102 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



information to insoluble alpha emitters is seriously complicated by dosimetry and the 

 relative biological effectiveness (RBE) of alpha particles for most actinides. Recent 

 reviews on plutonium and other actinides in the environment have very little to say about 

 ecological effects; rather, they dwell primarily on distribution and behavior (Romney and 

 Davis, 1972; Martell, 1975; Hakonson, 1975; Hanson, 1975). 



There are three basic approaches to the study of ecological effects of transuranics: 

 (1) direct experiments in which radionuclides are applied at various levels to study 

 systems, (2) observations of populations that occupy contaminated areas, and (3) model- 

 ing and extrapolation from applicable research data. Each approach has inherent 

 advantages and shortcomings. The direct experimental approach might enjoy a relatively 

 high degree of credibility and accuracy, but it has not been used with transuranics for 

 reasons of safety and lack of public acceptance. Examination of contaminated areas is 

 quite feasible and has been done at such places as the Nevada Test Site, Enewetak, and 

 Rocky Flats (in Colorado). This approach is less than ideal, however, because of the usual 

 lack of good experimental control and the common presence of more than one 

 potentially toxic substance, which lead to uncertainties in data interpretation. The third 

 approach can be used when needed with existing data, but accuracy may be poor because 

 of the complexity and uncertainty associated with parameter values. 



Ecosystems can probably tolerate higher levels of radioactivity from most, if not all, 

 of the transuranics than from the more biologically mobile fission products, such as ^^Sr 

 and '^^Cs. Low solubility, lack of essential nutrient analogues, and the virtual lack of 

 penetrating radiations for most transuranics form the basis for this opinion. However, 

 critical experiments to make this comparison have not been done; there is some 

 concern that biological incorporation of long-lived transuranics in the environment may 

 slowly increase with time, and it is known that very low levels may be carcinogenic. 



Direct Experiments 



The literature dealing with effects of ionizing radiation on plants and animals is massive. 

 Important reviews and bibUographies include the BEIR report (National Academy of 

 Sciences-National Research Council, 1972), the UNSCEAR report (United Nations, 

 1972), and the bibliography by Sparrow, Binnington, and Pond (1958). The vast majority 

 of this literature, however, is based on laboratory studies with X- or gamma radiation. A 

 far smaller body of literature exists on radiation effects on natural populations. Whicker 

 and Fraley (1974) reviewed field studies dealing with the effects of ionizing radiation on 

 terrestrial plant communities, and Turner (1975) prepared a similar review for native 

 animal populations. This literature also is restricted primarily to X-and gamma radiation, 

 but it provides a substantial basis for understanding dose— effect relationships. 



A major problem in applving this information to transuranics is that of determining 

 the equivalent dose to critical tissues which would result from a given level of 

 contamination. Of the 17 transuranic nuchdes listed as being of some importance in the 

 nuclear industry to the year 2000 (Energy Research and Development Administration, 

 1976), 13 are alpha emitters with generally infrequent emission of weak (mostly <0.07 

 MeV) photons. The other 4 are beta emitters with accompanying weak photon emissions. 

 Alpha— weak-photon emitters include the particularly important nuclides Pu, Pu, 

 ^"^^ Am, ■^'^^Cm, and ^'*'*Cm. Alphas from these nuclides have energies of 5 to 6 MeV and 

 ranges in air and biological tissue of roughly 4 cm and 40 jum, respectively (Walsh, 1970), 

 which lend considerable complexity to the problem of dosimetry. 



