Nuclear Weapons

"You know what uranium is, right? This thing called nuclear weapons like lots of things are done with uranium including some bad things."

- Donald J. Trump - Feb 17, 2017

To fully understand the Cold War, one needs to understand the nature of the threats that faced the world, chief among them were nuclear weapons. That is where this page is centred.

The term explosion is a general one for an accelerated release of energy generating extreme temperatures, releasing of gases and expanding volume.

Chemical explosions

Chemical explosions may be supersonic as in the case with detonations using high explosives like Pentaerythritol tetranitrate (PETN) or subsonic and initiated by deflagration (combusting materials via heat transfer) of low explosives like gunpowder, as seen in fireworks. There are many chemical reactions that will release energy. They are known as exothermic reactions. If the reaction proceeds slowly, the released energy will be dissipated and there will be few noticeable effects other than an increase in temperature. On the other hand, if the reaction proceeds very rapidly, then the energy will not be dissipated. Thus, a great quantity of energy can be deposited into a relatively small volume, then manifest itself by a rapid expansion of hot gases, which in turn can create a shock wave or propel fragments outwards at high speed. Chemical explosions may be distinguished from other exothermic reactions by the extreme rapidity of their reactions. In addition to the violent release of energy, chemical explosions must provide a means to transfer the energy into mechanical work. This is accomplished by expanding product gases from the reaction. If no gases are produced, then the energy will remain in the products as heat

Most chemical explosions involve a limited set of simple reactions, all of which involve oxidation (reaction with oxygen). On general terms, an explosion results from the very rapid release of a large amount of energy within a limited space and short timespan. Energy may be broadly classified as potential or kinetic. Potential energy is energy of configuration, position, or the capacity to perform work. For example, the relatively unstable chemical bonds among the atoms that comprise 2,4,6-trinitrotoluene (TNT) possess chemical potential energy. Potential energy can, under suitable conditions, be transformed into kinetic energy, which is energy of motion. When a conventional explosive such as TNT is detonated, the unstable chemical bonds are converted into bonds that are more stable, producing kinetic energy in the form of blast and thermal energies. This process of transforming a chemical system's bonds from lesser to greater stability is exothermic in other words it is a reaction which produces heat energy.

In 1945 the whole of the nature of explosives changed, from the outermost parts of the atom, to its centre, the nucleus. All earlier explosives had depended upon a chemical reaction between a fuel and an oxidiser. The first such explosive was gunpowder, in which the fuel is a mixture of sulfur and carbon, with potassium nitrate as the oxidiser. It was invented by the Chinese in the middle of the 9th century AD. Initially it contained too small a concentration of potassium nitrate to be an explosive (about 25%). In the middle of the following century the first formulary for gunpowder and descriptions of its uses were published for the military. By this time the highest level of potassium nitrate had risen to about 50%, still not an explosive. However by 1280 explosive gunpowder was in use, this required a mixture of 75% potassium nitrate, 15% carbon and 10% sulfur, in fact the biggest man made explosion for hundreds of years killed 100 guards at a bomb store in Weiyang in 1280CE, resulting from the explosion of stored gunpowder. The Chinese had discovered that by varying the proportion of the three ingredients one could produce powder for fuses, pyrotechnics, rockets and explosives. It was three hundred years before any significant advance was made in the world of explosives, and that was merely the replacement of potassium nitrate with the cheaper, though not so good, sodium nitrate. The sodium nitrate version was fine for use in mining, but not for military purposes, the reason being that sodium nitrate is deliquescent meaning that it absorbs water from the air to the extent of dissolving, meaning it cannot be stored for very long. Potassium nitrate does not have this defect.

The first major development of a new explosive was that of nitroglycerine. Nitroglycerine was synthesised by the Italian chemist Ascanio Sobrero in 1847. He considered nitroglycerine to be far too dangerous to be of any practical use. Sobrero is quoted to have said "When I think of all the victims killed during nitroglycerine explosions, and the terrible havoc that has been wreaked, which in all probability will continue to occur in the future, I am almost ashamed to admit to be its discoverer." The problem was that at room temperature it was unstable, and could be set off by the slightest shock. The safety was improved by adding it to a number of other substances, the most famous product of which was Dynamite, made by adding it to a form of diatomaceous earth known as kieselguhr, it was invented by Alfred Nobel in 1867, following the death of his brother in a nitroglycerine explosion. Numerous other explosives were produced by adding a whole variety of ingredients, producing products such as gelignite, cordite and ballistite. The difference between gunpowder and these nitroglycerine based explosives is that the oxidiser forms part of the core compound itself.
By the end of World War II, other explosives had been introduced, including TNT (2,4,6-trinitrotoluene) , RDX (Royal Demolition eXplosive, cyclonite, hexogen, 3,5-trinitro-1,3,5-triazine) and HMX (High Melting eXplosive, Octogen, cyclotetramethylene-tetranitramine), but they were all to be outclassed in terms of power by nuclear weapons that were thousands, and then millions of times more powerful.

Atoms and elements

The idea of atoms is not a new one, it is a very old idea, appearing in many ancient cultures such as Greece and India. The word atomos, meaning "uncuttable", was coined by the ancient Greek philosophers Leucippus of MIletus (ca. 480 bc - 420 bc) and his pupil Democritus (ca. 460 bc -370 bc). However, these ancient beliefs were based on metaphysical reasoning rather than empirical evidence, and thus didn't find widespread acceptance in the scientific community. The idea disappeared into obscurity for centuries, but was brought up again by the English scientist John Dalton (1766 - 1844). His theory can be summarised as follows:

Elements are made of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties;
Atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole-number ratios e.g. 1:1, 1:2, 2:3, 1:3:3, and so on, to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.

Some of these have had to be modified as a result of later discoveries, but broadly peaking they are still true. There are just 94 naturally occurring elements, and approximately 24 that have been created artificially. Each element has characteristic chemical properties by which it can be distinguished from all the other elements. In the Rutherford-Bohr model atom diagram you can see that an atom consists of a central nucleus containing neutrons and protons, surrounded by electrons. Most of the volume of an atom is in fact empty space, and that relative to the size of the nucleus, the electrons are at a great distance, they are also much smaller and lighter.

If you look at the above simplified equation for the detonation of gunpowder you will note that to the left of the arrow the number of atoms of each element are exactly equal to the number of those on the right. This is a characteristic of a chemical reaction. The atoms themselves are not changed they are simply re-arranged. The properties of the substances on the left are different to those on the right. During the release of energy that occurs three solids become two solids and two gases. The gases, even at room temperature take up a very much greater volume than the solids on the left, and this is magnified by the fact that they are also at a very much higher temperature.

Particle	Symbol	Charge		Mass
Particle	Symbol	Unit	Coulombs	Number	Relative (amu)	Actual (kg)
Electron	e^-	1-	-1.6022 x 10^-19	0	0.00054858	9.1093837015 × 10⁻³¹
Proton	p⁺	1+	+1.6022 x 10^-19	1	1.007277	1.6726 x 10^-27
Neutron	n	0	0	1	1.008665	1.6495 x 10^-27

The electron was the first type of atomic particle to be discovered by J.J. Thomson (1856-1940) in 1897. The proton was discovered by Ernest Rutherford (1871-1937) in 1917, shortly after he had demonstrated and described the

nucleus. It was not until 1932 that James Chadwick (1891-1974) discovered the third particle, the neutron. The basic properties of the three particles are, for most purposes described in the table above (1 amu = 1 atomic mass unit, one unified atomic mass unit is approximately the mass of one nucleon - either a single proton or neutron, and is numerically equivalent to 1 g/mol ). However when we come to radioactivity and nuclear reactions things change a bit largely because of our old friend Albert Einstein (1879-1955), and his theory of relativity.

An atom of an element is the smallest particle which can exhibit the chemical properties of that element. For example, if single atoms of iron were broken up, the pieces would have the recognisable properties of quite different elements. Atoms are exceedingly small, far smaller than the limits of visibility under a light microscope, nevertheless, most of the matter contained in each atom is concentrated in a central nucleus which is about one ten thousandth of the size of the whole atom. A nucleus always carries one or more positive electrical charges and, in the normal state, it is surrounded by a cloud consisting of an equal number of negatively charged particles called electrons, so that the atom as a whole is electrically neutral. These electrons are commonly imagined as moving in orbits around the nucleus like planets around the sun, although this is a drastic over-simplification.

The chemical properties peculiar to each element are determined by the number of protons, i.e. the number of positive charges in the nucleus of each atom. Consequently, the elements can be numbered consecutively from the lightest element hydrogen (one proton with an electron in orbit around it) up to the largest atoms of the currently known heaviest element oganesson with 118 protons in its nucleus and therefore 118 electrons in orbit.

The elements may be listed in order of the number of protons (their atomic number), in a form known as the Periodic Table of the Elements. Elements in vertical groups have similar chemical properties.

Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.

The number of protons within the atom's nucleus is called its atomic number (Z) and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number (A), and each isotope of a given element has a different mass number.

Radioactive decay

Radioactive decay is a complex matter. Rather than a single thing it is a number of processes. Certain atoms are unstable or radioactive, in fact it is a property of the nuclei of those atoms. In radioactive decay the unstable nucleus emits sub-atomic particles. Such nuclei are said to be parent nuclei, and produce daughter nuclei during decay. The SI unit for measuring radioactive decay is the becquerel (Bq). formerly it was the Curie (Ci) If a quantity of radioactive material produces one decay event per second, it has an activity of one Bq. Since any reasonably-sized sample of radioactive material contains very many atoms, a becquerel is a tiny level of activity. The americium source in an ionisation type smoke detector has an activity of about 37kBq, and that is due to only 0.3g of the isotope.

The most common decay modes are alpha, beta, and gamma decay. The subatomic particle emitted in alpha decay is a positively charges alpha particle or helium nucleus, the subatomic particle emitted in beta decay is a negatively charged electron, and the subatomic particle emitted in gamma decay is a photon or gamma ray.

The neutrons and protons in a nucleus have various interactions. The strong nuclear force is the most powerful force over subatomic distances. The electrostatic force is significant. Also of importance is the weak force. The interplay of these forces is very complex. If the configuration of the particles in a nucleus shifts ever so slightly, the particles can possibly fall into a lower-energy arrangement, resulting in spontaneous nuclear decay. The resulting transformation changes the structure of the nucleus, it is a nuclear reaction, in contrast to chemical reactions, which concern interactions of electrons with nuclei.

Some nuclear reactions do involve external sources of energy, in the form of "collisions" with outside particles. However, these are not considered decay.

The decay process is entirely random, and it is impossible to predict when a particular nucleus will decay. Different radio-nuclides decay at different rates, commonly described in terms of half-lives. Given a sample of a particular radio-nuclide, the half-life is the time it takes for half the radio-nuclides to decay. The most active substances are quickly lost, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from 109 years for nearly stable nuclides, to 10^-6 seconds for highly unstable nuclides.

α-decay (alpha decay)

Alpha decay is a form of radioactive decay in which an atomic nucleus characterized by mass number A and atomic number Z ejects an alpha particle (which is a 4He nucleus) and transforms into a nucleus with mass number that is reduced by 4 and atomic number reduced by 2.

β-decay (beta decay)

Beta decay (sometimes called neutron decay) is a type of radioactive decay in which a β-particle or electron is emitted. A neutron alone in space will only last about ten minutes before ejecting an electron and leaving a proton behind. This is energetically possible, because the mass of the neutron exceeds the sum of the masses of the proton and electron.Beta decay

Heavier nuclei have more neutrons than protons, because the Coulomb repulsion makes it harder to bind protons. Coulomb repulsion is the repulsive force between two positive or two negative charges, as described by Coulomb's law. But neutrons are fermions and the Pauli exclusion principle requires that as we add more and more neutrons, the energy of the added neutrons will be higher and higher. So β-decay is favoured energetically.

In β-decay, the weak nuclear interaction converts a neutron into a proton while emitting an electron and an anti-neutrino. If the proton and neutron are part of an atomic nucleus the decay processes transmute one chemical element into another.

γ-decay (gamma decay)

Emission of γ rays is similar to emission of photons by excited states of atoms. The nucleus can be excited by having just emitted an α- or β-particle, or by colliding with another nucleus, or by being bombarded by neutrons. All these

events can lead to a nucleus which is excited, and electromagnetic radiation is emitted.

Photons with energies of more than 10 keV are frequently called gamma rays, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard x-rays. There is no physical difference between gamma rays and x-rays of the same energy, these are two names for the same electromagnetic radiation. Gamma rays are distinguished from x-rays by their origin. Gamma ray is a term for high-energy electromagnetic radiation produced by nuclear transitions, while x-ray is a term for high-energy electromagnetic radiation produced by energetic electrons. Because it is possible for some electron transitions to be of higher energy than some nuclear transitions, there is an overlap between what we call low energy gamma rays and high energy x-rays. γ rays are more penetrating than either alpha- or beta-radiation, but less ionizing.

Sometimes the radioactive decay of a sample can result in the release of multiple forms of radioactivity. For example, in the radioactive decay of radon-222, both alpha and gamma radiation are emitted, with the latter having an energy of 8.2 x 10⁻¹⁴ J per nucleus decayed. This may not seem like much energy, but if 1 mol of ²²²Rn atoms were to decay, the gamma ray energy would be 4.9 - 10⁷ kJ!

Positron emission (β+ decay)

Positron emission happens with nuclides in which the neutron:proton ratio is low. Positron decay is the conversion of a proton into a neutron with the emission of a positron.The neutron:proton ratio increases, and the daughter nuclide is more stable than the parent nuclide. The positron has the mass of an electron, but a positive charge. Thus, the overall mass of the nuclide doesn't change, but the atomic number is decreased by one, which causes a change in the elemental identity of the daughter isotope.

Electron capture

Electron capture occurs when one of the inner electrons in an atom is captured by the atom's nucleus. When electron capture occurs an inner shell electron combines with a proton and is converted into a neutron.

Types of Radioactive Decay, Simplified
Type of decay	Reason	Conversion	Emission	Nuclear change
Alpha decay	Nucleus is too heavy	Two protons and two neutrons split off from the nucleus	Alpha	Atomic number decreases by two, mass number decreases by four
Beta decay	Two many neutrons / too few protons	An electron is pulled off a neutron which then becomes a proton	Beta	Atomic number increases by one, mass number is unchanged
Gamma decay	Too much energy	Nucleus releases energy as high-energy radiation	Gamma photon	Nucleus becomes more stable, but is otherwise unchanged
Positron emission	Too many protons	A positron, or positive electron, is pulled off a proton, which becomes a neutron	Positron	Atomic number decreases by one, mass number is unchanged
Electron capture	Too many protons	An inner orbit electron is captured by the nucleus, converting a proton to a neutron, with the release of a neutrino	Neutrino	Atomic number decreased by one, mass number is unchanged

Nuclear Fission

The element uranium found in nature is a mixture of isotopes but most of it consists of atoms with 92 protons and 146 neutrons, i.e. a total of 238 mass units in each nucleus: hence, this isotope is referred to as ²³⁸U. Another isotope of

uranium, ²³⁵U, with 92 protons and 143 neutrons found in natural uranium to the extent of about O.7% is used in many nuclear reactors and nuclear weapons when much of the ²³⁸U has been removed, this process is known as enrichment, and the product is so-called enriched uranium. ²³⁸U is not fissile, but ²³⁵U is, fissile material is material capable of sustaining a nuclear fission chain reaction. Weapons grade uranium is enriched to better than 90% ²³⁵U.

The plutonium isotope ²³⁹Pu (94 protons plus 145 neutrons) is an artificial one produced in a nuclear reactor from ²³⁸U and the isotope of uranium 233U is produced similarly from thorium. All three of the above isotopes ²³⁵U, ²³³U, and ²³⁹Pu, can be used as explosive charges in nuclear weapons.

Uranium and plutonium are heavy metals near the end of the consecutive list of elements. At the other end of the list, the lightest element hydrogen has two additional rarer isotopes all three of which have nuclei containing only one proton. One of these is called deuterium because its nucleus contains two units (one proton plus one neutron), and the other tritium because of its three units (one proton plus two neutrons). Both deuterium and tritium are used directly or indirectly as the nuclear explosive charge in a thermonuclear or hydrogen bomb.

The isotopes ²³³U, ²³⁵U, and ²³⁹Pu are radioactive and their atoms disintegrate by expelling alpha or beta particles or gamma rays from the nuclei. But there is another way in which these atoms can break up. When they capture, or are hit by a neutron each nucleus splits up into two not quite equal fragments. At the same time one to three other neutrons are released. The fissile charge even in a small nuclear weapon although it may weigh only a few kilos, contains multiple billions of atoms and these do not all split up in quite the same way, causing fission with several different daughter products. One of the first described fission processes, involved ²³⁵U splitting into ⁹⁰Kr (Krypton) and ¹⁴⁴Ba (Barium). The products of typical fission detonations contain about 200 different radioactive isotopes of about 35 elements.

Chain reactions

If more than two neutrons are released during decay, then a chain reaction may occur, given that other conditions are favourable. This was suggested first by Leo Szilard (1898-1964) some time before fission was demonstrated. In the case of ²³⁵U the number of neutrons released is 2.3 on average.

Critical sizes of fissile charges

When a piece of fissile material is below a certain critical size, a few of the atoms are continually undergoing fission but more neutrons escape from its surface than are produced in fission and an increasing chain of fissions is not built up. If several pieces of fissile material, totaling more than the critical amount, are suddenly brought together inside a strong container, or tamper, a nuclear detonation results. The critical size depends upon a number of factors including the element and which isotope is involved, the purity of the material, its density i.e. whether it is solid metal or in porous, spongy form and also upon the nature of the container and whether it absorbs neutrons or can reflect them back into the fissile charge. Commonly you will hear the term "critical mass", this is a fundamental error in thinking, derived from poorly written articles and school textbooks. It is not a mass that is critical, it is a combination of factors, as can be seen previously. In fact early experiments during the development of the first atomic bombs were dogged by the variability of the materials being used. In particular this was a problem with plutonium, as some samples were of one allotropic form and some of another. In fact there are six allotropes of 239Pu with densities varying from 16.51 - 19.86g/cm3. Purity of the desired isotopes is also a major problem, even small amounts of impurities lead to major differences in the fissile properties of the material, generally because they absorb neutrons into their nuclei without fissioning. If critical mass does have a meaning, it refers to the mass of a perfect sphere of uniform density, in specific conditions. Published information suggests that an unconfined sphere of pure 235U metal of about 170mm diameter and weighing about 52 kilograms would be a critical amount. The increasing mechanical complication of bringing together, rapidly and simultaneously, a number of sub-critical pieces of fissile material sets a practical limit to the power of nuclear fission weapons.

Unlike a chemical reaction nuclear fission results in different atoms being formed. Each fission releases energy, in the case of ²³⁵U this is 202.5 MeV. This may not sound much, but each 1 gram of 235U completely fissioned would release 1 megawatt of of energy.

Nuclear fission weapons

There are two principal types of fission weapons the gun type and the implosion type, the second being considerably more complex. Fundamentally the design objective for a fission weapon is to assemble a super-critical mass of fissionable material as rapidly as possible.

Gun type weapon

In the gun type weapon two pieces of fissionable material, each rather more than half sub-critical are used, one being designated the "bullet" and the other the "target". When they are brought together they form a super-critical quantity. The bullet is fired down a "barrel" by conventional high explosive, when the bomb reaches the appropriate altitude, triggered by electronics which detect height. This results in the combined components forming a super-critical mass. The casing of the weapon is extremely strong, the idea being to contain the force of the explosion until it builds to the desired degree. Various versions of this basic idea have been developed, for example the first British weapon used two hemispheres, thus minimising the quantity of fissile material required to achieve super-criticality. The version shown in the diagram is of what appears to be the simplest form, but requires some 30% more fissile material. The bomb used on Hiroshima was of this type. The gun type weapon can only be used with 235U as 239Pu suffers from a condition known as pre-detonation, in other words it will start a chain reaction prior to becoming fully assembled into the super-critical mass and produce a far lower level of energy than is required. Pre-detonation is caused by the presence of 240Pu.

Implosion type weapon

Because of the short time interval between spontaneous neutron emissions found in plutonium due to the decay by spontaneous fission of the isotope 240Pu, Manhattan Project scientists devised the implosion method of assembly in which high explosives are arranged to form an imploding shock wave which compresses the fissile material to super-criticality. When the high explosive is detonated, an inwardly directed implosion wave is produced. This wave acts upon the heavy metal tamper which in turn compresses the sphere of fissionable material. The decrease in surface to volume ratio of this compressed mass plus its increased density is then such as to make the mass super-critical. In more recent weapons the fissionable material is usually surrounded by a tamper consisting of 238U. The HE is exploded by detonators timed electronically by a fusing system, which may use altitude sensors or other means of control. The design of the explosive charges and their firing is complex and it was a major issue for the Manhattan Project engineers. Specialised components had to be developed to construct the firing circuitry.The bomb which was used for the Trinity test and that which was dropped on Nagasaki were both of the implosion type, as was the first British weapon. The initiator is a small component which acts as a source of neutrons, typically this is composed of beryllium and polonium.

Nuclear fusion and thermonuclear weapons

A temperature of many million degrees centigrade is reached in the detonation of a nuclear fission weapon. At this temperature atoms are stripped of most of their surrounding cloud of electrons and the nuclei move at very high speeds experiencing many collisions with one another. Under these circumstances the nuclei of the rarer hydrogen isotopes deuterium and tritium have enough kinetic energy to overcome the repulsive forces between their single positive electrical charges and they are able to fuse together. The energy released in the fusion of these two nuclei is about one-twelfth of that released in the fission of a single ²³⁵U nucleus, but on an equal weight basis, the fusion energy is about two and a half times as large as the energy of fission of ²³⁵U. In the process of fusion a neutron is released at a very high speed from each pair of reacting nuclei and it has enough energy to split the commoner atoms of ²³⁸U. Thus, if ²³⁸U metal is used as the bomb case in a thermonuclear weapon the quantity of fission products will be increased many Teller-Ulam configurationtimes. This type of weapon is the fission-fusion-fission type or so-called 'dirty' bomb. At one time it was thought that deuterium and tritium as isotopes of the gaseous element hydrogen would have to be liquified at a very low temperature and maintained there for containment in a thermonuclear weapon. This is inconvenient although it has been reported that the first American H-bomb tested in 1952 was of this type. In later weapons the deuterium is combined chemically with the metal lithium in the form of a white powder, forming lithium deuteride (LiH sometimes written as LiD), which may be turned into a ceramic material. Each neutron (l mass unit) released by the triggering fission bomb splits a lithium atom (6 mass units) into the non-radioactive gas helium (4 mass units) and tritium (3 mass units) and the latter fuses with the deuterium atoms present in the compound. There is no limit, other than the convenience of delivery, to the size of a fusion or thermonuclear weapon and it is claimed that lithium deuteride is less costly than fissile materials such as ²³³U, ²³⁵U or ²³⁹Pu. The largest weapon tested by the former USSR was 50MT, the so-called Tsar-Bomba, but it is believed that they developed a 100MT device. Helium gas, the main product of a thermonuclear detonation, is not radioactive (hence the expression 'clean' bomb) but the very high speed neutrons which are also emitted collide with other atoms, e.g. they collide with the nitrogen atoms in the atmosphere and release a very intense and penetrating form of gamma radiation (flash), and they may induce intense radioactivity in some of the ground material if the weapon is burst on the ground or at low altitude but this decays rapidly in a few days, initially.