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1、Chapter 6. Nuclear Weapons1.History of Weapons Development2.Nuclear Explosions 3.Uranium and Nuclear Weapons 4.Plutonium and Nuclear Weapons 5.Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of Pluton
2、iumReactor-Grade Plutonium as a Weapons Material1934: Italian physicist Enrico Fermi learns how to produce nuclear fission. Race to develop weaponized nuclear reactions. 1942: US Manhattan Project led by Robert Oppenheimer develops fission weapons. 1945: Little Boy and Fat Man dropped on Hiroshima a
3、nd Nagasaki. WWII ends. Over 100,000 dead. 1949: USSR tests its first nuclear weapon.1952: US develops first fusion bomb (H-Bomb). 450 times more powerful than Nagasaki bomb. 1952: UK develops its own nuclear weapon. 1960: France develops nuclear weapon. 1964: China develops nuclear weapon.1968: US
4、USSR China France, UK sign nuclear non-proliferation treaty (NPT). 189 countries now party to the treaty. Yet, others have developed nuclear weapons: Israel, Pakistan, India, North Korea, (South Africa). Iran may be pursuing nuclear weapons capabilities, but claims program is peaceful.During the 196
5、0s, it became possible for nuclear weapons to be delivered anywhere in the world.Comparison of years of achieving nuclear weapons and civilian nuclear electric power, for acknowledged nuclear-weapon countries.History Of Nuclear WeaponsNuclear weapons were symbols of military and national power, and
6、testing nuclear was often used both to test new designs as well as to send political messages. There are at least 29,000 nuclear weapons held by at least seven countries, though 96% of these are in the possession of just two the United States and the Russian Federation.Nuclear Weapons in USAbout 12,
7、000 nuclear weapons are deployed in 14 states. Five states: New Mexico, Georgia, Washington, Nevada, North Dakota which account for 70 percent of the total. The others are in Wyoming, Missouri, Montana, Louisiana, Texas, Nebraska, California, Virginia, Colorado.Overseas, about 150 U.S. nuclear weapo
8、ns are at 10 air bases in seven countries: Belgium, Germany, Greece, Italy, the Netherlands, Turkey and Britain.The United States is believed to be the only nation with nuclear weapons outside of its borders. The number of U.S. nuclear weapons in Europe has greatly decreased in the early 1980s. Atom
9、ic bombing of Hiroshima and Nagasaki The United States Army Air Force dropped two atomic bombs on the Japanese cities of Hiroshima and Nagasaki on August 6 and August 9, 1945 during World War II.There goal was basically to secure the surrender of Japan.At least 120,000 people died immediately from t
10、he attacks.Thousands of people died years after from the effects of nuclear radiation.About 95% of the casualties were civilians.Japan sent notice of its unconditional surrender to the allies on August 15, a week after the bombings.These bombings were the first and only nuclear attacks in the world
11、history.Hiroshima and Nagasaki Cont The role of bombings in Japans was to make them surrender.The U.S. believed that the bombing ended the war sooner.In Japan, the general public tends to think that the bombings were needless as the preparation for the surrender was in progress.The survivors of the
12、bombings are called hibakusha, a Japanese word that literally translates to “bomb-affected people.”The suffering of the bombing is the root of Japans postwar pacifism, and the nation has sought the abolition of nuclear weapons from the world ever since.Aftermath Attack On JapanThe nuclear attacks on
13、 Japan occurred during hot weather. So it was more effected toward the people.Many people were outside and wearing light clothings. This ladys skin is burned in a patterns corresponding to the dark patterns of her kimono.The dark sections of clothing absorbed more heat and burnt her to her flash.So
14、basically darker cloths would make it worst.Aftermath ContThis was the effect of Nagasaki it left a heavy destruction at high blast. This bomb created a smoke that would basically harm people.The smokestacks happen from the open at the top. The blast wave may have traveled down the stacks bringing p
15、ressures toward were it blast.The blast was so powerful it ruin almost most of the country.William Schull, Atomic Bomb Joint Casualty Commission (ABCC)Contrary to what is commonly supposed, the bulk of the fatalities at Hiroshima and Nagasaki were due to burns caused either by the flash at the insta
16、nt of the explosion or from the numerous fires that were kindled, and were not a direct consequence of the amount of atomic radiation received. Indeed, the the ABCC estimated that over half the total deaths were due to burns and another 18% due to blast injury. Nonetheless, ionizing radiation accoun
17、ted for a substantial number of deaths, possibly 30%.Atomic Bomb Survivor Excess Cancer 5467Total Cancer Excess 437 (421) Population of Survivors Studied 86,572Total Cancers observed after the Bomb 8,180 Total Cancers Expected without Bomb 7,743Excess Leukemia 104Excess Tumor 334437+=The delayed eff
18、ectsduring the period from 1950 through 1990(白血病）Chapter 6. Nuclear Weapons1.History of Weapons Development2.Nuclear Explosions 3.Uranium and Nuclear Weapons 4.Plutonium and Nuclear Weapons 5.Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup
19、of a Chain ReactionExplosive Properties of PlutoniumReactor-Grade Plutonium as a Weapons MaterialProducing Bomb MaterialsSeparate 235U (0.7%) from natural uranium:gas diffusion of UF6centrifuge of UF6 gasthermal diffusion of UF6 gas electromagnetic separationProduction of 239Pu by the reaction238U(n
20、, 2b)239Pu2.1 Basic Characteristics of Fission BombsBomb Material: Separating 235U by gas Diffusion One diffusion unit the diffusion plant The blue spot is a personhttp:/www.npp.hu/uran/3diff-e.htmUreys research group: gas molecules with different molecular mass could be separated by a diffusion met
21、hod: Lighter molecules pass through membranes containing pin holes faster than heavier molecules235UF6 is 389, 238UF6 392 Since the molecular weights differ so little the industrial operation is a long and laborious process.1000 unitsBomb Material: Separating 235U by Electromagnetic methBomb Materia
22、l: Separating 235U by Electromagnetic methodThe principle of this method is the same as the mass spectrometry for chemical analysis.This is still a very important method for chemical analysis today.$ 300 million for coilscentrifuge methodthe centrifuge method feeds UF6 gas into a series of vacuum tu
23、bes 1 to 2 meters long and 15-20 cm diameter, each containing a rotor. When the rotors are spun rapidly, at 50,000 to 70,000 rpm, the heavier molecules 238UF6 increase in concentration towards the cylinders outer edge. There is a corresponding increase in concentration of 235UF6 molecules near the c
24、enter. Enhanced concentration is further achieved by inducing an axial circulation within the cylinder. The enriched gas is drawn off and goes forward to further stages while the depleted UF6 goes back to the previous stage. Isotope Separation by Plasma CentrifugeA vacuum arc produces a plasma colum
25、n which rotates by action of an applied magnetic field. The heavier isotopes concentrate in the outer edge of the plasma column resulting in an enriched mixture that can be selectively extractedFission Energy for War and PeaceNew Methods of Isotope Separation1.In the cyclotron resonance method a rad
26、iofrequency field selectively energizes one of the ionized isotopes in magnetically confined plasma; isotopes are differentiated and the more energetic atoms are collected. 2.In the laser induced selective ionization method, the laser is tuned to selectively to ionize U235, but not U238. An electric
27、 field extracts the ions from the weakly ionized plasma and guides them up to collecting plates. Energy Yield of Nuclear WeaponsTNT, Trinitrotoluene三硝基甲苯The explosive yield of TNT is considered to be the standard measure of strength of bombs and other explosivesThe energy yield of nuclear weapons is
28、 commonly expressed in kilotons (kt) or megatons (Mt) of high explosive (TNT) equivalent.1 kt of TNT = 1012 cal = 4.18 1012 J.Estimate the energy released by the fission of 1.0 kg of 235U. (3.15e-11 J) 1000 g = 8.06e13 J (per kg). About 86% of the energy is in the kinetic energy of the fission fragm
29、ents themselves. Complete fission of 1 kg of 235U would give a prompt explosive yield of about 71013 J, or 17 kt. 1 mol235 g6.023e231 mol235U92 142Nd60 + 90Zr40 + 3 n + QQ = (235.043924 - 141.907719 - 89.904703 - 3x1.008665) = 0.205503 amu (931.4812 MeV/1 amu) = 191.4 MeV per fission(1.6022e-13 J /
30、1 MeV) = 3.15e-11 J This amount of energy is equivalent to 2.21010 kilowatt-hour, or 22 giga-watt-hour. This amount of energy keeps a 100-watt light bulb lit for 25,000 years.Actual yields in nuclear weapons are less than 17 kt/kg of fissile material, because a bomb will disassemble without complete
31、 fissioning of the material.the worlds first nuclear bomb, in the Trinity test in NewMexico in July 1945, 6.1 kg of plutonium = a yield of 18.6 ktModern bombs are efficient, approaching 40%Sixteen hours ago an American airplane dropped one bomb on Hiroshima, Japan, and destroyed its usefulness to th
32、e enemy. That bomb had more power than 20,000 tons of T.N.T. . Harry S. Truman 1The minimum quantity for a sustained chain reaction to take place is called the critical mass or critical size, which depends on the moderator, chemical and physical states, shape etc. 2.3 Critical Mass for Nuclear Weapo
33、nsestimates of critical masses for 239Pu and 235U must be verified by experiments. These experiments were extremely dangerous, because an accidental assembly of a critical mass would lead to an explosion. The experiment to determine the critical mass was called tickling the dragons tail. Louis Sloti
34、nthe average distance: the average distancesuch a particle travels before it interacts.mean-free-path lengthCritical Mass With and Without ReflectorsProperties of fissile materials for nuclear weapons: fission cross section, neutrons per fission, mean free path at 1 MeV, and critical mass and radius
35、. = A/NAf = 17 cm.Rc, one-half of the mean free path . Larger , lower Rc,Mc = 4R3/3 60 kgthe fissile material to be isotopically pure, or almost so, and in metallic form.The reflector returns escaping neutrons to the fissile volume by one or more scattering events; this can reduce the critical mass
36、by a factor of 2 or 3Materials for the reflector: uranium and tungsten.Thicknesses: 4 - 15 cmTypes of Nuclear BombFor an effective nuclear weapon, the critical mass must be assembled quicklygun-type bombReducing Critical Masses by Implosion 1/V 1/3M 1/2Nominal Numbers for the Critical MassThe actual
37、 critical mass for nuclear weapons cannot be precisely stated. depends on design variables: the compression achieved in implosion, the tamper used, and the isotopic purity of the material. Different designs give different resultsthe nature of advanced designs and the resulting sizes are not made pub
38、lic by weapons builders.it suffices to use the nominal numbers :about 10 kg for weapons-grade uranium 5 kg for weapons-grade plutonium.2.3 Buildup of a Chain ReactionWhen a critical mass is assembled, neutrons from the natural fission process initiate a chain reaction. The number of nuclei undergoin
39、g fission reactions increases rapidly leading to an explosion. The energy released in fission reactions blow the fission material apart, and at some point, the chain reaction stops.In a nuclear weapon, dispersion of the fuel begins before the developing chain reaction reaches its maximum design leve
40、l. the time between successive fission generations must be shortThe chain reaction in a bomb therefore relies on fast neutronsThe time rate of change in the number of fission neutrons NK: effective multiplication factor, is the delayed neutron fraction is the mean time between successive fission gen
41、erationsN = N0etit would take 80 generations to go from one fission in the first generation to 1.2 1024 fissions in the last, consume all the fuel 1 kg U = 2.5 1024 nucleithe chain reaction would develop completely in a time on the order of one-millionth of a secondChapter 6. Nuclear Weapons1.Histor
42、y of Weapons Development2.Nuclear Explosions 3.Uranium and Nuclear Weapons 4.Plutonium and Nuclear Weapons 5.Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of PlutoniumReactor-Grade Plutonium as a We
43、apons Materiallow-enriched uranium (LEU): 0.7120%highly enriched uranium (HEU): 20%weapons-grade uranium: 90%Uranium bombs can be made over a wide range of enrichments, but the mass of uranium required is greater forlower enrichment.high enrichments are used in 235U bombs:1.preferable for building a
44、 compact bomb2.it requires more separative work to obtain a 37-kg critical mass at 60% enrichment than to obtain the smaller critical mass (roughly 18 kg) at 90% enrichment.the critical mass for 60% enrichment is 22 kg of 235U (37 kg of U), whereas only 15 kg of 235U (and U) are required at 100% enr
45、ichment4. Plutonium and Nuclear Weapons4.1 Explosive Properties of PlutoniumDifficulty predetonation(预爆）:The isotope 240Pu has a half-life of 6564 years. Its primary decay mode is by alpha-particle emission, but it also sometimes decays by spontaneous fission.This fission produces neutrons that may
46、cause premature initiation of a chain reaction in a bomb before the fissile material has been fully compressed.With predetonation, the weapon gives a much smaller explosion than designed; in other words, it “fizzles.”1. 233U has the largest value of , the number of fission neutrons produced per ther
47、mal neutron absorbed, and hence is the best prospect for a thermal breeder reactor. A breeder reactor needs an of at least two since one neutron is needed to sustain the chain reaction and oneneutron must be absorbed in the fertile material to breed a new fissile fuel atom. Fertile materials are tho
48、se such as 232Th and 238U that, upon thermal neutron absorption, may yield fissile materialsProperties of different grades of plutonium: isotopic abundances, neutron emission from spontaneous fission (SF), and decay heat from radioactive decay.Solution 1 different grades of plutoniumThe neutron emis
49、sion rate from spontaneous fission is about 910 (g.s)1 for 240PuThe number of neutrons from spontaneous fission is considerably less in weapons-grade plutonium and still less in “supergrade” plutonium.The probabilities of premature detonation are correspondingly reducedAlthough 240Pu creates problem
50、s due to spontaneous fission, it does not greatly increase the required mass of 239Pu for criticality.the fission cross section for 240Pu is about 1.5 b at 1 MeV and a chain reaction is possible in pure 240PuFor 238U, the fission cross section even at 1 MeV is low and a fast neutron chain reaction i
51、n 238U is impossible, It contributes little to the fission yield but absorbs neutrons.a 238U contaminant means that a greater mass of 235U is required for criticality.Solution 2: bring together a critical mass very quicklyThe critical mass is inversely proportional to the square of the density, and
52、compression can change the massfrom subcritical to supercritical. In a typical bomb, the implosion shock wave has a speed of about 5000 m/sthe implosion proceeds at a rate such that the bomb goes from initial supercriticality to full compression in a period of about 105 sM 1/2neutrons from spontaneo
53、us fission can trigger a chain reaction before the plutonium is fully compressed, at any time after the material becomes supercritical (k 1). The energy generated by fission then reverses the implosion, the system expands, the multiplication factor drops, and thechain reaction eventually ceases. If
54、the reversal occurs relatively early, when the compression and multiplication factors are low, the chain reaction willnot be rapid enough to have embraced much of the plutonium before it is terminated.41Nuclear Fusion BombsA thermonuclear Bomb consists of explosives, fission fuel, and D, T, and Li.A
55、 thermonuclear bomb begins with the detonation of small quantities of conventional explosives. The explosion starts fissionable chain reaction that heats to 1e7 K to ignite a chain of fusion reactions.2D + 3T 4He + n + 17.6 MeVn + 6Li T + 4He ( = 942 b)n + 7Li T + 4He + n ( = 0.045 b)A neutron bomb
56、is a fusion bomb designed to release neutrons.A cobalt bomb is a dirty bomb to kill using radioactive 60Co.Fusion42Nov. 1, 1952, the first H-bomb Mike tested,mushroom cloud was 8 miles across and 27 miles high;the canopy was 100 miles wide, 80 million tons of earth was vaporized. H-bomb exploded Mar
57、. 1, 1954 at Bikini Atoll yielded 15 megatons and had a fireball 4 miles in diameter.USSR H-bomb yields 100 megatons.H-bombChapter 6. Nuclear Weapons1.History of Weapons Development2.Nuclear Explosions 3.Uranium and Nuclear Weapons 4.Plutonium and Nuclear Weapons 5.Nuclear Weapons related IssuesBasi
58、c Characteristics of Fission BombsCritical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of PlutoniumReactor-Grade Plutonium as a Weapons Material Fission produces a charge equivalent to 500,000 TONS of TNT. Fusion produces a charge equivalent to 50,000,000 TONS of TNT. Rad
59、iation effects last decades. Technological obstacles: high-grade radioactive materials do not occur naturally. Delivery systems must not damage explosive materialWeapons Delivery SystemsEarliest weapons were simply gravity bombsdropped from airplanes. Ballistic missiles reduce risk of interception.
60、Inter-continental Ballistic Missiles (ICBMs). “Hardened” missile silos (导弹发射井） Mobile missile launchers. Submarine-launched Ballistic Missiles (SLBM). Multiple independent re-entry vehicle (MIRV).多弹头分导导弹Mutually Assured DestructionMAD doctrine asserts that nuclear war would berationally impossible s
61、ince both countries would bedestroyed. Perhaps nuclear weapons preserved peace during Cold War? MAD depends on: second-strike capability Inability to defend against nuclear attack Protection against accidental launch Rational enemyM.A.D.Hardened ICBM silos and SLBMs reduced chancethat weapons would
62、be destroyed in a first strike. Anti-ballistic missile systems. Defensive? Yet, undermines MAD. 1972: ABM treaty restricts development of antiballisticmissile technology. 2002: US withdraws from treaty, claiming threat fromrogue nations. Current issues with Russia. However, ABM technology still not
63、very effective. MIRVs and decoys difficult to deal with. Fears of a new arms race (Death Star?)Threat of Proliferation: Former USSRFormer Soviet republics had nukes upon independence. Concern about weakness in the states.Republics agree to give up nuclear weapons, return them to Russia. Alexander Le
64、bed, former secretary of Russian Security Council, claimed in 1997 interview that about 100 weapons are unaccounted for. Controlled radioactive substances also missing.Proliferation: Nuclear Terrorism Concern that groups not deterred by MAD could obtain a nuclear weapon. If a terrorist attacks, wher
65、e do you retaliate? Dirty-bomb would release radioactive material. Some states may provide technology. Or insecure facilities could be undermined by terror groups.Arms LimitationUS-Russia Agreements: SALT I & II with USSR/Russia. Strategic Arms Reduction Treaty (1991) Anti-Ballistic Missile Treaty (
66、1972-2002) Agreements on testing Partial Test-Ban Treaty (1963) Comprehensive Test Ban Treaty (1996) Proliferation agreements Nuclear Non-proliferation Treaty (1968) Monitoring Agency International Atomic Energy Agency (IAEA)51Adebayo Amusu www.iearn.orgAlexander May Chapter 6. Nuclear Weapons1.Hist
67、ory of Weapons Development2.Nuclear Explosions 3.Uranium and Nuclear Weapons 4.Plutonium and Nuclear Weapons 5.Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of PlutoniumReactor-Grade Plutonium as a Weapons Material
