Introduction to Atomic Energy
This series of four lectures, with minor demonstration and exhibit components, is intended to provide basic knowledge of the atomic energy field for the interested layman, in accordance with Dr Seaborg's prescription.
Slides for Lecture 1
Slides for Lecture 2
The energy of radioactive decay is accompanied by a decrease in atomic mass, and the energy thus released is of a much higher order than in chemical changes. (The 5660 tonnes of hydrogen and oxygen would show a reduction in mass of one gramme after combustion, the same amount as would be found in the decay of 45 kilogrammes of radium to radon.) Unfortunately, controlling this release is beyond human power, leading Lord Rutherford to describe atomic energy (in the state of science as it was then) as "moonshine".
Because of electrostatic repulsion, reactions between light nuclei are quite limited in scope, involving only the lightest elements except under truly extreme conditions. Even in the heart of the Sun, the proton-proton reaction proceeeds only slowly, a major factor in fixing the lifetimes of stars. "Catalyzed" fusion, proposed by Bethe and Weiszacker, has a higher activation energy but proceeds much more rapidly.
Very heavy nuclei can actually be caused to break up into lighter nuclei by incident neutrons, with the liberation of significant energy, mostly in the form of the motion of the new nuclei as they repel each other. The release of additional neutrons by this reaction gives rise to a controllable process. On a purely intellectual level, this deserves to be regarded as one of the greatest accomplishments of mankind.
Unlike radioactive decay, the results of fission are not fixed. The nucleus splits into two main fragments, usually two of different mass numbers (occasionally a third, most often a helium nucleus), with the release of one to three free neutrons. Note the logarithmic scale : the abundances are over all fissions, rather than over all products, and therefore total to 200%.
The "chain reaction", made possible by the emission of more than one neutron per fission, becomes "critical" (self-sustaining) when each fission triggers at least one additional fission. The "multiplication factor" in the scenario shown is 1.02 ; note that, over the course of one thousand "generations" of fission (which could occur in a second), this results in an overall multiplication of almost four hundred million.
The criterion for "fuel breeding", the complete replacement of fissile material consumed, is that, after the neutron necessary to cause another fission, at least one neutron is left over to be absorbed in fertile material. The high thermal neutron yield of uranium-233, combined with the high thermal neutron capture cross-section of thorium-232, make this an attractive system. Breeding with uranium-238 requires working with fast neutrons, which is much more difficult.
The number of collisions required to slow down a neutron from fission to thermal energy, and the chance that it will be absorbed along the way, are the primary factors which define the value of a moderator. Ordinary hydrogen has by far the greatest slowing-down power, but its capture cross section is too high for use with natural uranium.
Neutron capture and beta decay define the interrelations of the fission fuels. For purposes of this chart, "long-lived" is in comparison to the time a fuel element typically remains in civilian power reactor (from months to about three years), and can be understood as "ten years minimum". Alpha decays are not shown because the half-lives are all considerably longer than that, extending into the billions of years.
Slides for Lecture 3
It is a common misconception that hyperboloidal cooling towers have something to do with atomic power. They have been in use at least since the 1930s, anywhere that the supply of water for cooling is limited. These belong to a coal-fired power plant, while many atomic plants (such as Comanche Peak) do not use them.
As of the last day of 2012, there were 437 central-station power reactors in the world, accounting for 373 GW of generating capacity, and 2350 billion kilowatt-hours of actual energy over the course of that year. This compares to 1370 billion kWh generated in the world as a whole in 1956, the year before the first American atomic power plants came on line.
"Light" (ordinary) water is a strong moderator, and the fuel elements for light-water reactors must be designed to prevent "burnout", where the power produced per unit surface area exceeds what the coolant can carry away. This is achieved by dilution and subdivision. Low-enriched fuel (in which 238-U serves as the diluent) is packaged as small-diameter pellets of uranium oxide ceramic, sheathed in zirconium metal, and made up into bundles with gaps between the pins too small for the water to have much moderating effect. Enriched or fully-enriched fuel is alloyed with zirconium, or dispersed as oxide powder into zirconium metal, and clad with "clean" zirconium in the form of thin plates. Zirconium is used because of its small neutron absorption cross-section ; oxide fuel is used because it suffers less than metal fuel from physical damage due to radiation and fission products, allowing greater burnups.
The power plant at Shippingport, Pennsylvania, was originally developed as the "Large Ship Reactor", and was operated by the US Navy for Duquesne Power and Light, from 1957 onward. It used an unusual core with fully-enriched "seed" elements (red) and natural-uranium "blanket" elements (gray), to reduce the cost of reprocessing. Criticality was impossible without both in place, and a substantial fraction of the power was produced in the blanket, by fission of both 235-U and converted plutonium. In blue is the "thermal shield", a non-structural steel element used to absorb gamma radiation which would otherwise heat the reactor vessel.
The pressurized-water reactor is the most common power reactor type in the world, with 273 in service (as of the last day of 2012). Generally speaking, it uses low-enriched uranium fuel, and is not optimized for high conversion ratio, although the Shippingport plant operated successfully as a uranium-thorium breeder in the period 1977-1982 (with a core of very different design than that shown above).
The boiling-water reactor, developed principally by General Electric, is a modification of the PWR design, intended to be more suitable for a stationary power plant on land. The "void fraction" of steam in the core makes it strongly self-regulating, but the short-lived radioactivity of nitrogen-16 (produced by neutron bombardment of oxygen-16) makes for maintenance headaches. With 84 in service, it is the second-most common type.
The Canadian Deuterium-Uranium reactor (™ AECL) typically uses natural uranium as fuel, with a burnup of 7500 MWd/t, although slightly-enriched uranium obtained from spent LWR fuel is an possibility which has been given much attention, especially in South Korea where a variety of plants is in use. Its pressure-tube design allows refuelling at power, and dumping the moderator while continuing coolant flow. 48 units are in service.
The Advanced Gas-Cooled Reactor is a British design, derived from the earlier MAGNOX type (which used natural uranium metal clad in an alloy termed "magnesium non-oxidizing"). AGR uses graphite as moderator and carbon dioxide as coolant, and produces higher-quality steam than LWRs can achieve, in a boiler enclosed within the concrete vault which holds the reactor. Counting the one MAGNOX reactor still operating at Wylfa (which is licensed through the end of 2015), 15 are in service, all in Britain ; low-enriched uranium oxide is used, clad in stainless steel, and typical burnup is 30 000 MWd/t (5000 for MAGNOX).
Because carbon dioxide attacks graphite chemically at high temperatures, designs for high-temperature gas-cooled reactors have generally employed helium, often at quite high pressures. This entails serious engineering challenges, and the Ft St Vrain plant (pictured) further suffered from leaks of water from its steam system into the helium loop. None is currently in service, although there are numerous proposals. American nuclear engineer Rod Adams has advocated using nitrogen gas at near-atmospheric pressure to drive a gas turbine of largely standard design, reducing the scope of the problem. Uranium-thorium fuel elements have generally been employed, with high conversion ratios to obtain high burn-up of the fuel elements, which generally use either oxide or carbide mixed with or encased in graphite ; the pebble-bed reactor is a design which circulates the fuel to obtain better performance.
The "high-power channel reactor", a Soviet design using graphite moderator and boiling light water coolant, has become infamous for its starring role in the Chernobyl disaster. In addition to reactivity characteristics which make it difficult to control, it is extremely complex, and also very large, contributing to the decision not to provide RBMK plants with proper containment vessels. Low-enriched oxide fuel is used, clad in zirconium, with a typical burnup of 15 000 MWd/t. 15 are in service, all in Russia.
Liquid metal coolants (generally either sodium or a lead-bismuth alloy) are very attractive for achieving high power densities, especially for fast-neutron reactors, which are required for plutonium breeding. Unfortunately, they have proven hard to handle. Two are now in operation in the world, and a variety of projects exists ; effectively all have been prototypes, so it is hard to give any definitive description. The "LAMPRE" at Los Alamos was an example of a fluid-fuel reactor in which the fuel did not circulate, being a plutonium alloy held in "thimbles", around which the sodium coolant circulated.
A variety of electromagnetic machines, such as "stellarators", "tokamaks", and "whatsitrons", have been used to attempt to produce nuclear fusion in low-density ionized gas (plasma). Great sums have been spent, and much has been learned about plasma physics, but little progress toward practical power generation has been made.
Another approach, "inertial confinement", generally involves blasting targets containing fusion fuel with lasers. The National Ignition Facility was built by the United States partly to allow testing thermonuclear reaction dynamics without setting off atomic bombs, but has achieved little for either power or weapons development.
Unlike the billion-dollar machines, "fusors" (originally designed by television pioneer Philo Farnsworth), which are often built by hobbyists, reliably achieve fusion, although at densities far too low to provide power. Development of this type of machine, using the boron-proton fission reaction, holds out some promise of plentiful power at exceedingly low prices, owing to the elimination of the energy-conversion equipment which dominates a typical power plant, but it is as yet unknown whether this can be achieved.
The increasing use of energy, since the invention of the steam engine circa 1700, has brought many benefits to mankind ; one would even be justified in saying it has been the greatest force for the liberation of humanity. But most of that energy comes from limited and dirty sources. The great value of nuclear fission is that its use has much less in the way of evil consequences than is true for most other sources capable of supplying energy on the scale needed for industrial civilization, its fuel supply is immense, and it is already in use.
Slides for Lecture 4
Randall Munroe, creator of the on-line cartoon feature XKCD, has compiled this useful reference chart of comparative radiation doses and exposures. This is the part dealing with low values.
There remains considerable debate about what factors the dose (in Gray, which is J/kg) various types of incident radiation should be modified by to obtain the biological equivalent dose in Sievert. This figure represents American practice. 0.025 eV is the nominal energy of a thermal neutron ; for purposes of this chart, X-rays and gammas are assumed to start at 100 eV, "fast protons" (for which the weighting factor is 10) at 1 keV, and heavy ions and alphas at 10 keV. In practice, alphas are rarely encountered much below 1 MeV, because of their great mass. The International Commission on Radiation Protection (2007 standard) weights protons generally at 2, down from 5 in 1990, and neutrons according to a function not plotted here.
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A variety of claims have been made, especially since the ill-considered announcement of Pons & Fleischmann in 1989, about the release of energy from deuterium fusion or other reactions, under conditions more closely approximating Earthly sea-level than the heart of a star, and without gamma or neutron radiation. Most of these have involved extraordinary claims about new solid-state physics, have gone directly against accepted physics, or have exhibited deceptive intent. The "E-Cat" device of Andrea Rossi is one of the last-named. A series of "experiments" and "demonstrations" have been presented with different, but all equally implausible results. After giving up on the claim that he had transmuted nickel to copper, he is now claiming to promote the lower isotopes of nickel, apparently with neutrons somehow stolen from lithium ; but aside from the unlikelihood of all these reactions going without any gamma emission, the distribution of isotopes in the supposed fuel residue is inconsistent with such a process.
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