Draft White Paper 

COMMENTS ON THE NIOSH WHITE PAPER, “TBD 6000 

REVIEW BY SC&A, DETERMINATION OF SETTLING TIME”

REVISION 0 

Contract Number 200-2009-28555

Prepared by 

William C. Thurber 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400

Vienna, Virginia 22182 

October 2013 

Disclaimer 

This document is made available in accordance with the unanimous desire of the Advisory Board 

on Radiation and Worker Health (ABRWH) to maintain all possible openness in its 

deliberations. However, the ABRWH and its contractor, SC&A, caution the reader that at the 

time of its release, this report is pre-decisional and has not been reviewed by the Board for 

factual accuracy or applicability within the requirements of 42 CFR 82. This implies that once 

reviewed by the ABRWH, the Board’s position may differ from the report’s conclusions. Thus, 

the reader should be cautioned that this report is for information only and that premature 

interpretations regarding its conclusions are unwarranted.
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 2 of 8

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.

S. Cohen & Associates:

Technical Support for the Advisory Board on

Radiation & Worker Health Review of

NIOSH Dose Reconstruction Program

Document Description:

Comments on NIOSH White 

Paper – Determination of Settling 

Time

Effective Date: 

Draft – October 7, 2013

Revision No.:

0 – Draft

Comments on the NIOSH White Paper, “TBD 6000 

Review by SC&A, Determination of Settling Time”

Page 2 of 8

Task Manager: ___________________ Date:

 William C. Thurber

Supersedes:

N/A

Project Manager: _________________ Date:

 John Stiver, MS, CHP

Reviewer:

 John Mauro

 John Stiver

Record of Revisions

Revision 

Number

Effective 

Date Description of Revision

0 (Draft) 10/07/2013 Initial issue
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 3 of 8

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.

TABLE OF CONTENTS 

1.0 Introduction ......................................................................................................................... 4

2.0 NIOSH Response ................................................................................................................ 4

3.0 Selection of the Appropriate Airborne Concentration ........................................................ 5

4.0 Selection of the Appropriate Terminal Velocity ................................................................. 6

5.0 Calculation of Equilibrium Settling Times ......................................................................... 6

6.0 References ........................................................................................................................... 8
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 4 of 8

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.

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

Determination of Settling Time”

1.0 INTRODUCTION 

SC&A has been tasked on numerous recent occasions with reviewing dose reconstructions, site 

profiles, and a petition evaluation report that relied on Battelle-TBD-6000 (Battelle 2011) to 

provide the bases for completed dose reconstructions or possible future dose reconstructions. 

These reviews provided additional insight into how TBD-6000 is used in practice and revealed 

some potential issues associated with using that document that were not apparent when the 

document was originally reviewed and proposed changes were discussed. Based on this recent 

experience, SC&A provided supplementary comments on TBD-6000 (SC&A 2013). One of the 

areas discussed in SC&A 2013 was the appropriateness of using a terminal settling velocity of 

0.00075 m/s for small diameter particles and assuming that equilibrium between particle settling 

and removal was achieved in 30 days or less. 

SC&A concluded that: 

The assumption that equilibrium between deposition and removal processes is 

reached in 30 days as specified in TBD-6000 should be re-examined. The 

analyses presented here based on average settling rates in Main Bay of the 

Hanford Melt Plant Building indicate that 33–37 days are required to reach 

equilibrium based on 24 hr/day of settling and 40–44 days based on 12.4 hr/day 

of settling. The 95th percentile of the 15 Main Bay samples is 69 days to reach 

equilibrium. Based on these calculations, the claimant-favorable approach would 

be to use a value of 70 days to achieve an equilibrium surface concentration 

where the rate of deposition equals the rate of removal. However, if the settling 

time for dust is 12.4 hr, the time to reach equilibrium would increase by 20% 

suggesting an upper bound of 84 days. 

2.0 NIOSH RESPONSE 

In August 2013, NIOSH issued a response in which they questioned the calculational approach 

taken by SC&A to estimate equilibrium settling times for airborne particles (NIOSH 2013). 

NIOSH recalculated equilibrium settling times for both Simonds Saw and Steel and the Hanford 

Melt Plant Building (Adley 1952). We will focus here on the Adley data, since they represent a 

more comprehensive dataset.1

In the August 2013 White Paper, NIOSH used the following equation to calculate the 

equilibrium settling time: 

 

1

 We have some questions about the appropriateness of using data from Simonds Saw and Steel. As 

described in SRBD 023579 – Summary Report of Three Surveys – air samples taken before and after the floor was 

washed with a high pressure hose showed little difference. This suggests that much of the contamination was 

embedded in the surface rather than simply deposited on the surface. 
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 5 of 8

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.

 Settling time = Surface contamination ÷ airborne concentration ÷ 0.00075 m/s 

In this equation, 0.00075 m/s is the theoretical terminal settling velocity for 5-μ AMAD 

spherical particles. For the Hanford Melt Plant, “surface contamination” was based on plate 

settling data from Table XIII of Adley. The settling data, listed in units of mg/ft2

/day, were 

multiplied by the applicable number of days the plate was exposed and converted to μg/m2. 

From the 20 plate samples (office samples omitted), NIOSH developed statistics for the 

geometric mean, the average, and the 95th percentile assuming a lognormal distribution. It 

should be noted that this procedure combines plate settling results from the shop, the furnace 

room, the burnout room, and the main bay area, even though each of these areas were 

structurally separate and experienced different ventilation conditions. NIOSH results are 

included in Table 1 (extracted from Table 1 of NIOSH 2013). 

Table 1. Equilibrium Settling Time – Hanford Melt Plant 

Distribution Parameter Surface Contamination 

(μg/m2)

Airborne Concentration 

(μg/m3)

Settling Days

Geometric Mean 1,839,873 534 53.1

Average 2,350,865 8,436 4.3

95th Percentile 5,645,961 26,757 3.3

Source: NIOSH 2013, Table 1 

NIOSH calculated the “airborne concentration” using air sampling data included in Tables II 

through VII of Adley. We have several concerns about this calculation of the “airborne 

concentration:”

 Adley Table V includes air samples from outdoor burning of uranium, which do not 

appear relevant to the calculation of settling time based on indoor plate samples 

 As with the surface contamination, all air samples are lumped together, even though they 

were taken in physically separated areas 

 Some high concentration air samples do not contribute significantly to surface deposition 

We have two additional concerns about the calculation of equilibrium settling times as 

summarized in Table 1 of the NIOSH White Paper: 

 We believe that the airborne value should be adjusted to a time-integrated value, since the 

deposition on the plates occurred for only a portion of each operating day 

 We question the use of 0.00075 m/s in calculating the settling time 

Each of these points is discussed in the ensuing paragraphs. 

3.0 SELECTION OF THE APPROPRIATE AIRBORNE 

CONCENTRATION 

As described above, the air samples from Adley Tables II through VII were used by NIOSH to 

calculate the airborne concentration. During the period of the air sampling (September 1949 
Effective Date:

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NIOSH White Paper – Determination of Settling Time

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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.

through October 1951), the Melt Plant was operating on a single shift basis and the measured 

concentrations are spot values taken during single shift operation. In order to obtain a timeintegrated value, these spot values must be adjusted. To accomplish this, we assumed that the 

plant was on a 5-day schedule for 52 weeks per year. We further assumed that, on a daily basis,

settling occurred over a 10.7-hour period. This implies that at the start of each shift, the air 

concentration instantaneously ramps up to the measured value and that it takes about 5 hours 

after the end of the shift for the concentration to fall to zero (i.e., a particle falling from a height 

of 10 m at a settling velocity of 0.00052 m/s requires 5.3 hours to reach the surface). Using these 

assumptions, the measured air concentration multiplier is 0.32 (10.7/24 × 260/365 = 0.32). 

4.0 SELECTION OF THE APPROPRIATE TERMINAL VELOCITY 

NIOSH assumed that the terminal settling velocity to use in calculating the equilibrium settling 

time was 0.00075 m/s, the terminal settling velocity for spherical 5-μ AMAD particles. As 

described in Adley (1952, page 48), the particles were clearly not spherical. Since the goal is to 

calculate settling times from measured surface concentrations, it is more appropriate to use a 

settling velocity that takes into account the fact that the actual particles are non-spherical. In its 

Supplementary Comments on Revision 01 of Battelle TBD-6000 (SC&A 2013), SC&A provided 

the basis for adjusting the terminal settling velocity to account for slip between air molecules and 

for lack of sphericity of the particles. The adjusted settling velocity was 0.00052 m/s. 

5.0 CALCULATION OF EQUILIBRIUM SETTLING TIMES 

SC&A has taken essentially the same data as used by NIOSH to develop Table 1 in their August 

white paper, but modified it based on the foregoing discussion. Rather than lumping all the 

surface contamination values together and all the airborne values together, as was done by 

NIOSH, we used disaggregated data based on the rooms or areas where the measurements were 

made. We also eliminated a few of the airborne values based on descriptions in Adley (1952) as 

to the relevance of particular values. For example, data for Operation E in the furnace room 

were eliminated based on a statement in Adley (1952, page 22): 

Thus Operation E is an example of a process that is an exposure to the operator, 

but is not a contributor to general atmospheric dustiness. 

The average air concentrations calculated by SC&A are summarized in Table 2. Office data 

were excluded from this analysis. 

All of the measured air concentrations in Table 2 were multiplied by a factor of 0.32 to account 

for the fact that the Melt Plant was operating and generating dust on a single shift basis. The 

terminal settling velocity was adjusted to 0.00052 m/s to account for slip and particle shape. 

Based on the available information, it was not possible to calculate separate air concentrations 

for winter and spring samples. Calculated equilibrium settling times are summarized in Table 3. 
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 7 of 8

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.

Table 2. Average Air Concentrations 

Adley Source Location Average Air Concentration 

(dpm/m3)

Table II Furnace Room 723*

Table III Saw Room 1,752*

Table IV Burnout Room 11,849

Table VI Oxide Burning 28,631

Table VII – Operation B Storage Bay Activities 1,291

Table VII – Operation C Rod Straightening Activities 5,358*

Table VII – Operation D General Air Samplesa1,569

Table VII – Operation E Autoclave Platform 440

a – Used only in calculating Main Bay Average 

* – Adjusted based on comments in Adley 1952 

Table 3. Recalculation of Equilibrium Settling Times Based on Average 

Adjusted Air Concentrations 

 Location Settling Rate

mg U/ft2/day

Settling 

Rate

μg/m2/day

Measured Surf 

Conc.

μg/m2

Adjusted Air 

Concentration

μg/m3

Days to

Equilibrium

Furnace Room – Winter 0.22 2,376 375,408 151 55.4

Furnace Room – Spring 0.89 9,612 1,124,604 151 166.1

Burnout Room – Winter 5.72 61,776 9,760,608 2,469 88.0

Saw Room – Winter 1.14 12,312 1,945,296 365 118.6

Saw Room – Spring 0.88 9,504 1,111,968 365 67.8

Main Bay – Winter 1.23 13,296 2,100,768 2,980 15.7

Main Bay – Spring 1.84 19,872 2,325,024 2,980 17.4

From this table, it can be seen that the time to reach equilibrium appears to be strongly dependent 

on whether one is dealing with a large open area (i.e., the Main Bay) subject to a variety of air 

currents or smaller enclosed rooms where shifting air currents are less likely. The NIOSHproposed use of 30 days to reach equilibrium appears appropriate for large open areas at the 

Hanford Melt Plant, but understates by a factor of 2 to 5 the time to reach equilibrium in more 

enclosed settings. For perspective, the Melt Plant was about 200 ft long and 90 ft wide (Gerber 

1996), and the Main Bay area occupies about 11,000 to 12,000 square feet of the plant. The 

smaller enclosed rooms are estimated to be 500 to 1,500 square feet in area. 
Effective Date:

October 7, 2013

Revision No.

0 – Draft

Document Description: Comments on

NIOSH White Paper – Determination of Settling Time

Page No.

Page 8 of 8

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.

6.0 REFERENCES

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.

Battelle 2011. Site Profiles for Atomic Weapons Employers that Worked Uranium Metals. 

Battelle-TBD-6000. Rev. 01. National Institute for Occupational Safety and Health (NIOSH); 

Cincinnati, Ohio; Division of Compensation Analysis and Support (DCAS); effective June 17, 2011; 

SRDB Ref ID: 101251

Gerber M.S. 1996. The Plutonium Production Story at the Hanford Site: Processes and Facility 

History. WHC-MR-0521. Westinghouse Hanford Company, Richland, Washington.

NIOSH 2013. TBD 6000 Review by SC&A, Determination of Settling Time, White Paper 

prepared by David Allen (DCAS), Cincinnati, Ohio. August 2013.

SC&A 2013. Supplementary Comments on Revision 01 of Battelle TBD-6000. Contract No. 

200-2009-28555, Rev. 0. Prepared by W.C. Thurber. SC&A, Inc., Vienna, Virginia. May 2013.