Actinides and Decay Products

Actinium is the last (bottom) member of group 3 (IIIB) of elements in the periodic table and the first of the actinide series of metallic elements that share similar chemical and physical characteristics. Actinium is also closely related in its characteristics to the element lanthanum, which is located just above it in group 3. The elements in this series range from atomic number 89 (actinium) through 103 (lawrencium). Actiniums most stable isotope is actinium-227, with a half-life of about 22 years. It decays into Fr-223 by alpha decay and Th-227 through beta decay, and both of these isotopes are decay products from uranium-235. 

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

To adequately treat these many applications and to illustrate the specific ways in which decay data make useful, if not crucial, contributions to them is a task that lies beyond the space and time limitations of this paper. We have thus chosen to limit the scope of this presentation to the discussion of several selected examples, drawn mostly from the area of fission-reactor physics. These include the results of recent significant developments in actinide-nuclide decay data and, in the spirit of this symposium, decay data of fission-product nuclides off the line of p stability and some of the problems and challenges they present to both experimental capabilities and nuclear theory. 

Hermann, O.W. and Westfall, R.M. (1995) ORIGEN-S Scale System Module to Calculate Fuel Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Association Source Terms,... 

Table 2 Half-lifes, mode of production, and decay mechanisms for the most widely used isotopes of the light actinides ...

All actinides are radioelements and only Th and U have half-lives long enough to justify neglecting their radioactivity in some special chemical or technical operations. Ac and Pa are present in small amounts as decay products of U and Th (Table 11.3). Extremely small amounts of Np and Pu are produced in U by neutrons from cosmic radiation 10 1, Pu/ U 10" " 1. Harkin s rule is... 

Because there are few data on the results of human exposure to actinides, the health effects of these radioelements are more uncertain than those discussed above for ionizing radiation, radon, and fission products. Americium accumulates in bones and will likely cause bone cancer due to its radioactive decay. Animal studies suggest that plutonium will cause effects in the blood, liver, bone, lung, and immune systems. Other potential mechanisms of chemical toxicity and carcinogenicity of the actinides are similar to those of heavy metals and include (i) disruption of transport pathways for nutrients and ions (ii) displacement of essential metals such as Cu, Zn, and Ni ... 

Previous discussion indicated that for both spent fuel and reprocessed waste, the chief sources of radioactivity for the first 10 to 100 y are the fission products °Sr and Cs. For later times out to 10 y, isotopes of Am, Pu, and Np are successively the dominant contributors to radioactivity. Assuming, as is likely, that corrosion failure of metal-waste packages in a geologic repository will not be significant before 10 to lO y, both Sr and - Cs with 30 y half-lives, will have decayed to insignificance. The actinides and long-lived fission products and Tc remain important at later times (cf. Krauskopf 1988). The U.S. program assumes that 1, Tc, and the actinides and... 

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