Nuclear demolition

Nuclear demolition was a new concept in so-called 'controlled demolition' that first appeared towards the end of the 1960s when building code requirements made it necessary to provide satisfactory proposals that could form the basis of eventual demolish of proposed modern tall steel frame buildings.

Atomic demolition

Modern nuclear demolition - as a variety of controlled demolition - has little in common with so-called atomic demolition, which existed as far back as the 1950s. In an atomic demolition, special small or medium-caliber atomic demolition munitions (SADM or MADM) are used. The detonation of such munitions produce the classic atmospheric nuclear blast and a large part of the explosive energy is spent on creating the well-known atomic air-blast wave, thermal radiation, penetrating ionizing radiation, an Electromagnetic Pulse (EMP) and considerable radioactive contamination of the surroundings. Only a relatively small part of such an explosion is therefore useful for the intended job – ie the demolition of the target structure. The vast majority of the energy released will create unwanted devastation around the demolished object.

Judged from this point of view, atomic demolition has never been a viable option demolition works in ordinary civil engineering environments. Such demolition work is always performed using conventional explosives and steel cutter charges placed at key load-bearing points throughout the structure. The method provides a high 'explosive-efficiency factor' – meaning that a very high proportion of the available explosive energy is directed at the actual demolition, and very little on collateral damage to the surroundings.

Atomic demolition was therefore only an option in time of real emergency – for example, wartime - when time is at a premium and the necessary weeks of calculation and preparation time need for conventional controlled demolition is out of the question.

Apart from 'collateral damage' considerations, there is also a financial barrier to the routine use of atomic demolition in civil infrastructure environments. Any suitable atomic device would currently cost at least 2 million US dollars and a precisely wrought SADM or MADM probably much more. In other words simple cost renders their routine use prohibitive.

However, cost becomes less of an issue as the size of the demolition project increases. Were all other things about equal, it would clearly be preferable to spend say 10 million US dollars on a single thermo-nuclear demolition charge than say a few hundred millions of US dollars on manually disassembling an enormous half-kilometer tall steel structure that could take years.

Difference between atomic and nuclear demolitions

The main difference between the two is that an old atomic demolition munition (“mini-nuke”) produces an atmospheric nuclear explosion with all the unacceptable collateral damage described above. The modern 'nuclear' approach employs a much larger thermo-nuclear charge buried deep underground, producing a typical deep underground explosion with very different physical properties.

Properties of an atmospheric nuclear explosion

About 99% of entire initial energy release of any nuclear explosion is in a form of X-rays. The remaining 1% is divided between gamma-rays, visible spectrum rays (that cause the initial blinding white flash known as a “nuclear flash”), neutrons, alpha-, beta-, and some other elementary particles. The collective term for this energy release is “primary radiation”. So, in its initial stage, any and every nuclear blast produces nothing but primary radiation, which in turn consists of 99% X-rays.

In atmospheric conditions X-rays can not travel very far from a nuclear blast hypo-center, because they are quickly stopped and absorbed by air atoms. At maximum, X-rays can only travel tens of meters before complete absorption by surrounding air. That is why about 99% of the energy of an atomic blast is spent on heating surrounding air in only tens of meters around the hypo-center of the blast. This relatively small area becomes superheated and begins to radiate heat in the form of visible light similar to the Sun, but much more intense). This process is called "thermal radiation". This extremely overheated area represents a secondary effect of an atmospheric nuclear blast, traditionally called a “nuclear fireball”. This is a quite distinct and separate phenomenon from the initial “nuclear flash”, which is pure white in color and lasts for only a split of a second. The thermal radiation is orange-yellow in color and can last for several seconds. The two are the result of very different processes.

The first belongs to the “primary radiation” of a nuclear explosion and exists irrespective of physical environment of the detonation; the second belongs to the secondary effect of an atmospheric nuclear explosion – ie to its “nuclear fireball” which can only be created in atmospheric conditions. The same applies to the well-known air blast wave or “shock wave”. An air-blast wave is created by the same secondary effect of an atmospheric nuclear blast, the fireball. Nuclear fireballs expand in the atmosphere while energy is being passed from inner “hotter” zones to outer “colder” zones creating air movement towards periphery. At some point, the expanding front of highly compressed air detatches from the nuclear fireball border and travels on its own at supersonic speed – smashing everything in its path. This is how the air-blast wave actually occurs.

Properties of a deep underground nuclear explosion

Scale of 150kt nuclear explosion in granite

Both of main the destructive factors commonly associated with nuclear explosions – the air-blast wave and thermal radiation – can exist exist only in atmospheric conditions. If a nuclear explosion occurs deep underground, there can be no “nuclear fireball” because of a total absence of surrounding air. Consequently, there can be no air-blast wave nor thermal radiation.

So what happens during an underground nuclear explosion?

Firstly, we must distinguish between deep and shallow underground. The broad brush distinction is between a detonation where the ground level surface is not broken and thus no crater is formed and one which breaks surface and forms a crater. While the physical properties of a deep underground nuclear explosion are distinctly different from an atmospheric one, the physical properties of a shallow sub-surface nuclear blast are mixed and comprise elements of both.

When a nuclear (or a thermo-nuclear) charge is detonated deep underground, its entire energy is released in the form of '“primary radiation” – exactly as described above. This radiation is absorbed by the surrounding soil or rock. So, while in atmospheric conditions the entire energy release initially heats air in a radius of several tens of meters, in deep underground conditions it heats some surrounding earth and rock to both melting and evaporation points. Deep underground nuclear explosions produce cavities, the sizes of which directly depend on two primary factors:

1. actual yield of a nuclear explosion; and
2. density of surrounding materials.

For example, a one kiloton detonation could evaporate/melt the following quantities (in tons) of various materials:

Rock type	Specific mass of vaporized material (in tons per kiloton yield)	Specific mass of the melted material (in tons per kiloton yield)
Dry granite	69	300 (±100)
Moist tuff (18-20% of water)	72	500 (± 150)
Dry tuff	73	200 - 300
Alluvium	107	650 (±50)
Rock salt	150	800

A 150 kiloton thermo-nuclear charge detonated sufficiently deep in granite rock will create a cavity of roughly 100 meters in diameter

“Damaged” and “crashed” zones

There are two distinctly different phases in the creation of a cavity during a deep underground nuclear explosion. It is important to understand them to grasp the mechanism of a modern nuclear demolition:

Phase 1 - A cavity is created and it reaches its final “primary size”. This happens when the entire energy of the detonation has been absorbed by surrounding rock and the maximum possible quantity of rock has evaporated. So, a cavity of a “primary size” results exclusively from the vapourisation of earth and rock rock.

Phase 2 - The enormous gaseous pressure of the "primary size cavity" expands it further in all directions and it expands from its final “primary size” to its final “secondary size”. It shall be understood that while “primary size” of the cavity results exclusively from disappearance of evaporated rock, it is not so when it comes to its “secondary size”. This phase 2 expansion occurs at the expense of neighboring areas of rock which are still solid resulting further tight compression

The diagram illustrates the processes involved.

Physical processes of a deep underground nuclear explosion

The expansion of cavity from its “primary size” to its “secondary size” produces at least two zones of extreme disintegration of the surrounding physical structure. The one immediately adjacent to the cavity is called the “crashed zone”; the next is called the “damaged zone”. Everything within the “crashed zone” is completely “pulverized” – i.e. reduced to a unique sort of 'petrified' state that is found nowhere else in nature. This physical material state is absolutely unique with no real parallels in any known process other than a deep underground nuclear detonation.

In practical terms a physical structure within a nuclear "Crashed Zone" appears to remain “intact”, retaining its former shape and color. But in reality its internal structure has been transformed to a sort of petrified and extremely fragile state such that the slightest further mechanical jarring can cause it to disintegrate to a microscopic dust. This metamorphosis occurs to any solid material within such a nuclear “crashed zone” – steel, stone, wood, glass, organic materials of any kind, - all will be similarly petrified and transformed. They will retain their shape and color for a while, but reduce to microscopic dust under the slightest mechanical pressure. Typically, the size of such particles is in the order of 100 microns - corresponding approximately to the thickness of an average human hair.

The zone beyond the “crashed zone” is called the “damaged zone”. Material in the "Damaged zone" is also pulverized, but to a lesser and decreasing extent with distance from the hypocentre. Debris will range in size from millimeters to centimeters and much larger pieces and fragments in its outer areas.

Distributions of damaged and crashed zones and principles of a modern nuclear demolition

In the above diagram all three zones – an actual cavity in the middle, a “crashed zone” and a “damaged zone” are shown as being ideally round and concentric. However, this would apply only when a detonation occurs “ideally” deep in a uniformly dense medium – for a 150 kiloton yield detonated in granite rock the depth needs to be at least a half-kilometer. In such a case, resistance of surrounding rock would be equal from every direction and the cross-section form of the cavity and zones would be circular. For a shallower detonation (but not sufficiently shallow to qualify as "Shallow sub-surface") the picture will be slightly different. The material above would offer less resistance than that from the sides and below and the primary cavity will expand most towards the area of least resistance. The major part of the expansion will thus be directed upwards and cavity and zones will not be ideally spherical. They will rather be elliptical in cross section – comparable in shape to an egg with its sharper end facing upwards.

The upper areas of such “crashed” and “damaged” zones play the major part in an actual nuclear demolition.

Practical example of a nuclear demolition scheme

Position of a nuclear demolition charge in the above picture is exactly in the middle of a cavity.

Let’s consider a practical example scheme for the nuclear demolition of a skyscraper built on granite rock that has its lowest underground foundations at -27 meters (i.e. 27 meters below the Earth surface). This scheme will look like this:

There is an upper limit on the yield-size of thermo-nuclear munitions that can be used for civil demolition purposes. It is set by the “Peaceful Nuclear Explosions Treaty of 1976” at 150 Kt . We know that a 150 kiloton deep underground nuclear explosion creates a cavity of final “secondary” size roughly 100 meters in diameter (50 meters radius) if it is detonated in granite. We can not position our nuclear charge such that the upper end of the cavity would reach the surface, because we do not want to cause severe radioactive contamination to surrounding areas. All we want is for it to demolish the structure above.

To do so we have to calculate the depth of the charge precisely, such that the upper end of the cavity should just reach the deepest underground foundations of the structure to be demolished - and no higher. With the lowest level of the structure foundations at 27 meters deep in this case, and an expected cavity radius of 50 meters, the charge must be positioned at a depth of 77 Meters (50+27) below the surface (see picture). The result will then be, not only the petrification and pulverisation to dust of the bulk of the building above, but the subsidence of any un-pulverised debris back into the extremely hot underground cavity such that it will be at least partially filled and the vast majority of the building will simply disappear. This melted "backfill" will also go some way to providing a sort of "Sarcophagus" seal that will help prevent the spread of any radio-active contamination.

That is the theory.

Nuclear demolition dynamics

The dynamics of a nuclear demolition

Now let’s consider the demolition dynamics of the above scheme.

Because the demolition charge is not positioned “ideally deep”, the resulting cavity, “crashed” zone and a “damaged” zone will not be “ideally round” either. They will all be extended upwards, approximating to an egg-shape such that their horizontal radii will be significantly decreased compare to their vertical radii. The process is shown graphically in this illustration.

As the cavity expands from its “primary” size to its “secondary” size, it produces “crashed” (inner) and “damaged” (outer) zones that also expand proportionately. Before the lowest extremity of the structure foundations are reached by the upper end of the egg-shaping cavity, they are reached successively by the borders of the expanding “damaged zone”, and the expanding “crashed zone”. Each will propagate upwards through the building structure in a manner governed by the density and construction materials of the structure. the distance of the propagation will be greater than through solid granite. Typically, the “damaged zone” (outer shock-wave boundary) might extend up to 350 meters or so, while the “crashed zone” (inner shock-wave boundary) might extend to about 300 meters – thus completely pulverizing at least 300 meters of the structure to fine powder.

This is the goal of a modern nuclear demolition scheme.

Nuclear demolition of relatively low-rise tower buildings

Nuclear demolition of a relatively low rise tower building

As illustrated above, nuclear demolition of a building less than 300 meters tall should be relatively straightforward, because its entire height will fit into the “crashed zone” zone of the detonation. Regardless of building construction type and strength, it will be reduced to dust in total. Immediately following the detonation it will become petrified, retaining its normal appearance but with a constituency not unlike a dried “sandy castle” and ready to simply crumble under its own weight.

The drawing illustrates how such a building fits entirety within the “crashed zone” (shown in blue). For perhaps a few seconds it will look “normal”, as if nothing has happened. But collapse will soon begin, normally from the bottom, where gravitational pressure of the remaining structure is most intense, and proceed with it descending neatly into its footprint. The detonation cavity which reaches the lowest underground foundations of the building is intended to contribute to this process. It will consume most of the building, melting any remaining un-petrified metal in the process. All that will eventually remain is a relatively small pile of debris represented by the lowest parts of the building perimeter that might be on the periphery of the pulverization process due to aspects of its construction. This is especially applicable to a building where the lowest parts of the above-ground perimeter overlap its underground foundation footprint, such that they are not fully enveloped by the propagation of the "crashed" and "damaged" zones.

Nuclear demolition of tall buildings

Caption here

Demolition of very tall buildings is more problematical. The “crashed zone” of a 150Kt nuclear detonation will only extend to a height of about 300 Meters. In a taller building, this will result in a relatively undamaged and heavy top section, with a narrow “damaged zone” beneath it. These zones will produce debris – ranging in size from the millimeter/centimeter scale to relatively big sections all perched on top of a “crashed zone” extending down to ground level. So, the net result of such a nuclear demolition project should be an undamaged building’s top that will lose its support base and crash downwards through the pulverized structure, scattering some debris and masses of fine dust until it reaches the ground.

Practically it might look like is shown on this illustration

It is clearly theoretically possible to completely pulverize structures taller than 300 meters but it would require a nuclear charge greater than 150Kt in TNT yield which, as mentioned above is not allowed by the terms of the “ Peaceful Nuclear Explosions Treaty of 1976”.

In any event, the standard 150Kt thermo-nuclear scheme described above is likely to be justified even on buildings taller than 300 meters, because to remove the resulting debris will still be more expedient than manual dis-assembly and disposal such huge structures.

See Also

• Traces of Tritium at the WTC Complex - American Chemical Society 1 October 2002.
• The Production and Dissolution of Nuclear Explosive Melt Glasses at Underground Test Sites in the Pacific Region - IAEA 6 November 1998
• The Containment of Soviet Underground Nuclear Explosions - U.S. DEPARTMENT OF THE INTERIOR, GEOLOGICAL SURVEY, OPEN FILE REPORT 01-312