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I am not a nuclear engineer, physicist, or technician, and am in no way officially connected with the nuclear industry. I do not hold myself up as any sort of expert on the subject; but have developed a keen interest in nuclear technology, and in the many ways that it might extend the survival of civilization. I have researched through books, the Internet, and occasionally by talking to guides, technicians, and even a scientist or two, at the various nuclear sites open to the public. I have gathered these researches together here, as a sort of a layman's guide to nuclear technology. What is most remarkable about this deep mysterious subject, is that it is not all that deep and mysterious, nor is it difficult to grasp the basics. There is no advanced math here, no deep physics, and nothing very abstract or confusing - this section was written so that even I can understand it. The reader who slogs all the way to the bottom of the page will know more about nuclear technology than most of the legislators, regulators, activists, and political hacks who presume to dictate how the industry should be run.
The discovery of nuclear fission, and then
fusion, seemed to hold out the Biblical prospect of both the blessing
and the malediction. Nuclear power was the answer to the world's energy
needs. Compared with coal, and oil, it was cheap, safe, and did not pollute.
These are virtues which it still has today. A pound of uranium has something
like the energy content of 3,000,000 tons of coal. It is actually cheaper
to produce power in a nuclear plant, than in a coal powered plant. When the
nuclear power generating industry first began, supporters predicted
energy that would be too cheap to meter. It is
also, despite the emotional rantings of the ignorant and fearful, quite a bit
safer. Far fewer people have been injured per kilowatt generated by
nuclear power, than is the case with coal powered plants. More people
have been killed in rail crossing accidents with trains transporting
coal to power stations, than have been killed in the entire history of
the nuclear program. Coal is also, like most minerals, slightly
radioactive. More radiation is put into the environment from the smoke
and gases generated by a coal powered plant, than is produced by any
nuclear plant - much more. Unfortunately, nuclear energy production has never
realized anything like it's full potential, in this country. It now
accounts for something like twenty percent of out total energy
The Cast of Characters
The basic element (if you will pardon the pun) of the
modern nuclear industry is uranium. As an element, and a metal, Uranium has
a number of interesting properties. The metal is nearly as hard as steel,
and heavier than lead, being nearly as dense as gold. It tarnishes very easily,
and though it is a lustrous silvery metal, it will quickly turn black on
exposure to air. Powdered uranium may glow in the dark, will sporadically
burst into flames, and can even react with water, in the manner of sodium.
It is pretty strange stuff. One of the more interesting characteristics,
of Uranium, as well as most other unstable elements, is that it is slightly
warm to the touch.
The various isotopes of uranium, are examples of the two types of major nuclear materials. The first type is the fissionable material, which is what most people think of, when they think of nuclear materials. The fissionable natural isotope of uranium is 235. This is the type of material that will split to create energy, and will have a critical mass. The critical mass of a fissionable is the weight of the substance which, when formed into a sphere, would support a chain reaction. There are ways to initiate chain reactions with less material, either through compression, or through the use of outside sources of radiation; but the critical mass of a given fissionable, is a pretty good guide as to how usable it would be as an explosive, or as a source of energy.
Generally, a fissile material will decay at a certain rate, defined by its half life. The half life of a material is the amount of time that it would take for half of a given sample to decay. As an example, with a half life of a bit more than 24,000 years, a one pound sample of plutonium 239, would, after 24, 000 years, only contain a half pound of P239, the rest having transmuted into U235, through alpha decay. This decay is spontaneous, and apparently random, and has nothing to do neutron inspired fission. Once critical mass is attained, things are different. Neutrons are given off as part of the normal decay process, but generally have no effect on the surrounding atoms. Because these neutrons are fast moving, and have no charge, they can pass by, and even pass through, other atoms without affecting them, and simply pass out of the mass of fissionable which produced them. As the mass gets larger, there is more material to pass through, and the likelihood of a strike increases. Eventually, there is a point at which there is so much material, that a strike is virtually certain. This strike will split a nucleus, and cause it to release further neutrons, which will strike other nuclei and cause even more neutrons to be released, to strike other nuclei, and a chain reaction occurs. This considerably speeds up and changes the nature of the transmutation of the mass. Instead of years, centuries, or millennia, we are talking microseconds. We are also talking a change in the materials produced. Rather than giving off alpha particles, as is the case with natural nuclear decay, neutron struck atoms tend to cleave more nearly in half. The critical mass is determined by the density, stability, and neutron cross section of the element in question. Some fissionables, like U238, are so stable, that they have no critical mass.
The most commonly used fissionable materials are uranium 235, and plutonium 239. The element numbers indicate the isotope, by enumerating the atomic weight. The atomic weight of an element is essentially the total number of neutrons and protons. Thus uranium, with an atomic number of 92 (92 protons) with have 143 neutrons in isotope 235, and 146 neutrons for isotope 238. Uranium 235, and plutonium 239 are the most commonly used, because they are unstable enough to have a critical mass. These materials will spontaneously create, and sustain chain reactions, when critical mass is accumulated. Thus a large enough chunk of plutonium will immediately begin to produce significant amounts of heat, as well as radiation. There is something almost frighteningly simple about this. You can take two small pieces of plutonium, and they will be somewhat warm to the touch. Slap them together, and they will immediately grow hot enough to boil water, or char your hands.
Natural uranium does not have much of the 235 isotope. As a matter of fact, the 235 isotope is very nearly at trace element levels, comprising only about 0.7% of a refined sample of naturally occurring uranium. This is pure metallic uranium, refined from ore, and separated out, just as iron or gold might be separated out into their pure metallic forms. Still, for most uses, the pure uranium must further be separated into its various isotopes. The process is generally done by weight, and is known as enrichment. Enriched uranium is higher in U235 than natural uranium. The degree of enrichment depends upon the intended use of the material. In addition to the enriched material, there is a residue, which is virtually pure U238. This is commonly known as depleted uranium, because the valuable, and sought after U235 has been taken out.
The second type of nuclear material is the fertile material. Fertile nuclear material is generally not usable as a source of energy; but may be converted into something that is. A good example of this is uranium 238. This material, though slightly radioactive, can not be made to support a chain reaction, and has no critical mass. When exposed to radiation, as in the walls of a reactor, and struck by neutrons, uranium 238 is converted into uranium 239, which quickly decays into neptunium 239. With a half life of around 56 hours, neptunium 239 quickly decays into plutonium 239. Thus Uranium 238 is the major source of plutonium 239. A major fertile metal is thorium, which is abundant, and easily converted.
There are many other useful radioactive materials; but they are pretty much limited to the role of supporting players for the main characters. Some are good neutron sources, while others are neutron reflectors. A list of all of the major nuclear materials is below, in table form. There are a number of other fissile, and fusile materials; but most of these are either too dangerous, or in too short supply to be considered useful.
The chart above, though a useful guide, is only somewhat accurate. This is due to the changing nature of radioactive elements, and to their changing densities, and varying states. Hot uranium expands, and greatly reduces it's density, over that of cold uranium. These substances are always changing, and thus the nuclear properties of any given amount of any such substance are dynamic, rather than static. As an example, a sample of tritium loses half of it's radioactivity, every twelve years or so, as it decays into helium. In contrast, the radioactivity of a sample of uranium 235 increases greatly, as it decays into thorium 231, which is around 2.6 x 10^11 more radioactive, and as the amounts of uranium 236 increase. So suddenly, after a few years, your sample of mildly radioactive uranium 235, is an extremely radioactive sample of uranium 236, thorium, protactinium, and a few other unhealthy substances. This also happens, though to a lesser extent, with the far more common uranium 238.
Eventually, all of the fissile elements end up as stable elements, at the end of what are generally called their decay series', all having found their lowest levels, as it were. It might seem that there should be a variety of different elements into which the fissiles can transform themselves, and indeed, there are a number of intermediate fissile elements in each series; but in the end all are transmuted into one of only four stable elements. These are known as the four natural decay series. I have included a great set of charts, taken from a physics website, showing the members of these four series, which together include all naturally occurring radioactive elements. These charts show how the decay progresses in each series. Though it is possible to create other radioisotopes, outside of what is show in these charts (strontium 90, cobalt 60, and many others), these are not natural decay products, and are made by bombarding elements with neutrons to take them up, and increase their atomic weight or number, whereas the natural decay series' always travel in a downward direction, toward lower atomic numbers, and lower atomic weights.
These series' end with the stable elements having atomic weights of 206, 207, 208, and 209. These are all isotopes of lead, except for 209, which is bismuth. The reason that there are four, and only four decay series, is due to the nature of nuclear decay. There are essentially two ways that a nucleus can decay. There is alpha decay, and beta decay. The reasons for this are known, to a degree; but are beyond the scope of this page. Because these are the only two natural ways for radioactive elements to decay and transmute, the limit of four distinct series is a given, with the number of series defined by the size of the alpha particle, which itself consists of four nuclear particles.
Of the two types of natural decay, beta decay makes no change in atomic weight; but does change the atomic number. Such changes are known as transmutations, and change one element into another element. Thus, when tritium, an isotope of hydrogen, beta decays, it transmutes into helium-3, a light helium isotope. The atomic weight remains approximately the same; but the atom itself is transmuted from element number one (hydrogen) into element number two (helium). In this case, the change occurs because one of the three neutrons in the tritium atom, decays into a proton. In doing so, this neutron gives off a beta particle, which is essentially a high speed electron. To be even more explicit, one of the down quarks in the neutron beta decays, giving off a beta particle, and an antineutrino. Because a neutron is composed of two down quarks, and one up quark, and a proton is composed of one down quark and two up quarks, this has the effect of changing a neutron into a proton.
Alpha decay, as can be seen in the element chart above, is far more common. Alpha decay also transmutes the element from which it is produced; but unlike beta decay, alpha decay also changes the atomic weight of the element. Alpha decay occurs, when the atoms of an element give off alpha particles. An alpha particle is essentially a helium nucleus, consisting of a pair of neutrons, and a pair of protons.
You might think that other types of decay would be possible; but under normal circumstances, they are not. There is no proton decay, nor do fissile nuclei commonly give off neutron/proton pairs or single neutrons, mostly due to energy thresholds required for particles to escape the nucleus. These can all be made to happen, in accelerators and other such places; but do not generally occur in nature. So every naturally occurring radioactive substance will appear on one of the four charts above, and all will eventually end up as one of the stable elements shown at the bottom of the charts. Thus the four natural decay series' take their forms. In every one of these reactions, the nucleus is attempting to get to the lowest possible energy state.
Fission of U-233: 17.8 kt/kg
1 kt. fission of 0.241 moles of material (1.45x10^23 nuclei)
We are in the midst of
the nuclear era, though this is a fact that we scarcely seem
to notice. The end is nowhere in sight, despite the efforts
of the anti nuclear activists. There is no end in sight for
the simple reason that we are now dependant on nuclear
technology, in a number of different ways. Everyone is
pretty much aware of the use of nuclear weapons, and nuclear
power generation, and is somewhat aware of the use of
radioactive materials in medicine; but there are scores of
other uses. Nuclear materials are used in smoke detectors,
food processing and sterilization, materials testing, and a
number of manufacturing processes.
Still, all of
these events were mere stepping stones --- experiments which
produced no usable results, and had no effect on the larger
world, outside of the laboratory. These are all examples of
science, of exploration and proof. An era does not start
until the engineers get involved, and create devices which
take the discoveries of the scientists out of the
laboratory, and put them into the world of common
experience. This did not happen until the forties. On
For myself, as for most others, the nuclear era began
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