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4.5 Thermonuclear Weapon Designs

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 ****

4.7 Speculative Weapon Designs

**** Unfinished ****

4.8 Simulation and Testing

**** Unfinished ****