ISIS
Nuke Proofs Part 2 – the science of the small, clean neutron bomb
By Ian Greenhalgh on July 21, 2016
Small,
clean nukes can be made from waste fuel by anyone with a decent metalworking
shop and a bit of know-how - a truly scary scenario in this age of terrorist
bomb attacks.
Editors
note by Ian Greenhalgh:
Once
again, Jeff delivers a truly devastating information bomb, one that will lay
waste to entire paradigms that have become increasingly anachronistic in the
modern age. For almost everyone, when they think of nuclear weapons they
picture the 1950s newsreels of colossal hydrogen bombs blasting immense
mushrooms clouds into the stratosphere and spreading deadly fallout over huge
areas. That type of bomb still exists, but are unlikely to ever be used, they
have existed for almost 70 years and no-one has ever dared to use one and I
doubt anyone ever will.
However,
those are very obsolete and outdated weapons, what we need to be concerned
about today is four or five generations evolved from those old megatonne
H-bombs – a totally different type of weapon that does not require a critical
mass and therefore requires only a tiny amount of fissile material by
comparison and best of all doesn’t produce a huge amount of nasty fallout and
ionising radiation. In fact, they produce no EMP, virtually no harmful fallout
and only a small amount of radiation that is gone within hours. The
yield of these bombs is, of course, much smaller, in the low kilotonne range,
starting around 0.5kt and scaling upwards by adding more fissile
material. All these factors make these weapons far more usable than the
old megatonne monsters.
Now for
the really scary part – these small clean neutron bombs are also very simple
and cheap to manufacture, so much so that even non-state actors such as Islamic
State could feasibly manufacture them in a decent machine shop.
Then it
gets even worse – you can make these things out of the used fuel rods from
nuclear reactors – something that is common and, given the political situation
in some countries that contain nuclear reactors, quite readily available on the
black market – certainly far easier to obtain than any ‘weapons grade’
material, which has been closely monitored for decades.
This
scenario totally undermines all the years of political anti-proliferation
negotiations, all the treaties and international monitoring agencies, all of it
is now irrelevant due to the new designs of the 4th and 5th generation as
described here by Jeff.
by Jeff
Smith
Note
below 1KT all charts stop. There is a reason why. Below 1KT you don’t need a
“critical Mass” because you just hold it together longer. 32 neutron chain
reaction is equal to 1KT or more in yield. So a much smaller amount will still
go bang.
Critical
Mass
Scientists
define criticality as a measure of the ability of nuclear material to sustain a fission chain reaction. If a
system is subcritical, it cannot sustain a fission chain reaction. If a
system is supercritical, the fission chain reactions grow greatly. A
system that is “critical” is the bounding case – this means that it
sustains a chain reaction with a constant rate. The critical mass of
fissile material depends on many factors:
·
Purity of material
·
Shape of material
·
Density of material
·
Temperature of material
·
Surrounding materials
The
“bare sphere critical mass” of weapons-grade uranium and weapons-grade
plutonium is approximately 52 kg and 10 kg (respectively).
The
critical mass of fissile material informs the most fundamental question of
nuclear weapon design. The question which Scientists have asked for the
first time at the beginning of the Manhattan Project and which every
nuclear weapon state has asked since then is: how much fissile material is
necessary for a nuclear weapon? The class of nuclear weapon design (see
section nuclear weapon
design ) determines the
amount which is needed. For a
gun-assembled design, more fissile material is necessary than for an implosion device, for example.
gun-assembled design, more fissile material is necessary than for an implosion device, for example.
The
International Atomic Energy Agency defines “significant quantities” of uranium
and plutonium as 25 kg and 8 kg (respectively). However, the figures which
are below show that it is possible to produce reasonable nuclear yields
with much less nuclear material than this, even if the
technical capability is low.
However,
research by a Polish Scientist, S. Kaliski, suggests that even smaller nuclear weapons are possible.
What
this means is if you use a very small amount of PU or an enriched fuel rod full
of Pu and compress it in a two point gun implosion system it will still go
bang. Any amount under 3KG will produce a yield up to 1KT. if velocity is high
enough and compression holds together long enough. The PU is the original
source for the neutrons so no neutron initiator is needed. A thick iron or
nickle casing will be strong enough at this explosive compression level to work
for low yields.
It is
necessary for the combination of choice of propellant and length/thickness of
the gun barrel to give sufficient acceleration to the projectile before
insertion. Because it is desirable to minimize weight and length of the
weapon, insertion velocities are limited to velocities which are below one
km per second.
Because
it is probable that the target and projectile will be close to a critical mass
before assembly, it is probable that a critical configuration will be
obtained before the complete insertion of the projectile in the target, or
possibly before the projectile reaches the target. The probability of this
occurring increases at the same time as the mass of the target
and projectile increases. As soon as a critical mass assembles, there is a
chance of pre-detonation.
Thus it is desirable to have as a high insertion velocity as possible to minimize the risk of a “fizzle”.
Thus it is desirable to have as a high insertion velocity as possible to minimize the risk of a “fizzle”.
Because
of the inherent pre-detonation risk during assembly, the choice of fissile
material is limited. Plutonium undergoes spontaneous fission and thus is a
neutron source itself – this greatly raises the risk of pre-detonation.
This is not the case for uranium, and so it becomes the preferred
material. However, utilizing natural or depleted uranium for the
tamper around the target can provide significant background neutrons. Thus
it is necessary to avoid them. Implosion devices do not suffer in the
same manner, because the implosion timescale is a lot shorter than the
assembly time for a gun device.
Neutron
Initiation
When it
is assembled, the supercritical configuration needs neutron initiation to begin the fission chain reaction. It
is possible to achieve this with internal neutron initiators or
external neutron initiators. Reliance on background neutrons is acceptable
with regard to gun designs because of the relatively slow assembly time
(in comparison with implosion devices), and it is possible to improve the
reliability of this method if the projectile can be made to rest in
the target without recoil – this increases the supercritical window for
neutron initiation.
Small
Size
To create
a low-yield minimum mass/volume weapon it should have an efficient
fissile material (for example plutonium). It should also need a low mass
implosion system and a thin beryllium reflector surrounding the fissile
material. Because volume increases together with the cube of the radius, a
thick layer of explosive or reflector around the core adds a lot more mass
than the mass of the core itself.
If a
beryllium reflector with thickness of a few centimetres is to be used then the
radius of a plutonium core is reduced by approximately half of the
thickness of the reflector. Because of the large difference in density
between these materials, it is possible to achieve large savings in mass.
However, at some point, when one increases the thickness of the reflector it
begins to gather more mass than it saves; this point represents the point
of smallest total mass for the system.
Minimum
mass and minimum volume designs generally are similar to one another. Using
a hollow core of fissile material would add only a little to the overall
volume.
Minimum
Fissile Content
Another
way to minimize size is by means of a small explosion in the most efficient way
as possible. It is possible to do this by means of applying the same principles
that one can use for a high efficiency design, but simply by means of
reducing the amount of fissile material to bring down the yield. The
larger mass of the implosion system and the tamper and reflector in this
case will lead to larger overall mass and volume, although less fissile material
is used.
It is
possible to compress 1 kg plutonium to produce a yield in the range of
approximately 1 kiloton using a more complicated flying plate design for
the implosion system. The use of this design determines the types of
fissile material which is to be used. The high compression suggests
accurate neutron initiation timing. It is possible to produce yields greater
than 1 kiloton with this system as a result of fusion boosting.
The
M388 nuclear projectile had a version of the American W54 warhead, which was a
very small fission device with a yield less than one kiloton. The W54
weighed approximately 23 kg, it has a selectable yield which was equal to
10/20 tons. This is very close to the minimum size and yield for a fission
warhead. The complete round weighed only 34 kg.
The W54
warhead likely was an almost minimum diameter for a sphere implosion
device (the Americans conducted tests of a 25 cm implosion system
however).
The
test devices for this design fired during Operation HARDTACK Phase II (tests codenamed Hamilton
and Humboldt on 15 October and 29 October 1958) weighed only 16 kg.
These devices had dimensions 28 cm by 30 cm. Humboldt had PBX-9404 as the
explosive.
Polish
Scientists – Kaliski
In the
late years of the 1970s, Polish scientists described an advanced
“bi-conical” configuration which was able to compress small amounts of
uranium or plutonium by factors of five to seven. It could also reduce the
critical mass to between 50 and 100 grams. Fissile cores of 200-400 grams
could be able to produce one or two kiloton yields. This method
is potentially an increase of an order of magnitude with regard to
efficiency in comparison with
traditional implosion methods which are used in nuclear weapons.
traditional implosion methods which are used in nuclear weapons.
Neutron
Initiation Theory
In
order for nuclear weapons to function successfully it is necessary for the
fission chain reaction to initiate at the correct time. When the system
becomes supercritical, a neutron is necessary in order to begin the
fission process. This “window” for successful neutron initiation differs
depending on the design of the weapon. Gun assembly devices stay in
a supercritical state during a relatively long time – a time which is
sufficient for a background neutron to initiate the fission chain reaction.
However
with regard to implosion devices this neutron initiation window is much
smaller, because the interval during which the bomb is near optimum
criticality is relatively short. Although theoretically it is possible to
initiate the fission chain reaction by means of a singular neutron, it is
an advantage for an initiator to at least emit several neutrons at the
optimum period, because it is possible to capture a singular neutron without
causing fission.
A
method for initiating the fission chain reaction is to use a continuous neutron
emitter: a material which has a high spontaneous fission rate, or an alpha
emitter together with beryllium. Although the neutron production method is
stochastic, they are produced with a specific average rate. As a result
there will be in uncertainty with regard to the initiation time, which in
turn leads to a high degree of variability in the performance of the device,
i.e. yield.
An
improved version of a continuous/spontaneous neutron emitter is that which
can produce a burst of neutrons at an exactly-defined time to in order to
maximise the performance of the device (yield) but at the same time to
reduce variability. These so-called “internal initiators” can be inside
the device, or external designs, which are positioned outside of the high
explosive.
Internal Neutron Initiation
Polonium-Beryllium
(Po-Be) initiators were employed in the first nuclear weapons. They
had the codename “Urchin”. The neutrons which are necessary in order to
initiate the fission chain reactions are produced as a result of a mix of
an alpha emitter, such as polonium, with beryllium.
In
order for this to be an improvement to a continuous neutron source, it is
necessary for the alpha emitter and beryllium to remain unmixed. This is
apart from the moment when neutrons are desired, when they are required to
be rapidly mixed. Fortunately, of all forms of ionising radiation alpha
particles penetrate the least, and it is relatively easy to block them.
A
suitable alpha emitter needs a compromise between high alpha particle activity
in order to guarantee the production of a sufficient quantity of neutrons,
and a sufficiently long half life in order to avoid frequent replacement
of the initiator. As a result polonium-210 is an obvious choice for alpha
emitters.
The
need for carefully timed, fast, efficient mixing is insured by means of the
design of the initiator component.
The
initiator is placed at the center of the fissile mass, and it uses the arrival
of the shock wave to drive the mixing process. This insures that the
entire mass becomes highly compressed and emits neutrons where they can be
most effective.
Tritium/Deuterium
Initiators
An
alternative to the mixing of an alpha emitter with beryllium is to apply the
high temperatures and densities which it is possible to achieve near to
the center of an implosion, in order to ignite Deuterium-Tritium (D-T)
fusion reactions. Very small quantities of deuterium and tritium are
necessary and they are found in a small high-pressure sphere at the center
of the fissile core.
The
tritium in a DT initiator is radioactive and has a half-life of approximately
12 years. This is a lot longer than polonium and than other potential
radioactive alpha emitters which it is possible to use in the Urchin
initiators. Thus it can be stored during a longer period.
External
Neutron Initiators (ENIs)
An
alternative from a source of neutrons inside the nuclear device is when an
external neutron initiator (ENI) is positioned outside of the device and
it is not necessary even to place it near to the fission assembly. Bomb
casings, cruise missiles and re-entry vehicles permit the ENI positioning
to be virtually anywhere in the weapon.
ENIs
employ a miniature particle accelerator (this is often called a neutron tube).
This accelerates deuterium and tritium together to generate high-energy
neutrons by means of a fusion reaction. The tube contains an ion source
and ion target at opposing ends in a short vacuum tube. The application of
a large current leads to the emission of ionized hydrogen from the source.
A large voltage then accelerates this in the direction of the target, where
– if there is sufficient energy – some of the deuterium and tritium ions
undergoes fusion, which generates high-energy neutrons.
Internal
neutron initiators are operated by means of the imploding device. It is
necessary that ENIs have an exactly-timed electric signal which has a
sufficiently high voltage and current to operate – this must be similar to
the conditions necessary to operate an exploding bridgewire (EBW)
detonator.
Binding
energy per nucleon determines the stability of an atomic nucleus. The
binding energy is the total energy which is necessary to split the nucleus
into constituent parts.
A
nucleus can try to increase its stability (and thus the binding energy per nucleon)
if it undergoes nuclear fission or nuclear fusion. During these processes,
the splitting (fission) or merging (fusion) of the nuclei of the atoms
releases nuclear energy.
Generally
the fusion of two nuclei with masses which are lower than iron (which,
together with nickel, has the largest binding energy per nucleon) releases
energy. However the fusion of nuclei which are heavier than iron absorbs
energy. One can see the opposite with regard to the reverse process,
nuclear fission. Generally this means that fusion occurs for
lighter elements only (e.g. hydrogen isotopes). Normally fission only
occurs for heavier elements
(e.g. uranium and plutonium).
(e.g. uranium and plutonium).
To
understand the design of nuclear weapons, it is useful to know the important
similarities and differences between fission and fusion. The two reactions
approximately generate a million times more energy than comparable
chemical reactions. This means that nuclear bombs are a million times over
more powerful than conventional bombs.000
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