Editors note by Ian Greenhalgh:
Once again, Jeff Smith 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However, those are very obsolete and outdated weapons, what we need to be concerned about today is four or five generations evolved from those old megatonne H-bombs – a totally different type of weapon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.
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.
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.
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”.
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.
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.
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.
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).
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.