Review of SRS Refined Construction Trade 

Worker Stratification Comparisons for 

Plutonium

Bob Barton, SC&A

Savannah River Site/SEC Issues Work Groups Meeting

December 5–6, 2019
Background

 Key Question: Do subcontractor construction trade 

workers (CTWs) have the same exposure potential as 

prime contractor CTWs?

 NIOSH analysis compares the relative magnitude of 

plutonium results for the two CTW groups

 First presented to SRS Work Group in October 2017

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Background (cont.)

 NIOSH October 2017 evaluation concluded: 

 “for most years there is little difference in the 95 percentile 

urinary excretion between DuPont CTWs and Subcontractor 

CTWs. The exception appears to be in the later 1970s and 

1980s. This observation is somewhat supported by 

contemporary interviews with subcontractor CTWs. 

Subcontractor CTW indicated that they were called in for 

more contaminated work to save the exposure of the onsite 

CTWs. For some years (1977-1979 and 1984-1986) this 

appears to be the case in that the 95th percentile of the 

subcontractor CTWs is a factor of 2-5 higher.” 

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Background (cont.)

 Due to noted differences between DuPont CTWs and 

subcontractor CTWs in the years 1977, 1979, 1985, and 

1986, NIOSH pursued a more rigorous analysis

 Urinalysis data evaluated for the years 1974, 1977, 1980, 

1983, and 1986, and intakes were derived for two time 

periods (1973–1978 and 1979–1987)

 Results presented in 2019 white paper, “Savannah River 

Site Plutonium Construction Trade Worker Stratification 

Refinement”

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Calculated Intakes Based on NIOSH’s 2019 Pu 

Stratification Evaluation

Years,

solubility 

type

Geometric 

mean 

DuPont 

CTW intake 

rate 

(dpm/d)

Geometric 

mean 

subcontract 

CTW intake 

rate (dpm/d)

Geometric 

mean ratio 

(DuPont/ 

Sub)

95th 

percentile 

DuPont CTW 

intake rate 

(dpm/d)

95th 

percentile 

subcontract 

CTW intake 

rate (dpm/d)

95th 

percentile 

ratio 

(DuPont/ 

Sub)

1973–1978,

M 0.7732 0.325 2.38 14.349 8.00 1.79

1973–1978,

S 15.71 6.97 2.25 268.7 169.4 1.59

1979–1987,

M 1.426 1.293 1.10 16.215 19.17 0.85

1979–1987,

S 26.38 22.65 1.17 279.2 326.1 0.86

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Background (cont.)

 Refined 2019 analysis concludes:

 “As can be seen by examination of the tables above, the geometric 

mean (GM) of the results for the DuPont CTWs are higher than that 

for the subcontractor CTWs for all years evaluated. For the 1973 

through 1978 period, the 95th percentile intake results for the 

DuPont CTWs are higher as well. For the 1979 through 1987 period, 

the subcontractor CTWs do have a higher 95th percentile due to 

the higher geometric standard deviation (GSD) of the data....

NIOSH believes it is reasonable to combine all CTWs into a single 

stratum for assignment of intakes in the SRS internal dose 

coworker study.” 

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SC&A Review – General Comments

 Finding 1: In SC&A’s opinion, the conclusion that 

subcontractor construction trade workers had higher 

excretion rates and derived intakes at the 95th 

percentile is significant from the standpoint of 

stratification because the 95th percentile is what is 

proposed for assignment to unmonitored subcontract 

construction workers. 

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SC&A Review – General Comments (cont.)

Observation 1: The evaluation in NIOSH’s 2019 white paper 

was limited to 5 years during the DuPont Era (1972–1989), 

and additional data (beyond previously captured NOCTS data) 

were coded for only subcontractors during 3 of these 5 years. 

Given that subcontract construction workers showed higher 

derived intakes at the 95th percentile from 1979 through 

1987, a more rigorous analysis during the SEC period may be 

appropriate to determine the extent to which such 

differences exist in other years, which may further prompt 

the need for stratification. 

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SC&A Review – Subcontractor Determination

 Subcontractors were identified by payroll ID in NIOSH 

analysis

 SC&A examined 35 randomly selected claims that were 

identified as subcontractors

 13 of 35 (~37%) exhibited evidence that the subcontractor 

designation may have been incorrect (Finding 2)

 Remaining 22 of 35 all exhibited evidence of employment 

with subcontractors

 Additional review of 25 claims designated as prime 

contractors all appear to be correctly categorized

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SC&A Review – Additional Data Coding

 Revised NIOSH evaluation included additional data 

coding for nonclaimant subcontractors in 1974, 1983, 

and 1986. 

 All other evaluated data were based solely on claimant 

records.

 Key Question: Does the addition of nonclaimant data 

for subcontractors in these years bias the results?

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SC&A Review – Additional Data Coding (cont.)

 Observation 2: The urinary excretion rates for claimant 

subcontractors at the average and lognormal geometric 

mean suggest a higher exposure potential than 

nonclaimant subcontractors for the years in which 

nonclaimant data were used to supplement the evaluation 

(1974, 1983, and 1986). Because nonclaimant data 

dominate the evaluated subcontractor population in these 

years, this could have a significant effect on the combined 

subcontractor analysis and subsequent comparison to 

prime contract workers.

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SC&A Review – Regression Analysis

 Hardcopy data often only contained an unnormalized 

bioassay result (activity per measurement disc rather than 

activity per volume)

 Conversion of unnormalized results to volumetric sample 

not straightforward without knowledge of aliquot amount

 NIOSH assumes a linear relationship to convert activity per 

measurement to activity per volume

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SC&A Review – Regression Analysis (cont.)

 SC&A also identified situations in which a zero or negative 

result given in activity per disc resulted in a positive 

reported volumetric result

 Raises the question of whether a linear regression or any 

meaningful numerical relationship would actually apply 

(SC&A Observation 3)

 Some of these data pairs were found to be transcription 

errors: What quality assurance assessment was applied? 

(SC&A Finding 3)

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SC&A Review – Comparison of Actual Bioassay to 

Linearly Fit Bioassay for 1986

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SC&A Review – Regression Analysis (cont.)

 Finding 4: The use of a regression analysis to convert raw 

bioassay results (dpm/disc) to a volumetric result 

(dpm/1.5 L) has not been technically justified and does not 

appear scientifically defensible. 

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Conclusions

 Refined analysis is limited to a handful of years.

 Evidence suggests payroll ID numbers alone may not 

be sufficient to correctly identify subcontractors.

 Quality assurance of additional data coding is not 

addressed.

 Use of linear regression to convert urinalysis values to 

a normalized volume is questionable.

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Conclusions (cont.)

 As SC&A indicated previously, the NIOSH analysis still 

shows that subcontractors had higher intakes at the 

95th percentile for 1979–1987.

 Therefore, SC&A does not agree that the NIOSH 

analysis demonstrates subcontractor CTWs and prime 

contractor CTWs are part of the same exposure strata.

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Potential Path Forward

 Expand analysis to remaining years of SEC-00103.

 Utilize all available logbook data (not just claimant 

data for prime contractors).

 Document quality assurance on transcription errors.

 Confirm subcontractor designations beyond payroll ID 

methodology.

 Discontinue use of regression methods for samples 

with unnormalized results and reevaluate using only 

normalized results.

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Questions?

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