Transuranics

Type of physical science: Chemistry

Field of study: Chemistry of the elements

The transuranic elements are those elements that are more massive than uranium. They are synthesized in the laboratory and are rarely found in nature. These elements undergo radioactive decay, and many have been used in nuclear energy and weapons.

Overview

The advent of particle accelerators has made it possible to synthesize elements with atomic numbers greater than 92. These elements, called the transuranium or transuranic elements, are all radioactive and were first made in the laboratory. Between 1940, when neptunium (atomic number 93) was first isolated, and 2010, twenty-one additional transuranium elements were discovered, and another four were reported discovered but had not yet been confirmed by the International Union of Pure and Applied Chemistry (IUPAC).

Transuranium elements are very rarely, if ever, found in nature. The effect of these elements on civilization has been as profound as the atomic bomb. Plutonium, first isolated in 1940, was later used in the atomic bomb dropped on Nagasaki.

In 1934, Enrico Fermi and others found that neutron irradiation of uranium produces a considerable number of radioactive substances. After bombarding uranium with neutrons, Fermi and his colleagues believed they had created elements 93 and 94, which they called ausenium and hesperium. However, their investigations had led to the discovery of the fission process rather than transuranium elements. Instead of creating heavier elements, they had split the uranium atoms to produce lighter elements. Subsequent work showed that practically all the radioactivities previously ascribed to transuranium elements were actually the result of fission products.

Several experimenters measured the energies of two main fission fragments by observing the distances they traveled from each other as a result of their mutual recoil as the nucleus exploded. Edwin McMillan of the University of California noted another radioactive product of the reaction, with a half-life of 2.3 days, that did not escape from the thin layer of fissioning uranium. In the spring of 1940, it was deduced by chemical means that this product was an isotope of an element with the atomic number 93, arising from the emission of an electron from uranium 239. This new element was named neptunium, as Neptune is the next planet beyond Uranus, for which uranium had been named. Neptunium was found to be very much like uranium in chemical properties.

Neptunium is unstable and will eject another electron to become plutonium, atomic number 94, as was discovered in 1941. Plutonium would later be used for the second atomic bomb. Neptunium was the second synthetic element isolated, after plutonium. Early studies of neptunium were limited to experiments using traces of the short-lived isotope neptunium 239. Neptunium 237 was discovered in 1942 and found to be sufficiently long-lived to make work with weighable amounts possible. This isotope is the decay product of uranium 237, which has a half-life of seven days and emits electrons. It can also be created from uranium 238; the first weighable amounts were produced by bombarding large amounts of uranium 238 with high-speed neutrons from a cyclotron. Neptunium 237 is an alpha emitter and has a half-life of two million years. It chemically resembles promethium, which is directly above it on the periodic table.

Plutonium, named for the planet Pluto, was first synthesized on December 1, 1940, by bombarding uranium oxide with sixteen-million-electronvolt deuterons from a cyclotron. Alpha radioactivity was found to grow into the chemically separated element 93 (neptunium) fraction during the following weeks, and this alpha radioactivity was chemically separated from neighboring elements, especially the elements 90 to 93 inclusive. These experiments, which gave solid proof of the identification of element 94, showed that this element had at least two oxidation states, distinguishable by their precipitation chemistry and by the fact that it required stronger oxidizing agents to oxidize element 94 to the upper state than was the case for element 93. The first successful oxidation of element 94 took place on February 23 and 24, 1941. The public announcement of the discovery of plutonium was withheld until after World War II due to the use of plutonium in the nuclear-weapons project. Much of the work was done in secret.

In its highest oxidation state, plutonium is similar to uranium in its (VI) oxidation state, and in a lower oxidation state it is similar to thorium (IV) and uranium (IV) oxidation states. Normal, stable plutonium in its (IV) oxidation state as an ion would form insoluble compounds or stable complex ions analogous to those of similar ions. Oxidation-reduction cycles, in which the element is said to be oxidized if it loses one or more electrons and reduced if it gains one or more electrons, have been applied to the separation process for plutonium and other transuranium elements. In the case of plutonium, a substance was used that carried plutonium in one of its oxidation states but not in another. By this principle, a carrier could be used to carry plutonium in one oxidation state and thus separate it from uranium and its fission products. Then the carrier and the plutonium could be dissolved, the oxidation state of plutonium changed, and the carrier reprecipitated, leaving plutonium in solution. The oxidation states of plutonium could again be changed, and the cycles repeated. With this type of procedure, only a contaminating element having chemistry nearly identical to that of plutonium would fail to separate if a large number of oxidation-reduction reaction cycles were employed.

The successful operation of the reactor and plutonium-extraction plant at Oak Ridge, Tennessee, led to the availability of the first milligram and then the first gram of plutonium in 1944. The availability of milligram amounts of plutonium led to the discovery of the plutonium (III) oxidation state. Earlier tracer work at the University of California in 1941 had established the existence of a lower oxidation state (IV or III state) and a higher state (VI). Ultramicrochemical work in late 1942 and early 1943 defined the existence of the (IV) and (VI) oxidation states. The existence of the (V) oxidation state was established in the late summer of 1944.

Plutonium, which has four oxidation states, has a chemistry as complex as any other element. It is unique among elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentrations. As a metal, its properties are also unique. Its six forms have melting points that range from room temperature to 640 degrees Celsius. Its appearance resembles that of lead. Plutonium has isotopes with atomic masses ranging from 232 to 246.

Applications

The most important transuranium element in human affairs, plutonium has enormous nuclear properties. Nuclear reactors use plutonium 239 as breeders, which are used in conjunction with the abundant uranium 238. Such a system "burns" uranium 238 indirectly through the medium of fissionable plutonium 239, formed by the absorption of neutrons in uranium 238. A fissionable isotope such as plutonium 239 gives rise to an amount of heat energy equivalent to approximately ten million kilowatt-hours per pound when it completely undergoes the fission reaction.

Due to its alpha radioactivity, plutonium is one of the most dangerous known poisons. Plutonium 239 undergoes fission with thermal neutrons, showing that all neutrons emitted in the process are eligible to cause further fissions, establishing the great value of this isotope. This led to the wartime plutonium project for the production of plutonium for nuclear weapons. The plutonium project, one part of the Manhattan Project, sought to cause the conversion of uranium to plutonium on a large scale and to separate plutonium 239 from uranium and the other radioactive elements produced by fission. The first successful operation of the chain reaction, opening the atomic age, took place under the western stands at the Stagg football field at the University of Chicago.

In summary, plutonium was discovered in December 1940, the first compound was isolated in August 1942, and the unusual properties of bismuth phosphate as a carrier for plutonium were described in December 1942. The bismuth-phosphate separation process was successfully launched in the pilot plant at Clinton Laboratories in Oak Ridge, Tennessee, in December 1943. By the following month, metal from the pile in Oak Ridge was being processed in the plant at a rate of 0.3 ton per day. Several grams of plutonium were produced over the next months.

In remote Washington State, there were two principal types of plants at the Hanford Engineering Works: those that formed plutonium within uranium by a nuclear chain reaction and those that separated the plutonium from the uranium and the fission products. The responsibility for making a weapon out of the plutonium produced at Hanford and the uranium 235 produced at Oak Ridge fell to the Los Alamos National Laboratory in New Mexico. Here, the plutonium 239 and uranium 235 were purified for use in a nuclear weapon. Only four and a half years after the discovery of plutonium, it was used in a nuclear bomb.

After the completion of the wartime Metallurgical Laboratory, where the most essential part of the investigations was concerned with the chemical processes involved in the production of plutonium, attention turned to the problem of synthesizing and identifying the next transuranium elements. Curium 242 was produced in the summer of 1944 by bombarding plutonium 239 with thirty-two million electronvolts of helium ions. It is highly radioactive and has a short half-life, making chemical investigations with it in macroscopic concentrations difficult. The identification of element 95, americium, followed in late 1944 as a result of the bombardment of plutonium 239 with neutrons. These two elements have stable (III) oxidation states that greatly resemble the chemical properties of the rare-earth elements; thus, they were most difficult to study. Americium was first isolated in the form of a pure compound as a hydroxide in the fall of 1945, at the Metallurgical Laboratory. Americium 243 has a half-life of 7,950 years and curium 244 has a half-life of 19 years, sufficient for study of these isotopes.

Transuranic elements up to number 103 all are members of a chemical family similar to the lanthanides, a series of rare earths ranging from lanthanum (atomic number 57) to lutetium (atomic number 71) in which the 4f electron shell is filled. The second rare-earth series, the actinides, in which the 5f electron shell is filled, begins with actinium (atomic number 89) and includes the natural elements thorium, protactinium, and uranium as well as the transuranic elements up to lawrencium, atomic number 103.

Since the transuranic elements are all of synthetic origin, their atomic masses generally depend on the source material, as this determines what isotopes are involved. Suitable isotopes are available for study of neptunium through einsteinium (atomic number 99). Study of the chemical properties of the elements fermium (atomic number 100) and above is limited to the use of trace amounts.

Metallic protactinium, uranium, neptunium, and plutonium have complicated structures. Americium was the first actinide to display a similarity to lanthanide metals in its crystal structure. As mentioned above, the metallic properties of plutonium are unique. The interatomic distances deduced from crystal-structure data indicate the presence of 6d electrons in these metals; however, from the standpoint of chemical reactivity, they behave more like the lanthanide metals than like metals that fill their outer 5d electron shell, such as tantalum, tungsten, rhenium, osmium, and iridium.

The most important prerequisite to the process of making transcurium elements was that sufficiently large amounts of americium and curium had to be available for neutron bombardment. Berkelium, with atomic number 97, was discovered in December 1949 as the result of the bombardment of milligram amounts of americium 241 with thirty-five million electronvolts of helium ions accelerated in the 152-centimeter cyclotron. Berkelium 243 has a half-life of 4.5 hours. Californium, atomic number 98, was discovered in February 1950 by the bombardment of milligram amounts of curium 242 with a thirty-five-million-electronvolt stream of helium ions. The isotope, which has an atomic mass of 243, has a half-life of forty-four minutes.

Elements 99 and 100 were discovered in debris from the detonation of the Ivy Mike thermonuclear device which took place in the Pacific Ocean in November 1952. Debris from the explosion was collected first on filter paper attached to drone airplanes that flew through the clouds and later from surface material from nearby atolls. This led to positive identification of isotopes of elements 99 and 100, einsteinium and fermium. Einsteinium 253 has a half-life of twenty days. Fermium 255 has a half-life of sixteen hours. Mendelevium, atomic number 101, was formed in 1955 by bombarding einsteinium 253 with a helium-ion beam. Mendelevium 256 has a half-life of ninety minutes and decays by electron capture into fermium 256.

Four new transuranic elements were discovered in the 1990s: darmstadtium (atomic number 110), whose most stable isotope, darmstadtium 281, has a half-life of 11 seconds; roentgenium (111), whose isotope roentgenium 281 has a half-life of 26 seconds; copernicium (112), whose isotope copernicium 285 has a half-life of 29 seconds; and flerovium (114), created by bombarding plutonium 244 with calcium 48 nuclei, whose most stable isotope, flerovium 289, has a half life of only 2.6 seconds. Flerovium was discovered by Russia's Joint Institute for Nuclear Research, which also discovered element 116, livermorium, in collaboration with the Lawrence Livermore National Laboratory in 2000. Livermorium was first created by irradiating curium 248 with ions of calcium 48. Its most stable isotope, livermorium 293, has a half-life of approximately sixty milliseconds.

Context

The transuranic elements comprise the entire set of elements containing more than ninety-two protons. Because their lifetimes are all short compared with the age of the earth, most are not found on earth naturally, and all were first observed in a laboratory. Both plutonium 239 and neptunium 237 have been found in nature in very small concentrations in uranium-containing ores. These isotopes are formed continuously as the result of the interaction of neutrons with uranium 238. Plutonium 239 has been found in pitchblende, a uranium ore, at concentrations of one part per trillion of uranium. This concentration is approximately what would be expected from the absorption of the available neutrons by the uranium ore. Similar, although somewhat smaller, concentrations of neptunium 237 are also found by the action of neutrons on the uranium.

It has been speculated that all nuclei with more than about 60 nucleons were produced by absorption of up to 180 neutrons by iron nuclei in the very short time of the explosion of a supernova. A supernova is a massive star that explodes at its death. In the process, elements more massive than iron (atomic number 26) are created. Supernovas may also create some transuranic elements. Several supernovas have lost half their luminosity in fifty-five days; thus, this is their half-life, the same as the half-life of californium 254. This isotope may be produced in the supernova explosion as the result of a large flux of neutrons. Similarly, supernova 1987A had a half-life of seventy-seven days, the same as cobalt 56.

If a heavy nucleus is given enough energy to overcome electric repulsion of another nucleus, the collision could produce a compound nucleus containing as many nucleons as the sum of the colliding nuclei. This compound nucleus may contain many more nucleons than any naturally occurring nucleus. For example, some of the heavier transuranium elements have been produced by collisions between the lighter transuranium elements and boron nuclei stripped of their electron cloud. There are also proposals to produce some of the elements by collisions between heavy, stripped nuclei such as copper and uranium nuclei. Compound nuclei with more than three hundred nucleons could then decay to the metastable ground states, still more massive than the first transuranic elements.

Transuranic, or TRU, waste is defined by the US government as "waste containing more than 100 nanocuries of alpha-emitting transuranic isotopes per gram of waste, with half-lives greater than 20 years." It typically contains americium 241 and plutonium 238, 239, 240, and 241. In the United States, TRU waste constitutes a separate category of radioactive waste, with specific regulations for its disposal.

Principal terms

ALPHA RADIOACTIVITY: helium nuclei of two protons and two neutrons with no electrons, carrying a +2 charge, emitted as a radioactive decay; may be used to bombard elements to create heavier elements

ATOMIC MASS: the sum of the atomic number (number of protons) and the number of neutrons in the nucleus of an atom

ATOMIC NUMBER: the number of protons found in the nucleus of an atom; each element has a unique atomic number

CYCLOTRON: a device that accelerates ions in a spiral path using electrical and magnetic fields until they emerge at great speed

DEUTERON: the nucleus of a deuterium, which is an isotope of the hydrogen atom that has one proton and one neutron and carries a positive charge

ELECTRONVOLT: the unit of energy equal to the energy acquired by an electron falling through a potential difference of one volt

HALF-LIFE: the time required for the concentration of a reactant to decrease by half

ISOTOPES: atoms having the same atomic number but different atomic masses; different isotopes of the same element have different physical and chemical properties

NUCLEON: the particles in an atom's nucleus, namely protons and neutrons

OXIDATION: the process by which an atom loses one or more electrons in a reaction, with the electron transferred to an adjoining atom in the chemical reaction; the atom that gains an electron is said to be reduced

Bibliography

Bernstein, Jeremy. Plutonium: A History of the World's Most Dangerous Element. Ithaca: Cornell UP, 2009. Print.

Chang, Raymond. Chemistry. 2nd ed. New York: Random, 1984. Print. An undergraduate-level college text that covers all areas of chemistry. Gives the reader a solid understanding of some of the simpler chemical processes and an in-depth study of chemical reactions, including oxidation-reduction. Most useful in the nomenclature for the transuranium elements.

Feinberg, Gerald. What Is the World Made Of? Atoms, Leptons, Quarks, and Other Tantalizing Particles. Garden City: Doubleday, 1977. Print. Addresses the building blocks of nature, including the atom. Briefly covers nuclear synthesis of elements. Brings up the issue of whether superheavy elements may exist as a result of collisions between heavy elements.

Goldsmith, Donald. Supernova! The Exploding Star of 1987. New York: St. Martin's, 1989. Print. Discusses the creation of elements, all of which heavier than iron (atomic number 26) are believed to have been created in supernova explosions. Argues that cobalt 56 caused supernova 1987A to glow, as this isotope had the same half-life as the supernova's luminosity.

Ley, Willy. The Discovery of the Elements. New York: Delacorte, 1968. Print. Uses the periodic table to show how elements, including the transuranics, were discovered. Shows how each fits into the periodic table and uses this to predict properties of yet-to-be-discovered transuranium elements.

Reed, Bruce Cameron. The History and Science of the Manhattan Project. Heidelberg: Springer, 2014. Print.

Rhodes, Richard. The Making of the Atomic Bomb. 25th anniv. ed. New York: Simon, 2012. Print.

Seaborg, Glenn T. The Transuranium Elements. New Haven: Yale UP, 1958. Print. A valuable source by one of the scientists who worked with the Plutonium Project. Dated, but very clear in much of the discussion. Some of the text is too technical for the general reader.

Stewart, Richard Burleson, and Jane Bloom Stewart. Fuel Cycle to Nowhere: US Law and Policy on Nuclear Waste. Nashville: Vanderbilt UP, 2011. Print.

Taylor, John G. The New Physics. New York: Basic, 1972. Print. Discusses some then-new concepts in physics, such as the idea that californium may have been created in supernova explosions, as the half-life of historical supernovas is the same as californium 254.

Weart, Spencer R. Scientist in Power. Cambridge: Harvard UP, 1979. Print. Discusses the early research into plutonium and how various nations, especially France, worked toward the atomic bomb.

Detectors on High-Energy Accelerators

Fission and Thermonuclear Weapons

The Periodic Table and the Atomic Shell Model

Radioactive Nuclear Decay and Nuclear Excited States