1 1 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



that passed through the impactor. The material at each impactor stage had been 

 characterized as being within a certain particle size range. Since the stage at which a 

 particle is deposited is a complex function of actual particle size, shape, and density, the 

 unit of size measurement used by them was the activity median aerodynamic diameter 

 (AMAD), which is the diameter of a unit-density sphere with the same settling velocity as 

 a plutonium-bearing particle in a population divided so that the radioactivity of all the 

 larger particles equals that of the smaller ones. 



They found that the two fabrication facilities produced the largest AMAD (4.0 and 

 2.7 ^m) and the recovery facihty produced the smallest AMAD (0.3 fim). The two 

 research and development facilities produced intermediate size particles. 



In 1975 Sanders (1976: 1977; 1978; 1979) began a study of plutonium-bearing 

 particles in various parts of the chemical separations process exhaust system at the 

 Savannah River Plant using autoradiographic tecliniques to record both fission-fragment 

 and alpha-particle tracks. 



Methods and Materials 



Particle Collection 



Particles are collected by drawing a fraction of exhaust air through membrane filters. 

 These filters are polycarbonate films that are 47 mm in diameter and 5 jum thick with 

 3 X 10^ 0.1-/.im-diameter pores per square centimeter, which gives a filter porosity of 

 0.024. The filters are supported in a polycarbonate aerosol holder.* Air is drawn through 

 the holder by a small diaphragm pump with a Vitonf diaphragm at a rate of 4 liters/min 

 to give a face velocity at the filter of 3.8 cm/sec. At this flow the total efficiency for 

 particle collection by the processes of impaction, diffusion, and interception, calculated 

 according to Spurny et al. (1969) is 100% for all particles with diameters of 0.001 )um 

 (the diameter of gas molecules) or larger. 



Arrangement of the air-sampUng system is shown in Fig. 1. As particles accumulate on 

 the membrane filters, membrane porosity and airflow are reduced. Integrated airflow is 

 measured with a dry-type test meter J in series with the diaphragm pump to determine the 

 fraction of the exhaust sampled. When nitrogen dioxide is present, exhaust gas is passed 

 through two gas-drying towers between the filter and the pump. The first tower contains 

 indicating Drierite§ to remove moisture from the air and save the Ascarite^ in the second 

 tower. The self-indicating Ascarite, in turn, absorbs nitrogen dioxide to protect the pump 

 and the dry test meter. A small flowmeter is mounted on the exhaust side of the dry test 

 meter to give an indication of the instantaneous flow rate through the system. Air from 

 the meter is fed back into the exhaust system to prevent its release to the service area. 



Film Preparation 



Figure 2 shows the procedure for converting the particle-containing filter membrane to a 

 polycarbonate film. After air has been sampled, the radioactivity retained on each filter is 



*The aerosol holders and membrane filters were produced by Nuclepore Corporation, Pleasanton, 

 Calif., and obtained from them or Bio-Rad Laboratories, Richmond, Calif. 

 fTrademark of E. I. du Pont de Nemours & Company, Inc. 

 rfManufactured by the American Meter Division of Singer. 

 §Trademark of W. S. Hammond Drierite Company, 

 f Trademark of Arthur H. Thomas Company. 



