TRANSURANIC AND TRACER SIMULANT RESUSPENSION 245 



dependency on wind speed was made for the 7-/im-diameter particles. This approximation 

 was for the 0.3-m height at sampling site AB. For the three data points taken at the 0.3-m 

 height, ■^^^Pu concentrations increased with the 5.9 power of wind speed. The 

 uncertainty in this exponent is too large to make a valid comparison between airborne 

 plutonium and soil concentrations. 



The July 1973 plutonium resuspension experiment at Rocky Flats showed resus- 

 pension of both ^^*^Pu and ^■^^Pu. However, all airborne plutonium concentrations were 

 significantly below MPC's in air. Since '^^^Pu was collected on each particle cascade 

 impactor stage, the suggestion is that most plutonium was attached to soil particles when 

 the plutonium was resuspended. 



Respirable Plutonium Concentrations at Hanford. Extensive data were obtained on 

 airborne radionuclide concentrations around resuspension sites studied (Sehmel, 1977c). 

 These concentrations were expressed both in microcuries per cubic centimeter of filtered 

 air and microcuries per gram of airborne solids. Tliis report summarized ranges of data 

 collected but did not detail data for each experiment. 



Airborne plutonium concentrations for both ^^*Pu and ^^^Pu measured (Sehmel, 

 1977c) in resuspension experiments are shown in Fig. 7 and are compared with Hanford 

 300 Area fallout levels (Thomas, 1976) approximately 30 km distant. The data 

 represented experiments conducted over various time periods. For each data symbol the 

 vertical line is plotted at the mid-time of the resuspension experiment, and the 

 experiment duration is shown by horizontal lines drawn at both maximum and minimum 

 measured airborne concentrations. Airborne peak plutonium concentrations at resus- 

 pension study sites were significantly greater than 300 Area fallout levels (Thomas, 

 1976), and airborne ^^^Pu concentrations, in general, were greater than airborne ^^^Pu 

 concentrations. However, althougli resuspension was and is still probably occurring at 

 these sites, measured airborne concentrations were significantly less than MPC's 

 (International Commission on Radiological Protection, 1959). 



Airborne plutonium concentrations at the U-Pond Area tended to remain constant as 

 a function of time. This constancy indicates that the weathering (or fixation) half-life for 

 surface contamination available for resuspension at this site is on the order of years. Tliis 

 is much greater than the 35 to 40 days often quoted (Wilson, Thomas, and Stannard, 

 1961) in Uterature on resuspension. However, the year weathering half-life at the U-Pond 

 could be a manifestation of some resuspension surface renewal process since this is an 

 active waste-disposal site. The explanations are unclear for differences in weathering 

 half-life. 



The maximum airborne ^^^Pu concentration measured was 8 x 10~'^ juCi/cm^ near 

 the Hanford meteorological station (HMS) tower on Jan. 11, 1972. All other plutonium 

 concentrations except one were at least one order of magnitude lower. This one 

 exception was measured 6.1 m above ground at the U-Pond during October 1973. In 

 comparison with other October data, the concentration for this sample was about one 

 and one-half orders of magnitude greater than any other sample. We hypothesized that 

 some more-active-than-normal particles or clusters of particles (hot) were resuspended 

 and collected on this filter. 



Nonrespirable Airborne Plutonium Fluxes at Rocky Flats and Hanford 



Nonrespirable airborne plutonium fluxes were calculated for both ^^*^Pu and ^^^Pu. The 

 Rocky Flats data (Sehmel, 1976a) are shown in Figs. 8 and 9. 



