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Chapter 6. Nuclear Weapons. History of Weapons Development Nuclear Explosions Uranium and Nuclear Weapons Plutonium and Nuclear Weapons Nuclear Weapons related Issues. Producing Bomb Materials Energy Yield Critical Mass for Nuclear Weapons Buildup of a Chain Reaction.

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Chapter 6. Nuclear Weapons

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Chapter 6. Nuclear Weapons

  • History of Weapons Development

  • Nuclear Explosions

  • Uranium and Nuclear Weapons

  • Plutonium and Nuclear Weapons

  • Nuclear Weapons related Issues

Producing Bomb Materials

Energy Yield

Critical Mass for Nuclear Weapons

Buildup of a Chain Reaction

Explosive Properties of Plutonium

Reactor-Grade Plutonium as a Weapons Material

  • 1934: 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 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 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 1960s, 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 Weapons

Nuclear weapons were symbols of military and national power, and 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 US

  • About 12,000 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 weapons 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 from more than 6,000 in the early 1980s.

Atomic 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 the 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 history.

Hiroshima and Nagasaki Cont…

  • The role of bombings in Japan’s 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 are called hibakusha, a Japanese word that literally translates to “bomb-affected people.”

  • The suffering of the bombing is the root of Japan’s postwar pacifism, and the nation has sought the abolition of nuclear weapons from the world ever since.

Aftermath Attack On Japan

  • The nuclear attacks on Japan occurred during hot weather.

  • So it was more effected toward the people.

  • Many people were outside and wearing light clothing's.

  • This lady's 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 darker cloths would make it worst.

Aftermath Cont…

  • This 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 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 instant 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 accounted for a substantial number of deaths, possibly 30%.

The delayed effects

Atomic Bomb Survivor Excess Cancer

during the period from 1950 through 1990

Population of Survivors Studied86,572

Total Cancers observed after the Bomb 8,180Total Cancers Expected without Bomb 7,743

Total Cancer Excess 437 (421)

Excess Tumor 334

Excess Leukemia 104





Chapter 6. Nuclear Weapons

  • History of Weapons Development

  • Nuclear Explosions

  • Uranium and Nuclear Weapons

  • Plutonium and Nuclear Weapons

  • Nuclear Weapons related Issues

Producing Bomb Materials

Energy Yield

Critical Mass for Nuclear Weapons

Buildup of a Chain Reaction

Explosive Properties of Plutonium

Reactor-Grade Plutonium as a Weapons Material

2.1 Basic Characteristics of Fission Bombs

Producing Bomb Materials

Separate 235U (0.7%) from natural uranium:gas diffusion of UF6centrifuge of UF6 gasthermal diffusion of UF6 gas electromagnetic separation

Production of 239Pu by the reaction238U(n, 2b)239Pu

Bomb Material: Separating 235U by gas Diffusion


235UF6 is 389, 238UF6

 One diffusion unit the diffusion plant 

Urey's research group: gas molecules with different molecular mass could be separated by a diffusion method: Lighter molecules pass through membranes containing pin holes faster than heavier molecules

The blue spot is a person

Since the molecular weights differ so little the industrial operation is a long and laborious process.

~1000 units

Bomb Material: Separating 235U by Electromagnetic method

Bomb Material: Separating 235U by Electromagnetic meth

The 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 coils

centrifuge method

the centrifuge method feeds UF6 gas into a series of vacuum tubes 1 to 2 meters long and 15-20 cm diameter, each containing a rotor.

When the rotorsare spun rapidly, at 50,000 to 70,000 rpm,

the heavier molecules 238UF6 increase in concentration towards the cylinder's outer edge.

There is a corresponding increase in concentration of 235UF6 molecules near the center.

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 Centrifuge

A vacuum arc produces a plasma column 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 extracted

New Methods of Isotope Separation

  • In the cyclotron resonance method a radiofrequency field selectively 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 from the weakly ionized plasma and guides them up to collecting plates.

Fission Energy for War and Peace

Energy Yield of Nuclear Weapons

TNT, Trinitrotoluene


The explosive yield of TNT is considered to be the standard measure of strength of bombs and other explosives

The energy yield of nuclear weapons is 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.

235U92142Nd60 + 90Zr40 + 3 n + Q

Q = (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 amount of energy is equivalent to 2.2×1010 kilowatt-hour, or 22 giga-watt-hour. This amount of energy keeps a 100-watt light bulb lit for 25,000 years.

(3.15e-11 J) 1000 g

= 8.06e13 J (per kg).

1 mol235 g

6.023e231 mol

About 86% of the energy is in the kinetic energy of the fission fragments themselves. Complete fission of 1 kg of 235U would give a prompt explosive yield of about 7×1013 J, or 17 kt.

Actual yields in nuclear weapons are less than 17 kt/kg of fissile material, because a bomb will disassemble without complete fissioning of the material.

the world’s first nuclear bomb, in the Trinity test in New

Mexico in July 1945,

6.1 kg of plutonium => a yield of 18.6 kt

Modern bombs are efficient, approaching 40%

2.3 Critical Mass for Nuclear Weapons

The minimum quantity for a sustained chain reaction to take place is called the critical mass or critical size, whichdepends on the moderator, chemical and physical states, shape etc.

Louis Slotin

estimates 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 dragon's tail.

the average distance: the average distance

such a particle travels before it interacts.

mean-free-path length

Critical Mass With and Without Reflectors

Properties of fissile materials for nuclear weapons: fission cross section, neutrons per fission, mean free path at 1 MeV, and critical mass and radius.

the fissile material to be isotopically pure, or almost so, and in metallic form.

λ = A/ρNAσf = 17 cm.

Rc, one-half of the mean free path λ. Larger ν , lower Rc,

Mc = ρ4πR3/3 ≈ 60 kg

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 3

Materials for the reflector: uranium and tungsten.

Thicknesses: 4 - 15 cm

Types of Nuclear Bomb

For an effective nuclear weapon, the critical mass must be assembled quickly

gun-type bomb

Reducing Critical Masses by Implosion

λ ~1/ρ



Nominal Numbers for the Critical Mass

  • The actual 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 results

  • the nature of advanced designs and the resulting sizes are not made public 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 Reaction

When a critical mass is assembled, neutrons from the natural fission process initiate a chain reaction. The number of nuclei undergoing 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 level.

the time between successive fission generations must be short

The chain reaction in a bomb therefore relies on fast neutrons

The time rate of change in the number of fission neutrons N

K: effective multiplication factor,

β is the delayed neutron fraction

τ is the mean time between successive fission generations

N = N0eαt

it 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 nuclei

the chain reaction would develop completely in a time on the order of one-millionth of a second

Chapter 6. Nuclear Weapons

  • History of Weapons Development

  • Nuclear Explosions

  • Uranium and Nuclear Weapons

  • Plutonium and Nuclear Weapons

  • Nuclear Weapons related Issues

Producing Bomb Materials

Energy Yield

Critical Mass for Nuclear Weapons

Buildup of a Chain Reaction

Explosive Properties of Plutonium

Reactor-Grade Plutonium as a Weapons Material

low-enriched uranium (LEU): 0.71–20%

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 for

lower 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% enrichment

high enrichments are used in 235U bombs:

preferable for building a compact bomb

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.

4. Plutonium and Nuclear Weapons

4.1 Explosive Properties of Plutonium

Difficulty – 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 cause premature initiation of a chain reaction in a bomb before the fissile material has been fully compressed.

With predetonation, the weapon gives a muchsmaller explosion than designed; in other words, it “fizzles.”

1. 233U has the largest value of η, the number of fission neutrons produced per thermal neutron absorbed, and hence is the best prospect for a thermal breeder reactor. A breeder reactor needs an η of at least two since one neutron is needed to sustain the chain reaction and one

neutron 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 thermal neutron absorption, may yield fissile materials

Solution 1 different grades of plutonium

Properties of different grades of plutonium: isotopic abundances, neutron emission from spontaneous fission (SF), and decay heat from radioactive decay.

The neutron emission rate from spontaneous fission is about 910 (g.s)−1for 240Pu

The 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 reduced

Although 240Pu creates problems 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 240Pu

For 238U, the fission cross section even at 1 MeV is low and a fast neutron chain reaction in 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 quickly

The critical mass is inversely proportional to the square of the density, and compression can change the mass

from subcritical to supercritical.

In a typical bomb, the implosion shock wave has a speed of about 5000 m/s

the implosion proceeds at a rate such that the bomb goes from initial supercriticality to full compression in a period of about 10−5 s


neutrons from spontaneous 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 the

chain reaction eventually ceases.

If the reversal occurs relatively early, when the compression and multiplication factors are low, the chain reaction will

not be rapid enough to have embraced much of the plutonium before it is terminated.

Nuclear Fusion Bombs

A thermonuclear Bomb consists of explosives, fission fuel, and D, T, and Li.

A 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 MeV

n + 6Li  T + 4He ( = 942 b)

n + 7Li  T + 4He + n ( = 0.045 b)

A neutron bomb is a fusion bomb designed to release neutrons.

A cobalt bomb is a dirty bomb to kill using radioactive 60Co.


Nov. 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 megatons and had a fireball 4 miles in diameter.USSR H-bomb yields 100 megatons.


Chapter 6. Nuclear Weapons

  • History of Weapons Development

  • Nuclear Explosions

  • Uranium and Nuclear Weapons

  • Plutonium and Nuclear Weapons

  • Nuclear Weapons related Issues

Basic Characteristics of Fission Bombs

Critical Mass for Nuclear Weapons

Buildup of a Chain Reaction

Explosive Properties of Plutonium

Reactor-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 obstacles:

  • – high-grade radioactive materials do not occur naturally.

  • – Delivery systems must not damage explosive material

Weapons Delivery Systems

Earliest weapons were simply gravity bombs

dropped from airplanes.

• Ballistic missiles reduce risk of interception.

• 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).


Mutually Assured Destruction

MAD doctrine asserts that nuclear war would be

rationally impossible since both countries would be


• Perhaps nuclear weapons preserved peace during ColdWar?

• MAD depends on:

– second-strike capability

– Inability to defend against nuclear attack

– Protection against ‘accidental’ launch

– ‘Rational’ enemy


  • Hardened ICBM silos and SLBMs reduced chance

  • that weapons would be destroyed in a first strike.

  • • Anti-ballistic missile systems.

  • – Defensive? Yet, undermines MAD.

  • • 1972: ABM treaty restricts development of antiballistic

  • missile technology.

  • • 2002: US withdraws from treaty, claiming threat from

  • ‘rogue’ nations. Current issues with Russia.

  • • However, ABM technology still not very effective.

  • – MIRVs and decoys difficult to deal with.

  • – Fears of a new arms race (Death Star?)

Threat of Proliferation: Former USSR

  • Former Soviet republics had nukesupon independence.

  • – Concern about weakness in thestates.

  • –Republics agree to give upnuclear weapons, return them toRussia.

  • • Alexander Lebed, former secretaryof Russian Security Council,claimed in 1997 interview that about100 weapons are unaccounted for.

  • – Controlled radioactivesubstances also ‘missing.’

Proliferation: Nuclear Terrorism

  • Concern that groups not deterred by MADcould obtain a nuclear weapon.

  • – If a terrorist attacks, where do you retaliate?

  • • ‘Dirty-bomb’ would release radioactivematerial.

  • • Some states may provide technology.

  • • Or insecure facilities could be underminedby terror groups.

Arms Limitation

  • US-Russia Agreements:

  • – SALT I & II with USSR/Russia.

  • – Strategic Arms Reduction Treaty (1991)

  • – Anti-Ballistic Missile Treaty (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)

Adebayo Amusu

Alexander May

Chapter 6. Nuclear Weapons

  • History of Weapons Development

  • Nuclear Explosions

  • Uranium and Nuclear Weapons

  • Plutonium and Nuclear Weapons

  • Nuclear Weapons related Issues

Producing Bomb Materials

Energy Yield

Critical Mass for Nuclear Weapons

Buildup of a Chain Reaction

Explosive Properties of Plutonium

Reactor-Grade Plutonium as a Weapons Material

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