Chapter 6 



Precipitation on the Ocean Bottotn 



61 



wastes by solids naturally occurring in sea water 

 or by solids formed by interaction of non-radio- 

 active components of the wastes with sea water 

 constituents. 



Certain generalizations can be made with re- 

 gard to the formation of a solid phase — a 

 precipitate, by the interaction of radioactive 

 constituents with sea water components. Pre- 

 cipitation may occur when the solubility product 

 of a substance has been exceeded. Funda- 

 mentally, in order to be able to predict when 

 this condition has been met, knowledge of the 

 ionic activities of the species involved must be 

 known. Ionic activity is used here in the thermo- 

 dynamic sense, and is not related to activity in 

 the radioactive sense. Unfortunately practically 

 nothing is known about ionic activities of fission 

 product elements in sea water. The theoretical 

 approach through this route appears, therefore, 

 to be impractical. 



The mass of radioactive elements that might 

 be introduced into the ocean from any expected 

 level of power production or foreseeable use 

 of bombs, will be small when compared to the 

 quantities of similar elements already in the 

 ocean. Thus, it is to be expected that chemical 

 precipitation of radioisotopes will occur only in 

 ocean regions where precipitation occurs nor- 

 mally. This process includes precipitation in 

 the usual sense and co-precipitation — the proc- 

 ess in which similar elements are simultaneously 

 removed from solution. For example, during 

 the precipitation of calcium carbonate, stron- 

 tium, a minor element, usually is co-precipitated 

 and carried along with the calcium carbonate. 



Sorption processes involving inactive solids 

 provide another set of mechanisms that may pro- 

 duce radioactive solids. The solids that are 

 present in sea water or might be produced from 

 inactive waste components are generally finely 

 divided, have large area to volume ratio, and 

 are charged. The sorption of radioactive and in- 

 active dissolved constituents onto the solids, in 

 the ratio of their relative concentration, is fa- 

 vored by these characteristics. Thus, in cases 

 where an element normally present in sea water 

 is known to be taken up by suspended solids it 

 can be expected that radioisotopes of the same 

 or chemically similar elements will also be taken 

 up. 



The oceans contain inorganic and organic, 

 living and dead suspended solids — all have 



sorption properties and may remove active and/ 

 or inactive constituents from solution. 



Settling characteristics 



The sinking of particles in the sea is usually 

 described in terms of Stokes' Law which as- 

 sumes, in its simplest form, smooth, rigid, 

 spherical particles of a stated diameter and den- 

 sity, sufficiently widely spaced so as not to im- 

 pede one another. It provided an adequate de- 

 scription of the behavior of these solids with a 

 restricted particle size range. For particles larger 

 than about 100 microns (0.1 mm) the law must 

 be modified to take into account turbulence 

 around the particle that has a net effect of re- 

 ducing the settling rate. Also, particles of col- 

 loidal and near-colloidal dimensions, less than 



TABLE 1 Settling Velocity of Quartz Spheres 

 (In Distilled Water) 



Settling 



Diameter velocity 



, '- V Time to settle 



(mm) (microns) (m/day) 1000 m 



1.0 1000 14,000 0.07 days 



0.1 100 800 1.25 " 



0.01 10 8 125 



0.001 1 0.08 34 years 



1/1024 0.98 0.07 39 



1/2048 0.49 0.02 137 



1/4096 0.25 0.004 685 



1/8192 0.12 0.001 2,740 



about a half micron, settle at a rate less than 

 predicted by Stokes' Law, presumably because 

 of charge interaction between particles and dis- 

 solved components. 



Table 1 gives the settling velocities for par- 

 ticles of a stated size in distilled water, has been 

 calculated from Stokes' Law and is subject to 

 the criticisms noted above. 



This table is a highly simplified and idealized 

 picture of the actual settling properties of solids 

 that normally occur in the oceans, and especially 

 of particles in the small size range. Particles in 

 this range probably will be the main concern 

 when considering the deposition of fission prod- 

 ucts. They are also in the size range that will 

 permit ocean circulation to alter markedly any 

 predicted location of deposition or of time to 

 reach the bottom. 



The density and shape factors that effect 

 settling characteristics are important when con- 

 sidering organic solids or living organisms. 

 The density approaches that of sea water which 



