Transcript Nuclear Physics and Bombs

Nuclear Physics and Bombs
Lupei Zhu
FALL 2004
EASA-130 Seismology and Nuclear Explosion
Topics
 Elements
and atoms
 Radioactive decay of atoms
 Fission and fusion
 Nuclear energy
 Nuclear bomb designs
 Nuclear explosion phenomena
 The Big-Bang theory
Basic Knowledge about Atom
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The smallest particle (10-10 m or 0.1
nm) of an element that still retains
the characteristics of the element.
An atom has a very small size (10-15
m) nucleus surrounded by
negatively charged electrons.
Nucleus consists of protons
(positively charged) and neutrons.
Proton and neutron have about the
same mass that is very larger than
electron’s. So an atom's mass is
essentially the mass of its nucleus.
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Periodic Table
– The number of protons (the atomic number) determines which
element it belongs to. Atoms with the same atomic number but
different number of neutrons are called isotopes, e.g., 235U and
238U.
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Our Universe:
92% H; 7% He
Most abundant
elements in the
Earth: O, Mg, Si, Fe
Most other light
elements have
escaped.
Peak at iron.
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Radioactive Decay of Atoms
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Some elements will spontaneously turn into other elements.
This is called radioactivity and was discovered in 1896.
It happens randomly and the probability only depends on the
structure of the nucleus (isotope).
Scientists use half-life to describe the probability of decay.
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Radioactivity Example
 Example: 238U  206Pb + 8 4He + 6ß
 The half-life T1/2 of 238U is about 4.5 billion years.
 The graph shows the number of 238U and 206Pb at time t
300
250
200
N206(t) = N0-N238(t)
150
100
N238(t) = N0/2 t/T
Number
of Atoms
50
0
0
2
4
Time in half-life
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6
Radioactive Dating
40
35
30
25
20
Ratio
15
10
5
0
0
206Pb/238U
= 2t/T - 1
2
4
6
Time in half-life
This can be used in the opposite way: if we can count
how much daughter isotope in the sample as compared to
the parent isotope we can get the age of the sample
t = T log2 (206Pb/238U + 1)
This method is called radioactive dating.
By using it, we find that Earth is ~4.5 Ga old.
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Fission Process
Heavy elements are split into lighter ones.
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Fusion Process
Light elements are combined into more heavy ones
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Nuclear Energy
Nucleons in the nucleus are bound tightly together by the socalled “strong force”. The nuclear binding energy is the energy
required to break them apart. Therefore it is possible to release
large amount of energy by breaking an nucleus to form a different
nucleus that is bound more tightly, i.e., has higher binding energy.
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Nuclear Energy
The left figure shows the
binding energy per nucleon
of different atoms.
Iron is the most stable
element.
Large amount of energy
(yield) is released by fusion
of light elements into
heavier elements or by
fission of heavy elements
into lighter elements.
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Yield of a 50 kg Uranium Bomb?
Fission of one 235U atom releases energy of
235 * 1 MeV/nucleon = 3.8 10-11 J.
 235 gram of 235U has 6.0 1023 atoms (Avogadro’s
number). A 50 kg U-bomb has a yield of
50,000/235*6.0 1023*3.8 10-11 = 4.8 1015 J.
 The yield of 1 kt of TNT is 4.2 1012 J. So the yield
of the 50 kg nuclear bomb is 1100 kt.
 The first Uranium bomb, the Little Boy, used 64
kg Uranium and its yield is ~15 kt. The first
generation A-bombs are not efficient (1-2%).
 US electric power consumption is about 3 1018 J in
2000.
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Chain Reaction
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For 235U and 239Pu, one fission takes one neutron
and produces three neutrons on average. The
produced neutrons can be used to split more 235U
and start a chain reaction.
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Nuclear Power Plants and Bombs
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To sustain the chain reaction, it is necessary to have many fissionable
atoms around to catch neutrons. The smallest amount of fissile
material is called critical mass.
The critical mass depends on the density of the material and how easy
for 235U to capture a neutron.
Inside a nuclear power plant, the reaction is controlled by absorbing
neutrons and moderating their speed. Slow neutrons can be captured
more easily so nuclear power plants can use low-grade uranium (a few
per cent in 235U) as fuel.
In contrast, nuclear explosions are uncontrolled chain reaction. The
fission material need to be supercritical and high grade (enriched).
The minimum mass is 47 kg for 235U and 16 kg for 239Pu in normal
condition.
The mass can be reduced substantially by using tamper and increasing
material density (e.g. the Fat Man bomb only used 6 kg of 239Pu).
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Fission Bomb Designs
A fission device:
1. a subcritical
system that can
be made
supercritical
quickly;
2. a strong
neutron source
to initiate the
supercritical
system.
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Fission Bomb Designs
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Fission Bomb Designs
Boosted weapon: a small amount of deuteriumtritium mixture is placed in the center of the
sphere of fissile material.
 When the primary explosion happens, it produces
enormous pressure and temperature at the center.
This causes the deuterium and tritium mixture to
undergo fusion and releases lots of neutrons in the
center of the fissile sphere, greatly increasing the
overall fission energy release.
 Boosting can increase yields by a factor of ten
(400 kt).
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Fusion
To achieve fusion enormous temperatures are
required. It takes about 50,000,000 degrees to get
deuterium (2H) to fuse with tritium (3H). The
required temperatures are higher for heavier
elements.
 At present, it appears that only a fission device is
capable to initial fusion in a H-bomb. Other
technology, such as laser, may be possible.
 Once started, the energy released by fusion will
sustain the process if enough fusion material are
available.
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Thermonuclear Bomb
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Material for Nuclear Bombs
Usable materials are 235U, 239Pu, 2H, and 3H.
 235U is less than 1% in natural uranium. It can be
enriched by using gas-diffusion or gas-centrifuge.
 239Pu is virtually nonexistent in nature and can be
obtained by bombarding 238U with neutrons in
nuclear reactors.
 2H can be enriched by conventional methods.
 3H can be produced by neutron irradiation of
lithium.
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Making a Nuclear Bomb
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Making a nuclear bomb requires a high degree of
competence in various disciplinary.
Given enough time and supply, any nation or group with
competent people should be able to produce a crude, heavy
nuclear device.
Delivery the weapon on a sophisticated platform is not
easy. Nuclear tests are likely to be needed in order to
reduce the size and weaponize.
Other possibilities are to steal or purchase existing nuclear
weapons, technology, or experts.
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Nuclear Explosion Phenomena
The explosion happens in micro-seconds.
 Can you use the nuclear physics you've learned to
explain what's happening here?
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The forms of nuclear energy
The energy released in the fission/fusion reactions
are in the forms of kinetic energy of particles
(neutrons and other produced elements) and highenergy gamma rays (photons).
 In the atmosphere, the particles collide with air
molecules to raise the temperature to 10 million
degrees (thermal energy)
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Nuclear Explosion Phenomena
Hot air radiates electro-magnetic waves in a wide
spectrum from infra-red to visible and to X and
gamma rays (thermal radiation and EM pulse).
 The EM waves travel at speed of light.
 The high temperature also causes the pressure of
the air around the explosion to increase to a
million atmospheric pressure (bar).
 The highly-pressured air expands at speeds larger
than the sound speed and generates shockwave.
 For atmospheric explosions, shockwave takes
about 50% of the yield and thermal radiation 35%.
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Nuclear Explosion Phenomena
The fireball rises through the atmosphere in the
form of a “mushroom” cloud.
 Radioactive products of the nuclear explosions
deposit around the area.
 Some enter the upper atmosphere with the plume
and fall to the ground over a large area.
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New Generation Weapons
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Neutron bomb: enhance neutron radiation with minimum
radioactive fallout and shockwaves.
Reduced radiation weapons (RRW): maximize the
electromagnetic pulse to destroy electronic equipment.
“doomsday bomb”: It has been hypothesized to produce a
Cobalt bomb (coat the outside a thermonuclear device with
a tamper of Cobalt). Neutrons produced in the fusion
reaction will change Cobalt to Cobalt-60, which is a very
radioactive atom with a half-life of 5.6 years. Such a bomb
with enough Cobalt may kill all life by spreading
dangerous radioactive material over the world.
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Where Are Elements Made?
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The light elements hydrogen and helium were created during the
Big Bang and the elements between hydrogen and iron can be
fused together inside stars. Heavier elements form during massive
stellar explosions called supernovae.
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Fusion in Stars
 Stars
like the Sun were mostly made of H
initially and have a temperature of ~107 K.
So, H to He fusion reaction is going on in
stars (main sequence).
 How did it get started in the first place?
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The Birth of a Star
As hydrogen clouds
condense, pressure
and temperature at the
center increase. This
lead to the ignition of
H fusion.
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The Fate of a Star
When most H is fused into He, fusion stops and
and the star starts to collapse under gravity.
 For stars with mass less than the Sun, they become
brown dwarf and eventually end up as cold, dead
bodies in space.
 For stars like the Sun, the gravitational force can
squeeze the center and make it hot enough to start
fusion of He. Star starts to swell into a red-giant.
 Elements from Li up to Fe are produced.
 Eventually all fusion fuel are burnt out. It
collapses to form a white dwarf.
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Supernova
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The crab nebula
More Massive stars tend
to explode in a supernova.
Elements heavier than Fe
are produced in explosion.
A small central core
remains to form a neutron
star.
If the mass is is large
enough, the core continue
to shrink to form a black
hole
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How Much Time Do We have?
Sun is radiating energy at a rate of 4 1026W.
 It has a mass of 2 1030 kg (1H).
 The main fusion process in the Sun is
combining 1H to 2H, which releases 1.4 MeV of
energy. The amount of H in the Sun can keep
the fusion process last for about
1033*6 1023 1.4*1.6 10-13/4 1026=3 1017s=10Ga.
 At a age of 4.5 Ga, the Sun is in its middle age
(no crisis yet).
 The
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