SC&A Evaluation of Residual Dose 

Reconstruction Methods at 

Simonds Saw and Steel 

Bob Barton 

Health Physicist 

S. Cohen and Associates 

Contractor to: 

Advisory Board on Radiation Worker Health/ABRWH 

Centers For Disease Control and Prevention 

July 29, 2014 

Idaho Falls, Idaho 
SC&A Review of Proposed Methods of 

Assigning Residual External Dose 

Original Concerns: 

1. What measurement data are available during 

this period? 

2. How do the proposed dose assignments 

compare with the available measurements? 

3. How long is the worker assumed to be exposed 

at the chosen external dose levels? 

2 
Available External Measurements 

• Gamma/Penetrating Radiation Measurements Taken 

During Six Survey Activities Spanning 1957–2007

– 79 measurements available from the 1957, 1976, 1980, 

1984 and 1999 survey reports (ranging from “Not 

Detected” to a maximum of 300 mR/hr). 

– Over 2,000 measurements taken during the 2007 USACE 

survey (maximum value of 63 mR/hr) 

3 
Available External Measurements (cont). 

• Beta/Non-Penetrating Measurements at 3 feet only 

available from the 1957 survey report (results shown 

below): 

Survey Location 

(Approx. Volume of Contamination) 

Beta @ 3 Feet 

(mreps/hr) 

10” Bar Mill Bed (75 ft2 – 1⁄2 in. thick) 1.0–1.7

Front of Shear (10 ft2 – 1 in. thick) 0.4

Between Plates on Mill Floor (not given) 0.05

Forge Area (not given) 0.2

4 
Evaluation of Chosen Dose Assignments

• Proposed Gamma/Penetrating Value: 80 mR/hr

– Represents the maximum observed measurement during the 

1957 survey

– Of the 79 measurements observed during the 1957–1999 surveys 

only 4 exceeded 80 mR/hr

– Highest values were associated with a localized hot spot which 

decreased to levels below 80 mR/hr within ~10 feet of the area

– The 95th percentile of the 1957–1999 data is ~66.5 mR/hr

– None of the 2,000+ measurements from 2007 exceeded 80 

mR/hr; the 95th percentile of measurements taken in AEC work 

areas is ~11.3 mR/hr (within the range of background for the 

Lockport area).

5
Evaluation of Chosen Dose Assignments 

(cont).

• Proposed Beta/Non-Penetrating Value: 1.35 mreps/hr

– Represents the midpoint of the 1957 measurements in the 10” Bar 

Mill Bed area

– Measurements in other plant areas ranged from 0.05 to 0.4 mreps/hr

• Beta/Gamma Dose Ratios:

– Table 3.10 of TBD-6000 prescribes a beta/gamma dose ratio of ~100

– 2013 SC&A study using MCNP calculated a beta/gamma dose ratio of 

~45

– Both values assume an infinitely thin layer of contamination (i.e., no 

self attenuation)  actual beta/gamma dose ratios are likely much 

lower

– Proposed Beta/Gamma ratio (1.35/.08 = 16.9) is within range of the 

calculated bounding values and so represents a realistic dose ratio

6
Worker Exposure Duration

• TBD assumes a work-day exposure duration of 10 hours 

during the operational period and 8 hours during residual 

period

• Insufficient evidence exists to warrant a shorter work day 

during the residual period

• NIOSH agrees that the exposure duration utilized during the 

residual period should be consistent with the operational 

period

• New dose reconstruction method assumes proposed 

external exposure assignments (80 mR/hr penetrating and 

1.35 mreps/ hr non-penetrating) are assigned as a constant 

for 10 hours per work day (2,500 hours per year). 

7
Summary of External Dose Evaluation

• Proposed dose reconstruction method assigns dose by 

selecting measured values in the upper end of observed 

survey results

• Beta/gamma dose ratio of chosen values is comparable with 

theoretical ratios assuming an infinitely thin layer of 

contamination

• Doses are assigned as a constant assuming a 10-hour work 

day, which is consistent with the operational period

• SC&A/Work Group recommend acceptance of the proposed 

method as plausible, scientifically defensible and claimant 

favorable

8
SC&A Review of Proposed Methods of 

Assigning Residual Internal Dose

• Original Concerns

1. Assignment of exposure duration (8 hours versus 

10).

2. How is the dust loading established for the 

beginning of the residual period?

3. How is the dust loading established at the end of 

the residual period? 

9
Selection of Dust Loading at the 

Beginning of Residual Period

• Ideally, the source term available for inhalation can be 

characterized immediately following operations

• Air sampling data at the site are available from 1948–1954, but 

not from 1955–1957

• NIOSH evaluated 21 general air samples available from 2 rolling 

days in 1954 (as close to the end of operations as feasible)

– Chosen samples represent an actual uranium rolling 

operation  likely a significant overestimate of actual 

residual airborne levels

– Samples taken during a period where industrial dust control 

measures had been removed or were rendered ineffective

10
Selection of Dust Loading at the End of 

the Residual Period

• 2007 USACE Survey contained an extensive evaluation of all 

areas of the site including former AEC operational areas 

(Buildings 3, 6/8, and 24)

• Proposed dust loading calculated based on the highest 

observed 95th percentile contamination measurement in these 

operational areas (67,000 dpm/100 cm2in the southern 

portion of Building 24)

• Calculated dust loading assumes a resuspension factor of 10-6, 

breathing rate of 1.2 m3/hr, and 10 hours per day

11
Selection of Dust Loading at the End of 

the Residual Period (cont.)

• 95th percentile surface contamination in Building 24 was a 

factor of 1.2 higher than the observed 95th percentile 

contamination during the 1999 survey of the same location

• Proposed intake rate is a factor of 4.5 higher than the 

previous intake rate derived from USACE’s Exposure Point 

Concentrations (EPC)

• ORAUT-OTIB-0070 recommends a source depletion factor of 

0.00067 d-1; the proposed methodology results in a 

calculated depletion factor that is roughly 25% of this value 

(i.e., more claimant favorable)

12
Selection of Dust Loading at the End of 

the Residual Period (cont.)

• 2007 USACE survey performed breathing zone analyses for 

work activities likely to generate airborne contamination 

including:

– Brush clearing activities using the hydro-axe

– Boring activities in Buildings 6 and 8

– Radiological survey work conducted on the Building 24 roof 

trusses

• Survey concluded: “Breathing zone sample results 

demonstrated that airborne contamination during site 

activities was minimal. The maximum value for the 

breathing zone samples equated to 0.2 DAC-hrs [~ factor of 

10 lower than proposed value], with the majority below 

detection limits.”

13
Summary of Internal Dose Evaluation

• 10-hour work day has been adopted for consistency with the 

operational period

• Airborne dust loading at the beginning of the residual period 

based on general air samples taken during a uranium rolling 

operation as close to the end of operations as feasible

• Airborne dust loading at the end of the residual period is 

based on the highest 95th percentile surface contamination 

value among buildings utilized in AEC work

14
Summary of Internal Dose Evaluation (cont.)

• Calculated source depletion factor of the proposed method 

compares favorably with the recommended value in ORAUTOTIB-0070

• Breathing zone analyses conducted during the 2007 survey 

are bounded by the proposed intake rate

• SC&A/Work Group recommend acceptance of the proposed 

method as plausible, scientifically defensible, and claimant 

favorable

15
Questions?

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