Radioactivity of the Actinides

1 Actinide Radioactivity in Uranium and Uranium-Plutonium Fuel 

The important actinides in irradiated uranium fuel are uranium, neptunium, plutonium, americium, and curium, which are produced according to the reactions of Fig. 8.5. U, 

Reactor type Fuel PWR Uranium (3.3% U) PWR Uranium and recycled plutonium HTGR U, thorium, and recycled uranium LMFBR Uranium and recycled plutonium 

Decay of forms Np, which is important because its ( , y) and (n, 2n) reactions lead to Pu and Pu. Also, Np is an important long-term constituent of radioactive wastes, particularly because its transport through some geologic media is not as delayed as that of other actinides and because of the toxicity of radionuclides in its decay chain, especially U, Th, and Ra. 

Although only small quantities of Pu are formed, its half-life of 86 years is long enough that Pu persists in plutonium recovered for recycle and is short enough that is the greatest contributor to the alpha activity of plutonium in irradiated fuel. Although the quantities and activities of 2.85-year Pu are relatively small, its decay daughter can 

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Differences in the availability and radioactivity of the actinides lead to a great disparity in the lengths of the following discussions of the individual elements. For instance, both thorium and uranium have been... 

Due to the high radioactivity of the actinides, we would expect that their use for materials applications would be limited. However, a relatively benign form of uranium, known as depleted uranium (DU), has been widely used in applications... 

When these isotopes become available, chemical studies will be greatly simplified, and the complications introduced by the radioactivity of the actinide elements will be substantially minimized. The longest-lived isotopes of berkelium, californium, and einsteinium are still fairly short-lived substances, and macroscopic amounts have a tremendous associated radioactivity. Nevertheless, it should eventually be possible to prepare and study the solid halides of the actinide elements through the element einsteinium using weighable amounts of reactants. This remains for the future, however. The special experimental problems associated with highly radioactive substances are considered below. 

The radioactivity of the actinides along with their limited availability makes their experimental investigation in... 

Studies of transition intensities are sparser for actinide ions than for lanthanide ions. A reason for this is of course the radioactivity of the actinides, which makes their manipulation troublesome. Special safety precautions are necessary during the manipulation of these compounds. Actinide-doped crystals are darkened by radiation damage of the crystal lattice, and solutions containing actinide ions are often subject to... 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Plutonium [7440-07-5] Pu, element number 94 in the Periodic Table, is a member of the actinide series and is metaUic (see Actinides and transactinides). Isotopes of mass number 232 through 246 have been identified. AH are radioactive. The most important isotope is plutonium-239 [15117-48-3] Pu also of importance are Pu, Pu, and Pu. 

Thorium [7440-29-1], a naturally occurring radioactive element, atomic number 90, atomic mass 232.0381, is the second element of the actinide ( f) series (see Actinides AND transactinides Radioisotopes). Discovered in 1828 in a Norwegian mineral, thorium was first isolated in its oxide form. For the light actinide elements in the first half of the. series, there is a small energy difference between and 5/ 6d7 electronic configurations. Atomic spectra... 

Every known isotope of the actinide elements is radioactive and the half-lives are such that only possibly Pu could... 

The actinoid elements (or actinides An) constitute a series of 14 elements which are formed by the progressive filling of the 5/ electron shell and follow actinium in the periodic table (atomic numbers 90-103). All of the isotopes of the actinide elements are radioactive and only four of the primordial isotopes, Th, and " " Pu, have a sufficient long half-life for there to be any of these left in nature. 

Transuranic elements Elements of atomic number >92. All are radioactive and produced artificially all are members of the actinide group. 

Preparation and handling of actinides are very difficult, because of their scarcity, radioactivity, toxicity and reactivity. The actinides isotopes are unstable and they transform spontaneously into other elements by a and (3 decay or by fission. The chemical toxicity of the actinides is similar to the toxicity of other heavy elements. The radio toxicity is extremely high giving radiation damage in cells. The safe handling of actinides samples needs hermetically closed containments (glove boxes) maintained under low pressure with respect to the laboratory to avoid the risk of dispersion and inhalation of the particles if a break of containment occurs. 

Neptunium is the first of the subseries of the actinide series known as the traiisuratiic elements—those heavy, synthetic (man-made) radioactive elements that have an atomic number greater than uranium in the actinide series of the periodic table. An interesting fact is that neptunium was artificially synthesized before small traces of it were discovered in nature. More is produced by scientists every year than exists in nature. 

AH the isotopes of americium belonging to the transuranic subseries of the actinide series are radioactive and are artificially produced. Americium has similar chemical and physical characteristics and is hofflologous to europium, located just above it in the rare-earth (lanthanide) series on the periodic table. It is a bright-white malleable heavy metal that is somewhat similar to lead. Americiums melting point is 1,176°C, its boiling point is 2,607°C, and its density is 13.68g/cm. ... 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

This chapter is an overview of what magnetic nuclear relaxation can bring to our knowledge of the actinide ions and their use in the nuclear industry. Most results are quite recent and the field is wide open. The author hopes this chapter will attract the attention of NMR specialists who should not be distraught by the experimental difficulties that always accompany the handling of radioactive materials. On the other hand, nuclear chemists and physicists will hopefully discover that NMRD is an interesting supplement to their favorite spectroscopic techniques. 

Another difficulty arises from the chemical properties of the actinide metals. They are chemically reactive, rapidly corroded by moist air, pyrophoric, and, when in the molten state, dissolve common crucible materials. The radioactivity of short-lived isotopes of Am and Cm makes their long-term storage difficult small amounts can be stored successfully under ultrahigh vacuum. Large amounts of isotopes such 238pu with a Ti/2 of only 87.7 years are best stored under a pure inert... 

These two long rows of elements are traditionally moved to the base of the chart so the more important, lighter elements may be closer together for clarity. These two rows of metals each reflect the progressive addition of 14 electrons into an /-type subshell. The lanthanides occur in only trace amounts in nature and are often called rare earths. All of the actinides have large, unstable nuclei that undergo spontaneous radioactive decay. 

Symbol Fm atomic number 100 atomic weight 257 a man-made transuranium radioactive element of the actinide series electron configuration [Rn]5/i27s2 oxidation state -1-3 sixteen isotopes are known most stable isotope Fm-257, ti/2 100.5 days. 

Apatite is being considered as a barrier that will prevent the leakage of radioactive nuclei from the radioactive waste storage. Because of the similarity in the chemical and spectral features REE have been chosen as a model of the fission products of the actinides. For this reason it is of importance to recognize whether the elements are incorporated in the bulk of the barrier, or adsorbed on the surface where they can be subjected to leaching out (Martin et al. 1996 Martin et al. 1999a Martin et al. 1999b). 

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