Chapter 13 



Large-Scale Biological Experiments with Tracers 



137 



naturally in the sea and (2) only the uptake 

 of carbon at the phytoplankton level is meas- 

 ured. It seems feasible to improve on the ex- 

 periments in bottles by conducting experiments 

 in small lagoons, or by employing larger partly- 

 enclosed volumes in the open sea. 



From experience with such experiments in 

 bottles, it can be shown that there is sufficient 

 uptake of carbon by the phytoplankton, if 

 grown in a concentration of 0.3 micro-curie per 

 liter for one day, to measure it if a one liter 

 sample is filtered and the radioactivity of the 

 filtered plants determined in a counter of 20 

 per cent efficiency. By increasing either the 

 counting time or the volume of water filtered, 

 the initial concentration of C^* can be decreased 

 correspondingly. 



For an experiment in a lagoon, we might 

 use a body of water of, say, 500 meters long 

 by 200 meters wide with an average depth of 

 10 meters, giving a volume of 1x10^ cubic 

 meters or 1 x 10^ liters. By filtering 100 liters 

 of water for phytoplankton, C^* at a concen- 

 tration in the water of 3x10"^ curies per liter 

 would suffice, or 3 curies for the experiment. 

 Since there is probably between a 50 per cent 

 and 90 per cent loss at each step up the food 

 chain, correspondingly larger volumes would 

 have to be strained for the higher forms, but 

 this is a simple problem by the use of standard 

 nets, etc. 



To get improved measurements of the uptake 

 of carbon by phytoplankton in the open sea, 

 and the passage of carbon to the smaller grazing 

 organisms, it is suggested that a moderately 

 large rubber tank open at the surface be em- 

 ployed to isolate a piece of the top of the sea, 

 yet have a sufficiently small surface-to-volume 

 ratio that the processes will more nearly ap- 

 proach normal conditions than is obtained in 

 bottle experiments. We might employ such an 

 apparatus of 20 meters diameter by 10 meters 

 deep, having a volume of tt 10^ cubic meters, 

 or ttXIO^ liters. By filtering 10 liter samples 

 for phytoplankton, with 20 per cent efficient 

 counting equipment, we would need to provide 

 about 3x10"^ curies per liter, or a total of 

 about 1/10 curie of O*. 



So?77e cost and logistic considerations 



For the two experiments with C^*, discussed 

 immediately above, the problems of handling 

 the amounts of activity involved present no 

 particular difficulty. Since C^* is a pure beta 



emitter, the shielding problem for even the 

 experiment requiring 3 curies is a simple matter. 

 The cost of the isotope, however is fairly high; 

 at present about $30,000 per curie. This might 

 be reduced somewhat if the present demand 

 were to increase. The cost, notwithstanding, 

 however, the information to be gained is well 

 worth the outlay. 



In the case of an experiment using gamma 

 emitters in the slow-mixing layer below the 

 thermocline, where about 100 curies would be 

 required, it is suggested that mixed fission 

 products from wastes from processing of reactor 

 fuel elements be used. A large quantity of such 

 wastes will be available, probably at no charge. 

 If one used HNO3 salted waste product from 

 a natural uranium-plutonium reactor, after 100 

 days "cooling," the reactor waste will contain 

 about 200 curies/gallon. Approximately half 

 a gallon will be needed, requiring about 10" 

 of lead shielding for transportation and han- 

 dling. A cubical container will require 10.05 

 cubic feet of lead, weighing 7,175 pounds. 

 This is feasible to handle by freight and on 

 shipboard. 



For the kilocurie quantities required for an 

 experiment in the upper mixed layer of the 

 sea, the handling problem reaches a different 

 order of magnitude. It becomes quite infeasible 

 to handle waste liquids in the volume required. 

 It may be possible, because of the much higher 

 activity per unit volume to employ slugs of 

 U^^^ from a reactor, which, after 30 per cent 

 burning and 100 days "cooling" have about 

 2x10^ curies per kilo of fairly long term 

 gamma activity. Even then some 2/10 kilos 

 of "used" U^^^ would be required. The prob- 

 lems of transporting and handling this are 

 somewhat difficult as are methods of dissolving 

 and liberating the material at sea, but probably 

 feasible. Further detailed consideration needs 

 to be given to this problem. It may, of course, 

 be that the use of an explosive reaction — a 

 small nuclear detonation for oceanographic and 

 biological experimental purposes — is the only 

 logistically feasible method. 



REFERENCES 



Revelle, R., T. R. Folsom, E. D. Goldberg, 

 and J. D. Isaacs. 1955. Nuclear Science 

 and Oceanography. United Nations Inter- 

 national Conf. on Peaceful Uses of Atomic 

 Energy, Geneva, Paper no. 277:22 pp. 



