Transcript Introduction to Nuclear Weapons

Introduction to Nuclear
Weapons
Physical Science
I. Nuclear Physics
A.
Key Concepts
1.
The atom:
Nucleus
surrounded by
electrons (a.k.a.
beta particles)
2. The Nucleus: Protons and
Neutrons
a.
b.
Electro-magnetism
holds electrons in
orbit (electrons are
negatively
charges, protons
are positive)
“Strong nuclear
force” holds
protons and
neutrons together
(137 times as
strong as electromagnetism)
3. Elements
a.
b.
c.
d.
Definition: Elements are atoms with the
same # of protons in nuclei (their atomic
number)
Change # protons = change element
Atomic weight = protons + neutrons +
electrons (trivial weight)
Change # neutrons but not protons =
same element but different atomic weight
 isotope (Carbon-12, Carbon-13,
Carbon 14, etc.)
4. The novelty of nuclear weapons
a.
b.
Chemistry – Elements are combined into
compounds (atoms become molecules),
which can release electro-magnetic energy
as heat, light, etc. ALL weapons before 1945
use chemistry – explosives, napalm, toxins,
etc.
Nuclear weapons use the strong nuclear
force for destruction  inherently more
powerful than any possible chemical reaction
(by weight)
B. Fission: Splitting a Nucleus
1.
2.
Heavy nuclei are unstable – Put too
many protons together and they repel
each other. Too many (or too few)
neutrons can increase this repulsion.
Spontaneous fission: Unstable heavy
nuclei can randomly fission – break into
two smaller nuclei (different elements).
3. Induced fission

Throw a neutron at an unstable nucleus
and:
It might escape (pass by without being
captured by nucleus)
 Be absorbed into the nucleus
 Trigger fission of the nucleus into two nuclei
(shown)

4. The Fission Chain Reaction
a.
b.
c.
d.
e.
More energy is required to hold one heavy nucleus
together than two moderate-sized nuclei.
Therefore, splitting a heavy nucleus releases a great deal
of energy (strong nuclear force).
If neutrons cause fission, and fission creates more
neutrons, a chain reaction may ensue. Small initial
energy (a few neutrons) cascades to trillions of split
nuclei.
Uncontrolled chain reaction = fission explosion. Requires
Critical Mass (enough nuclei close together for neutrons to
be more likely to hit nuclei than fly out of the mass without
hitting anything)
Critical mass varies by element, isotope, shape (spheres
work best), and density (so compressing sub-critical mass
can make it “go critical” and explode)
Example: Chain Reaction in U-235
C. Fusion: Combining Nuclei
1.
2.
3.
It takes more energy to hold two light
nuclei together than a single moderatesized nucleus.
Therefore, forcing two light nuclei
together into one nucleus generates
energy.
In general, fusion produces more energy
than fission (which means bigger bombs)
Curve of Binding Energy: Note energy
increase in fusion (light elements)
compared to fission (heavy elements)
4. The problem of fusion
a.
b.
c.
d.
Fission is easy – just throw some neutrons at
inherently-unstable nuclei and they split
Fusion is hard – Hydrogen doesn’t just
randomly slam into itself with the energy level
of the sun’s core. About 100 million degrees
required to overcome strong nuclear force.
All efforts to create controlled fusion use more
energy to force the nuclei together than they
extract from fusion
BUT we do have one tool to generate huge
amounts of uncontrolled energy – a fission
chain reaction! (Even this just barely provides
enough energy – limiting fusion weapons to
very light elements like hydrogen)
II. Weapon Design
A.
The most basic fission weapon (aka
atomic bomb) – The U-235 weapon
1.
2.
U-235 is fissile – Only low-energy neutrons
are needed to split the nucleus. Other types
of uranium (U-238, the most common type)
require very high-energy neutrons for fission
(= nearly impossible to create a chain
reaction)
Critical mass of U-235 = 50 kg (about 110
pounds) in a sphere.
Advantage of U-235 over U-238
3. The gun-type nuclear weapon
a.
b.
Principle = Quickly mash two sub-critical
pieces of U-235 together into one piece
above critical mass. Detonation ensues.
Simplified design:
4. Barriers to building a
gun-type weapon
a.


Getting the U-235
99.3% of Uranium is U-238.
Must enrich uranium to
increase % of U-235
Combine uranium with fluorine
to make uranium hexafluoride
gas (“hex”). Then put hex in a
container surrounded by a
membrane. Slightly more U235 will diffuse out than U-238.
Also useful…
Gas Centrifuges


Since U-235 is
lighter than U-238,
spinning hex
rapidly pulls the U238 to the edge
and leaves more
U-235 in the
middle
US cascade of
centrifuges

b. The danger of “fizzle”






Difficult to eliminate the last U-238 from the U-235 (Hiroshima
bomb was 80% U-235 / 20% U-238)
U-238 spontaneously fissions, generating neutrons
Danger = chance that U-238 will start a partial chain reaction
just before critical mass is reached. Blows U-235 apart before
most of it has a chance to fission. Result = small explosion.
Solution = assemble critical mass so quickly that U-238 is
unlikely to spontaneously fission at the wrong moment (we now
know Hiroshima bomb had just under a 10% chance of fizzle –
the U-238 in the weapon spontaneously fissioned about 70
times/second)
Similar problem makes U-233 gun-type bombs difficult to build
(contaminated with U-232, which fissions too rapidly) and Pu239 ones impossible (contaminated with Pu-240)
More complex designs reduce – but do not eliminate – chance
of fizzle. DPRK test probably fizzled (very small blast)
c. Safety problems
i.
ii.
iii.
Accident-prone: Two subcritical masses
kept in close proximity to explosives
Accidental moderation: Seawater
moderates (slows) neutrons, and slower
neutrons are more likely to cause fission
before escaping the core. Result = drop
bomb in seawater = potential detonation!
Terrorist’s dream: Easy to use U-235 to
improvise a nuclear device
B. The Basic Implosion-Type
Fission Weapon
1.
Why bother?
a.
b.
c.
d.
e.
Desire to use Pu-239 (can be made using
nuclear reactors, so no separation necessary)
Compressing material takes 1/10 the time of
slamming it together (helps prevent fizzle)
Less fissile material is required if it can be
compressed
Much safer – accidental detonation can be
made impossible
Allows flexibility: some or all charges can be
detonated, compressing material to different
degrees
Advantage
of Pu-239

2. The basic components
a.
b.
c.
d.
Subcritical mass of Plutonium (any isotope), U233 (rarely), U-235, Np-237 (similar to U-235
but easier to obtain), or Am-241 (theoretically)
surrounded by explosives  nearly all
designs use Pu-239 or U-235
Explosives are shaped, layered, and timed to
generate a spherical shock wave
Neutron initiator supplies neutrons to begin
fission at right moment – too soon causes
fizzle, but so does too late (material rebounds
after compression)
Tamper between explosives and Pu-239 helps
to reflect neutrons and hold compression for a
moment or two to maximize yield
Simplified Implosion Design
3. Maximizing Efficiency (Proportion
of material that fissions before the
whole thing blows itself apart into
sub-critical pieces)
a.
b.
c.
Neutron reflector: Surrounds fissile material
below tamper to bounce stray neutrons back
into the core
Levitating core: Empty space between tamper
and core to allow tamper to build up
momentum (standard in today’s weapons)
External neutron trigger (particle accelerator
outside the sphere) – also useful if you want to
put something else in the center of the core….
C. Boosted Fission Weapons:
Using Fusion to Increase Power
1.
2.
3.
4.
Problem: Most fissile material wasted (only 1%-20% fission
before it blows itself apart – Hiroshima bomb was 1.4%
efficient). More neutrons needed!
Solution = fill core with isotopes of H that fuse easily:
Deuterium (D or H-2 -- 1 proton, 1 neutron) and Tritium (T or
H-3 -- 1 proton, 2 neutrons) can fuse into He-4 (2 protons, 2
neutrons), creating energy and 1 extra neutron. Fusion
energy generated is trivial in these weapons, but…
The “boost”: Extra neutrons hit the fissile material and cause
more of it to fission before blowing itself apart. Result = much
larger explosion (about double the explosive power).
Advantages: Higher yield for equal mass – which also means
weapons can be miniaturized (up to a point), “dial-a-yield”
through control of D/T injected into center.
Schematic of Primary Part
of Boosted Fission Weapon
Hollow core, where D (H-2) and T (H-3) are injected for boosting.
Fissile material (U-235 or Pu-239)
Beryllium reflector (2 cm)
Tamper (tungsten or
uranium) (3 cm)
High explosive (10 cm)
Aluminum case (1 cm)
C. Boosted Fission Weapons:
Using Fusion to Increase Power
1.
2.
3.
4.
Problem: Most fissile material wasted (only 1%-20% fission
before it blows itself apart – Hiroshima bomb was 1.4%
efficient). More neutrons needed!
Solution = fill core with isotopes of H that fuse easily:
Deuterium (D or H-2 -- 1 proton, 1 neutron) and Tritium (T or
H-3 -- 1 proton, 2 neutrons) can fuse into He-4 (2 protons, 2
neutrons), creating energy and 1 extra neutron. Fusion
energy generated is trivial in these weapons, but…
The “boost”: Extra neutrons hit the fissile material and cause
more of it to fission before blowing itself apart. Result = much
larger explosion (about double the explosive power).
Advantages: Higher yield for equal mass – which also means
weapons can be miniaturized (up to a point), “dial-a-yield”
through control of D/T injected into center.
D. Staged Fusion Weapons:
The Thermonuclear or
Hydrogen Bomb
1.
Parts:
a.
b.
c.
The “primary stage” – A
fission device
The “secondary stage” –
designed to fuse when
bombarded with
radiation
The casing: Usually
made of U-238
2. Inside the Secondary
Radiation channels filled with polystyrene
foam surround the capsule
 The capsule walls are made of U-238
 Spark plug of plutonium boosts fusion

3. Radiation Implosion
a.
b.
c.
d.
Primary ignites  high-energy X-Rays
X-Rays fill the radiation channels, turn
polystyrene to plasma
Tamper is heated  outside ablates
(vaporizes – think of an inside-out rocket).
Ablation compresses the nuclear fuel.
Plasma helps keep the tamper from blocking
the radiation channels, increasing duration of
compression
4. The fusion explosion
a.
b.
c.
Compressed fuel must still be heated
Plutonium “spark plug” in center of fusion
fuel is compressed, becomes supercritical and fissions (raises temperature
inside case)
Result = huge pressures and
temperatures produce fusion, which
releases far more energy than fission
PLUS “fast fission” of spark plug from
fusion-produced neutrons
5. The fuel
a.
b.
c.
Early designs (first US test) used
deuterium and tritium – but this
required cryogenic machinery (D
and T are gases at room
temperature)
Modern designs use solid Lithium
Deuteride instead. Enriched fuel
(lots of Li-6) much more effective.
The fusion process: Neutrons from
fission turn some D into T, which
then fuse together, generating more
neutrons. Some D and T also
fuses with Lithium (but this
generates less energy).
E. Enhanced Fusion Weapons
1.
2.
Fission-Fusion-Fission designs: Make the
bomb case out of U-238 or even U-235 and
it will detonate when neutrons from the
fusion capsule hit it, greatly enhancing yield
(doubling power is easy)
Multi-stage weapons: Use the secondary
stage to compress a tertiary stage, and so
forth. Each stage can be 10-100 times
larger than previous stage (= unlimited
explosive potential)
III. Detonation Parameters
A.
Yield – A measure of explosive power
1.
2.
Expressed as kt or Mt of TNT
Measures power not weight – 20 kt weapon
is equivalent to detonating 20,000 TONS of
TNT all at once. 1 Mt means the equivalent
of a million tons of TNT detonating at once.
Examples: “Tiny” to Huge
Oklahoma City non-nuclear bomb (.002 Kt)
 Davy Crockett nuclear rifle (.01 kt)
 British tactical nuclear weapon (1.5 kt)
 The nuclear cannon (15 kt)
 Hiroshima (15 kt) and Nagasaki (20 kt)
 Max pure fission: Orange Herald (720 kt)
 Chinese (3 Mt) and British (1.8 Mt) H-Bombs
 Largest deployed weapon (25 Mt)
 Tsar Bomba, the largest bomb tested (58 Mt)

Comparative fireballs by yield
B. Height: Air-Burst vs. Ground-Burst
Zones of destruction (1 Mt weapon)
Groundburst (energy concentrated at ground
zero):
Airburst (energy distributed over wider area):
IV. Effects of Nuclear Weapons
A.
Prompt effects
1.
Thermal and
visible radiation
(heat and light)
a.
b.
Initial pulse =
1/10 second (too
quick for eyes to
react). Few
killed, but many
blinded
Second pulse =
most heat
damage, lasts up
to 20 seconds for
large weapons
c. Biological
effects
i. “Flash burns” –
Most prominent
on exposed
areas (i.e. dark
areas of kimono
worn by this
victim)
Burns 1.5 miles from hypocenter in
Nagasaki
Add 20% for 1st degree burn range,
subtract 20% for 3rd degree burn range
ii. Blindness: Most far-reaching
prompt effect

Flash
blindness
(temporary)
and retinal
burns
(permanent)
from light
focused on
retina
iii. Fire Storms





Heat ignites flammable materials
If large enough area burns, it creates its own
wind system, sucking in oxygen to feed the
flames
Natural example in Peshtigo, WI (1871): “A wall
of flame, a mile high, five miles (8 km) wide,
traveling 90 to 100 miles (200 km) an hour,
hotter than a crematorium, turning sand into
glass.”
Firestorms in Hiroshima (but not Nagasaki),
Dresden, Tokyo in World War II.
Result: Large numbers of people not burned by
nuclear detonation will be burned by subsequent
firestorms sweeping through city
2. Blast
damage
a.
b.
c.
Heat of fireball
causes air to
expand rapidly,
generating a
shock wave
Shock wave hits
and damages
buildings, and is
followed by…
Low-pressure
area follows and
sucks everything
backwards (blast
wind)
Note the Mach Front:
1 Mt
d. Biological Effects

Few likely to die from blast wave itself, but
flying debris may kill many
Lung damage occurs at about 70 KPa (double
the pressure needed to shatter concrete
walls)
 Ear damage begins at 22 KPa (as brick walls
shatter)


In general, heat will kill anyone close
enough to experience primary blast
damage. Crushed buildings will kill many
outside this zone.
3. Ionizing Radiation
For most weapons, immediate radiation
(gamma rays and neutrons) will only kill
those very close to the explosion
 More on biological effects later…

D e s tru c tive R a d iu s (k m ) o f B la s t E ffe c ts , B y Y ie ld o f W e a p o n
50
45
40
D is ta n c e (k m )
35
30
T herm al
25
B last
R adiation
20
15
10
5
0
2.5
5
10
25
Y ield (K t)
250
2500
25000
Hiroshima Health Dept Estimates
Short-Term Fatalities At Hiroshima
Burns
Debris
Radiation
4. Electromagnetic
Pulse (EMP)

High-altitude
nuclear bursts
generate
magnetic fields
over large
areas (induces
current in
transistors and
integrated
circuits)  fried
electronics
B. Fallout
1.
Definition: Radioactive particles fall to
earth (fission products, contaminated soil
and debris sucked up by explosion)
2. Dangers of Ionizing Radiation
Alpha radiation
a.
i.
ii.
iii.
Composed of Helium nuclei (2 protons, 2
neutrons)
Little danger unless inhaled or ingested –
stopped by a piece of paper (or skin)
Very destructive if inhaled or ingested (only
known example = Alexander Litvinenko,
poisoned with alpha-emitter Po-210)
b. Beta radiation
i.
ii.
iii.
Consists of electrons emitted by
radioactive atoms
Can burn exposed skin – stopped by
clothing, skin, and goggles
Effective range is only a few feet, so
exposure to radioactive dust is most
likely source of damage (no known
fatalities from beta exposure at
Hiroshima or Nagasaki)
c. Gamma radiation
i.
ii.
Extremely high energy photons emitted
by the detonation and fallout
Penetrating power is high. Needed to
reduce exposure by half:
d. Neutron radiation
i.
ii.
iii.
iv.
Produced by blast itself, insignificant in fallout
Induces radioactivity (alpha, beta, gamma) in
materials it encounters
Shielding requires light elements (hydrogen,
lithium)
Enhanced-Radiation Weapons, aka “Neutron
Bombs” -- permit fusion-produced neutrons to
escape, killing people even in armored
vehicles (explosions still level civilian
structures)
e. Measures of Radiation
i.
ii.
iii.
iv.
Measurements of exposure: 100 rad = 1
gray
Relative biological effectiveness (RBE):
alpha = up to 20, neutron varies,
beta/gamma/X-Rays = 1
Measures of effect: rad * RBE = rem,
gray * RBE = sievert
Since gamma exposure is likely to be
source of most radiation poisoning, rad
usually = rem and gray usually = sievert
f. Radiation Poisoning (Acute
Radiation Syndrome)
i.
Triggered by
cumulative exposure
– hourly dose * hours
exposed
ii. LD 50
is 4.5
Grays
g. Danger of Internal Absorption
Strontium-90 is chemically similar to
Calcium  incorporated into bones
 Iodine 131 is absorbed by the thyroid
 Cesium 137 is chemically similar to
potassium and absorbed throughout the
body

3. Distribution of Fallout
Fallout = “point-source pollutant”
(exposure almost always decreases with
distance)
a.
i.
ii.
Key variables = speed and direction of wind.
Closer to source usually more dangerous –
but downwind “hot spots” are possible
1 Mt Surface Burst: Cumulative and
Hourly Radiation Exposure
“Hot Spots” from Castle Bravo Test
b. US-USSR Predictions

Immediate Deaths:
Fallout (1977 estimates):
Fallout (1990 Estimate)
Fallout (USSR Estimate)
4. Half-Life
a.
b.
c.
Definition: Time for 50% of a radioactive
substance to decay
Short half-life: These isotopes are very
radioactive but don’t last long
Long half-life: These are less radioactive
but also long-lived
Example: 100 KT Surface Blast,
Fort Hood Main Gate

100 KT = larger than ordinary fission
bomb, smaller than largest Russian
weapons
15 psi:
Virtually all
dead
5 psi: 50%
dead, 45%
injured
2 psi: 5%
dead, 45%
injured)
1 psi: 25%
injured
Compare: 1 MT Surface Blast
Compare: 20KT Surface Blast
100 KT Surface: Fallout
54321Possible
hour: Lethal
hours:
Zone
Lethalof
and 50% Lethal
Sickness
C. Global Climate
1.
2.
3.
4.
“Nuclear Winter” – Controversial theory
that nuclear war would cause serious
global cooling
Key variable = soot and smoke from fires
ignited by nuclear weapons
Targeting cities or heavily forested areas
increases risk (major assumption)
Recent model (2006) suggests 10-year
cooling cycle from “small” nuclear war
(100 Hiroshima-sized bombs used on
population centers)
5. Objections
a.
b.
c.
Model assumes carbon lofted into
stratosphere – but this process is only
confirmed for very small particles (diesel
soot)
Model assumes urban targeting – bases
may be more logical targets
Standard objections to climate modeling