核技術(shù)導(dǎo)論英文課件：Chapter 6 Nuclear Weapons

1、Chapter 6. Nuclear WeaponsHistory of Weapons DevelopmentNuclear Explosions Uranium and Nuclear Weapons Plutonium and Nuclear Weapons Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of PlutoniumReactor

2、-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 and Nagasak

3、i. 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 USSR China

4、 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 1960s, it bec

5、ame 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 testing nu

6、clear 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 12,000 nuclea

7、r 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 weapons are at

8、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. Atomic bombing

9、 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 the attacks

10、.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 history.Hi

11、roshima 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 bombings a

12、re 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 Japan occ

13、urred 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 basically

14、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 pressures t

15、oward 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 instant of the

16、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 accounted for a

17、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 effectsduring

18、 the period from 1950 through 1990(白血?。〤hapter 6. Nuclear WeaponsHistory of Weapons DevelopmentNuclear Explosions Uranium and Nuclear Weapons Plutonium and Nuclear Weapons Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionE

19、xplosive 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, 2b)239Pu2.1 Basic

20、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 method: Lighter molecul

21、es 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 Material: Separating 235U b

22、y 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 tubes 1 to 2 meters lo

23、ng 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 center. Enhanced conc

24、entration 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 column which rotates by a

25、ction 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 SeparationIn the cyclotron resonance method a radiofrequency field sele

26、ctively energizes one of the ionized isotopes in magnetically confined plasma; isotopes are differentiated and the more energetic atoms are collected. In the laser induced selective ionization method, the laser is tuned to selectively to ionize U235, but not U238. An electric field extracts the ions

27、 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 commonly expressed in k

28、ilotons (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 fragments themselves. Complet

29、e 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 / 1 MeV) = 3.15e-11 J This

30、 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 fissioning of the mater

31、ial.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 the enemy. That bomb had m

32、ore 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 Weaponsestimates of critical

33、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 Slotinthe average distance: t

34、he 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. = A/NAf = 17 cm.Rc, on

35、e-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 by a factor of 2 or 3Mat

36、erials 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 critical mass for nucle

37、ar 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 public by weapons builders.

38、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 undergoing fission reactions incr

39、eases 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 level. the time between succ

40、essive 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 generationsN = N0etit would

41、 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 WeaponsHistory of Weapons DevelopmentNu

42、clear Explosions Uranium and Nuclear Weapons Plutonium and Nuclear Weapons Nuclear Weapons related IssuesProducing Bomb MaterialsEnergy Yield Critical Mass for Nuclear WeaponsBuildup of a Chain ReactionExplosive Properties of PlutoniumReactor-Grade Plutonium as a Weapons Materiallow-enriched uranium

43、 (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:preferable for building a compact bombit requires more separa

44、tive 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% enrichment4. Plutonium and Nuclear Weapon

45、s4.1 Explosive Properties of PlutoniumDifficulty predetonation(預(yù)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 cause premature initiation of a chain

46、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 thermal neutron absorbed, and hence is the

47、 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 those such as 232Th and 238U that, upon t

48、hermal 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 emission rate from spontaneous fission is

49、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 problems due to spontaneous fission, it does

50、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 in 238U is impossible, It contributes l

51、ittle 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 compression can change the massfrom su

52、bcritical 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 spontaneous fission can trigger a chain reactio

53、n 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 the reversal occurs relatively early,

54、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 thermonuclear bomb begins with the de

55、tonation 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 is a fusion bomb designed to release n

56、eutrons.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. 1, 1954 at Bikini Atoll yielded 15 m

57、egatons and had a fireball 4 miles in diameter.USSR H-bomb yields 100 megatons.H-bombChapter 6. Nuclear WeaponsHistory of Weapons DevelopmentNuclear Explosions Uranium and Nuclear Weapons Plutonium and Nuclear Weapons Nuclear Weapons related IssuesBasic Characteristics of Fission BombsCritical Mass

58、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. Radiation effects last decades. Technological obsta

59、cles: 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. Inter-continental Ballistic Missiles (ICBMs). “H

60、ardened” missile silos (導(dǎo)彈發(fā)射井） Mobile missile launchers. Submarine-launched Ballistic Missiles (SLBM). Multiple independent re-entry vehicle (MIRV).多彈頭分導(dǎo)導(dǎo)彈Mutually Assured DestructionMAD doctrine asserts that nuclear war would berationally impossible since both countries would bedestroyed. Perhaps n