CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Read online

  Ideally, before being stored, a fuel rod needs to be reprocessed. It still contains some useful fuel, which can be reused. Also, some radioactive isotopes produced by fission can be separated and used as radiation sources in medicine or research. Others are pure waste and need to be stored out of contact with the natural environment.

  Natural uranium was used in the earliest reactors—but because it is used up rapidly, enriched uranium is preferred, in which the fraction of is increased by an enrichment process. The chemistry of different isotopes is practically the same, so non-chemical separation must be used, with gaseous compounds such as (uranium hexafluoride). In such a gas, molecules with are about % lighter than those with , and therefore, at a given temperature they move faster and diffuse more rapidly through porous partitions. Alternatively, a specially designed centrifuge, with a rapidly spinning shaft, may spin the gas and cause heavier molecules to be concentrated in the outer layers.

  In either case, because the separated isotopes are so close in mass, the difference in concentration is very small. Therefore, uranium separators must be connected in a cascade of many units feeding each other, with the enriched fraction advancing to the next level and the depleted fraction recycled to an earlier one. (Completely depleted uranium is sometimes used for armor-piercing ammunition because it is very dense and at bullet-speeds packs a lot of kinetic energy.)

  Usually most of reactor fuel still consists of the more abundant isotope . Neutron absorption makes this isotope unstable and after some nuclear changes it turns into plutonium , an artificial element with protons. Plutonium is also a suitable nuclear fuel, and part of the energy released in a nuclear reactor comes from the fission of plutonium produced there.

  Reprocessing nuclear fuel is a difficult task because spent fuel is too radioactive for humans to handle directly. All devices involved in reprocessing—including those that pull out used fuel rods and transport them—are operated by remote control, and when discarded many must be stored safely (like the spent fuel) for long periods. One reason partially spent fuel must be removed from reactors and reprocessed is that some fission products absorb neutrons and thus reduce efficiency ("poison the reactor").

  Currently, the United States has stopped reprocessing spent fuel fresh from power stations, and allows it to cool down in pools located near reactors, but reprocessing is about to be resumed. France which gets most of its electric energy from fission, Russia and other countries, do maintain successful reprocessing centers.

  Tidbits

  Nuclear reactors were recognized early as ideal power sources for large submarines, because they needed no air and required only infrequent refueling.

  Fission reactors were also designed for powering spacecraft. The United States launched SNAP 10-A in 1965, but it was shut down after 43 days due to malfunctions. Soviet Russia launched many reactors (#13), which were later detached and boosted to a higher orbit, with a lifetime of centuries. That program ended when the reactor on Cosmos , powering an ocean-surveillance radar, failed to detach. The satellite with its reactor crashed on 24 January 1978 into a frozen lake in Canada, creating strong protests and ending the use of reactors in space.

  In addition, the radioactive heat produced by plutonium is used in radioisotope thermal generators (RTGs) to power space probes to the outer parts of the solar system, too far from the Sun for solar cells to generate sufficient power. RTGs gradually lose power after 20-30 years, and, of course, they never return to the Earth's neighborhood.

  Nazi Germany also tried to develop nuclear energy during World War II, on a much more limited scale than the allied powers. However, graphite was regarded as unsuitable, as samples tested for moderator were not pure enough and absorbed too many neutrons. Heavy water was chosen instead, a by-product of hydroelectric power stations in Norway. However, the Norwegian underground effectively sabotaged its production there.

  Review Questions

  For this problem, first solve problem (#5) in the preceding section. Assuming a nucleus releases in a fission event (counting some secondary processes; the total averages ), how many tons of TNT are needed to obtain the energy yielded by complete fission of gram ?

  Compile a glossary, defining briefly in alphabetical order in your own words: Barn (unit), cascade for isotope enrichment, chain reaction (nuclear), critical mass,

  cross section (for nuclear interaction), delayed neutrons, enrichment (of uranium),

  fission (nuclear), fission fragments, fuel rods, graphite, heavy water,

  isotope separation by centrifuges, isotope separation by porous partitions, photon,

  plutonium, "poisoning" of a nuclear reactor, prompt neutrons, reprocessing of nuclear fuel, thermal neutron.

  Review Answers

  For this problem, first solve problem (#5) in the preceding section. Assuming a nucleus releases in a fission event (counting some secondary processes, see #10; the total averages ), how many tons of TNT are needed to obtain the energy yielded by complete fission of gram ?

  [If is Avogadro's number, one gram of contains atoms. By (b) of the preceding problem (#5), each atom yields (). The total energy released is:

  By (c) of the preceding problem (#5), gram of TNT holds kilocalories or .

  So the energy released is the same as:

  ]

  Compile a glossary, defining briefly in alphabetical order in your own words: barn (unit)

  Area of , unit of nuclear cross section.

  cascade for isotope enrichment

  Teaming of many isotope separators for enrichment.

  chain reaction (nuclear)

  A fission reaction in which each fission produces at least one additional fission.

  critical mass

  A mass of nuclear fuel sufficient for a chain reaction.

  cross section (for nuclear interaction)

  Equivalent target area in a nucleus for an incoming particle to produce a reaction.

  delayed neutrons

  Neutrons emitted from fission with second delay.

  enrichment (of uranium)

  Technology raising the fraction of the isotope.

  fission (nuclear)

  The splitting of an atomic nucleus into two large fragments.

  fission fragments

  Nuclei of lighter elements, produced by nuclear fission.

  fuel rods

  Rods containing fuel, inserted into a nuclear reactor.

  graphite

  A form of carbon, used as a moderator in nuclear fission.

  heavy water

  Water in which deuterium replaces hydrogen.

  isotope separation by centrifuges

  Separation of an isotope by gas centrifuge.

  isotope separation by porous partitions

  Separation of an isotope by gas flow through porous partitions.

  photon

  A packet of energy in which an electromagnetic wave is absorbed.

  plutonium

  An artificial element with 94 protons, common nuclear fuel.

  "poisoning" of a nuclear reactor

  The accumulation of neutron-absorbing fission fragments, reducing or stopping fission in a reactor.

  prompt neutrons

  Neutrons emitted promptly from nuclear fission, about 98%.

  reprocessing of nuclear fuel

  Chemical separation of fission product from unburned nuclear fuel and artificial isotopes.

  thermal neutron

  A neutron slowed down by a moderator to thermal energies.

  Controlling the Nuclear Reaction

  It takes elaborate technology and design to get a nuclear chain reaction going. At the same time, the rate of fission cannot get too high. If more than neutron per fission initiates another fission event, the temperature will gradually rise. The energy release is never fast enough for the reactor to explode like a bomb (one advantage of using thermal neutrons), but if the reaction grows out of control, it may quickly destroy the reactor.

  Figure 3.5


  Nuclear Power Station

  Control is maintained by control rods of a material such as the metal cadmium, which has a high absorption cross section for neutrons. The rods are automatically pushed deeper into the reactor to reduce the rate of fission, or pulled out to maintain or increase it.

  Delayed neutrons allow for the control. About % of the neutrons released in fission are prompt neutrons, released very quickly, faster than the reaction time of automatic control machinery. However, % are delayed neutrons, which provide a very narrow margin for reactivity control. Reactors need to stay on the % margin between a fizzle and runaway fission. It is a very small margin, and because of its narrowness, any power reactor has multiple independent safety devices

  In case of an emergency, an emergency shutdown (a "scram") automatically pushes or drops the rods in all the way, as well as extra rods for emergency use, usually withdrawn. The chain reaction then stops immediately, but not the radioactive decay of fission fragments. The energy these release is much less than that of the fission process, but in the hours after shutdown enough heat is still produced to melt or damage parts of the reactor ("nuclear meltdown") so the flow of cooling water must be maintained.

  On 28 March 1979 the power reactor at Three Mile Island in Pennsylvania encountered a problem and shut down automatically, but because operators misinterpreted the behavior of the reactor and shut down safety controls that provide cooling in an emergency, it suffered a partial meltdown. In the United States and in most countries, reactors are enclosed in a thick concrete containment building so that even if meltdown occurred and contaminated fission products escaped the reactor (not the case at Three Mile Island), they are kept from spreading.

  Operator error was also the cause of a reactor accident at Chernobyl on 25 April 1986. One of the reactors in a power station supplying Kiev, the capital of the Ukraine, went "prompt critical", with its chain reaction sustained by the uncontrollable prompt neutrons alone. It had a graphite core, and the sudden heat release blew off the top of its enclosure. The core then caught fire, generating a smoke plume laced with radioactive fission products, contaminating a wide area around the station, which was evacuated (and remains so), and also spreading radioactive contamination over parts of Europe.

  Breeder Reactors

  Chain reactions are possible because a fission releases more than one new neutron. The fact that the number is typically makes possible a breeder reactor, in which each fission not only provides a neutron to continue the chain, but also an extra neutron to be captured by ordinary , turning it into plutonium to replace the used-up fuel. Such a reactor could, in principle, use almost all its uranium as fuel. Thorium could similarly be used to "breed" , another possible nuclear fuel; India in particular is interested in such a process, as it has large thorium deposits.

  The first commercial power reactor, a relatively small one, started operating in 1957 near Shippingport, outside Pittsburgh, Pennsylvania. It originally used a conventional fuel cycle based on and slowed-down ("thermal") neutrons. In 1977 it was however restructured to successfully "breed" thorium into , http://www.phy6.org/stargaze/Sthorium.htm. Power generation ended in 1982, after a run of 25 years, and the reactor was successfully decomissioned and buried in a distant site in Washington State

  Breeder reactors based on uranium are difficult to design and maintain, because the conversion of to plutonium is more efficient with fast neutrons (also used in nuclear bombs). They cannot be cooled by water (which slows down neutrons) but operate at high temperatures and are cooled by a metal above its melting point, e.g. liquid sodium. Some such "fast breeders" were built and ran successfully, but so far very few have been used for power generation.

  Tidbits

  The uranium mines of Gabon, Africa, have been supplying the French power system with nuclear fuel. In 1972, it was discovered that some uranium deposits from Oklo, Gabon, were slightly depleted in and contained an unusual variety of isotopes that might have come from nuclear fission. It is believed that about billion years ago, when the concentration of was higher (its half-life is about billion years), a natural fission process was sustained in some of the deposits for a long time (#14). It was caused by water leaking into the deposit and forming a natural moderator. The process was probably cyclical—heat generated by fission would drive out the water and stop the reaction until fresh water entered again.

  Review Questions

  Why is the nuclear power industry interested in elements such as deuterium , carbon , cadmium, thorium , uranium , and , and plutonium ?

  Compile a glossary, defining briefly in alphabetical order in your own words: Breeder reactor, cadmium, Chernobyl accident, containment building, control rods, fast neutrons, meltdown, Oklo phenomenon, prompt critical nuclear reactor, thorium cycle, Three Mile Island accident.

  Review Answers

  Why is the nuclear power industry interested in elements such as deuterium , carbon , cadmium, thorium , uranium , and , plutonium ? [Deuterium and carbon are preferred moderators in nuclear reactors.

  Deuterium and the related nucleus tritium are also candidates for controlled fusion.

  Thorium can absorb a neutron from uranium fission and turn into , a usable nuclear fuel.

  is a nuclear fuel found in nature as % of uranium. Natural or enriched, it can fuel nuclear reactors.

  Uranium enriched in is also used in nuclear bombs.

  is the most common isotope of uranium in nature.

  is an artificial isotope of element , produced (in steps) by neutron absorption in .]

  Compile a glossary, defining briefly in alphabetical order in your own words: breeder reactor

  A nuclear reactor producing new fuel by neutron capture.

  cadmium

  A metal used in reactor control, since it avidly consumes neutrons.

  Chernobyl accident

  The destruction in 1986 of a nuclear power reactor in Chernobyl, Ukraine.

  containment building

  A building with thick walls enclosing a nuclear reactor, confining any waste released in an accidental meltdown.

  control rods

  Rods loaded with cadmium thrust into a nuclear reactor, to control the rate of fission.

  fast neutrons

  Unmoderated neutrons from nuclear fission, useful in converting into , and also in nuclear bombs.

  meltdown

  Destruction of the core of a reactor by uncontrolled heat release.

  Oklo phenomenon

  Natural fission in uranium deposits, which occurred in Oklo, Gabon, about billion years ago.

  prompt critical nuclear reactor

  A nuclear reactor losing control, by maintaining a chain reaction with prompt neutrons alone.

  thorium cycle

  Nuclear power cycle using produced from Thorium.

  Three Mile Island accident

  A partial meltdown in 1979 of a nuclear power reactor at Three Mile Island, near Harrisburg, Pennsylvania.

  Final Note

  The United States had a project to release fission energy aimed at developing a nuclear bomb (also called "atomic bomb", a name used in fiction by H. G. Wells and Harold Nicholson). After 1942, it was led by General Leslie Groves of the U.S. Corps of Engineers, who previously supervised the construction of the Pentagon building in Washington. Groves sent (#15) Major John H. Dudley to find an isolated desert site for the base of the project (Los Alamos, New Mexico, was chosen) and to hide its purpose (he referred to it as the "Manhattan Engineer District." Gradually, this became known as the Manhattan Project.

  Figure 3.6

  This painting by Gary Sheehan illustrates the world's first self-sustaining nuclear chain reaction that took place on a squash court beneath Stagg Field on the University of Chicago campus.

  The first controlled nuclear reaction was achieved on December 1942. The reactor was a near-spherical "pile" of pure graphite (carbon) bricks, in which cans of uranium oxide were embedded at fixed intervals, and holes were also left for contr
ol rods (one of which is being manipulated by the man standing in the center of the image). It was located in a closed space under stadium seating (torn down since then) at the University of Chicago, and the project was led by the Italian physicist (and Nobel laureate) Enrico Fermi. After a successful chain reaction was achieved (kept at a low level, since no cooling was provided), Arthur Compton, one of the leaders of the project, reported by telephone to James Conant in Washington, chairman of the National Defense Research Committee. The project was secret, so he had to improvise. He said (from #15, abbreviated):

  "You'll be interested to know that the Italian navigator has just landed in the new world…."

  Conant replied: "Were the natives friendly?"

  Compton: "Everyone landed safe and happy."

  More than 60 years have passed since then and nuclear energy has had an enormous impact. It now supplies most of the electricity in France, and great amounts in the United States, Germany, the United Kingdom, Spain, Russia, and other countries. It can light and heat our homes—but is also capable of frightening destruction, and nuclear waste needs to be held safe for thousands of years. Handle with care.

  Note: This material was in part taken from the Web collection "From Stargazers to Starships," listed at #1 and #12 below. Additional information may be found there.

  References / Further Reading

  Overview of discoveries related to atoms and nuclei, http://www.phy6.org/stargaze/Ls7adisc.htm.

  Ions in water solutions, http://www.phy6.org/Education/whposion.html.

  Electrons "boiled off" a hot wire in vacuum, http://www.phy6.org/Education/welect.html.