Nuclear Fission, Nuclear Fusion and the Teller-Ulam theory...If your not interested in Nuclear Weapons...Ignore this Posting.
MIRVman
did you do in the War Daddy!
I was a 'NUKE PUKE" in the Airforce and I was a member of an unique
Nuclear Weapons Fraternity of brothers who were poised 24/7 to react to
a Broken Arrow or Bent spear type incident or accident.
We also worked on the side wherein if a "Device" such as a Suitcase
Nuke, or other configuratuion was discovered, we would be sent in to
"Render it Safe" So I spent time at Clark Field, Philippine's then
"TDY" to Vietnam for duties. Now you know why my Group here is so
heavily rendered in Bomb discussions.
So here are the non-classified details of how a weapon works, and why
it has the potential for masss distruction
Nuclear bombs involve the forces, strong and weak, that hold the
nucleus of an Atom together, especially atoms with unstable nuclei.
There are two basic ways that nuclear energy can be released from an
atom:
Nuclear fission - You can split the nucleus of an atom into two smaller
fragments with a neutron. This method usually involves isotopes of
uranium (uranium-235, uranium-233) or plutonium-239.
Nuclear fusion -You can bring two smaller atoms, usually hydrogen or
hydrogen isotopes (deuterium, tritium), together to form a larger one
(helium or helium isotopes); this is how the Sun produces energy.
In either process, fission or fusion, large amounts of heat energy and
radiation are given off.
Designs of Nuclear Bombs
To build an atomic bomb, you need
:
A source of fissionable or fusionable fuel
A triggering device
A way to allow the majority of fuel to fission or fuse before the
explosion occurs (otherwise the bomb will fizzle out)
The first nuclear bombs were fission devices, and the later fusion
bombs required a fission-bomb trigger. I will discuss the designs of
the following devices:
Fission bombs
Gun-triggered fission bomb (Little Boy), which was detonated over
Hiroshima, Japan, in 1945
Implosion-triggered fission bomb (Fat Man), which was detonated over
Nagasaki, Japan, in 1945
Fusion bombs
Teller-Ulam design of a hydrogen fusion bomb, which was test-detonated
on Elugelap Island in 1952
Fission Bombs
A fission bomb uses an element like uranium-235 to create a nuclear
explosion , if you understand the basic process behind radioactive
decay and fission. Uranium-235 has an extra property that makes it
useful for both nuclear-power production and nuclear-bomb production --
U-235 is one of the few materials that can undergo induced fission.
If a free neutron runs into a U-235 nucleus, the nucleus will absorb
the neutron without hesitation, become unstable and split immediately.
Picture a U-235 nucleus with a neutron approaching from the top. As
soon as the nucleus captures the neutron, it splits into two lighter
atoms and throws off two or three new neutrons (the number of ejected
neutrons depends on how the U-235 atom happens to split).
The two new atoms then emit gamma radiation as they settle into their
new states.
There are three things about this induced fission process that make it
interesting:
The probability of a U-235 atom capturing a neutron as it passes by is
fairly high. In a bomb that is working properly, more than one neutron
ejected from each fission causes another fission to occur. This
condition is known as supercriticality.
The process of capturing the neutron and splitting happens very
quickly, on the order of picoseconds (1*10E-12 seconds).
An incredible amount of energy is released, in the form of heat and
gamma radiation, when an atom splits. The energy released by a single
fission is due to the fact that the fission products and the neutrons,
together, weigh less than the original U-235 atom.
The difference in weight is converted to energy at a rate governed by
the equation
e = m * c^2. A pound of highly enriched uranium as used in a nuclear
bomb is equal to something on the order of a million gallons of
gasoline.
When you consider that a pound of uranium is smaller than a baseball
and a million gallons of gasoline would fill a cube that is 50 feet per
side (50 feet is as tall as a five-story building), you can get an idea
of the amount of energy available in just a little bit of U-235.
In order for these properties of U-235 to work, a sample of uranium
must be enriched . Weapons-grade uranium is composed of at least
90-percent U-235.
Critical Mass
In a fission bomb, the fuel must be kept in separate subcritical
masses, which will not support fission, to prevent premature
detonation. Critical Mass is the minimum mass of fissionable material
required to sustain a nuclear fission reaction. This separation brings
about several problems in the design of a fission bomb that must be
solved.:
The two or more subcritical masses must be brought together to form a
supercritical mass, which will provide more than enough neutrons to
sustain a fission reaction, at the time of detonation.
Free neutrons must be introduced into the supercritical mass to start
the fission.
As much of the material as possible must be fissioned before the bomb
explodes to prevent fizzle.
To bring the subcritical masses together into a supercritical mass, two
techniques are used:
Gun-triggered
Implosion
Neutrons are introduced by making a neutron generator. This generator
is a small pellet of polonium and beryllium, separated by foil within
the fissionable fuel core. In this generator: The foil is broken when
the subcritical masses come together and polonium spontaneously emits
alpha particles. These alpha particles then collide with beryllium-9 to
produce beryllium-8 and free neutrons. The neutrons then initiate
fission.
Finally, the fission reaction is confined within a dense material
called a tamper, which is usually made of uranium-238. The tamper gets
heated and expanded by the fission core. This expansion of the tamper
exerts pressure back on the fission core and slows the core's
expansion. The tamper also reflects neutrons back into the fission
core, increasing the efficiency of the fission reaction.
Gun-Triggered Fission Bomb
The simplest way to bring the subcritical masses together is to make a
gun that fires one mass into the other. A sphere of U-235 is made
around the neutron generator and a small bullet of U-235 is removed.
The bullet is placed at the one end of a long tube with explosives
behind it, while the sphere is placed at the other end. A
barometric-pressure sensor determines the appropriate altitude for
detonation and triggers the following sequence of events:
*The explosives fire and propel the bullet down the barrel.
*The bullet strikes the sphere and generator, initiating the fission
reaction.
*The fission reaction begins.
*The bomb explodes.
Little Boy was this type of bomb and had a 14.5-kiloton yield (equal to
14,500 tons of TNT) with an efficiency of about 1.5 percent. That is,
1.5 percent of the material was fissioned before the explosion carried
the material away.
Implosion-Triggered Fission Bomb
Early in the Manhattan Project the secret U.S. program to develop the
atomic bomb, scientists working on the project recognized that
compressing the subcritical masses together into a sphere by implosion
might be a good way to make a supercritical mass.
There were several problems with this idea, particularly how to control
and direct the shock wave uniformly across the sphere. But the
Manhattan Project team solved the problems. The implosion device
consisted of a sphere of uranium-235 (tamper) and a plutonium-239 core
surrounded by high explosives. When the bomb was detonated, this is
what happened:
*The explosives fired, creating a shock wave.
*The shock wave compressed the core.
*The fission reaction began.
*The bomb exploded.
Fat Man was this type of bomb and had a 23-kiloton yield with an
efficiency of 17 percent. These bombs exploded in fractions of a
second. The fission usually occurred in 560 billionths of a second.
Modern Implosion-Triggered Design
In a later modification of the implosion-triggered design, here is what
happens:
*The explosives fire, creating a shock wave.
*The shock wave propels the plutonium pieces together into a sphere.
*The plutonium pieces strike a pellet of beryllium/polonium at the
center.
*The fission reaction begins.
*The bomb explodes.
Fusion Bombs
Fission bombs worked, but they weren't very efficient. Fusion bombs,
also called thermonuclear bombs, have higher kiloton yields and greater
efficiencies than fission bombs. To design a fusion bomb, some problems
have to be solved:
Deuterium and tritium, the fuel for fusion, are both gases, which are
hard to store.
Tritium is in short supply and has a short half-life so the fuel in the
bomb would have to be continuously replenished.
Deuterium or tritium has to be highly compressed at high temperature to
initiate the fusion reaction.
First, to store deuterium, the gas could be chemically combined with
lithium to make a solid lithium-deuterate compound. To overcome the
tritium problem, the bomb designers recognized that the neutrons from a
fission reaction could produce tritium from lithium (lithium-6 plus a
neutron yields tritium and helium-4; lithium-7 plus a neutron yields
tritium, helium-4 and a neutron).
That meant that tritium would not have to be stored in the bomb.
Finally, It was recognized that the majority of radiation given off in
a fission reaction was X-rays , and that these X-rays could provide the
high temperatures and pressures necessary to initiate fusion.
Therefore, by encasing a fission bomb within a fusion bomb, several
problems could be solved.
Teller-Ulam Design of a Fusion Bomb
To understand this bomb design, imagine that within a bomb casing you
have an implosion fission bomb and a cylinder casing of uranium-238
(tamper). Within the tamper is the lithium deuteride (fuel) and a
hollow rod of plutonium-239 in the center of the cylinder. Separating
the cylinder from the implosion bomb is a shield of uranium-238 and
plastic foam that fills the remaining spaces in the bomb casing.
Detonation of the bomb caused the following sequence of events: The
fission bomb imploded, giving off X-rays. These X-rays heated the
interior of the bomb and the tamper; the shield prevented premature
detonation of the fuel. The heat caused the tamper to expand and burn
away, exerting pressure inward against the lithium deuterate. The
lithium deuterate was squeezed by about 30-fold.
The compression shock waves initiated fission in the plutonium rod. The
fissioning rod gave off radiation, heat and neutrons. The neutrons went
into the lithium deuterate, combined with the lithium and made tritium.
The combination of high temperature and pressure were sufficient for
tritium-deuterium and deuterium-deuterium fusion reactions to occur,
producing more heat, radiation and neutrons.
The neutrons from the fusion reactions induced fission in the
uranium-238 pieces from the tamper and shield. Fission of the tamper
and shield pieces produced even more radiation and heat. The bomb
exploded.
All of these events happened in about 600 billionths of a second (550
billionths of a second for the fission bomb implosion, 50 billionths of
a second for the fusion events). The result was an immense explosion
that was more than 700 times greater than the Little Boy explosion: It
had a 10,000-kiloton yield.
Consequences of Nuclear Explosions
The detonation of a nuclear bomb over a target such as a populated city
causes immense damage. The degree of damage depends upon the distance
from the center of the bomb blast, which is called the hypocenter or
ground zero.
The closer one is to the hypocenter, the more severe the damage. The
damage is caused by several things:
*A wave of intense heat from the explosion
*Pressure from the shock wave created by the blast
*Radiation
*Radioactive fallout , clouds of fine radioactive particles of dust and
bomb debris that fall back to the ground
At the hypocenter, everything is immediately vaporized by the high
temperature (up to 500 million degrees Fahrenheit or 300 million
degrees Celsius). Outward from the hypocenter, most casualties are
caused by burns from the heat, injuries from the flying debris of
buildings collapsed by the shock wave, and acute exposure to the high
radiation.
Beyond the immediate blast area, casualties are caused from the heat,
radiation, and fires spawned from the heat wave. In the long-term,
radioactive fallout occurs over a wider area because of prevailing
winds. The radioactive fallout particles enter the water supply and are
inhaled and ingested by people at a distance from the blast.
Health Risks
Scientists have studied survivors of the Hiroshima and Nagasaki
bombings to understand the short-term and long-term effects of nuclear
explosions on human health. Radiation and radioactive fallout affect
those cells in the body that actively divide (hair, intestine, bone
marrow, reproductive organs). Some of the resulting health conditions
include:
*Nausea, vomiting and diarrhea
*Cataracts
*Hair loss
*Loss of blood cells
*These conditions often increase the risk of:
*Leukemia
*Cancer
*Infertility
*Birth defects
Scientists and physicians are still studying the survivors of the bombs
dropped on Japan and expect more results to appear over time.
In the 1980s, scientists assessed the possible effects of nuclear
warfare (many nuclear bombs exploding in different parts of the world)
and proposed the theory that a nuclear winter could occur. In the
nuclear-winter scenario, the explosion of many bombs would raise great
clouds of dust and radioactive material that would travel high into
Earth's atmosphere. These clouds would block out sunlight.
The reduced level of sunlight would lower the surface temperature of
the planet and reduce photosynthesis by plants and bacteria. The
reduction in photosynthesis would disrupt the food chain, causing mass
extinction of life (including humans). This scenario is similar to the
asteroid hypothesis that has been proposed to explain the extinction of
the dinosaurs.
Proponents of the nuclear-winter scenario pointed to the clouds of dust
and debris that traveled far across the planet after the volcanic
eruptions of Mount St. Helens in the United States and Mount Pinatubo
in the Philippines.
Nuclear weapons have incredible, long-term destructive power that
travels far beyond the original target. This is why the world's
governments are trying to control the spread of nuclear-bomb-making
technology and materials and reduce the arsenal of nuclear weapons
deployed during the Cold War.
I will continue to discuss other weapon configurations, the light at
the end of the Tunnel will align , once I've described the destructive
powers the Nuclear countries have at their disposal, and what we as
individuals have to either Fear or Fight.
MIRVman