MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 46 7 



For most applications to NTS, the grass data of Selunel et al. (1973) appear to be the 

 best analog. The trend of these data, in the range of 2 to 12 m/sec air velocity and 1- to 

 100-jum particle diameter, is approximately 



^=3x10-^ dp (3) 



where Vj is the deposition velocity (cm/sec), U is the wind velocity (cm/sec), and dp is 

 the particle diameter (jum). For lO-jLtm particles, Eq. 3 yields values similar to Healy's 

 results for neutral atmospheric stability. 



Resuspension Factor. The resuspension of plutonium from soil is often expressed as the 

 ratio of air concentration (microcuries per cubic meter) to surface soil concentration 

 (microcuries per square meter). Many such measurements have been made at NTS (Mork, 

 1970; Anspaugh and Phelps, 1974) and in the vicinity of Rocky Flats, Colo. (Volchok, 

 1971). The measured magnitudes of this ratio range generally from 10~^ to 10~^ ' m~^ . 

 To estimate "acceptable soil concentrations," Anspaugh (1974) used a value of 10~^ 

 m~^ for NTS. These ratios are, to say the least, extremely variable with respect to time 

 and environmental factors, such as wind speed and direction, rainfall, and disturbances 

 affecting aerodynamic properties of soil surfaces. Other factors affecting this ratio are the 

 aerodynamic properties of plutonium-bearing particles and their susceptibility to saltation 

 and resuspension. There is evidence that the ratio tends to decrease with time after fallout 

 contamination of soil (Anspaugli et al., 1973; Anspaugh, 1974; Kathren, 1968). 

 Anspaugh et al. (1975) have proposed a model in which the air/soil ratio decreases as a 

 function of time from a maximum of lO"'* to a minimum of 10~^ m~^ , i.e., 



^=10-Vxp[-k(f)'^] +10-^ (4) 



where Ca = air concentration (juCi/m'*) 



Css = soil surface concentration (juCi/m^) 

 k = 0.15day-'^ 

 t = time from deposition (days) 



This model is consistent with data collected over the years at NTS. 



Resuspension Models. Many attempts have been made to develop mathematical models 

 to simulate resuspension (Amato, 1971; Mills and Olson, 1973; Killougli and McKay, 

 1976). Most of these are based on models of wind erosion developed by Bagnold (1960) 

 and, as a function of wind speed, take the form 



Ca = K (U - Ut)' ^ (5) 



where Uy is a tliieshold wind speed (m/sec) and K is a constant (sec/m^). 



Others (Sehmel and Orgill, 1973; Shinn and Anspaugh, 1975) have used a power-law 

 expression of the form 



Ca = K U" (6) 



--SS 



