Since the various design elements of a thermonuclear weapon combine to form
a complex integrated system, discussing the design space of these weapons
involves complicated tradeoffs between design objectives and has many
possible design variations.
In an attempt to address this in some kind of orderly fashion I first sketch
out several basic structures for the overall weapon, in rough order of
increasing sophistication (Subsection 4.5.1 Principle Design Types).
Following this, I address a series of possible tradeoffs and the issues
connected wit each.
4.5.1 Principle Design Types
The descriptions of weapon designs, and the developmental sequence described
is speculative, but it is consistent with all facts about weapons, weapon
development programs, and physics of which I am currently aware.
4.5.1.1 Early Designs
The earliest radiation implosion designs seem to have used a single large
cylindrical chamber encompassing both the primary and cylindrical secondary.
The casing was hemispherical at one end, where the primary sphere was
located. The thermonuclear weapon was integral to the bomb casing itself -
i.e. the ballistic shell of the bomb was the support structure for the
radiation case, and the physical structure that held the entire
thermonuclear device together.
Both the US and UK initially used casings made of steel, which were lined
with lead or lead bismuth alloy to form the radiation case (probably 1-3 cm
thick). The secondary pusher, which made up the inner wall of the radiation
channel, was made of either natural uranium or lead (possibly as a lead-
bismuth alloy). Operational bombs probably all used uranium tampers to
maximize yield, but some test devices were equipped with lead tampers to
hold down yield and fallout production. A massive radiation shield (uranium
or lead) was located between the primary and secondary to prevent fuel
preheating by the thermal radiation flux. A boron neutron shield was used in
some designs to reduce neutron preheating.
The secondary stage consisted of the exterior pusher/tamper, a standoff gap,
and a cylinder filled with fusion fuel. Lithium deuteride, highly enriched
in Li-6, was the preferred fuel for maximum yield but early shortages in
lithium enrichment capacity lead to the deployment of bombs containing
partially enriched lithium (40% and 60% Li-6 in the U.S.), or natural
lithium. Down the axis of the fusion fuel cylinder was a solid (or nearly
solid) rod of plutonium or HEU for the spark plug.
The design approach of these early bombs followed that of Mike and the test
devices exploded during Castle: the use of a standoff gap to create the
necessary gradual compression required a large diameter (Mike was 80 inches
wide, all of the Castle series devices had diameters from 54 to 61.5
inches). The rapid energy release from the primary followed by a relatively
lengthy implosion required a thick casing for radiation containment, making
the entire bomb very heavy. Mike weighed an anomalous 164,000 pounds, but
even the Castle devices all weighed in between 23,500 and 40,000 lb.
These early bombs were thus quite massive, and had high yields. The Mk 17
and Mk 24 (the weaponized version of Castle Romeo, using unenriched lithium
deuteride) had a diameter of 61.4 inches and a weight of 42,000 lb (yield:
15-20 megatons). The relatively compact and light Mk 15, whose development
was completed somewhat later (and used 95% Li-6 deuteride), still had a
diameter of 34.6 inches and weighed over 7,000 lb (yield: 3.8 megatons). And
all of these weapons *were* bombs, since no missile could carry them. In
fact, only the very largest aircraft could carry them - one per plane.
Although the primaries used in these bombs were much improved over early
fission designs, they were still relatively massive initially. The TX-5
primary used in the Mike device still weighed in at well over 1000 kg, and
the comparatively thick tamper and explosive layers delayed the escape of
both photons and neutrons significantly, by up to 100 nanoseconds.
4.5.1.2 Modular Weapons
During the fifties the diameter of the bomb casing and the primary shrank as
US and Soviet weapons became more compact, partly driven by improved primary
designs. Lighter weight megaton-range weapons were desired for greater
flexibility in the types of aircraft that could carry them, and for
increased payload. Light weight high yield weapons were especially important
for the early ICBMs, which had limited payloads, and low accuracy. Only a
light weight, high yield weapon would give a reasonable chance of destroying
a designated target when carried by an ICBM. It was also useful if the same
basic weapon design could be used in different weapon systems (bombs,
ballistic missiles, cruise missiles, etc.)
This led to a modular approach to the weapon system. Instead of the
aerodynamic casing of the delivered munition, the electronics, and the
"physics package" being a single integrated entity - these three things were
separated. The nuclear warhead proper (the "physics package") was self-
contained, except for a cable connector to the electronics that detonated
the explosives, and fired the neutron generator. The electronics package was
separate, and could be different for each type of weapon (especially
important for the varying fuzing requirements). These two components could
then be fitted into different bomb or missile bodies to create multiple
types of deployable systems.
Since the warhead casing no longer needed to withstand the environmental
rigors of the completed weapon, it could be made out of lighter and less
rugged materials. This led to the use of a light casing (aluminum alloy, or
even plastic) that was lined with a high-Z material to form a radiation
case.
4.5.1.3 Compact Light Weight Designs
More efficient implosion systems and the advent of boosting made primaries
more compact and less massive without sacrificing yield of efficiency. At
this point (which occurred in the U.S. around 1955-1956), there seem to have
been different development paths available.
One path followed the existing design principles, harnessing the increased
temperatures and pressures generated by boosted light weight primaries
through greater radiation confinement by increasing the thickness of the
radiation case at the primary end. This evolved into a separate radiation
case for the primary, a spherical shell of uranium (for example) surrounding
the high explosive shell of the implosion system, with an aperture for
releasing the radiation into the secondary radiation chamber (the chamber
made by lining the external casing). The energy absorbed by the primary case
wall at a high temperature was reradiated as the temperature in the chamber
dropped. This made confinement and channeling of the thermal radiation more
effective. Baffles or other barriers could be added to modulate the energy
transfer into the secondary radiation case.
It appears that an alternate path may have been followed by the US starting
with the Hardtack I test series (although possibly first pioneered in
Redwing). According to statements made by LLNL scientists Wood and Nuckolls,
and LASL Director Bradbury, new design ideas were introduced at this time
that extended the Teller-Ulam concept. This coincides with the development
of the very light W-47 warhead for the Polaris missile (600 lb weight and
600 kt yield, later increased to 800 kt). I speculate that the design
approach introduced here was the use of modulated primary energy release.
4.5.1.4 Two Chamber Designs
At some point, the development trend toward a separate radiation case around
the primary lead to a full two chamber design for the weapon, with some
means of regulating radiation flow between the chambers (like a temporary
radiation barrier). With better control over the radiation flux around the
secondary, a reduced standoff with a reduced secondary diameter (and perhaps
a lighter pusher/tamper) became possible.
This could also be conveniently combined with a spherical secondary design.
This has been described as the "peanut design" - two spherical hollow
chambers joined at the waist, with a primary sphere in one, and a spherical
sphere in the other. Alternatively, a two chamber - spherical secondary
design can be used with a modulated primary.
This approach offers the inherent advantages of spherical implosion - a
smaller radius change for compression in 3-dimensions to attain a given
density compared to two. Smaller radius change translates directly into
faster implosion, an important consideration in a smaller, lighter, higher
pressure weapon design which would be prone to disassemble faster.
In a spherical secondary the radiation shield between the primary and
secondary would evolve into a baffle between the two chamber to prevent the
primary from directly (and thus unevenly) heating the side of the secondary
facing it, forcing the radiation flux to diffuse into the channel around the
secondary.
The primary in a two-chamber design may be effectively encased in a heavy,
close fitting uranium shell that can act as an implosion tamper. By trapping
the explosive gases, this shell can act as the wall of a spherical piston,
forcing the expanding gases to transfer all of their energy to the inward
moving beryllium/plutonium shell, and minimizing the amount of explosive
required. Such a primary may use a thin uranium or tungsten tamper between
the beryllium and plutonium shell layers to enhance inertial confinement of
the fissile mass.
4.5.1.5 Hollow Shell Designs
It was pointed out earlier that it is difficult to efficiently compress more
than the outermost layers of a solid cylindrical or spherical fuel mass. In
any case, only the outermost layers actually *need* to be compressed, since
they contain the lion's share of the fuel mass. It would be logical then to
dispense with the idea of using a solid fuel mass in the center, and only
use a hollow shell of fuel in the first place. A hollow spark plug shell
could be nested directly inside the fuel shell, but a second tamper layer
may be included between the two.
A hollow shell could be used with either a cylindrical secondary (making it
"totally tubular"), or with a spherical design.
Several advantages are obtained with this approach.
The fuel near the center that would be inefficiently compressed is
eliminated, improving overall fuel utilization.
The addition of the dense second tamper or spark plug on the inner side of
the fuel layer can also directly enhance compression. Whenever a shock
reaches the inner side of the fuel, it will be reflected back into the fuel
at higher pressure, compressing the fuel further. If the compression
gradient is continuous, it will tend to "pile up" at the inner interface,
with the same effect of compression enhancement. The dense inertial tamper
on the inner side of the fuel layer will also help keep it at a constant
high density.
Finally, the hollow shell design allows the spark plug to accelerate to very
high velocities before it goes critical. The implosion velocity at
criticality could be even higher than the average maximum implosion velocity
for the secondary, due to the effects of thick shell collapse and
convergence. An implosion velocity exceeding 1000 km/sec is conceivable.
This is so fast that densities much higher than those achieved by high
explosive systems would be attained before energy production from fission
becomes high enough to halt implosion. Even relatively small masses of
fissile material (< 1 kg) could be fissioned efficiently.
Hollow shell secondaries would be essential for use with primaries that rely
on modulated energy release to create efficient compression.
4.5.1.4 High Yield and Multiple Staged Designs
The first thermonuclear devices were high yield by most any standard (10.4
Mt for Ivy Mike, 15 Mt for Castle Bravo). But they were also very heavy, and
difficult to push to even greater yields. High yield weapons with greater
yield-to-weight ratios, providing even higher yields in deliverable packages
were desired.
As a rough approximation, we can say that the amount of energy required to
implode a secondary is proportional to its mass, since the primary
energy/secondary mass ratio defines the achievable implosion velocity. The
yield of the secondary should also be roughly proportional to its mass. Thus
there is a roughly proportional relationship between the primary and
secondary yields, using similar design principles.
From available data (based on known trigger tests, and fizzles where only
the primary fired), it appears that this range can be from 10-200, with 30-
50 being more typical ratios.
If a very large yield is desired, then we must obviously have a very large
primary. Large fission primaries are expensive, heavy, and potentially
dangerous (due to the large amount of fissile material present). Even in
very heavy weapons, the yield of the primary is limited to no more than a
few hundred kilotons, limiting total yield to a maximum of 10-20 megatons.
The high yield designs actually developed (mostly in the fifties and early
sixties) seem to have used refined versions of the basic thermonuclear
weapon design approach, as described above, with the addition of multiple
staging to achieve even higher yields. The relatively light weight W-53 9 Mt
warhead/bomb deployed by the US (still in service!), was one of the highest
yield warheads the US ever deployed, and probably is a 3 stage weapon.
This is really large enough for almost any conceivable destructive use
(except maybe blowing up asteroids). Nonetheless, military requirements for
even larger weapons have been drafted, and in the case of the Soviet Union,
actually built, tested, and deployed. At one point in the mid-fifties the US
military requested a 60 megaton bomb! This military "requirement" was
apparently driven by the fact that this was the highest yield device that
could be delivered by existing aircraft. The Soviets eventually went on to
develop a 100+ megaton design (tested in a 50 megaton configuration). To
make such megaweapons, a bigger driving explosion is required to implode the
main fusion stage. This has led to the design of three stage weapons, where
a thermonuclear secondary is the main driving force to implode a gigantic
tertiary stage.
Building gargantuan bombs is not the only motivation for adopting three
stage weapons however. If the fusion neutrons are not harnessed to cause
fission in the tamper (either because the bomb is intended to be very clean,
or very dirty) then the ratio in yields between stages is correspondingly
reduced - to a range of something like 10 to 15. This limits the practical
maximum yield to 3 to 5 megatons. It may be doubted whether even this is
much of a limitation since out of a current arsenal of over 10000 warheads,
the US only has 50 bombs with yields over 3 megatons. In the fifties however
this seemed unacceptably small, so "clean" weapons were deemed to require
three stage design.
Three stage design can provide other advantages though. By offering the
weapon designer additional freedom in design, it may be useful even if the
bomb is not especially large, clean, or dirty. For example, in optimizing a
weapon to minimize weight for a given yield, a designer can consider which
type of driver for the main stage is the lightest - a large fission primary
or a compact two stage device. If weapon-grade fissile material is very
precious, then a two-stage driver might be chosen simply to minimize the
over utilization of this material.
In a three stage weapon the radiation cases for the secondary and tertiary
might be kept separate initially. The primary would implode the secondary
but a barrier would prevent energy from reaching the tertiary. This barrier
could be designed to ablate away during the secondary implosion, so that
when the secondary energy release occurred, it would have become
transparent.
Alternatively it may be useful to harness a portion of the primary's energy
to create an initial weak compression shock in the tertiary to enhance
compression efficiency.
4.5.2 "Dirty" and "Clean" Weapons
Whether to make a fission-fusion weapon into a fission-fusion-fission weapon
is one of the most basic design issues. A fission-fusion weapon uses an
inert (or non-fissionable) tamper and will obtain most of its yield from the
fusion reaction directly. A fission-fusion-fission weapon will obtain at
least half of its yield (and often far more) from the fusion neutron induced
fission of a fissionable tamper.
The basic advantage of a fission-fusion-fission weapon is that energy is
extracted from a tamper which is otherwise deadweight as far as energy
production in concerned. The tamper has to be there, so a lighter weapon for
a given yield (or a more powerful weapon for the same weight) can be
obtained without varying any other design factors. Since it is possible to
do this at virtually no added cost or other penalty, compared to an inert
material like lead, by using natural or depleted uranium or thorium there is
basically no reason not to do it if the designer is simply interested in
making big explosions.
Fission of course produces radioactive debris - fallout. Fallout can be
reduced by using a material that does not become highly radioactive when
bombarded by neutrons (like lead or tungsten). This requires a heavier and
more expensive weapon to produce a given yield, but is also considerably
reduces the short and long term contamination associated with that yield.
This is not to say that the weapon is "clean" in any commonsense meaning of
the term. Neutrons escaping the weapon can still produce biohazardous
carbon-14 through nitrogen capture in the air. The primary and spark plug
may still contribute 10-20% fission, which for a multi-megaton weapon may
still be a megaton or more of fission. Significant contamination may also
occur from the "inert" tamper radioisotopes, and even from the unburned
tritium produced in the fusion stage. Reducing these contributions to the
lowest possible level is the realm of "minimum residual radiation" designs
discussed further below.
During the fifties interest in both the US and USSR was given to developing
basic design that had both clean and dirty variants. The basic design tried
to minimize the essential fission yield by using a small fission primary,
and spark plug sizes carefully chosen to meet ignition requirements for each
stage, without being excessive (note that although only part of the spark
plug will fission to ignite the fusion stage, the essentially complete
fission of the remainder by fusion neutrons is inevitable). These weapons
appear to have all been three-stage weapons to allow multi-megaton yields
(even in the clean version) with a relatively small primary. The dirty
version might simply replace the inert tamper of the tertiary with a
fissionable one to boost yield.
The three-stage Bassoon and Bassoon Prime devices tested in Redwing Zuni (27
May 1956, 3.5 Mt, 15% fission) and Redwing Tewa (20 July 1956, 5 Mt, 87%
fission) are US tests of this concept. Clearly though, the second test was
not simply a copy of the first with a different tamper. The fusion yield
dropped from 3 Mt to 0.65 Mt, and the device weight increased from 5500 kg
to 7149 kg between the two tests. The inference can be made that the
tertiary in the first used a large volume of relatively expensive (but
light) Li-6D in a thin tamper, which was replaced by a heavier, cheaper
tertiary using less fusion fuel, but a very thick fissionable tamper to
capture as many neutrons as possible.
The 50 Mt three stage Tsar Bomba (King of Bombs) tested by the Soviet Union
on 30 October 1961 was the largest and cleanest bomb ever tested, with 97%
of its yield coming from fusion (fission yield approximately 1.5 Mt).
Assuming a primary of 250 kt (to keep the fissile content relatively low for
safety reasons), we might postulate secondary and tertiary stages of 3.5 Mt
and 46 Mt respectively. This fusion stages would require 1700 kg of Li6D (at
50% fusion efficiency), and something like 250 kt of fission for reliable
ignition. If the initial spark plug firings were 25% efficient, later
fission would release another 750 kt - placing the total at 1.25 Mt (close
enough to the claimed parameters to match within the limits of accuracy).
This was a design though for a 100-150 Mt weapon! A lead tamper was used in
the tested device, which could have been replaced with U-238 for the dirty
version (thankfully never tested!).
4.5.3 Maximum Yield/Weight Ratio
Except for safety, the weight of a weapon required to provide a given yield
is the most important design criterion. In the years since the first nuclear
weapon was exploded, far more money has been spent in building nuclear
weapon delivery systems than in the weapons themselves. The high cost of
delivery for what is basically a rather small package is due to the fact
that nuclear delivery systems are generally intended to be used only once.
Clearly this is true for missiles, but it is true for bombers as well since
recovery and reuse is not part of their nuclear mission profile.
Since the cost of the delivery vehicle is much greater than the cost of the
warhead, making the warhead as light as possible for the intended yield
quickly came to dominate the weapon design process. this is normally
expressed in terms of the yield-to-weight (YTW) ratio (kt/kg).
Naturally it is easier to get a high ratio for a larger bomb. The highest
ratio for any warhead in the US arsenal is the 9 Mt Mk-53/B-53 bomb, which
happens to be the oldest weapon in service (operational since 1962), but
also the largest. At 4000 kg, it has a ratio of 2.25 kt/kg. The Tsar Bomba,
as tested, had a ratio of 1.7 kt/kg (its weight was 30 tonnes). As
*designed* it had a ratio of 3.4-5 kt/kg!
Table 4.5.3-1. Yield-to-Weight Ratios of Current US Weapons
Weapon YTW Ratio Yield(kt)/Weight(kg) In Service Date
Mk-53 2.25 9000/4000 1962
W-88 1.5 475/330
W-80 1.31 170/130
B-83 1.10 1200/1090
W-87 1.0 300/300
W-78 0.96 335/350
W-76 0.61 100/165
The much earlier W-47 warhead seems to have achieved ratios of 2.2-2.7
kt/kg. However YTW ratio is no every thing. The W-87 and W-88 are said to
use reduced amounts of expensive nuclear materials (deemed important when
ambitious expansion of the US nuclear arsenal was planned in the early
eighties) which, coupled with the much larger payloads of the MX and trident
II missiles, may account for the reduced (but still quite respectable) YTW
ratios of these warheads.
Part of optimizing the YTW ratio is careful weight management. Very light
weight primaries, the use of light weight weapon cases, and multiple
radiation cases are innovations to minimize weight. Since the tamper is one
of the heaviest parts of the weapon, squeezing as much energy out of this is
very important too.
The end of surface testing of nuclear weapons after the atmospheric test ban
treaty effectively removed "cleanliness" as a significant concern for
designers. Complaints about fall-out vanished, and so did the ability of the
international community to monitor weapon design through fall-out analysis.
The cost-effectiveness of lighter weapons put great pressure on designers to
extract weight saving however they could, and it is likely that the idea of using non-fissile tampers disappeared very quickly. There is scant evidence that so-called "clean" designs were ever deployed in any quantity.
The fission yield of the tamper can be increased even further by adding
slow-neutron fissionable material to it. Basically this means using enriched
uranium instead of natural or depleted uranium.
Highly enriched uranium is definitely known to be used in U.S. weapons.
About half of the U.S. inventory of weapons-associated HEU is less than
"weapons grade" (<93.4% that is). The probable use of most or all of this
uranium (generally with an enrichment of 20-80%) was in thermonuclear weapon
tampers.
The W-87 Peacekeeper warhead (to be redeployed on the Minuteman-III) has a
current yield of 300 kt, that can be increased to 475 kt by adding a HEU
sleeve or rings to the secondary. Whether this represents an actual addition
to the existing secondary, or whether it replaces an existing unenriched
sleeve is not known. The W-88 Trident warhead is a closely related design,
and has a current yield of 475 kt indicating that it is already equipped
with this addition. The 175 kt yield difference amounts to the complete
fission of 10 kg of U-235.
Now, once one considers using substantial amounts of HEU in the secondary,
the question of why the fusion fuel is needed at all arises. The answer: it
probably is not essential. The idea of imploding fissile material is what
set Stanislaw Ulam on the path to that led eventually to thermonuclear
weapons. But with the availability of large amounts of HEU, and the trend
toward smaller weapon yields (compared to the multimegaton behemoths of the
fifties), the Ulam's idea of using radiation implosion to create a light
weight high-efficiency pure fission weapon returns as a viable possibility.
It is an interesting question whether all modern strategic nuclear weapons
*are* in fact thermonuclear devices!
4.5.4 Minimum Residual Radiation (MRR or "Clean") Designs
It has been pointed out elsewhere in this FAQ that ordinary fission-fusion-
fission bombs (nominally 50% fission yield) are so dirty that they merit
consideration as radiological weapons. Simply using a non-fissile tamper to
reduce the fission yield to 5% or so helps considerably, but certainly does
not result in an especially clean weapon by itself. If minimization of
fallout and other sources of residual radiation is desired then considerably
more effort needs to be put into design.
Minimum residual radiation designs are especially important for "peaceful
nuclear explosions" (PNEs). If a nuclear explosive is to be useful for any
civilian purpose, all sources of residual radiation must be reduced to the
absolute lowest levels technologically possible. This means elimination
neutron activation of bomb components, of materials outside the bomb, and
reducing the fissile content to the smallest possible level. It may also be
desirable to minimize the use of relatively hazardous materials like
plutonium.
The problems of minimizing fissile yield and eliminating neutron activation
are the most important. Clearly any MRR, even a small one, must be primarily
a fusion device. The "clean" devices tested in the fifties and early sixties
were primarily high yield strategic three-stage systems. For most uses (even
military ones) these weapons are not suitable. Developing smaller yields
with a low fissile content requires considerable design sophistication -
small light primaries so that the low yields still produce useful radiation
fluxes and high-burnup secondary designs to give a good fusion output.
Minimizing neutron activation form the abundant fusion neutrons is a serious
problem since many materials inside and outside the bomb can produce
hazardous activation products. The best way of avoiding this is too prevent
the neutrons from getting far from the secondary. This requires using an
efficient clean neutron absorber, i.e. boron-10. Ideally this should be
incorporated directly into the fuel or as a lining of the fuel capsule to
prevent activation of the tamper. Boron shielding of the bomb case, and the
primary may be useful also.
It may be feasible to eliminate the fissile spark plug of a MRR secondary by
using a centrally located deuterium-tritium spark plug similar to the way
ICF capsules are ignited. Fusion bombs unavoidably produce tritium as a by-
product, which can be a nuisance in PNEs.
Despite efforts to minimize radiation releases, PNEs have largely been
discredited as a cost-saving civilian technology. Generally speaking, MRR
devices still produce excessive radiation levels by civilian standards
making their use impractical.
MRRs may have military utility as a tactical weapon, since residual
contamination is slight. Such weapons are more costly and have lower
performance of course.
This leads to another reason why PNEs have lost their attractiveness - there
is no way to make a PNE device unsuitable for weapons use. "Peaceful" use of
nuclear explosives inherently provides opportunity to develop weapons
technology. As the saying goes, "the only difference between a PNE and a
bomb is the tail fins".
4.5.5 Radiological Weapon Designs
This is the opposite extreme of an MRR. Earlier several tamper materials
were described that could be used to tailor the radioactive contamination
produced by a nuclear explosion - tantalum, cobalt, zinc, and gold. Uranium
tampers produce contamination in abundance - but quite a lot of energy too.
In some applications it may be desired that the ratio of contamination to
explosive force be increased, or tailored to a narrower spectrum of decay
times compared to fission by-products.
Practical radiological weapons must incorporate the precursor isotope
directly into the secondary. This is because the high compression of the
secondary allows the use of reasonable masses of precursor material. In an
uncompressed state, the thickness of most materials required to capture a
substantial percentage of neutrons is 10-20 cm, leading to a very massive
bomb. A layer of 1 cm or less will do as well when compressed by radiation
implosion.
Some radioisotopes that would be very attractive for certain applications
are difficult to produce in a weapon. A case in point is sodium-24, an
extremely prolific producer of energetic gammas with a half-life of 14.98
hours. This isotope produces a remarkable 5.515 MeV of decay energy, with
two hard gammas per decay (2.754 MeV and 1.369 MeV) and might be desired for
very short-lived radiation barriers. The most obvious precursor, natural Na-
23, has a minuscule capture cross section for neutrons in the KeV range
(although it is a significant hazard from induced radioactivity in soil
after low altitude nuclear detonations). The best for precursor candidate
for Na-24 is probably magnesium-24 (78.70% of natural magnesium) through an
n,p reaction.
4.6 Weapon System Design
4.6.1 Weapon Safety
Due to their enormous destructive power, it is extremely important to ensure
that nuclear weapons cannot explode at either their full yield, or at
reduced yield, unless stringent and carefully specified conditions are met.
Weapons must be resist:
* malicious tampering,
* human error,
* component or systems failure (either inside or outside the weapon),
and
* accidental damage.
To meet these requirements elaborate provisions for weapon safety are
required. This issue has been of major concern since the first nuclear
weapons, and many of the major advances in weapon design are related to
weapon safety.
Weapons are invariably designed with a series of disabling mechanisms, all
of which must be successfully overridden before an explosion can occur.
These include locking mechanisms requiring special keys or codes, redundant
safeties that must be removed to arm the weapon, environmental sensing
switches (disabling mechanisms that are overridden only when the weapon has
experienced environmental conditions and stresses expected during
operational employment), and sophisticated fuzing systems to detonate the
device at the proper place and time. Often these multiple safety systems
require cooperation by more than one person to complete weapon arming.
Scenarios that must be addressed include:
* inappropriate activation of the weapon's firing system,
and
* detonation of the high explosives by means other than the firing system
(e.g. physical damage through fire or impact).
**** Unfinished ****
4.6.1.1 Safeties and Fuzing Systems
4.6.1.2 Accident Safety
4.6.2 Variable Yield Designs
**** Unfinished ****
4.6.3 Other Modern Design Features
**** Unfinished ****