3. ARTIFICIAL RADIOACTIVITY IN THE SEA 



Y. MlYAKE 



1. Artificial Radioactivity in Oceanic Waters 



At present the artificial radioactivity in the oceans consists mainly of the 

 fission products of 235 U, 239 Pu and 238TJ f rom tests of nuclear weapons. In 

 some areas a small amount of artificial radioactivity has been introduced into 

 the sea by atomic waste disposal. The spectrum of fission products varies from 

 one source to another according to the type of parent nuclides as well as the 

 kinetic energy of the fission neutrons. More significant are the changes in their 

 composition and activity with the lapse of time after the fission. 



Table I gives the percentage of the activity of the principal radioisotopes 

 produced by slow neutron bombardment on 235TJ (i kg) respectively 20 days 

 and one year after fission. In twenty years only a few nuclides remain ; 90 Sr- 

 90 Y and 137 Cs- 137 Ba occupy respectively 48% and 45% of the total activity. 

 The rest of the activity will consist of 147 Pm and 151 Sm. 



In addition, radioactive nuclides produced by neutron capture or other 

 nuclear reactions such as 3 H, "0, 3 5S, stQo, 58Co, 6oc , ™Z\\, ™M.\\, s 9 Fe, "Fe 

 have been detected in sea-water and marine products. 



The artificial radioactivity from nuclear tests has been introduced into the 

 oceans from the fallout, both immediate and delayed, and from direct injection 

 through the surface or sub-surface tests. Since the ocean covers 71% of the 

 earth's surface, of the stratospheric fallout, 71% falls into the oceans if the 

 stratospheric dust is distributed evenly in the air. But this may not be true 

 because the greater part of the stratospheric dust is accumulated in the same 

 hemisphere in which the tests are carried out. According to the observation on 

 the distribution of 90 Sr in the stratosphere (presentation by J. Spar at the 

 United Nations Scientific Committee on the Effects of Atomic Radiation, New 

 York, 1960), the total amount of 90 Sr in the Northern Hemisphere was 0.5 

 megacurie while it was only 0.2 megacurie in the Southern Hemisphere (Jan.- 

 Sept., 1959). As the oceanic areas are respectively 60.6% and 81.0% in Northern 

 and Southern Hemispheres, 66% of stratospheric fallout falls into the sea, 

 assuming the above ratio of 90 Sr distribution. However, it is well known that 

 the ground deposition has its maximum between 30°N and 60°N, where the 

 oceanic area occupies only 50% of the earth's surface. The 90 Sr deposition in 

 1958 between 30°N and 60°N was estimated to be 43% of the total or 59% of 

 that in the Northern Hemisphere ; therefore, the stratospheric fallout in the sea 

 might be about 61% of the total. 



In 1960, the total deposition of 90 Sr on the surface of the earth is regarded 

 to be roughly 6 megacuries, of which 3.6 megacuries of 90 Sr were introduced 

 into the ocean from the sky, which is equivalent to 10 mc/km 2 on the 

 average. If we assume the ratio of stratospheric dust is 2.5 : 1 between the 

 Northern and Southern Hemispheres, the depositions of 90 Sr on the ocean 

 surface in the two hemispheres is respectively 17 mc/km 2 and 5 mc/km 2 . 

 The level of activity of 90 Sr in sea-water depends largely upon the depth of the 



[MS received August, I960] 78 



