66 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Figure 2 shows the distribution of radioactive fallout (in millicuries per 100 square 

 miles) between 2 and 35 days following the Mike detonation explosion in the Marshall 

 Islands (Machta, 1964). Tlrese results represent only that fractional amount of the debris 

 which was contained in the troposphere. Since the residence time in the troposphere is on 

 the order of 20 to 40 days, the deposition rate was quite rapid and was confined mainly 

 to the hemisphere where the test took place; the higher concentration was near the 

 latitude where the explosion occurred. 



The distribution of debris of stratospheric origin is considerably different from that 

 of tropospheric origin. Most of the debris leaving the stratosphere does so through the 

 tropopause discontinuity, which occurs near the midlatitudes and is almost independent 

 of the latitude of the detonation. Empirical box models (Krey and Krajewski, 1970b), 

 which describe the movement of radioactivity from the upper to the lower stratosphere, 

 between hemispheres, from the stratosphere to the troposphere, and the deposition rate 

 on the earth's surface, have been developed and appear to be reasonably satisfactory. 



The movement of radioactive debris in the troposphere is influenced by all the forces 

 of the weather. Rain and snow will scavenge radioactive particles, which will cause the 

 debris to be distributed unevenly on the earth's surface. Recently, measurements have 

 shown that the scavenging of radionuclides by cirrus cloud ice particles resulted in major 

 depletion of radionuclides from atmospheric layers of 1 .3 to 2.8 km thick at about 10 Vr.- 

 (Young, Wendell, and Wogman, 1975). The mixing of air masses as they move west to 

 east across the United States and are orographically Ufted over mountain ranges can 

 increase the ground-level concentration of radionuclides on the downwind side. This 

 effect is presumably due to the downwind mixing of high-level air, which contains higher 

 concentrations of both cosmogenic and nuclear-weapons-produced radionuclides. This 

 effect is shown in Fig. 3 where the atmospheric concentrations of ^Be and ' "'^Cs for a 

 period of iVj yr for Quillayute, Wash. (48°N, 125°W), Richland, Wash. (46°N, 119°W), 

 and Rocky Flats, Colo. (40°N, 106°W), are compared. Storm systems originating in the 

 Aleutians move air masses over the Quillayute sampling site which are orographically 

 lifted several thousand feet by the Cascade Mountain range before they descend to the 

 Ricliland site. The air mass is again lifted by the Rocky Mountain range before it descends 

 to the Rocky Flats sampling site. The average annual air concentrations of ^Be and ' ^^Cs 

 during 1973 through early 1975 were 2.1 to 2.4 times as great at Richland and 2.9 to 3.1 

 times as great at Rocky Flats as those at Quillayute (Thomas, 1972). 



Production and Characteristics of Individual Transuranic Elements 



Because of their methods of production, the relative abundances of the transuranium 

 elements are considerably different in nuclear detonations than in reactor operations. In 

 nuclear detonations neutron capture occurs in extremely rapid succession, producing 

 uranium or plutonium isotopes of very liigh mass which rapidly decay to form a spectrum 

 of transuranium elements. In this case there is no opportunity for the decay of the 

 various uranium isotopes, which could break the chain of successive neutron capture. In 

 the reactor production of radionuclides, the neutrons are captured only one at a time, 

 and the resulting product may decay before additional neutron capture. In Table 7 the 

 amounts of the various transuranium elements resulting from the Mike nuclear test are 

 compared with those which result from nuclear power generation. It is immediately 

 evident that the transuranium elements resulting from nuclear energy generation are 

 much higher relative to ^^^Pu, particularly in the region just below and above 239 than 



