268 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



particles. This would correspond to a diameter for the larger, less-frequently resuspended 

 particle(s), which may be three times the diameter of normally resuspended plutonium 

 particles. Only since extensive air samples have been collected has the resuspension of 

 these unusually active particles been suggested. However, the frequency of their 

 resuspension appears to be very low. 



The presence of a more-radioactive-than-normal resuspended particle is also indicated 

 by tlie ^'"Am data shown in Figs. 18 and 19. The two positive ^'''Am air samples for 

 the HMS tower may indicate that larger-than-normal ^'^'Am particles or clusters of 

 particles were collected on these filters. 



As discussed earlier for resuspended particles at Rocky Flats, a more-radioactive- 

 than-average particle (or cluster of particles) was collected on the 2-/jm stage of a particle 

 cascade impactor. 



Resuspension Factors at Hanford 



Resuspension factors have been used to describe resuspension air concentrations. The 

 resuspension factor (expressed in units of meters"') is defined as the airborne 

 concentration of contaminant per cubic meter divided by the surface contamination level 

 per square meter immediately below the point where the airborne concentration was 

 measured. Often air concentrations for determining resuspension factors have been 

 measured from about 1 to \% m above ground. However, airborne concentrations are a 

 function of the upwind contamination level, not a contamination level immediately 

 below the air-concentration measurement site. It is the transport from upwind 

 contamination sites to the concentration measurement site that determines the airborne 

 concentration. 



Although the validity of resuspension factors is questionable, they were for a long 

 time the only method for estimating air concentrations. Consequently resuspension 

 factors were estimated from data obtained at Hanford (Sehmel, 1977c). Resuspension 

 factors and the basis for their calculation are shown in Table 9 for both ^^^Pu and 



1 ABLE 9 Resuspension Factors at Hanford 



Material 



Area 



Air concentration 



Minimum 



Maximum 



Surface contamination 

 (G), MCi/m' 



Minimum 



Maximum 



Resuspension 

 factor* range, 



m 



'Pu Inside diemical separation 

 areas (Corley, Robertson, 

 and Brauer, 1976) 



'Cs BC (Bruns, 1976;Mishima, 

 1973) 



7x10'* 8x10- 



4,9 X 10" 



2x10-" ZxlQ-'" 0.29 



1.2 X 10 



55.4 



6 X 10- 

 2 X 10-' 



to 



4 X 10-' 

 7 X 10- 



to 



■ Resuspension factor = 1 0' x /<j- 



'^^Cs; literature values are used as an indication of ground-surface contamination levels. 

 Airborne concentrations (Sehmel, 1977c) are shown in Fig. 9. Resuspension factors 

 calculated from air concentrations and ground -surface contamination levels are shown in 

 the last column. Resuspension factors for ^^^Pu range from 6 x 10^'° to 2 x 10~^ 

 m~ ' . Resuspension factors for '^''Cs range from 4 x 10 '' to 7 X 10"^ m"'. These 

 ranges, from 10"^' to 10~^ m~ ' , are within ranges reported in the literature (Mishima, 

 1964; Sehmel and Lloyd, 1976a). 



