Monday, January 26, 2015

Chemical Explosions Versus Nuclear Explosions: Where The Energy Comes From

The previous article focused on chemical explosives such as nitroglycerin, TNT, picric acid and cyclonites. This article focuses on what distinguishes a chemical bomb from a nuclear bomb by taking a close look at what is going on at the atomic level.

Nuclear Devices

A nuclear explosion is a very high-speed nuclear reaction. These explosions release energy from bonds that hold nuclei together inside atoms. The nuclear process in a nuclear device or bomb may be one of fission (splitting atomic nuclei) or a multistage combination of fusion (combining atomic nuclei) and fission, where fission initiates a fusion reaction (there are no pure fusion bombs in existence). We will explore in detail how fission and fusion weapons work in the next article. A chemical explosion, in contrast, is a chemical reaction in which the nucleus stays intact while electron bonds between atoms in the explosive compound are broken and new ones are created.

Explosive Yields of Nuclear Devices Compared to Chemical Explosives

A nuclear weapon yields far more explosive energy than any chemical weapon. Still, it is conventional to describe the amount of energy released by a nuclear detonation as TNT equivalent. For example, the Fat Man nuclear bomb that dropped on Nagasaki released the equivalent of about 22 kilotonnes (kt) of TNT. Fat Man, an implosion-style plutonium bomb, was not very powerful compared to many other nuclear devices, It was a thousand times less powerful than later large nuclear fission bombs that were developed, and about ten thousand times less powerful than the first hydrogen bombs.

Yet, as we touched on in the previous article, this nuclear explosion was ten times more powerful than the Halifax chemical explosion, one of the largest explosions in history. Hydrogen bombs (H-bombs or thermonuclear bombs) are the most powerful bombs of all. The first H-bomb tested was Ivy Mike. It was detonated in 1952 by the U.S in an atoll in the Pacific Ocean. This bomb was equivalent to 10.4 Mt (mega or million tonnes). If we compare this to the 22 kt yield of Fat Man, it is about 500 times more powerful. Ivy Mike's detonation is shown below right.

The Official CTBTO Photostream;Wikipedia
The Halifax explosion was devastating but it was not the largest chemical explosion on record. That distinction goes to the accidental N1 rocket launch explosion in 1969. This heavy-lift Soviet rocket exploded on the launch pad after a loose bolt was ingested into a fuel pump. Designed for very large payloads, it contained 680 tonnes of kerosene and 1780 tonnes of liquid oxygen. It is difficult to estimate how much of this explosion was detonation and how much of it was deflagration, as the two fuels were unmixed when it went off, but its energy was roughly equivalent to 7 kt of TNT (the Halifax explosion was equivalent to 2.9 kt). The photo below left gives you an idea of what this huge type of rocket looked like. The U.S. Saturn V, which it was built to compete with, is on the left for comparison.


The yields of various nuclear devices range enormously, from 0.01 kt (10 tonnes TNT equivalent) for low-yield tactical nuclear weapons to an utterly devastating 50 Mt. This was the yield of Tsar Bomba, the most powerful H-bomb (and therefore the most powerful artificial explosion) ever detonated. Russian-made, it was detonated in the Russian archipelago, Novaya Zemlya. A participant in the experiment felt the heat from it 270 km away. The shockwave broke windowpanes as far away as 900 km. The 8-minute video below, from the History Channel, discusses Tsar Bomba and provides a chilling account of what happens when a hydrogen bomb like this one detonates.

To get an idea of the variability in nuclear weapon yields, the graph below compares the yields of various nuclear weapons developed by the U.S. (Russian Tsar Bomba would be off this scale). It plots yield in kilotonnes (kt) against weapon weight in kilograms (kg). Note that the kt scale is a logrhythmic scale. (Little Boy, near fat Man below, is the nuclear bomb dropped on Hiroshima three days before Fat Man dropped on Nagasaki. It was a gun-type uranium fission bomb.)

A Brief Look at Nuclear Weapons and Warfare

We often associate nuclear explosions with terrifying mushroom clouds but actually both chemical and nuclear reactions can produce these kinds of clouds. Even naturally occurring volcanic eruptions can produce them. These clouds can form at any altitude from the sudden release and expansion of gases that are less dense than the air around them. The cloud is buoyant so it rises rapidly. Instability inside the cloud creates turbulent vortices that curl downward around the edges forming a vortex ring that draws up a central column or stem.

Wikipedia Commons
The central stem contains smoke, debris, which can include tonnes of dirt, and condensed water vapour. When the cloud reaches equal pressure with thinner air at higher altitude, it begins to disperse and debris drawn up begins to drift back down as fallout. If it is a nuclear bomb, this fallout will contain radioactive dust and debris: atoms in the dirt and debris are bombarded with fast moving neutrons emitted during the reaction. These atoms in turn, absorbing extra neutrons, also become unstable and radioactive (I will explain this process further on in this article).

Below, a horrific mushroom cloud forms after Fat Man detonated on Nagasaki, Japan in 1945.

Unique to nuclear explosions, and part of what makes them so deadly, is that in addition to the impact, the intense heat and the shock wave of the explosion, the reaction also scatters highly radioactive reaction products and fallout into the air, water and over the land.

We are lucky that only two nuclear bombs have been used in the history of warfare so far but for the people of Japan, the emotional scars from the bombings of Hiroshima and Nagasaki may never be healed. Sadly, at least 129,000 people died in the bombings but the real death toll will never be known, as many more deaths resulted from radiation exposure and there are reports of many cancers and birth defects that may be linked to the effects of radiation as well.

In both cities most of those killed were civilians, leading to continuing international debate about whether or not these bombings were necessary to end the war. Many people consider the acts immoral, a war crime and even a form of terrorism. Contributors to Wikipedia offer an excellent debate over these concerns.

Why Nuclear Explosions Yield Much More Energy than Chemical Explosions

Why do nuclear bombs yield so much more energy than even the most powerful chemical bombs? It has to with the energy in the bonds that are broken during the reaction. Chemical explosions are chemical reactions in which only the atom's electrons are involved. Nuclear explosions involve bonds between protons and neutrons inside the nuclei of the atoms. To understand these differences, let's take a close look at the atom.

An atom consists of elementary particles called protons and neutrons, which are tightly bound within a nucleus. Outside the nucleus, the atom's electrons are found within orbital clouds.

The most realistic depiction of an atom is perhaps the orbital cloud model. A helium atom model is shown below. It consists of two protons, two neutrons and two electrons. The protons and neutrons, confined in the relatively very tiny nucleus, are shown as red and purple dots. The shaded grey disc shows us where electrons might be.

An atom is indeed tiny. 1 angstrom (Å) is 10-10 metres, or 100,000 fentometres
(10-15 m). The most powerful electron microscope today can barely "see down" to about 1 angstrom, the size of an atom.

In reality the electron cloud, left, would be spherical. The best mathematical model of the atom is the quantum mechanical model, which the left diagram is based on. It is defined by probabilities and uncertainty. The darkness of the cloud above represents the probability of finding an electron in a particular location within it. It's more likely to be close to the nucleus (darkest) than far away from it (lightest). The nucleus is very tiny compared to the atom's size.

An atomic model that is easier to conceptualize is the Bohr model. Bohr models of four small fairly simple atoms are shown below. This is the model that best helps us to visualize how chemical bonds between the electrons work.

Two kinds of bonds hold an atom together. Nuclear bonds hold protons and neutrons together in the nucleus. Electron bonds keep electrons bound to the atom. Each of these types of bonds is carried out by its own type of fundamental force, of which there are four. The strong force, aptly named, holds the nucleus together and the electromagnetic force holds the electrons in the atom. This is the force of electricity, magnetism and electromagnetic radiation including light. Inside the atom, the electromagnetic force is more specifically experienced as an electrostatic force: This is the force that makes opposite charges attract each other (think of static cling). The positive charges of the protons in the nucleus attract the negatively charged electrons to the atom.

The two kinds of energy associated with these forces are nuclear binding energy and electron binding energy, respectively. The difference in the energies associated with these two forces is the key to what makes nuclear explosions so much more powerful than chemical explosions. The strong force is 137 times stronger than the electromagnetic force. Let's look more closely at how these binding energies work inside atoms.

Chemical Explosive Energy Is About Chemical Bond Energy

We can describe two basic levels of chemical bonding. Atoms bind together to create molecules and molecules bind together to create larger molecules. An example of an atomic bond between two hydrogen atoms (H) to create hydrogen gas (H2) is shown below right. Each atom has one electron and these two lone electrons can share a single electron orbital. The energy payoff for doing so is that they create a more stable lower-energy system.

Jacek FH;Wikipedia

Molecules can also bind with one another. Below, two acetone molecules bond together (dotted line).
The hydrogen bond in hydrogen gas is an example of a covalent bond. This is the strongest class of chemical bond. The atoms are held together tightly and a fair amount of force is required to break them. The acetone-acetone interaction directly left depicts a dipole-dipole molecular interaction between two acetone molecules. Each molecule is an electric dipole. It has a more negatively charged region and a more positively charged region. Two oppositely charged regions of two molecules are attracted to each other. This is the weakest type of bond. You might know that acetone is a liquid at room temperature (some nail polish removers contain it). This bond is the reason why acetone is a liquid and not a gas. The bond is very weak however, so acetone readily evaporates. A similar dipole-dipole bond acts between water molecules. Water too is a liquid at room temperature and it evaporates, but not as fast as acetone does because its dipole-dipole bonds are stronger.

Chemical bombs are about chemical reactions, in which atomic and/or molecular bonds are broken and created. These bonds have potential energy, called bond energy. Like electrons attracted to a nucleus, chemical bond energy is derived from electrostatic interactions, and therefore the reactions involved in a chemical explosion fall into the electromagnetic energy scale.

The Unique Chemistry of Explosive Compounds

Although chemical bonds all fall under the electrostatic interaction umbrella, they vary widely in strength, from weak dipole-dipole interactions to ionic bonds (such as NaCl or table salt) to very strong covalent bonds. The nitrogen ≡ nitrogen triple (covalent) bond is the strongest chemical bond of all. This means that nitrogen gas (N2) (two nitrogen atoms triple bonded to each other) is, for the most part, chemically inert. It is non-reactive at room temperature and pressure, and it is challenging for living organisms and industry to access all the useful energy in those very strong stable bonds. However, once nitrogen atoms are liberated they will also bond very strongly with various other atoms, creating a variety of energy-rich molecules, some of which are explosive.

Explosive compounds tend to have two prerequisites: First, they contain very strong covalent bonds. For example, many explosive compounds contain either a nitro group (-NO2) or an azide group (-N3). A nitro group is shown below left. "R" simply depicts the rest of the (organic) molecule. The solid/dotted lines depict very strong nitrogen bonds in this group.

An azide group (the three N's) is depicted by the two resonant structures shown below right. "Resonant" means that, in reality, these two powerful nitrogen-nitrogen bonds exist in a hybrid state between the two structures shown.

Second, explosive compounds are unstable. Both azide groups and nitro groups are kinetically unstable, or labile. This means that, even though there are strong (stable) covalent bonds present, the three-dimensional structure of the molecule as a whole is not stabilized by those bonds. Rather than stabilizing the molecule, separate regions of strong positive and strong negative charges make it twisty and unwieldy. This means that these groups lower the activation barrier to the potentially explosive decomposition reaction (this process is covered in the previous article). That is why these compounds are sensitive to mechanical shocks or temperature change. A bump (as in the case of nitroglycerin, a nitro-group compound) can be enough to overcome the activation barrier, triggering an explosive decomposition reaction. A large amount of chemical energy from those strong covalent bonds is released very rapidly.

Nuclear Devices: Where the Energy Comes From

A key difference between a chemical reaction and a nuclear reaction is in how the reaction is conserved, or balanced. Balancing a nuclear reaction equation offers excellent clues about where the energy comes from.

First we will look at how chemical reactions are balanced. To balance a chemical reaction we need to make sure the numbers of different atoms on the reactant (left) side of the reaction equals the numbers of atoms on the product (right) side of the reaction equation. No atoms must be lost or created. The two TNT reactions from the previous article, shown below, are balanced. For example, there are 14 carbon atoms in the reactant and 14 carbons atoms in the products in both cases.

2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C + energy
2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C + energy

The number of atoms is always conserved and mass is always conserved in these reactions. Energy is also conserved in chemical reactions, although measuring it in a practical sense can be challenging for explosive reactions (we will get into this a bit more later on in this article). The energy of an explosion is chemical bond energy transforming into the mechanical energy of the shock wave, the kinetic energy of gases, other reaction products and debris, and the thermal energy, or heat, released by the explosion.

Mass-Energy Equivalence

For nuclear explosions, we need to revise the conservation rules. Mass and energy are not conserved (as separate entities!) in a nuclear explosion. Instead, mass is converted into energy. To understand how this works, we need to move a bit beyond the pre-quantum era of chemistry to take into account Einstein's equation, E = mc2. Energy equals mass x the speed of light squared. This equation tells us that there is an equivalent relationship between energy and mass. We can still talk about conservation of energy BUT we need to define energy in a broader sense. All mass, according to Einstein, has an equivalent energy associated with it. Perhaps even more surprising is the fact that most of the mass of atoms, molecules and objects comes not from the intrinsic masses of all the elementary particles inside them, but from energetic interactions between those particles. For example, the mass of a proton is many times higher than the masses of the individual quarks that make it up. The energy associated with the strong force binds quarks together into protons and neutrons, and this energy contributes much of the proton's mass. The strong force is carried out by a large and unknown number of massless particles called gluons inside the proton. A similar arrangement is thought to exist inside a neutron.

This mass-energy equivalence rule, an update from pre-quantum chemistry, also holds true for chemical reactions but the energies at play are too small to translate into any significant changes in mass during the reaction. In other words, although a chemical explosion reaction releases bond energy, bond energy does not significantly contribute to the mass of the molecules involved. Even for a tightly bound azide group, the potential energy present in the strong nitrogen bonds do not contribute significantly to its mass. I recommend that you read the second answer in the question I've linked to here. It offers great insight into the differences in mass-energy between chemical reactions, nuclear reactions and particle interactions inside colliders.

Residual Strong Force = Nuclear Binding Energy

Nuclear reactions involve energy associated with the powerful strong force, which is a very significant contributor of mass to protons and neutrons, while chemical reactions involve changes in electron orbital energies and energies associated with chemical stability. Atom-bound electrons simply have far less energy available than the energy that is tapped during a nuclear reaction. Nuclear explosions are direct evidence for the incredible amount of energy that is tied up inside the nuclei of atoms as the strong force. This mass-energy equivalence update allows us to use new conservation rules for nuclear reactions. In a chemical reaction, the numbers and types of atoms must not change, and we use this to balance our equation. In a nuclear reaction, they can and do change. With a few exceptions, we instead balance the numbers of protons and neutrons on each side of the equation, rather than the number of atoms. As we will see, in a nuclear reaction, atoms themselves change. In a nuclear reaction, thanks to mass-energy equivalence, we can also track how much energy is released by measuring the reduction in mass.

A nuclear reaction does not access all the strong force within an atom. Neutrons and protons remain intact so the strong force holding quarks together inside them is not involved. There is tremendous energy in a nuclear explosion but not even that amount of energy is enough to do that job. To blast protons or neutrons apart you need the highly focused ultra-intense energy available inside a high-energy collider.

Instead, a nuclear reaction depends on the much weaker but still very powerful residual strong force. This is a residuum of the strong force that holds the nucleons (protons and neutrons) together inside the nucleus. The energy in this residual force is what becomes accessible during a nuclear reaction.

A Closer Look at Nuclear Reaction Conservation Laws

Nuclear reactions come in many different types. Nuclear bombs rely on explosive fusion reactions and/or explosive fission reactions. Nuclear reactors rely on a much slower and highly controlled rate of fission (they are the subject of a future article in this series). Radioactive decay reactions (and there are many kinds or modes of decay) are spontaneous fission reactions. The Sun, for example, is a continuous fusion reaction, fusing hydrogen into helium. The reaction rate is controlled by the balance of two opposing forces - (inward) gravity and (outward) hydrostatic pressure. Despite their differences, all of these reactions follow three basic conservation laws: Invariant mass, charge and baryon number must balance for each side of the equation.

Conservation of Invariant Mass

Invariant mass is the rest mass of an object. For example, the invariant mass of a proton is approximately 1.67 x 10-27 kg (that's the mass of the quarks and the strong force mass contribution). Particle masses, tellingly, are more often measured in energy units, so we can also describe the proton's invariant mass as about 938 mega electron volts (MeV/c2) (almost 1 TeV or 1 tera electron volt; the c2 is usually dropped) of energy. Particle physicists find this measurement much more useful. The mass/energy of the protons and neutrons involved in the reaction is conserved. The numbers of protons and neutrons (with a few exceptions), as well as their individual masses, stay the same. With no exceptions, the total baryon number is conserved, as we will see.

Conservation of Baryon Number

Baryon number makes it possible for us to keep track of all the protons and neutrons in the atoms involved in our nuclear reaction so we can balance our equation. This is one of the most important conservation laws of nature. As we now know, every proton and neutron is made up of three fundamental particles called quarks. Each quark is assigned the baryon number 1/3. Any particle made of three quarks is called a baryon, so protons and neutrons (nucleons) are baryons or baryonic composite particles, and each nucleon has a baryon number of 1. A hydrogen atom with just one proton, therefore, has the baryon number 1 (it is called hydrogen-1). Lithium, with three protons and three neutrons in its nucleus, has the baryon number 6 (it is called lithium-6).

Reflecting the requirement of balancing baryon number, nuclear reactions are written a bit differently than chemical reactions. Below is an example of a nuclear reaction called deuterium-lithium fusion (which hasn't been balanced yet; this example reaction is borrowed from Wikipedia - find it at the link above).

When lithium-6 and deuterium (hydrogen-2) are forced together under very high pressure, they undergo a fusion reaction (deuterium  or hydrogen-2 is a stable isotope of hydrogen, containing 1 proton and 1 neutron). The trick with fusion reactions, and part of the challenge in designing a future fusion reactor, is that, while atoms can be attracted to one another and react chemically, the atomic nuclei deep inside them do not interact under ordinary conditions. In fact, two (positively charged) nuclei will repel each other very strongly (this is called the Coulomb barrier) if they are forced close together. That is, unless they get close enough to come under the influence of each other's residual strong force. This is the force that ultimately fuses the two nuclei into one. Only extremely high temperature and/or pressure, such as deep inside the Sun, will force two nuclei close enough to fuse.

A note here: deuterium-lithium fusion is simply a sample problem. Although the fuel of the hydrogen bomb (also called H-bomb or thermonuclear weapon) is solid lithium deuteride, the explosive reaction itself is a deuterium-tritium fusion reaction, not shown here. Tritium (hydrogen-3) is an unstable isotope of hydrogen containing two neutrons and one proton. The primary fission part of the H-bomb releases neutrons that split lithium-6 into tritium and helium-4. The tritium then fuses with deuterium during the secondary fusion part of the bomb to create more helium-4.

Back to our sample equation, now we can balance it.

For the lithium atom, 6 is the mass number, which is the number of protons and neutrons in the atom. Because each proton and neutron has a baryon number of 1, mass number is equivalent to the baryon number. 3, below, is the atomic number, the number of protons in the lithium atom. Li is lithium's chemical symbol. We know that baryon number must be conserved in this reaction, so what is our missing product? It must have a baryon number 4 so it could have any combination of neutrons and protons and still observe baryon conservation. Only one stable nucleus fits, with a stabilizing balance of neutrons and protons, and that is another helium-4.

This reaction is called a fusion reaction because lithium and deuterium fuse into an excited and highly unstable beryllium-8 nucleus (not included in the reaction above). This nucleus (the large central vibrating mass below) lasts only 7 x 10-17 seconds before it decays through alpha decay into two very stable helium nuclei, shown below.

Not all nuclei are stable. Nuclear decay is the process in which an unstable nucleus changes into a more stable nucleus. An unstable nucleus has either too many protons or too many neutrons. Because it is unstable, it contorts and vibrates and has too much internal energy. A stable nucleus is in a lower energy state. The unstable nucleus must lose energy in order to change (a process called transmutation) into a stable one. An unstable nucleus eventually stabilizes by ejecting an alpha particle or a neutron, or one of its protons can change into a neutron or vice versa. The emission of the high-speed helium nuclei (alpha particles) above is an example of alpha emission. The unstable beryllium-8 nucleus decays through alpha decay into two alpha particles. During the process, other particles such as electrons may be emitted, as well as electromagnetic radiation (not shown in the above reaction).

What Radiation Is and Where It Comes From

This nuclear reaction happens to conserve atomic number as well as baryon number, but not all do. Two kinds of decay reactions - beta minus decay and beta plus decay - do not conserve atomic number. These reactions, by changing the number of protons in an atom, change the identity of the atom.

Here is a helpful rule of thumb: Changing the number of electrons creates a charged atom or ion, changing the number of neutrons creates a different isotope of an atom, changing the number of protons changes an atom into a different atom.

When atomic nuclei are split open (fission) or fused, very high energies are involved. These fission and fusion reactions not only create products that we can expect when we balance an equation, such as the two helium nuclei in the example above, but the energy released may high enough to allow the creation of altogether new particles as well. These particles include electrons, positrons (the electron's antimatter twin), and neutrinos. However, even the most powerful H-bomb doesn't have enough energy to create new neutrons and protons. Very high-energy (very fast) helium nuclei are called alpha particles. Very high-energy electrons are called beta particles. A nuclear reaction can create alpha, beta or free neutron (particle) radiation, as well as gamma, X-ray and infrared (heat) electromagnetic radiation (EM). This radiation carries off some of the energy released in the explosion. In addition, a nuclear explosion creates a blindingly bright flash - this is EM radiation in the visible wavelength range.

Beta Radiation From Nuclear Reactions

To understand beta radiation, we'll look at two examples: carbon-14 and magnesium-23.

Carbon-14 is a slightly unstable atom. It will eventually transmute into stable nitrogen-14 through beta minus decay, emitting an electron (e-) and an electron antineutrino. This electron has high velocity so it is called beta radiation. The antineutrino is harmless as all matter, even Earth, is invisible to neutrinos - they just pass right through it.

Magnesium-23 is unstable too but it transmutes into stable sodium-23 through beta plus decay instead, emitting a positron (e+) and an electron neutrino.

Positron radiation is considerably more dangerous than beta radiation, even though the two particles have similar energies. As soon as the positron comes close to an electron (and electrons are everywhere in all matter) the matter/antimatter pair will annihilate into energy in the form of two gamma rays. Unlike beta radiation, gamma rays have enough energy to penetrate right through the body's tissues, including bone, to damage cells and DNA. Only thick shielding such as lead can stop them. Beta particles can only penetrate skin-deep.

In both of these reactions, baryon number is conserved but atomic number is not. A neutron in carbon-14 decays into a proton, creating a stable nitrogen nucleus. A proton in magnesium-23 decays into a neutron.

Beta plus decay poses an interesting puzzle. This reaction might seem impossible when we consider than a neutron has just a bit more mass than a proton does. How does the reaction gain mass? This is not an endothermic reaction, meaning that it does not absorb energy from the environment. The answer is that a proton inside an unstable nucleus has binding energy at its disposal, which it uses to decay into a neutron. A very small amount of mass is gained but the system moves into a more favourable lower energy state. This reaction mechanism is impossible for an isolated proton, and current understanding is that isolated protons are stable. If there is any instability it is miniscule, with an estimated half-life of about 1032 years, far longer than the lifetime of the universe.

Radiation From Fission And Fusion (Hydrogen) Bombs

Both types of beta decay occur after a nuclear fission bomb detonates, as various radioactive fission products decay into more stable atoms. Some of this decay is rapid, on the scale of milliseconds, while other decays will happen much more slowly. For example, strontium-90 is an unstable isotope within nuclear fallout. It has a half-life of 28.8 years as it decays through beta minus decay into stable yttrium-90. It is very hazardous because it acts like calcium in the body when it's ingested, being deposited in bone where its presence can cause bone cancer. The cancer is caused by a continuous dose of beta radiation (high-speed electrons) emitted by strontium-90 as it decays.

A note on half-life: Half-life means that, for strontium-90 for example, on average, half of a given sample will still be strontium-90 after 28.8 years and half will have decayed into yttrium-90. Decay is a probabilistic process. When an individual atom will decay is totally random. This is why strontium-90 delivers continuous radiation. It doesn't all decay at one time.

One might guess that a hydrogen bomb, which operates by fusion, does not create any radioactive fission-product fallout. However, all known fusion bombs have two parts, one of which creates the fission reaction that sets off the fusion explosion. The fission reaction, like fission weapons, creates various radioactive products. As well, the fusion reaction itself creates some radioactive tritium (a beta radiation emitter), but even more deadly is the fusion reaction's tremendous output of radiation, which includes neutrons, gamma rays, X-rays as well as alpha and beta particles. Furthermore, a hydrogen bomb detonation close to the ground can activate atoms in the soil, etc. Through intense neutron radiation, a wide variety of radioactive isotopes can be created. This radiation induces radioactivity when atomic nuclei capture free neutrons, become unstable themselves and emit gamma rays and beta radiation and possibly more alpha and neutron radiation.

Calculating the Explosive Energy of Chemical Versus Nuclear Reactions

We know that the fusion reaction in the Sun releases a tremendous amount of energy, and we can assume that the deuterium-tritium fusion reaction inside the H-bomb also releases a lot of energy. We aren't dealing with the chemistry of chemical bonds, so how do we calculate the energy released in a nuclear explosion?

A bomb calorimeter, for example, can accurately measure the heat of combustion reactions, those that deflagrate (explored in the previous article). It does this by measuring the reaction's enthalpy. It measures the initial and final temperature and measures the masses and specific heat capacities of the reactants. For explosive chemical reactions, a bomb calorimeter isn't as accurate. Products of the explosion, after everything is cooled back to room temperature, are usually not those present at the moment of maximum temperature and pressure. Therefore, explosive energy output is most often calculated indirectly instead, either by carrying out a performance test tailored to the type of explosive or by calculating an estimate of its explosive power by taking into account the reaction's oxygen balance (described in the previous article), the heat of explosion of the reaction, the volume of products and the chemical potential of the explosive.

A Sample Calculation

Like chemical explosions, the energy output of nuclear explosions can only be indirectly measured. Fortunately we can utilize Einstein's mass-energy equivalence. Once again we will use the lithium-deuterium reaction as our example. We can get an accurate measurement of the energy released simply by comparing the invariant mass on the left side with the right side of the equation:

For this, we can look up the relative atomic masses (u) of the reactants and products. The value u is the average mass of the atoms of an element. It is useful because many elements found in nature consist of a mixture of different stable isotopes, and we can simply look it up for any element on Wikipedia and elsewhere. Relative atomic mass therefore gives us a mass that reflects that element's composition in nature.

6.015 u (lithium) + 2.014 (hydrogen-2) = 8.029 u

8.029 u is the mass that would balance this reaction if we treated it like a chemical reaction. However, when we calculate the mass of our two helium nuclei products, we get less mass:

2 x 4.0026 (helium-4) = 8.0052 u

8.029 - 8.0052 = 0.0238 u

This is the mass equivalent of the energy that is created per one atom each of lithium and deuterium during this reaction.

One atomic mass unit (u) is defined as 1/12 the mass of a carbon-12 atom which equals 1.66 x 10-27 kg. This is the average mass of a nucleon (proton or neutron). We'll take this as the mass per 1 u. Now that we can translate u into mass in kg, we can use Einstein's equation to find out how much energy that's equivalent to:

E = mc2 so E = 1u x c(speed of light)2

E = (1.66 x 10-27 kg) x (3 x 108 m/s)2

E = 1.5 x 10-10 kg(m/s)2

This is the formulation of a unit of energy called the joule (J):

E (per u) = 1.5 x 10-10 Joules

We can translate this energy into electron volts. 1 MeV (mega or million electron volts) equals 1.6 x 10-13 J

E = 1.5 x 10-10 Joules x 1MeV/1.6 x 10-13 J = 931 MeV of energy released per u.

Now we can figure out what the explosive yield of our reaction is per kilogram of reactant mixture:

To get our answer into joules/kg:

We can take 1.5 x 10-10 J/u and divide by 1.66 x 10-27 kg/u to get 90 x 1015 J/kg (or 90 PJ/kg). However, just a tiny portion of the expected product's mass is converted into energy. The easiest way to get that is to take a percentage of the product mass than went missing. We know this went to energy:

0.0238 u/8.029 u = 0.003 or 0.03%
90 PJ/kg x 0.03% = 3 x 1014 J/kg or 300 TJ/kg

The energy output of this reaction is 300 TJ/kg. To put this in perspective, one kg of TNT releases 4.2 x 109 joules of energy. Let's compare the two:

3 x 1014 J/kg divided by 4.2 x 109 J/kg = 7.14 x 104 

This means that the fusion of one kg of lithium-deuterium releases 70,000 times more energy than one kg of TNT. This doesn't mean that if you put lithium and deuterium together they will spontaneously explosively fuse in a monumental explosion. What triggers a nuclear explosion and how the rates of nuclear reactions are controlled is the subject of the next article in this series.


Mass and energy are equivalent. Mass is converted into energy during a nuclear explosion. The speed of light squared in E = mc2 means that enormous energy is locked up in a tiny amount of mass. The total number of nucleons does not change during a nuclear reaction.

The energy of the reaction comes from nuclear binding energy, which contributes significant mass to atoms. Nuclear binding energy is the residual energy of the strong force that holds quarks together inside each nucleon. These reactions bring home the fact that mass and energy are two sides of the same coin, a fact that was unknown before Einstein.

Nuclear reactions tap a source of energy that is unavailable to chemical reactions which involve only the electrons of the atoms and the associated electromagnetic force, a much weaker fundamental force.

In the next article, we will explore in greater detail how nuclear binding energy works and how fission and fusion reactions are triggered and controlled.

Sunday, January 18, 2015

The Science of Bombs

(This article and the next one are written as two parts of one question: What are chemical and nuclear explosions?  This article focuses on what an explosion is and how chemical explosives work. The next article, Chemical Versus Nuclear Explosives, explores what a nuclear explosion is and then compares nuclear explosions with chemical explosions.

A nuclear bomb delivers an incredibly powerful explosion, shown below.

The photo left is of the BADGER nuclear explosion in 1953, part of Operation Upshot-Knothole at the Nevada Test Site in the U.S.

What Is an Explosive?

Wikipedia describes it well: "An explosive device is a device that relies on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy."

Explosives are used for a wide variety of jobs from putting on a New Year's Eve fireworks show to obliterating an entire city. In this article we will explore how chemical explosives work.

Chemical Explosives

Almost all explosions, with the exception of nuclear bombs, are examples of chemical explosions. A nuclear explosion is not a chemical explosion. Instead it relies on the process of fission or fusion, something we will get into in Part 2. This means that all chemical explosives rely on chemical reactions, which are interactions between the electrons of atoms that are part of the explosive compound. Generally heat or a physical shock is required to trigger the explosive chemical reaction. This reaction is an extremely rapid decomposition of the compound, which usually releases a lot of gas and heat. However, not all decomposition reactions are explosive.

An Explosive Reaction is (Usually!) a Decomposition Reaction

A decomposition reaction is a chemical reaction in which a compound separates into either elements or simpler compounds. Under certain extreme environmental conditions such as heat, radiation, humidity or physical agitation, for example, a compound, which is normally chemically stable, will become unstable. An unstable compound may decompose. Explosive compounds vary widely in their chemical stability. They may take anywhere from decades to a mere fraction of a second to decompose.

There are three basic types of decomposition reactions: electrolytic, catalytic and thermal.

a) Electrolysis

An example of electrolytic decomposition is the electrolysis of water into hydrogen gas and oxygen gas. Water is stable under most conditions so it does not spontaneously decompose. An electrical current applied to water, however, will trigger the decomposition reaction:

2 H2O(l) → 2 H2(g) + O2(g)

b) Catalysis

This is sometimes a tricky concept to understand so I will give it some extra time here. Catalytic decomposition is a case where a catalyst lowers the activation energy of a reaction. In this case the reaction without the catalyst would be either non-spontaneous or it would happen at a very slow rate. A catalyst speeds up the rate of a reaction but it is not consumed or changed in the process. In order for any kind of reaction to take place, the reactant(s) must undergo a rearrangement of chemical bonds. The slowest step in this process is called the transition state. Here, the chemical species are neither reactant nor product. They are in an in-between state and energy is required to form it. This energy is called activation energy. If the reactants do not have as much energy as the activation energy they cannot undergo the reaction. A catalyst works by providing a new route with lower activation energy from reactant to product. It doesn't shift the equilibrium of the reaction but, at any given point in time, it allows a greater proportion of reactants to react, and this is how a catalyst speeds up the reaction rate.

For example, our cells make trace amounts of hydrogen peroxide (H2O2), which functions as part of our immune system and as a signaling chemical. It is also an oxidizer so too much of it in our cells can damage our DNA and proteins. Our cells regulate the level of H2O2 by producing an enzyme called peroxidase to catalyze its decomposition into water and oxygen before it has a chance to react with DNA or proteins.:

2 H2O2 → 2 H2O + O2

Peroxidase lowers the activation energy of the decomposition reaction shown above, which allows this slow spontaneous reaction to happen at a much faster rate.

c) Thermal Decomposition: Endothermal and Exothermal

Endothermic Decomposition

Many decomposition reactions are thermal, which means that heat is involved. Most decomposition reactions are endothermic, which means that the reaction absorbs heat from the environment. You can verify this for yourself. Although many of these reactions involve toxic chemicals, one basic reaction is very safe and easy to do. It is actually very similar to what goes on chemically when you are baking. Many recipes call for baking soda (a base) to be mixed with an acid, which can be lemon juice, for example. Even buttermilk is slightly acidic. The reaction creates tiny bubbles of carbon dioxide gas that add a light airy texture to your finished product. I have a great tall buttermilk pancake recipe that makes good use of this reaction.

In this case, baking soda is added to a citric acid solution in a cup (the complete method is at the link above, with some variations to try).

H3C6H5O7(aq) + 3 NaHCO3(s) → 3 CO2(g) + 3 H2O(l) + Na3C6H5O7(aq)

A thermometer placed in the cup records the falling temperature as the reaction absorbs heat. Three key things occur during this reaction: a loss of mass (which you can measure on a scale), an increase in volume (which can be used to blow up a balloon), and a decrease in temperature (which you can measure with a thermometer). The reaction takes place in the following brief video.

Exothermic Decomposition

Some thermal decomposition reactions are exothermic, which means they release heat into the environment. Explosive reactions fall into this category. Many exothermic reactions require heat to trigger them but once underway they release heat.

The Bhopal Disaster

A few of these kinds of reactions release a lot of heat very quickly, and this can potentially set up a positive feedback loop, which in turn may create a thermal runaway. In this case, an increase in temperature changes conditions that cause a further increase in temperature. The simple diagram below right gives you an idea of how it works.


In chemical engineering this event is usually accidental and undesirable. A horrific example of an accidental thermal runaway is the 1984 Bhopal disaster at a pesticide plant in India. Wikipedia cites three general causal factors: heat was not able to escape, contaminants accelerated the reaction and gas scrubbers were not functioning. A diagram of the reaction is shown below.


The chemical process is as follows: methylamine (1) reacts with phosgene (2) to form methyl isocyanate (MIC) (3), which is then reacted with 1-naphthol (4) to create carbaryl (5), more commonly known as Sevin, a well-known pesticide. It is legal for home house in Canada but I avoid using it because of its toxic effects.

Several issues at the plant led to a runaway reaction and a toxic gas cloud was released that contained, along with toxic MIC, phosgene, hydrogen cyanide, carbon monoxide and hydrogen chloride. Some of these gases are heavier than air so they stayed close to ground level where they were breathed in. Over 3700 people died and over half a million others were injured by the toxic gases.

Explosive Reactions

In some types of exothermic decomposition reactions, gas escapes so rapidly that a shock wave, or detonation, occurs. High explosives fall into this category. What makes a chemical reaction an explosive one? The reaction must exhibit:

1) Rapid expansion of volume (usually the result of rapid production of gases)
2) Release of heat
3) A rapid reaction rate
4) A reaction trigger, which starts the reaction

There are many different explosive chemicals. Wikipedia has gathered a list of them.  Wikipedia also has a list of mixtures and types of explosives. Even sugar, grain and coal dust are explosive under the right conditions. Here we will focus on a few well-known examples of explosive compounds such as nitroglycerin (dynamite), cyclonite (RDX and C-4 explosive), picric acid and trinitrotoluene (TNT). Each of these is an example of a high explosive. Following this, we will explore the differences between high and low explosives.


Nitroglycerin is a contact explosive, which means it requires a physical shock or friction to set it off. It is highly unstable, which means that any jar to it or shaking of it can set off its explosive decomposition, making its manufacture and transport very dangerous. Even while stored carefully, it spontaneously decomposes slowly over time: (rewrite)

4 C3H5N3O9(s) → 6 N2(g) + 12 CO(g) + 10 H2O(g) + 7 O2(g)

Dynamite, shown below, contains nitroglycerin but its sawdust construction makes it much less sensitive to shock. It requires a blasting cap to deliver the shock required to detonate it.

Image adapted from Pbroks13;Wikipedia

Still, old dynamite is hazardous because over time liquid nitroglycerin can seep out of the tube and dry into very unstable crystals. Freezing dynamite is not a good safety option. When dynamite is frozen (it freezes at +10°C) it is very stable. However, if it thaws too quickly, it becomes extremely unstable. Most manufacturers recommend storing dynamite under cool conditions for no longer than one year.

Cyclonite (RDX)

Cyclonites or cyclic nitramines is a family of chemically related compounds. RDX, for example, goes by the complex formula hexahydro-1,3,5-trinitro-1,3,5-triazine. Thermal decomposition of these compounds leads to various simple molecules such as HCN, NO, N2O, NO2, CO, CO2, H2O, H2CO, etc. The chemical processes involved in their decomposition are complex and not well understood and reaction products may vary greatly depending on the environmental conditions in which the reaction takes place. Despite their complex reaction chemistry, cyclonites tend to be very powerful and safe to use. You may have heard of the plastic explosive, C-4, for example. An explosive used by the U.S. military, it contains 91% RDX.

C-4 is a plastic moldable claylike explosive which is very stable and more powerful than dynamite. Below right, a crewman of the U.S. Navy inserts blasting caps into blocks of C-4 explosive.

C-4 is stable and it requires a combination of extreme heat and a shock wave to detonate (supplied by a detonator). This makes it fairly safe to store and use. Discovered in Germany in 1898 as a medical compound and a propellant, its explosive qualities were later explored and it was used widely in World War II. It currently has a wide variety of military and industrial applications.

Picric Acid

Picric acid, a yellow crystalline solid acid phenol (2,4,6-trinitrophenol), is a highly nitrated compound similar to TNT (explored next). It decomposes explosively, producing large volumes of carbon monoxide (CO) gas, steam and nitrogen gas.

One of the most powerful chemical explosions in history took place in Halifax Canada in 1917. The Norwegian SS Imo and the French SS Mont-Blanc collided in the harbour, detonating 2653 tonnes of various explosives, mostly picric acid, which were loaded on the SS Mont-Blanc. This steamship was chartered to carry the explosives from New York to France to supply the French war effort during World War 1. A fire aboard the ship detonated the explosives about 20 minutes after the collision (Picric acid detonates above 300°C). Over 2000 people died and much of Halifax was destroyed.

Below is a photo of Halifax from around 1900, before the explosion. Much of the area visible was destroyed.

Below a photo looks toward the harbor two days after the explosion. On the far side of the harbor you can just make out the steamship IMO, run aground. The SS Mont-Blanc was completely blown apart. Parts of her hull were launched almost 300 metres into the air as the blast ripped through the hull and cargo at more than 1000 m/s. Temperatures of about 5000°C and thousands of bars pressure accompanied the detonation (air pressure at sea level is about 1 bar). Fittings and hull parts flew up to 4 km in all directions.

Researchers estimate that the explosive force was equivalent to 2.9 kilotonnes of TNT. To put this in perspective that is about one tenth the power of the nuclear bomb dropped on Nagasaki, Japan, ending WW 2.

Various kinds of artillery shells were used in World War 1. Four types are shown below left.

From left to right they are a 90 mm fragmentation shell, 120 mm pig iron incendiary shell, 75 mm high explosive shell and a 75 mm fragmentation shell. The high explosive shell (second from the right) would have been filled with either picric acid or TNT and used as an armour-piercing shell.

Picric acid is more powerful than TNT but it is not as stable, which has lead to its disuse as an explosive. It is still used in fireworks to improve the tone of the colours and to provide the characteristic loud whistling noise. It is stored under water or in solution. If it dries out it becomes extremely hazardous, as it is then unstable and very sensitive to shock and friction, and a bomb squad may be required to dispose of it. It's not something you want to find in an old school chemistry cabinet. Picric acid solution was also once very common in first aid kits. You would soak gauze in it to treat burns. Over the decades these bottles have been known to dry out, leaving behind yellow highly unstable explosive picric acid crystals. If you come across an old metal first aid kit beware!


Finally, TNT (trinitrotoluene), a yellow powdery solid, is very stable compared to other explosive compounds. It is insensitive to shock and friction, impervious to water and has a very high spontaneous detonation temperature. This means it has high activation energy. TNT requires the fairly intense pressure wave from an explosive booster in order to detonate. There are many explosive blends of compounds in current use that contain TNT, many of which are oxygen-rich. The extra oxygen balances TNT's production of carbon so that more carbon monoxide is explosively released. making for an even more powerful explosion. It has many military and peacetime uses in engineering, construction, mining and quarrying.

When TNT (trinitrotoluene) detonates, two similar chemical reactions occur:

2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C + energy
2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C + energy

TNT decomposes into nitrogen gas, carbon monoxide, carbon and either water or hydrogen gas. This reaction yields 2.8 MJ (mega or million joules) of explosive energy per kilogram of reactant. To compare, gunpowder yields 3 MJ per kilogram (kg) and gasoline yields, on average, 47.2 MJ/kg. The energy yield of TNT is used as an explosive energy standard, even for nuclear weapon yields.

Surprising isn't it that gasoline yields significantly more energy than TNT does? We will see in a moment why our vehicle engines don't blow to smitherines.

High Explosives Versus Low Explosives

Detonation and deflagration are two examples of very rapid decomposition reactions. We tend to hear the word detonation but the word deflagration isn't in common use. Both are examples of explosive reactions. A low explosive deflagrates. A high explosive detonates.

Low Explosives

Technically, deflagration, characteristic of low explosives, is a combustion reaction that propagates through the process of heat transfer. Hot burning material heats up the layer of cooler material next to it and ignites it. We usually call this process "burning" as in the burning of a log in a fire pit, or gunpowder in a gun chamber, the blue flame of gas/air in a gas stove or even the blast of a rocket engine.

Like explosive decomposition reactions, the combustion reactions involved in deflagration are highly exothermic and happen very fast. Here, however, is a bit of a two-part tricky twist:

First, not all combustion reactions involve the production of heat (though they do produce energy) nor do they all happen rapidly. Combustion is perhaps better defined as an example of a redox (reduction/oxidation) reaction. For example, the cells in our bodies are continuously carrying out cellular respiration, which is a combustion or redox reaction. The simplified version of it looks like this:

C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + energy (ADP into ATP)

Our cells use glucose (from food) and oxygen (from air) to make not exactly heat but high-energy storage compounds called ATP, which deliver the energy that cells need to carry out all their functions. This combustion reaction can be considered a decomposition reaction because glucose, with the addition of oxygen, is broken down into simpler molecules.

Second, not all combustion reactions are decomposition reactions. In rocket (liquid fuel) engines the basic combustion reaction is a synthesis reaction, where water, as water vapour gas, is synthesized from liquid hydrogen and liquid oxygen:

2H2(l) + O2(l) → 2H2O(g) + energy

A schematic diagram of how a liquid-fuel rocket works is shown below.

Liquid hydrogen and liquid oxygen are mixed in the combustion chamber. The resulting combustion reaction produces a very rapidly expanding volume of water vapour gas, which shoots out the nozzle and provides the rocket's thrust.

With this tricky twist being acknowledged, for our purposes we can safely assume that most low explosive reactions are decomposition reactions that rapidly release large amounts of heat and kinetic energy.

Decomposition moves through the compound as a flame front. This definition has no lower speed limit; it may move very slowly. However it does have a top speed limit, which is the speed of sound. What defines a low explosive as low is that this wave front moves slower than the speed of sound. It is subsonic. In reality, combustion propagation speed falls along a spectrum. At the upper end of the limit, a propagation (flame) front can become highly turbulent and it can, often under unpredictable circumstances, become a supersonic front, or shock wave, something characteristic of the blast of a high explosive.  An internal combustion engine and fireworks are examples of low explosives (at least for the most part as we will see).

High Explosives

A high explosive detonates rather than deflagrates. In this case, the decomposition reaction moves through the compound as a much faster shock front. Here, the speed of propagation is always supersonic, faster than the speed of sound.

While the combustion reaction characteristic of low explosives consumes oxygen from the air in order to propagate, the faster reaction of a high explosive must consume the oxygen within the explosive compound itself during the redox reaction. Like most low explosive materials, high explosives tend to be organic compounds (organic compounds), composed of oxygen, carbon, hydrogen and nitrogen. If an explosive molecule contains just enough oxygen to convert all of its carbon into carbon dioxide and all of its hydrogen into water it is said to have zero oxygen balance. If it has more oxygen than is needed, it has positive oxygen balance. If doesn't have enough, it has negative oxygen balance. Pure TNT has negative oxygen balance, so it creates carbon-rich soot as a reaction product.

As we learned earlier, the sudden reaction triggered by any chemical explosive is an exothermic one. This means that the reaction releases energy (heat and kinetic energy) rather than absorbing it. The outward blast typical of a chemical bomb (a high explosive) is usually created by the explosive release of gas, which is formed as a product of the reaction. The supersonic moving front of gas and other material creates a shock wave that moves outward ahead of the gas front. In fact, the reaction itself propagates by using the shock wave. This is quite different from the heat transfer method of propagation observed during deflagration in low explosives. Here, compressive heating of the explosive material inside the shock wave drives the reaction forward. It is this shock wave that causes most of the damage of a high explosive.

It may seem that low explosives are the tamer gentler type of explosive. However, it does not necessarily mean that low explosives release less energy than high explosives. TNT, a high explosive that we explored earlier, yields far less energy per kilogram than gasoline, a low explosive. TNT contains 2.8 (MJ) mega joules of energy per kilogram (kg). (A note here: this number is not the same as TNT's heat of combustion, which is 14.5 MJ/kg. This latter figure requires that some carbon in TNT react with the oxygen in the air, a combustion reaction that occurs after the initial detonation reaction). Gasoline, on the other hand, contains on average 47.2 MJ/kg, about 17 times more energy than TNT. The key difference between the two is in their propagation (detonation/deflagration) velocities.

What makes TNT so damaging is its detonation velocity, about 7000 m/s, creating an intense supersonic pressure wave - a very sudden increase (up to 20 bar) followed by a decrease in air pressure, which tends to blow structures apart. (The speed of sound is 340 m/s in air.) Gasoline on the other hand does not detonate but deflagrates instead. Its reaction propagation rate is just 0.34 m/s, creating a smaller but more sustained pressure wave that pushes the pistons down in a car's engine. It does not create a supersonic blast wave that would blow up the whole engine. In fact, most deflagrates create an overpressure of about just 0.5 bar), reflecting their generally low propagation velocities. This table compares the detonation velocities of various chemical high explosives.

Fireworks Are Low (and Sometimes High) Explosives

Devices such as fireworks are technically classified as low explosives.

The power required to lift the firework into the air is provided by a black powder that contains mostly potassium nitrate (KNO3). This black powder recipe hasn't changed much since the gunpowder of ancient Chinese times. It deflagrates at 3 m/s, and in open air its heat and gas dissipate quickly without much fanfare. The trick is to confine it in the bottom of the firework shell. The trapped gas pressure builds up and when it escapes it hurls the shell high up in the air. This is exactly the same reason why holding a lit firework in your hands is so dangerous - you are creating an enclosure where pressure can build up explosively.

I've drawn a diagram of a typical fireworks shell, shown below. This one, I believe, would shoot up into the sky, burst in a bright white flash and then burst green and then red stars. The red and green star chemicals are examples of colour-creating compounds, of which there are many. For a more complete list of them, click here.

The firework is lit by a fuse, which may be one of two types. The fast-burning fuse (shown as a red line above) carries the flame to the lift charge (black powder zone, bottom). A slow fuse (the thick buff central tube) connects this zone to the colour content of the firework above. This part might even be a time delay fuse as when you see a firework shoot up, go dark and then suddenly burst into stars a few seconds later.

Some fireworks such as maroons and concussion fireworks, explode with a very loud bang. If you want to know more about various kinds of fireworks try this link at, which has a great glossary of fireworks. The bang is a shock wave. Whether the shock wave is subsonic or not will technically define it as a low or high explosive. Both kinds of fireworks, usually filled with a potent flash powder, are very dangerous and some of these flash powders, depending on which reference source you use, contain high explosive components, and some of the bangs they create verge on detonations. In addition to these, some other fireworks contain trace amounts of picric acid, which is a high explosive. Here, rather than causing as detonation, it is used to create a loud hissing sound and it can also be used as a colour enhancer.

Fireworks are fascinating and their design and usage is both a science and an art. However, there are illegal fireworks available online and at street corner vendors. Beware of these as some have been known to contain high explosives such as dynamite. Instead of shooting upward, a shell filled with high explosive would detonate creating a deadly blast in all directions. To learn how to use store-bought fireworks safely check Consumer Fireworks Safety in Canada. Towns and cities hire certified fireworks operators (pyrotechnicians) to set off and choreograph larger and more dangerous display fireworks. To tame your inner pyromaniac and turn it into a useful skill, check out this training site in Canada or try Pyrotechnics Guild International. To learn more about fireworks - their chemistry and colours - this webpage is a good source of information.

Deflagration and Detonation: Sometimes There is a Fine Line

Sometimes detonation happens by accident in a device that is designed to use deflagration. The exact mechanism for how this works is not well understood but an accidental transition from deflagration to detonation can be caused by partial confinement or obstacles in the way of the flame front. These obstructions can create unpredictable eddy currents that suddenly accelerate the flame front to supersonic speed. For example, in an engine it causes engine knocking. During normal combustion, the fuel/air mixture in the cylinder burns in an orderly controlled way and develops maximum pressure at the right time to push the piston down. Ideally, all the gas/air is burned each cycle. When an engine knocks (it's little detonations you are hearing), the cylinder is not firing correctly and small amounts of unburned gas/air remain. This mixture is outside of the boundary of the next flame front and therefore it is subjected to heat/pressure for a period longer than the gasoline is designed for. Detonation of that gas/air can then occur. A shockwave can rapidly increase pressure in the cylinder beyond its design limits. Repeated knocking can seriously damage or destroy an engine. The following 9-minute video not only explains how the pistons in your engine work but what knocking is and how to deal with it.

In a firearm accidental detonation can cause sudden excessive heating and a potentially lethal structural failure of the weapon. The key difference between a normal firing and a catastrophic detonation has to do with with how the explosion propagates outward. Because the pressure wave caused by the flame front during deflagration does not move faster than a normal (sound) pressure wave, it has time to be "pulled" toward any areas of lower pressure. In a firearm, this means that the bullet is pushed out of the chamber in front of the flame front toward the area of lower pressure further down the barrel. Detonation pressure is faster than the speed of sound so the explosion has no chance of being "pulled" toward any areas of lower pressure. The pressure of the blast front is equal (and much higher) in all directions. This means that the entire chamber of the gun experiences a shockwave, which it is not designed to handle, and that is why the gun can blow apart.

A Fascinating Case of Accidental Detonation

The 1989 U.S.S. Iowa disaster is a fascinating example of accidental detonation in a gun turret ? but even more fascinating for the Navy's handling of the accident. A detonation in the central #2 gun turret killed 47 of the turret crewmen aboard and severely damaged the gun turret itself. The explosion created a fireball as hot as 1600°C traveling at 610 m/s with a pressure wave of 280 bars that spread through all three of the gun rooms, releasing various toxic gases including cyanide from the burning polyurethane foam that lined the turret. The heat from the fire then ignited almost 1000 kg of powder bags in the powder handling area. Nine minutes later another explosion followed, most likely from the buildup of carbon monoxide from the first explosion. Carbon monoxide is highly flammable and readily forms an explosive mixture with air at room temperature. With exposure to heat it will explosively decompose into carbon and carbon dioxide. Below, the #2 turret is cooled with seawater shortly after exploding.

The Navy's initial investigation concluded that the explosion was deliberate. Normally, a mission-specific projectile would be rammed into the bore using a pneumatic ram. Then the powder propellant, which consists of 2-inch long rods of nitrocellulose embedded with diphenylamine (a stabilizer) bundled into bags (usually six are used), is rammed in behind the projectile and the breach is closed before firing.

Their theory was that a crewman, Clayton Hartwig, who died in the explosion, deliberately caused the explosion by inserting either a chemical or electronic detonator in between two of the powder propellant bags as the gun was loaded. Navy officers and investigators leaked to the media than Hartwig and another crewman, Kendall Truitt (who survived), were lovers, that their relationship soured and that Hartwig, suicidal over the breakup, deliberately caused the explosion by using a detonator. Truitt, married to a woman, acknowledged that he was sole beneficiary of a $100,000 life insurance policy on Hartwig. While the Navy focused intensely on the relationship details, the victim's families, the media and U.S. Congress soundly criticized the findings. The U.S. Senate and the U.S. House Armed Services Committees launched their own investigation and to assist them they hired Sandia National Laboratories to review the technical investigation. They found no evidence that any detonator was used and they concluded that the explosion was caused by over-ramming the propellant into the bore. This may have been the result of human error or a mechanical failure of the ram. They concluded that it likely compressed the powder to the point that the bags spontaneously ignited.

Before the technical investigation was completed, the Navy released its own report, finding that the explosion was a deliberate act "most likely" caused by Hartwig by using an electronic timer and that the powder bags were over-rammed under his direction to ensure that the explosion went off. The report was then quickly endorsed by top Navy officials and briefed to the media at the Pentagon.

Meanwhile the Sandia forensic investigation, still incomplete, continued, and its findings showed that the explosion could have been accidentally caused by over-ramming. It recommended a number of improvements to safety procedures. Meanwhile there was speculation about whether the powder itself was the cause of the explosion. Some sources claimed it had been exposed to temperatures as high as 32°C while it was stored for months on uncooled barges in New York River, and this may have made the powder unstable. Sandia requested a series of drop tests to investigate the possibility. A large number of variously designed drop tests done by both Sandia and the Navy found that the powder did detonate a significant number of times, indicating that it was sensitive to even fairly low over-ram pressures.

This story, in which the Navy was not only shown to fumble its own internal investigation but it appeared to try to manipulate the press as well, was the subject of intense media coverage. Several notable books were written on the disaster. Richard Schwoebel, who directed Sandia's investigation, wrote "Explosion Aboard the Iowa," published in 1999. In it, he recounts many of the problems involved with the Navy's investigation. Also in 1999, investigative journalist Charles Thompson published "A Glimpse Into Hell: The Explosion on the USS Iowa and Its Cover-Up" documenting his own investigation, and which is very critical of many of the Iowa's officers involved in the Navy's investigation. It was later made into a made-for-TV movie called "A Glimpse Into Hell."

The Hartwig family sued both the Navy and NBC News for damages and both suits were dismissed. Thirty-eight other Iowa victims sued the Navy and their suit was dismissed. Several Navy officers cited in Thompson's book sued both Thompson and his publisher, W.W. Norton. The suit against Thompson was dismissed and the case against Norton was later settled out of court.

In the next article we will explore the even more devastatingly powerful explosions of nuclear devices, and compare them to chemical explosions.