission must release additional neutrons each fission must release additional neutrons h fi i n m h fi i n m t r l t r l dditi n l n dditi n l n tr n tr n � � and promptly underwent and promptly underwent d d l l d d � material must use fissions efficiently (critical mass) material must use fissions efficiently (critical mass) an explosive chain reaction. an explosive chain reaction. � 235 235 U and U and 239 239 Pu are both fissionable materials, Pu are both fissionable materials, � but but 235 235 U is rare and must be separated from U is rare and must be separated from 238 238 U � and and 239 239 Pu is made by exposing Pu is made by exposing 238 238 U to neutrons. U to neutrons. Nuclear Weapons 15 Nuclear Weapons 16 Introductory Question Introductory Question (revisited) Little Boy Little Boy (revisited) Is it possible to have 100 tons of plutonium Is it possible to have 100 tons of plutonium � This bomb started as a This bomb started as a 235 235 U hollow sphere U hollow sphere � and not have it explode? and not have it explode? below critical mass (60 kg) below critical mass (60 kg) � A cannon fired a A cannon fired a 235 235 U plug through that sphere U plug through that sphere so that it exceeded critical mass so that it exceeded critical mass so that it exceeded critical mass so that it exceeded critical mass Yes Yes Y A. A. � and it promptly and it promptly No No B. B. underwent an underwent an explosive chain explosive chain reaction. reaction. Nuclear Weapons 17 Summary about Summary about Nuclear Weapons Nuclear Weapons � Nuclear energy is stored in atomic nuclei Nuclear energy is stored in atomic nuclei � Nuclear fission released electrostatic potential Nuclear fission released electrostatic potential � Each fission releases an astonishing energy Each fission releases an astonishing energy � Induced fission permits a chain reaction Induced fission permits a chain reaction � Fission bombs explode via a chain reaction Fission bombs explode via a chain reaction 3