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MEMO

TO: TBD-6000 Work Group

FROM: Bill Thurber and John Mauro, SC&A

DATE: October 14, 2013

SUBJECT: Response to Comments Regarding Time to Reach Equilibrium for the Buildup of 

Uranium on Surfaces

The purpose of the recent SC&A white paper (SC&A 2013) provided to the TBD-6000 Work 

Group was to ascertain whether or not the use of a 30-day settling period to achieve equilibrium 

between dust settling and removal was supported by the data in Adley et al. 1952. The 

equilibrium settling time was calculated using the equation:

Settling time = Surface contamination ÷ airborne concentration ÷ settling velocity 

The surface contamination was measured at the Hanford Melt Plant on plates exposed in various 

areas of the plant for 117 or 158 days. The air concentrations were averages of measurements 

taken over extended periods throughout the plant. The settling velocity was obtained based on 

the theoretical terminal settling velocity for 5 μm AMAD spherical particles (0.00075 m/s) and 

adjusting the value for particle shape and slip of the particles between air molecules. The 

adjusted settling velocity was 0.00052 m/s. Since the Melt Plant operated only one shift per day, 

the airborne concentrations were adjusted downward to reflect that fact. It should be emphasized 

that the initial purpose of these calculations was not to determine whether the TBD-6000 default 

settling value was a conservative construct in dose reconstruction, but rather to determine the 

best values from the Adley data. SC&A’s calculations showed that there was considerable 

variability in the calculated equilibrium settling times in various areas of the Melt Plant, as 

shown in Table 1.

Table 1. Recalculation of Equilibrium Settling Times Based on 

Average Adjusted Air Concentrations

 Location Days to Equilibrium

Furnace Room – Winter 55.4

Furnace Room – Spring 166.1

Burnout Room – Winter 88.0

Saw Room – Winter 118.6

Saw Room – Spring 67.8

Main Bay – Winter 15.7

Main Bay – Spring 17.4

Within the large main bay area of the plant, equilibrium was achieved in 16–17 days. In more

confined areas, times as high as 166 days were required to reach equilibrium.

Given this range of calculated equilibrium times, it is useful to consider how this information 

might affect dose reconstruction. For example, if the dose reconstructor was advised to use a 
Memo: Response to Comments 2 SC&A – October 14, 2013

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution.

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

default assumption that deposition occurred 24 hours per day, 7 days per week, then the effective 

air concentrations would be increased by a factor of 3.14 (24/10.7 × 7/5). Correspondingly, the 

days to equilibrium in Table 1 would be reduced by that factor. Under that scenario, use of a 

30-day settling time would be appropriate and sometimes conservative for most of the examples 

in Table 1. However, even with such a default assumption regarding the settling hours per week, 

some of the areas in the Melt Plant would still have required more than 30 days to reach 

equilibrium. For example, if one averaged the winter and spring values for the furnace room, the 

time to reach equilibrium would be 35 days using the default deposition time assumption. 

Similarly, the time to reach equilibrium in the saw room would be 33 days. However, use of a 

default value for the terminal settling velocity of 0.00075 m/s would further decrease these 

settling times by a factor of 1.4 (0.00052/0.00075). With these corrections, all of the Adley 

results could be explained with equilibrium settling times of less than 30 days. 

If the dose reconstructor is required to assume that deposition is continuous for 30 days to 

calculate the amount settled on a surface and to use a default value for the terminal settling 

velocity of 0.00075 m/s, then based on the information presented here, the amount deposited will 

not be understated.

Reference

Adley, F.E., W.E. Gill, and R.H. Scott, 1952. Study of Atmospheric Contamination in the Melt 

Plant Building, USAEC report HW-233352 (Rev.). Hanford Works, Richland, Washington. 

April 4, 1952.

SC&A 2013. Comments on the NIOSH White Paper, “TBD 6000 Review by SC&A, 

Determination of Settling Time,” Draft White Paper, Rev. 0. SC&A, Inc., Vienna, Virginia. 

October 7, 2013.