BENCHMARK I
Part 2
Nuclear
weapons are particularly destructive, with immediate effects including blast
and thermal radiation and delayed effects produced by ionizing radiation,
neutrons, and radioactive fallout. They are expensive to build, maintain, and
employ, requiring a significant fraction of the total defense resources of a
small nation. In a totalitarian state the leader must always worry that they
will be used against the government; in a democracy the possibility of an
unauthorized or accidental use must never be discounted. A nuclear detonation
as the result of an accident would be a local catastrophe.
Because
of their destructiveness, nuclear weapons require precautions to prevent
accidental detonation during any part of their manufacture and lifetime. And
because of their value, the weapons require reliable arming and fuzing
mechanisms to ensure that they explode when delivered to target. Therefore, any
nuclear power is likely to pay some attention to the issues of safing and
safety, arming, fuzing, and firing of its nuclear weapons. The solutions
adopted depend upon the level of technology in the proliferant state, the
number of weapons in its stockpile, and the political consequences of an
accidental detonation.
Nuclear Weapon Design
Fission Weapons
An
ordinary ÒatomicÓ bomb of the kinds used in World War II uses the process of
nuclear fission to release the binding energy in certain nuclei. The energy
release is rapid and, because of the large amounts of energy locked in nuclei,
violent. The principal materials used for fission weapons are U-235 and Pu-239,
which are termed fissile because they can be split into two roughly equal-mass
fragments when struck by a neutron of even low energies. When a large enough
mass of either material is assembled, a self-sustaining chain reaction results
after the first fission is produced.
The minimum mass of fissile material that can
sustain a nuclear chain reaction is called a critical mass and depends on the
density, shape, and type of fissile material, as well as the effectiveness of
any surrounding material (called a reflector or tamper) at reflecting neutrons
back into the fissioning mass. Critical masses in spherical geometry for
weapon-grade materials are as follows:
Uranium-235 Plutonium-239 Bare sphere: 56 kg 11 kg Thick Tamper: 15 kg 5 kg
The
critical mass of compressed fissile material decreases as the inverse square of
the density achieved. Since critical mass decreases rapidly as density
increases, the implosion technique can make do with substantially less nuclear
material than the gun-assembly method.
If
any more material is added to a critical mass a condition of supercriticality
results. The chain reaction in a supercritical mass increases rapidly in
intensity until the heat generated by the nuclear reactions causes the mass to
expand so greatly that the assembly is no longer critical.
Fission
weapons require a system to assemble a supercritical mass from a sub-critical
mass in a very short time. Two classic assembly systems have been used, gun and
implosion. In the simpler gun-type device, two subcritical masses are brought
together by using a mechanism similar to an artillery gun to shoot one mass
(the projectile) at the other mass (the target). The Hiroshima weapon was
gun-assembled and used 235 U as a fuel. Gun-assembled weapons using highly
enriched uranium are considered the easiest of all nuclear devices to construct
and the most foolproof.
Gun-Device
In
the gun device, two pieces of fissionable material, each less than a critical
mass, are brought together very rapidly to forma single supercritical one. This
gun-type assembly may be achieved in a tubular device in which a high explosive
is used to blow one subcritical piece of fissionable material from one end of
the tube into another subcritical piece held at the opposite end of the tube.
15-kt
weapon was airdropped on 06 August 1945 at Hiroshima, Japan. The device
contained 64.1 kg of highly enriched uranium, with an average enrichment of
80%. The six bombs built by the Republic of South Africa were gun-assembled and
used 50kg of uranium enriched to between 80 percent and 93 percent in the
isotope U-235.
Compared
with the implosion approach, this method assembles the masses relatively slowly
and at normal densities; it is practical only with highly enriched uranium. If
plutonium -Ñ even weapon-grade -- were used in a gun-assembly design, neutrons
released from spontaneous fission of its even-numbered isotopes would likely
trigger the nuclear chain reaction too soon, resulting in a "fizzle"
of dramatically reduced yield.
Implosion-Device
Because
of the short time interval between spontaneous neutron emissions (and,
therefore, the large number of background neutrons) found in plutonium because
of the decay by spontaneous fission of the isotope Pu-240, Manhattan Project
scientists devised the implosion method of assembly in which high explosives
are arranged to form an imploding shock wave which compresses the fissile
material to supercriticality.
The
core of fissile material that is formed into a super-critical mass by chemical
high explosives (HE) or propellants. When the high explosive is detonated, an
inwardly directed implosion wave is produced. This wave compresses the sphere
of fissionable material. The decrease in surface to volume ratio of this
compressed mass plus its increased density is then such as to make the mass
supercritical. The HE is exploded by detonators timed electronically by a
fuzing system, which may use altitude sensors or other means of control.
The
nuclear chain-reaction is normally started by an initiator that injects a burst
of neutrons into the fissile core at an appropriate moment. The timing of the
initiation of the chain reaction is important and must be carefully designed
for the weapon to have a predictable yield. A neutron generator emits a burst
of neutrons to initiate the chain reaction at the proper moment Ñ- near the
point of maximum compression in an implosion design or of full assembly in the
gun-barrel design.
A
surrounding tamper may help keep the nuclear material assembled for a longer
time before it blows itself apart, thus increasing the yield. The tamper often
doubles as a neutron reflector.
Implosion
systems can be built using either Pu-239 or U-235 but the gun assembly only
works for uranium. Implosion weapons are more difficult to build than gun
weapons, but they are also more efficient, requiring less SNM and producing
larger yields. Iraq attempted to build an implosion bomb using U-235. In
contrast, North Korea chose to use 239 Pu produced in a nuclear reactor.
Boosted Weapons
To
fission more of a given amount of fissile material, a small amount of material
that can undergo fusion, deuterium and tritium (D-T) gas, can be placed inside
the core of a fission device. Here, just as the fission chain reaction gets
underway, the D-T gas undergoes fusion, releasing an intense burst of
high-energy neutrons (along with a small amount of fusion energy as well) that
fissions the surrounding material more completely. This approach, called
boosting, is used in most modem nuclear weapons to maintain their yields while
greatly decreas-ing their overall size and weight.
Enhanced Radiation Weapons
An
enhanced radiation (ER) weapon, by special design techniques, has an output in
which neutrons and x-rays are made to constitute a substantial portion of the
total energy released. For example, a standard fission weapon's total energy
output would be partitioned as follows: 50% as blast; 35% as thermal energy;
and 15% as nuclear radiation. An ER weapon's total energy would be partitioned
as follows: 30% as blast; 20% as thermal; and 50% as nuclear radiation. Thus, a
3-kiloton ER weapon will produce the nuclear radiation of a 10-kiloton fission
weapon and the blast and thermal radiation of a 1-kiloton fission device.
However, the energy distribution percentages of nuclear weapons are a function
of yield.
Fusion Weapons
A
more powerful but more complex weapon uses the fusion of heavy isotopes of
hydrogen, deuterium, and tritium to release large numbers of neutrons when the
fusile (sometimes termed "fusionable") material is compressed by the
energy released by a fission device called a primary. Fusion (or ÔÔthermonuclearÕ
weapons derive a significant amount of their total energy from fusion
reactions. The intense temperatures and pressures generated by a fission
explosion overcome the strong electrical repulsion that would otherwise keep
the positively charged nuclei of the fusion fuel from reacting. The fusion part
of the weapon is called a secondary.In general, the x-rays from a fission
primary heat and compress material surrounding a secondary fusion stage.
It
is inconvenient to carry deuterium and tritium as gases in a thermonuclear
weapon, and certainly impractical to carry them as liquefied gases, which
requires high pressures and cryogenic temperatures. Instead, one can make a
ÒdryÓ device in which 6Li is combined with deuterium to form the
compound 6Li D (lithium-6 deuteride). Neutrons from a fission
ÒprimaryÓ device bombard the 6 Li in the compound, liberating
tritium, which quickly fuses with the nearby deuterium. The a particles, being
electrically charged and at high temperatures, contribute directly to forming
the nuclear fireball. The neutrons can bombard additional 6Li nuclei
or cause the remaining uranium and plutonium in the weapon to undergo fission.
This two-stage thermonuclear weapon has explosive yields far greater than can
be achieved with one point safe designs of pure fission weapons, and
thermonuclear fusion stages can be ignited in sequence to deliver any desired
yield. Such bombs, in theory, can be designed with arbitrarily large yields:
the Soviet Union once tested a device with a yield of about 59 megatons.
In
a relatively crude sense, 6 Li can be thought of as consisting of an alpha
particle ( 4He) and a deuteron ( 2H) bound together. When
bombarded by neutrons, 6 Li disintegrates into a triton ( 3
H) and an alpha:
6 Li + Neutron = 3 H + 3 He + Energy.
This
is the key to its importance in nuclear weapons physics. The nuclear fusion
reaction which ignites most readily is
2 H + 3 H =
4 He + n + 17.6 MeV,
or,
phrased in other terms, deuterium plus tritium produces 4He plus a
neutron plus 17.6 MeV of free energy:
D + T = 4 He + n + 17.6 MeV.
Lithium-7
also contributes to the production of tritium in a thermonuclear secondary,
albeit at a lower rate than 6Li. The fusion reactions derived from
tritium produced from 7 Li contributed many unexpected neutrons (and
hence far more energy release than planned) to the final stage of the infamous
1953 Castle/BRAVO atmospheric test, nearly doubling its expected yield.
Safing, Arming, Fuzing, and Firing (SAFF)
Safing
To ensure that the nuclear
warhead can be stored, handled, deployed, and employed in a wide spectrum of
intended and unintended environmental and threat conditions, with assurance
that it will not experience a nuclear detonation
Detonator
Alpha, Beta and Gamma Emitters and Their Effects on Living Beings.
Even some years ago
nobody knew exactly about how NW effects on a human being.
In 1945 atom bomb was
first used in Hiroshima. (12,5 thousand ton of trinitrotoluene).
Mankind has realized
unseen and unimaginable power of destruction, casualties and people caught by
alpha, beta and gamma emitters from the bomb.
ALPHA Ð twice ioned atom (nucleus) of
helium. DoesnÕt play much role in NW. But if a human is affected by radioactive
agents it begins to emit in and around the zone of exist. The period of living
alpha emitters in the air is short, but it is very difficult to get them away
of the organism.
BETA Ð electron, its
period in the air is short, it can be kept (stopped) by clothes.
GAMMA Ð quantum of
high frequency, has no barrier, enters through the organism, ruins our gene
system and causes mutation.
The Process of Nuclear Weapon Effect
- A bomb is launched (the type of explosion
can be in space, air, land-surface, underground, water-surface,
underwater);
- Appears a great cloud, which radiates
(light); the temperature Òof millions of SunsÓ rises (over Hiroshima it
was 100 meters in diameter);
- Light emission begins (in Hiroshima it was
2 km long);
- Alpha, beta and gamma (so-called entering
radiation) emission begins;
- High pressure erects;
- Smashing wave begins to proliferate;
- Dust and dirt rises from the ground;
- Radiation sits on (pollutes) all these
numerous objects;
- A huge cloud of substance with radioactive
agents gathers; and the wind blows it over the Earth and circulates danger
and lethal dose of radiation. Prohibition depends on the wind and
landscape. It can be of any shape, length and width.
A Historical Example of the Use of Nuclear Weapons
Nuclear
weapons have been used militarily only twice, at the end of World War II.
1)The
"Fat Man" atomic bomb that destroyed Nagasaki in 1945 used 6.2
kilograms of plutonium and produced an explosive yield of 21-23 kilotons [a
1987 reassessment of the Japanese bombings placed the yield at 21 Kt]. Until
January 1994, the Department of Energy (DOE) estimated that 8 kilograms would
typically be needed to make a small nuclear weapon. Subsequently, however, DOE
reduced the estimate of the amount of plutonium needed to 4 kilograms.
2) 15-kt weapon was airdropped on 06
August 1945 at Hiroshima, Japan. The device contained 64.1 kg of highly
enriched uranium, with an average enrichment of 80%.
At the time of the bombing, Hiroshima was a
prosperous city of nearly 320,000. The bomb exploded almost directly over
the center of the city. Two square miles of the city were completely
leveled by the bomb, and the intense heat generated by the explosion
started fires as far as two miles from ground zero. Distance
from Killed Injured Population 0 -1.0 86% 10% 31,200 1.0 - 2.5 27% 37% 144,800 2.5 - 5.0 2% 25% 80,300 Total 27% 30% 256,300
Ground Zero (km)
Though Fat Man was nearly twice as powerful as
Little Boy, its damage was less extensive, due partly to the geography of
the Nagasaki area and partly to the fact that the bomb was dropped about 2
miles off target. Distance
from Killed Injured Population 0 -1.0 88% 6% 30,900 1.0 - 2.5 34% 29% 27,700 2.5 - 5.0 11% 10% 115,200 Total 22% 12% 173,800
Ground Zero (km)
