Chapter 6. Nuclear Weapons
1.History of Weapons Development
2.Nuclear Explosions
Producing Bomb Materials
Energy Yield
Critical Mass for Nuclear Weapons
Buildup of a Chain Reaction
3.Uranium and Nuclear Weapons
4.Plutonium and Nuclear Weapons
Explosive Properties of Plutonium
Reactor-Grade Plutonium as a Weapons Material
5.Nuclear Weapons related Issues
•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 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 Studied
86,572
Total Cancers observed after the Bomb 8,180
Total Cancers Expected without Bomb 7,743
Total Cancer Excess
437 (421)
Excess Tumor
Excess Leukemia (白血病）
+
= 437
334
104
5467
Chapter 6. Nuclear Weapons
1.History of Weapons Development
2.Nuclear Explosions
Producing Bomb Materials
Energy Yield
Critical Mass for Nuclear Weapons
Buildup of a Chain Reaction
3.Uranium and Nuclear Weapons
4.Plutonium and Nuclear Weapons
Explosive Properties of Plutonium
Reactor-Grade Plutonium as a Weapons Material
5.Nuclear Weapons related Issues
2.1 Basic Characteristics of Fission Bombs
Producing Bomb Materials
Separate 235U (0.7%) from natural
uranium:
gas diffusion of UF6
centrifuge of UF6 gas
thermal diffusion of UF6 gas
electromagnetic separation
Production of 239Pu by the reaction
238U(n, 2b)239Pu
235
239
U
Pu
Bomb Material:
Separating 235U by gas Diffusion
392
 One diffusion unit
the diffusion plant 
235UF
6
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
is 389, 238UF6
The blue spot is a person
http://www.npp.hu/uran/3diff-e.htm
Since the molecular weights differ
so little the industrial operation is a
long and laborious process.
~1000 units
235U
Bomb
eparating
BombMaterial:
Material:SS
eparating235
Uby
byElectromagnetic
Electromagnetic method
meth
Uranium Isotope Enrichment by the
Electromagnetic Method.
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.
From a
particle
accelerator
238
235
UF6
collector
UF6
collector
$ 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 rotors are 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
1. 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.
2. 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
三硝基甲苯
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
+3n
+ 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
(3.15e-11 J) 1000 g
1 mol
235 g
6.023e23
1 mol
This amount of energy is
equivalent to 2.2×1010
kilowatt-hour, or 22 giga-watthour. This amount of energy
keeps a 100-watt light bulb lit
for 25,000 years.
= 8.06e13 J (per kg).
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
Sixteen hours ago an American airplane dropped one bomb on
Hiroshima, Japan, and destroyed its usefulness to the enemy. That
bomb had more power than 20,000 tons of T.N.T. ....
Harry S. Truman
1
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, which
depends on the
moderator, chemical and
physical states, shape
etc.
The Idea of a Guillotine for Critical Mass
Determination
Louis Slotin
Neutron
monitoring
devices
Releasing
mechanism
235
U or
239
Pu
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.
Mc = ρ4πR3/3 ≈ 60 kg
Larger ν , lower Rc,
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
The Implosion Arrangement
Ignition
points
Reducing
Critical
Masses by
Implosion
Chemical
explosive
239
Pu
Fission material is surrounded by chemical explosive
which is ignited at many points simultaneously. The
explosion forces pieces of 239Pu together and even
reduces the volume to reduce the critical mass.
λ ~ 1/ρ
V ~ 1/ρ3
M ~ 1/ρ2
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
1.History of Weapons Development
2.Nuclear Explosions
Producing Bomb Materials
Energy Yield
Critical Mass for Nuclear Weapons
Buildup of a Chain Reaction
3.Uranium and Nuclear Weapons
4.Plutonium and Nuclear Weapons
Explosive Properties of Plutonium
Reactor-Grade Plutonium as a Weapons Material
5.Nuclear Weapons related Issues
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:
1. preferable for building a compact bomb
2. 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 much smaller
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)−1 for 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
M ~ 1/ρ2
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.
41
H-bomb
Fusion
Nov. 1, 1952, the first Hbomb 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.
42 100
USSR H-bomb yields
megatons.
Chapter 6. Nuclear Weapons
1.History of Weapons Development
2.Nuclear Explosions
Basic Characteristics of Fission Bombs
Critical Mass for Nuclear Weapons
Buildup of a Chain Reaction
3.Uranium and Nuclear Weapons
4.Plutonium and Nuclear Weapons
Explosive Properties of Plutonium
Reactor-Grade Plutonium as a Weapons Material
5.Nuclear Weapons related Issues
• 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).
多弹头分导导弹
Mutually Assured Destruction
MAD doctrine asserts that nuclear war would be
rationally impossible since both countries would be
destroyed.
• 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’ enemy
M.A.D.
•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 nukes upon independence.
– Concern about weakness in the states.
–Republics agree to give up nuclear weapons, return
them to Russia.
• Alexander Lebed, 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, where do you retaliate?
• ‘Dirty-bomb’ would release radioactive material.
• Some states may provide technology.
• Or insecure facilities could be undermined by 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 www.iearn.org
Alexander May
51
Chapter 6. Nuclear Weapons
1.History of Weapons Development
2.Nuclear Explosions
Producing Bomb Materials
Energy Yield
Critical Mass for Nuclear Weapons
Buildup of a Chain Reaction
3.Uranium and Nuclear Weapons
4.Plutonium and Nuclear Weapons
Explosive Properties of Plutonium
Reactor-Grade Plutonium as a Weapons Material
5.Nuclear Weapons related Issues