Tag Archives: nuclear

The Nuclear Double Flash

Identification of a nuclear explosion uses a number of different methods. The Comprehensive Test Ban Treaty Organisation (CTBTO) runs a series of networks which listen for infrasound sound waves produced in the atmosphere by above-ground explosions; which monitor the oceans for underwater tests; and which monitor seismic activity to detect underground tests. The CTBTO also run a network of radionuclide sensors that sample the air to detect certain isotopes produced by nuclear explosions.

But if a nuclear weapon is ever used again as a weapon of war, the first notification will come from space-based networks (e.g. the US DSP or the Russian Oko) looking for the characteristic double flash of a nuclear detonation.

Watch the video above of the first two seconds of the Castle Bravo nuclear test. Do you notice anything unusual? Let’s take a look at a few individual frames.


Frame 01


Frame 11


Frame 49

The explosion begins bright, but then dims before becoming bright again: this is the nuclear double flash. It’s a little easier to see in the slowed-down excerpt below.

The variation in the brightness of the light emitted by a nuclear explosion follows a distinct pattern. It is possible to build light sources that are as powerful as nuclear explosions, or to produce light sources that have the same double flash characteristics, but not to produce a source with both characteristics. Thus the nuclear double flash is taken as irrefutable evidence that a nuclear explosion has taken place.


Note the logarithmic scale on both axes.*

As the nuclear explosion begins, the bomb and all of its components are heated to extremely high temperatures of around ten million kelvin. This causes these components to emit low-energy (“soft”) x-rays and high-energy (“hard”) ultraviolet waves. These x-rays and UV waves are absorbed by the air within a few metres of the device and this causes the air to be heated to temperatures of around one million kelvin, causing it to become incandescent and emit light. This is responsible for the initial, very fast (about three hundred millionths of a second after detonation) bright peak.

At the same time, the explosive shock wave itself (the hydrodynamic front) is expanding outwards and quickly compresses the air in front of it like a piston, causing it to become superheated. Inside this shock wave, the temperature is so high that the gas inside it become completely ionised (i.e. the gas becomes a plasma) and this makes the shock wave opaque to light. The brightness minimum is therefore caused by the shock wave “trapping” light behind it as it forms.

Light is still emitted because the shock wave itself is incandescent and is therefore emitting light outwards, ahead of itself, but this light is about one-tenth of the brightness of the preceding and following maxima. As the shock wave expands, it cools rapidly, and as it cools it becomes more transparent, allowing the light previously trapped behind it to escape. This is responsible for the second bright peak, which lasts much longer than the first because the full energy of the weapon is now being fully released, with nothing to block it. As the fireball expands it dissipates, and this is responsible for the gradual decrease in brightness.


As the graph above shows, the time of the first minimum and the time of the second maximum depend on the weapon’s yield. A larger yield means a more powerful initial release of energy, and a more powerful shock wave, and this shock wave then takes more time to “pass through” the initial hot region created by x-ray/UV absorption, and then also takes longer to cool down to the point at which is becomes transparent to the light that it has trapped behind it.

For a one kiloton device, the time between the minimum and the second maximum is only 30 milliseconds, too short a gap for the human eye to perceive, but bhangmeters aboard satellites can spot it (and by measuring the time interval get a rough idea of the weapon’s yield). For larger weapons, such as the 100 kT warheads aboard the UK’s Trident II D-5 missiles, the interval is long enough (0.3 seconds at 100 kT) for human beings to perceive.

* Taken from Guy E. Barasch, “Light Flash Produced by an Atmospheric Nuclear Explosion”, LASL-79-84, Los Alamos National Laboratory, 1979.

What Does “Fail-Safe” Mean?

The term “fail-safe” is often-used to refer to an object or a device, but it more properly refers to a condition. In this post I hope to explain what “fail-safe” actually actually means, with reference to how nuclear power station stay safe.

To “fail safe” means that in the event of a failure, the failure causes the device to fail in such a way that the device is rendered safe. In terms of deaths per gigawatt year nuclear power comes second only to hydroelectric power in terms of safety (Source: ExternE Externalities of Energy Project, European Commission). This is due to the incredible emphasis that is put on safety in nuclear power stations, and is a testament to nuclear power stations’ defence in depth concept.

Control Rods

One of the key parts of a nuclear reactor is the control rod assembly. When fission occurs in a fuel rod, neutrons are released and these neutrons go on to cause further fissions. The purpose of control rods is to “soak up” excess neutrons and prevent further fissions. Control rods are made of materials such as boron, cadmium and hafnium that have a large capture cross section, meaning that they have a high probability of capturing and absorbing neutrons.


The control rod assembly for the CROCUS research reactor.

If the control rods are raised out of the reactor the excess neutrons are not absorbed and further fission occurs and the reactor releases more thermal energy. If the control rods are lowered into the reactor the neutrons are absorbed, fission does not occur and the amount of thermal energy released is decreased.

Control rods fail safe by being held up by electromagnets. In the event of a power failure the electromagnets are no longer powered and thus the control rods will fall into the reactor, shutting it down. Whilst we cannot be sure that the power supply to the reactor will not fail, we can be sure that gravity won’t fail. If the control rods weren’t held up by electromagnets then we’d run the risk of a fail dangerous situation, with the control rods raised up out of the reactor and no way for them to be reinserted to shut down the reactor.

Moderator and Coolant

The neutrons released in each fission process are travelling too fast to cause further fissions. (Imagine trying to putt a golf ball – hit it too hard and it will just skip over the hole.) The job of the moderator is to slow these neutrons down so that they are travelling at the correct speed to continue the chain reaction process.

The moderators used in nuclear reactors vary between different designs, but graphite and light- and heavy-water are common.

The job of the coolant in the reactor is to take thermal energy away from the nuclear fuel and transfer it (via a heat exchanger) to a steam generator that then drives a turbine and generates electricity. If coolant leaks from a reactor whilst the nuclear fission process continues this leads to thermal energy not being removed from the fuel, and the fuel heating up to the point at which is gets so hot that it melts – a meltdown.

In some reactors (e.g. PWRs, BWRs, SCWRs) the coolant is the moderator, and the reactor will fail safe in the event of a coolant leak because a coolant leak is a moderator leak and the reactor cannot continue the fission process without a moderator. Other reactor designs, that do not use a combined moderator-coolant, have different safety features in place to cope with a coolant leak.

Bent Spears, Broken Arrows and Empty Quivers

In my research for a previous post I came across the US’s official list of nuclear weapons-related codewords, and they are some of my favourite codewords ever.

PINNACLE is a codeword (technically a flagword) that indicates that a message is of interest to the major command units of the military. It’s mentioned here because whilst it can be used on its own, it is often used, or must be used, in combination with the codewords listed below.

BENT SPEAR is used to report incidents involving nuclear weapons that are “of significant interest” but which are not categorised as NUCFLASH or BROKEN ARROW. The incident in which six AGM-129 cruise missiles with live 150 kiloton W80-1 nuclear warheads (which were supposed to have been removed) were loaded onto a B-52 Stratofortress bomber and left unguarded at Minot and Barksdale Air Force Bases was classified as a BENT SPEAR.

NUCFLASH is used to report incidents that could create a risk of nuclear war. This includes any incident involving the actual or possible detonation of a nuclear weapon, or any incident in which a nuclear-armed or nuclear-capable aircraft deviates from its approved flightplan. It also covers incidents with the possibility of, or the appearance of, a nuclear detonation or attack, such as a ballistic missile launch, the presence of cruise missiles on non-friendly aircraft that are not on an approved flight path, or objects from space reentering Earth’s atmosphere. A PINNACLE NUCLFASH report has the highest priority of any report in the US military.

BROKEN ARROW is used to report incidents involving US nuclear weapons that do not create the risk of nuclear war. This includes the nuclear or non-nuclear detonation of a US nuclear weapon, the burning or jettisoning of a nuclear weapon or radioactive contamination or other hazard from a US nuclear weapon. The incident in which a nuclear-armed Titan-II missile caught fire and exploded in its silo was classified as a BROKEN ARROW, as were a number of incidents in which B-52 bombers carrying nuclear weapons crashed. (The incident in the 1996 movie Broken Arrow would actually have been classified as EMPTY QUIVER.)

EMPTY QUIVER is used to report the seizure, theft or loss of a nuclear weapon. The incidents in which the USS Scorpion submarine sank with two eleven kiloton Mark 45 nuclear torpedoes aboard, or the incident in which an A-4E Skyhawk aircraft carrying a one megaton B43 bomb fell over the side of the aircraft carrier USS Ticonderoga would probably be classified as EMPTY QUIVER events.

DULL SWORD is used to report minor incidents involving nuclear weapons or systems which could impair their ability to be deployed. This includes damage to systems capable of carrying or deploying nuclear weapons but which are not carrying nuclear weapons at the time. FADED GIANT is used to report incidents involving military nuclear reactors, or any other military radiological incident that does not involve nuclear weapons.

Two less cool-sounding codewords are EMERGENCY EVACUATION and EMERGENCY DISABLEMENT. The EMERGENCY EVACUATION codeword is used when nuclear weapons have to be removed from their approved location at short notice, without advance planning, e.g. if an Air Force base or silo holding nuclear weapons was being overrun by enemy forces. EMERGENCY DISABLEMENT refers to the use of the weapon’s command disable system, in which a warhead is deliberately made inoperational, preventing its use by enemy forces. The method by which this is achieved is unknown, but it is thought to operate by destroying either the warhead’s power supply, the sensitive electronic components within the warhead, or another part of the warhead’s triggering system.

Unconventional Nuclear Weapons

When people think of nuclear weapons they tend to think of bombs and missiles, but there have been (and possibly still are) some more unusual nuclear weapons.

Conventional Nuclear Weapons

Most nuclear weapons states (NWSs) use ballistic missiles as the primary tool in their nuclear arsenals. These can be battlefield (with a range less than 100 km), tactical (range up to 300 km), theatre (up to 3500 km), intermediate (up to 5500 km) or intercontinental (beyond 5500 km). These ballistic missiles can be land-based, either in underground silos or aboard mobile launchers, or submarine-launched; NWSs prefer SLBMs because they give a secure second strike capability (the UK’s only nuclear weapons are SLBMs).


A Trident II SLBM (as used by the USA and the UK) ignites its first stage rocket motor.


A Russian SS-25 Sickle ICBM aboard a transporter erector launcher vehicle.

Ballistic missile forces are frequently supplemented by cruise missiles, usually air-launched by fighter or bomber aircraft. Cruise missiles differ from ballistic missiles in that they do not follow ballistic paths but rather fly point-to-point, close to the ground (in a process known as terrain hugging). Ballistic missiles use rocket engines and can operate outside of Earth’s atmosphere, whereas cruise missiles are powered by air-breathing fanjet or ramjet engines (and are therefore unable to operate outside of the atmosphere), and generate lift and steer just as aircraft do.


A French Mirage 2000N fighter carrying an ASMP-A nuclear-capable cruise missile on its centre hardpoint.

Only the United States and China still use air-dropped nuclear bombs as part of their nuclear deterrent. The nuclear weapons programmes of Israel, India, Pakistan and North Korea are shrouded in secrecy, so it is possible – but unlikely – that they possess bombs as part of their arsenals.


A B83 bomb, the most powerful weapon in the US arsenal at 1.2 megatonnes TNT equivalent.

Unconventional Nuclear Weapons

Nuclear Land Mines (AKA Atomic Demolition Munitions)

The conventional nuclear weapons listed above are all designed to be delivered to their target remotely, whereas atomic demolition munitions (ADMs) are designed to be transported and emplaced by soldiers in the field. The US and the USSR/Russia developed a number of ADMs but no current weapons are known, though it is alleged that Israel have placed ADMs in the Golan Heights to secure the area.

sadmSource: flickr/rocbolt

The most recent ADM, the US’s Special Atomic Demolition Munition (shown above) was decommissioned in 1989. It used the tiny W54 warhead, with a yield of 0.01-10 kilotons TNT equivalent, and had a mass of around seventy kilograms. It was designed to be carried in a backpack and emplaced by a team of paratrooper Special Forces.

Perhaps the strangest nuclear weapon of all falls into the ADM category. Blue Peacock was a 1950s British project to place ten kiloton nuclear landmines on the North German Plain to guard against a Soviet invasion from the east. The bomb’s designers were concerned that the cold weather would prevent the bomb’s electronics from operating correctly, and one suggestion was that live chickens be placed inside the bomb with a source of food and water and that the heat from the chicken’s bodies would be sufficient to keep the bomb operating correctly. The project was cancelled in 1958 before any bombs were placed.

So-called “suitcase nukes” also fall into the ADM category. A number of people have alleged that the US, USSR and Israel have produced suitcase nukes, but most nuclear scientists and engineers do not believe it to be possible to shrink a warhead – both in terms of size and mass – small enough to fit in a suitcase and be easily man-portable.

Nuclear Artillery

The W54 warhead used in the SADM was originally developed for the Davy Crockett recoilless rifle system. The Davy Crockett was fielded by US units between 1961-1971 and was designed to fire an M388 nuclear projectile containing a 0.01-0.02 kiloton W54 warhead up to four kilometres, and was envisaged as primarily an anti-personnel and area denial weapon due to the fallout it would produce.


An M388 nuclear projectile attached to the Davy Crockett system.

The US developed a number of other shell-firing nuclear artillery pieces: the M65 atomic cannon firing fifteen kiloton W9 and twenty kiloton W19 warheads; the M110 and M115 howitzers firing W33 warheads with a selectable yield up to forty kilotons; and the M109, M114 and M198 howitzers firing 0.072 kiloton W48 warheads. The USSR also produced a number of shell-firing nuclear artillery pieces, and the US, USSR, France and others operated nuclear rocket artillery pieces.

Nuclear Depth Charges

The UK, USA and USSR have all at one time fielded nuclear depth charges for use in anti-submarine warfare. The Mark 101 Lulu was an eleven kiloton nuclear depth charge fielded from 1958-1971, and the twenty kiloton B57 nuclear bomb (1968-1993) could also be used in this role.


A Mark 101 Lulu nuclear depth charge.

Air-to-Air and Surface-to-Air Rockets

Most nuclear rockets and missiles are air-to-surface or surface-to-surface, but some air-to-air and surface-to-air nuclear missiles have been created. Air-to-air nuclear missiles, such as the one-and-a-half kiloton AIR-2 Genie (1957-1985) and the 0.25 kiloton AIM-26 Falcon (1961-1972) were designed to guarantee a hit against incoming aircraft, and to destroy multiple aircraft with one device.


An F-106 Delta Dart fires an AIR-2 missile.

Surface-to-air nuclear missiles have also been used in anti-aircraft roles, but were, and are more commonly used in an anti-ballistic missile (ABM) role. The current Russian ABM system (A-135) uses ABM-3 Gazelle missiles with 10 kiloton nuclear warheads, and previously used much more powerful ABM-1 Galosh missiles with 2-3 megaton warheads. These ABM nuclear missiles are designed to destroy incoming ballistic missiles by damaging their electronic components via intense X-ray bombardment and neutron flux.

vnukovo-stitchSource: wikimapia/ogima

The image above shows the A-135 ABM complex at Vnukovo (click to enlarge). Blue control buildings are on the right, and the sliding silo covers are visible to the left. There are a total of twelve silos at this site, with a further fifty-six silos spread between four other sites surrounding Moscow.


Chrysopoeia is the artificial production of gold, long the goal of alchemists (particularly the transmutation of base metals, such as lead). Turning one element into another via chemical reactions is impossible, but it is possible using nuclear reactions.


Crystalline gold

Gold has only one stable isotope, gold-197, so any nuclear reaction aiming to produce gold must finish with producing gold-197. If your reaction produces another gold isotope, such as gold-196 or gold-198 this will decay over the course of a few days to form another element (platinum-196 or mercury-196 in the case of gold-196, and mercury-198 in the case of gold-198).

Producing gold from lead is impossible, but it is possible to turn mercury into gold. If mercury-196 (0.15% of natural mercury) is irradiated with slow neutrons it forms mercury-197 which then decays via electron capture to form stable gold-197. If mercury-198 (9.97% of natural mercury) is used instead, it can be irradiated with fast neutrons, causing it to lose a neutron and form mercury-197 and then gold-197 as above.

In 1980 Glenn Seaborg (after whom seaborgium is named) produced a tiny amount of gold (thousands of atoms) by bombarding bismuth-209 with carbon-12 and neon-20 atoms, but only formed radioactive isotopes of gold in the process.

Regardless of the method used, the production of gold in nuclear reactions is prohibitively expensive, costing many times the price of gold per unit mass produced.