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Appendix 1 Technical Note Appendix 1 Technical Note It is hoped that this technical note, written by a non-scientist, will be useful for non­ scientist readers, and that any scientist readers will find it not inaccurate (though much simplified). A-bombs and H-bombs Fission and fusion 1. An A-bomb - an atomic or fission bomb - derives its immense explosive energy from the fission or splitting of atoms of a very heavy element, uranium or plutonium. A hydrogen or H-bomb (or fusion bomb) derives a large part of its explosive energy - but not all - from the fusion of atoms of hydrogen, the lightest element. Since H-bombs require A-bombs in order to function, this note begins with fission and A-bombs. Sub-atomic particles - protons, neutrons and electrons 2. Atoms - like unimaginably small solar systems about one hundred-millionth of a centimetre in diameter - are mostly empty space. Each has at its centre a nucleus - the analogue of our sun - made up of sub-atomic particles called nucleons. The most important are protons (with a positive electrical charge) and neutrons (electrically uncharged); thus the nucleus as a whole carries a positive electrical charge. 3. The number of protons in an atom varies from one in the hydrogen atom to 92 in uranium, the heaviest element in nature. There are even more protons in the so­ called transuranic elements (including plutonium) which do not exist in nature. 4. In the nuclei of the lighter elements the number of protons and neutrons is generally about equal. But in nuclei of the heavy elements the proportion of neutrons is greater; for example, a uranium nucleus has 92 protons but more than 140 neutrons. It is this neutron surplus that makes nuclear fission possible (see paragraph 10 below). 5. The number of protons in the nucleus determines the chemical properties and iden­ tity of an element. So, for example, an atom with 6 protons - whatever the number of neutrons - must be carbon; an atom with 26 protons must be iron; one with 92 must be uranium. This proton number is called the atomic number, denoted by the symbol Z. 6. The total number of nucleons - protons plus neutrons - is the atomic mass number (symbol A). So if N denotes the number of neutrons, then Z plus N = A. 7. An element has only one atomic number Z and a single chemical identity; if Z changes it becomes a different element. But the atomic mass (A) may vary because the number of neutrons may vary. Thus, an element may contain atoms of varying atomic mass; 99.3% of natural uranium is uranium-238 and 0.7% is uranium-235. 8. These variants of different atomic mass are called isotopes, and they may be stable or unstable. The latter are called 'radioisotopes', because they are radioactive. 9. Outside the nucleus, sub-atomic particles called electrons move round it like planets orbiting round the sun. Electrons are, 1850 times lighter than protons. They carry a 229 230 Appendix 1 negative electrical charge, equal to the positive charge on the nucleus, so that gener­ ally the atom is electrically neutral. But it can, in various ways, acquire a net charge, negative or positive, and it is then called an ion. Nuclear fission 10. The neutron is highly effective in inducing nuclear changes. Being uncharged and rel­ atively very heavy, it is extremely penetrating, and it can move freely, unchecked by forces of electrostatic attraction or repulsion, until it collides with an atomic nucleus. 11. If certain heavy unstable elements, of which the most important are uranium (but see paragraph 16 below) and the man-made element plutonium, are bombarded by neutrons, some atoms may split into roughly equal fragments, creating new atoms of lighter elements. Because lighter nuclei contain lower proportions of neutrons, some neutrons then become surplus and are ejected from the nucleus at a speed of 10,000 miles per second. The number of surplus neutrons per fission in uranium- 235 averages 2.5 and these free neutrons may go on to collide with other uranium nuclei, causing further fissions, and creating a chain reaction. 12. At each fission, a large amount of energy is released, mainly in the form of kinetic energy of the neutrons and the fission fragments. Heat is generated as they are slowed down by interaction with the surrounding material. The fission products released (mostly radioisotopes from the middle of the periodic table) include stron­ tium-90, caesium-137 and iodine-131, which are important constituents of fall-out. 13. If every fission results in one new fission - if the multiplication rate (k) is 1 - then criticality is achieved and a chain reaction will be maintained at a steady rate. If k is less than 1, the reaction will die out. To keep it going, each free neutron must liber­ ate enough new neutrons to replace itself and to compensate for unproductive neu­ trons (see paragraph 16 below). 14. If k is more than 1 and each fission yields enough neutrons to cause more than one new fission, then the neutron population and the number of fissions will increase exponentially in a 'divergent chain reaction'. The rate of growth will depend upon the excess k; if each fission leads to 1.5 new fissions on average, the multiplication factor, k, is 1.5 and the excess k is 0.5. 15. Since atoms are mostly space and nuclei are so small, looking for a nucleus in an atom is like looking for a needle in an empty house. It is quite astonishing that a free neutron ever does hit a nucleus. Some nuclei present better targets than others, and the probability of a nucleus being effectively hit is called the nuclear cross­ section. This differs from element to element, and from isotope to isotope, and each nucleus has several different cross-sections (for different particle velocities and for different types of encounter - fission, absorption, elastic collision, and non-fission neutron capture: see paragraph 17 below). 16. The chance that a free neutron may be unproductive is therefore high. It may miss the target altogether. It may bounce off it, be absorbed, or be lost to the system by 'neutron escape' - a surface effect dependent on size and shape, being greater from a flat plane than a sphere and from a small sphere than a large one. Density of the fissile material is an important factor; when compressed, less material is required for criticality. The amount in which fission can produce enough free neutrons to main­ tain the neutron population is called the critical mass. Below this critical mass no chain reaction is possible. 17. In elastic collision the neutron's initial speed of 10,000 miles per second is slowed down to, say, one mile per second. The slow-moving, so-called thermal, neutrons Technical Note 231 are most likely to cause fissions in uranium-235 but cannot fission U-238. In natural uranium (or uranium with a high U-238 content) it is only the U-235 constituent that makes a chain reaction possible. However, U-238 atoms may absorb free neu­ trons by non-fission neutron capture to form a new element, plutonium (Pu). The isotope Pu-239, like U-235, is a fissile material. U-238 is not fissile itself but is called fertile because fissile material can be produced from it. A-bombs: atomic or fission bombs 18. Both nuclear reactors and atomic bombs use fission reactions in uranium and/or plutonium. In reactors, controlled chain reactions are needed to produce a steady energy release, and they can operate on natural uranium or uranium slightly enriched in U-235. But a fast, uncontrolled, chain reaction in pure or almost pure fissile material can be used as a super-explosive. 19. Assembling U-235 or Pu-239 in a more than critical mass can initiate a divergent chain reaction which releases tremendous amounts of energy in a period of time measured in fractions of micro-seconds. The super-critical mass can be created either by bringing two sub-critical pieces of fissile material together very rapidly in a gun-like device, or by implosion. The implosion method uses high explosive to compress a sub-critical mass of fissile material to an extremely high density at which it becomes super-critical. All this must be done with very great rapidity. Timing is crucial for, as we saw, the neutron population, and the number of fissions, increase exponentially in a divergent chain reaction so that most of the energy release occurs in the last few generations of fission. Therefore, the total yield will be disappointingly small if the system is blown apart by a premature explosion before the designed fission process is complete. There are various technical devices - such as neutron reflectors and heavy casing - which are used to delay disassembly and maximise the neutron population growth. 20. Atomic (or fission) bombs have vastly greater explosive power than chemical bombs. It would take 22,000 tons of conventional high explosive to produce an explOSion equal to the Nagasaki bomb 'Fat Man', which contained only a few kilograms of Pu-239 and only consumed about 1.25 kg of it. However, there is a limit to the explosive power obtainable from a fission bomb, because the critical mass imposes a limit on the amount of fissile material that can be used. But there is no theoretical limit to the power of fusion bombs, which can have multi-megaton yields. H-bombs: hydrogen, thermonuclear or fusion bombs 21. H-bombs work not just by fission in the heaviest elements but by fusing nuclei of the lightest element, hydrogen.
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