DISCLAIMER: This is a working document provided by the Centers for Disease Control and Prevention (CDC) 

technical support contractor, SC&A, for use in discussions with the National Institute for Occupational Safety and 

Health (NIOSH) and the Advisory Board on Radiation and Worker Health (ABRWH), including its Working Groups 

or Subcommittees. Documents produced by SC&A, such as memorandum, white paper, draft or working documents 

are not final NIOSH or ABRWH products or positions, unless specifically marked as such. This document prepared 

by SC&A represents its preliminary evaluation on technical issues.

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

MEMORANDUM

TO: Savannah River Site Work Group 

FROM: SC&A, Inc.

DATE: October 11, 2018 

SUBJECT: SC&A Review of RPRT-0070 Methods for Th-232 Dose Reconstruction 

Introduction and Background 

In May 2017, the National Institute for Occupational Safety and Health (NIOSH) released 

ORAUT-RPRT-0070, Evaluation of Method for Assessment of Thorium-232 Exposures at the 

Savannah River Site from 1972 to 1989 (NIOSH 2017a; hereafter “RPRT-0070). During the 

Advisory Board on Radiation Worker Health meeting on April 14, 2018, SC&A was tasked with 

evaluating RPRT-0070 as part of the Special Exposure Cohort review process. Prior to the 

release of this report, methods for thorium-232 (Th-232) dose reconstruction were described in 

ORAUT-OTIB-0081, Revision 03, Internal Coworker Dosimetry Data for the Savannah River 

Site (NIOSH 2016). In the prior methodology, coworker distributions were developed utilizing 

available urinalysis data for americium/curium/californium (Am/Cm/Cf) during the period of 

interest (October 1972 through the end of 1989) and stratified into two separate worker intake 

groups (construction trade workers and non-construction trade workers). The derived intakes 

based on the trivalent bioassay data from NIOSH 2016 are summarized in Table 1 for Type M 

and Type S thorium and the two separate intake groups.

Table 1. Summary of Derived Daily Intake Rates Based on Am/Cm/Cf Bioassay Data 

(October 1972–December 1989) as Provided in NIOSH 2016 

Worker intake 

group

50th percentile 

Type M intake 

(dpm/d)

50th percentile 

Type S intake 

(dpm/d)

95th percentile 

Type M intake 

(dpm/d)

95th percentile 

Type S intake 

(dpm/d)

Non-construction 

trade workers 4.8 67.6 44.5 626.2

Construction 

trade workers 5.2 72.7 58.2 874.8

The updated methodology presented in RPRT-0070 provided an alternate method for evaluating 

thorium exposures during the period 1981–1989 that is based on evaluation of routine air 

monitoring data and supporting estimates of potential intake using source term data. RPRT-0070 

indicates that use of the trivalent actinide bioassay model developed in ORAUT-OTIB-0081 

would still be used for the period from October 1972 through May 1980, when evidence suggests 
Memo – RPRT-0070 Review 2 SC&A – October 11, 2018 

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that major work with thorium material had ceased.1

1 It should be noted that the issue of the suitability of the internal bioassay monitoring program at the Savannah 

River Site for the purposes of reconstructing internal doses is the subject of continued Work Group discussions and 

research conducted by NIOSH, Oak Ridge Associated Universities Team (ORAUT), and SC&A.

 Beginning in mid-1980 or the start of 1981,2

NIOSH recommends assignment of intakes of thorium based on the censoring level (CL) for 

routine alpha air sampling data at the Savannah River Site (1×10-12 microcuries per cubic 

centimeter [μCi/cm3]) which would result in a daily intake rate of 14.6 disintegrations per minute 

per day (dpm/d). This value can be compared to the previous coworker intake values shown in 

Table 1. NIOSH postulates that the level of exposure based on the air sampling reporting level 

would bound potential exposures for the following reasons:

2 It is not clear from RPRT-0070 whether the newly developed thorium dose reconstruction method begins in 

June 1980 (when large-scale thorium production ceased) or is intended to begin in January 1981, as was indicated in 

final sentence of Section 1.0 of NIOSH 2017a.

1. Ninety-five percent of the sampled alpha air monitoring data for the 773-A Area were 

below the reporting level used in developing the internal exposure methods presented in 

RPRT-0070. 

2. Alternate methods of assessing potential intake methods, as presented in NUREG-1400 

(NRC 1993), would result in estimated intakes of thorium presented in Table 3-4 of 

RPRT-0070 as more than an order of magnitude lower than those based on the reporting 

level air sampling approach.

3. Estimated airborne concentrations of thorium based on the previously derived coworker 

intakes in NIOSH 2016 would result in air concentrations that are 6–7 times higher than 

the air sampling level of 1×10-12 presented in RPRT-0070. 

Each of these three important facets of RPRT-0070 are discussed below. 

Item 1: Discussion of the Air Sampling Data Evaluated in ORAUT-RPRT-0070 

To evaluate available alpha air sampling data for use in dose reconstruction, NIOSH elected to 

analyze a random sample of daily air sample results in the 773-A Laboratory Areas. The size of 

the appropriate random sample to be analyzed was determined based on a NIOSH statistical 

analysis of the gross alpha air sampling at the Plutonium Fuel Form Facility during the 1970s as 

presented in ORAUT-RPRT-0080, Potential Neptunium Exposure to Plutonium Fuel Facility 

Construction Workers in Building 235-F at the Savannah River Site (NIOSH 2017b). The intent 

of that report was to evaluate the exposure potential to workers involved in new construction that 

was occurring inside Building 235-F but outside the actual neptunium billet line. RPRT-0070 

does not discuss the applicability of that statistical analysis in in relation to the exposure 

potential in a different facility (773-A Laboratory Areas) during a different time period (the 

1980s). Such a discussion would appear necessary to demonstrate that the statistical analysis and 

derived random sample sizes in NIOSH 2017b are sufficiently analogous to derive thorium 

intakes for dose reconstruction as presented in NIOSH 2017a. 

Observation 1: The methods used in selecting an appropriate sample size for analysis were 

based on air sampling data from a separate facility during a separate timeframe. NIOSH should 

 
Memo – RPRT-0070 Review 3 SC&A – October 11, 2018 

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provide a discussion in RPRT-0070 to demonstrate that the statistical analysis of gross alpha air 

sampling in Building 235-F during the 1970s is sufficiently analogous of the conditions in the 

773-A Laboratory Areas during the 1980s to justify its use in defining a random sample size for 

exposure potential analysis to thorium (1981–1989). 

As shown in Item 1 above, NIOSH presents the compelling argument that over 95% of the 

evaluated air sampling results were below the CL of 1×10-12 μCi/cm3. This is best demonstrated 

in Figure 3-2, which provides the median percent of the air samples that are above the CL for 

4 years: 1975, 1981, 1984, and 1987. This figure is recreated as Figure 1 in this report for 

convenience. 

Figure 1. Figure 3-2 from NIOSH 2017a, Showing Percentage of Air Samples Below the 

Censoring Level (1×10-12 μCi/cm3) 

The period under evaluation in RPRT-0070 spans 1981 through 1989; therefore, the inclusion of 

1975 air sampling data does not appear to be relevant to the exposure conditions experienced 

during the 1980s. In addition, the references provided with Figure 3-2 (in this memo listed as 

Dupont 1981a–f, Dupont 1984a–e, and Dupont 1987a–c) only provide information on the 

samples that were above the CL and do not provide the total number of air samples taken during 

a given measurement period (i.e., the denominator). Therefore, it is unclear how the percentage 

of samples above the CL for a given monitoring period was determined. One possibility is that 
Memo – RPRT-0070 Review 4 SC&A – October 11, 2018 

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the full list of air sampling location codes was assumed to be the entirety of the air monitoring in 

operation during a given period. However, without actual totals provided to support Figure 3-2, it 

is not possible to verify that this was the approach taken. 

Observation 2: Because the air sampling methodology only applies to the period after May 1980 

(or alternately December 1980), it is not clear why NIOSH chose to analyze air sampling data 

during the year 1975 as shown in Figure 3-2 of RPRT-0070. Inclusion of such data is likely 

inappropriate. 

Observation 3: Given that the cited air sample references only report the values that were above 

the CL (1×10-12 μCi/cm3), SC&A requests clarification of how the total number of samples (i.e., 

the denominator) was determined as presented in Figure 3-2 of RPRT-0070. 

Although RPRT-0070 presents an analysis of the percentage of air sample data that exceeded the 

CL, it is equally important to consider the magnitude of the air samples that exceeded the CL and 

at what locations those measurements occurred to determine the relative impact to exposure 

potential to a given contaminant and/or work activity. Therefore, SC&A analyzed the available 

air sample results that reported alpha activities greater than the CL. Table 2 summarizes the 

sampled data from 1981, 1984, and 1987. As seen in the table, a large percentage of positive air 

sampling results reported that respiratory protection had been used (~84% for all 3 years 

combined). SC&A calculated the weighted air concentrations by area in Table 2 by assuming all 

monitoring periods for a given area in which a positive result was not reported were equal to 1⁄2 

the CL (0.5×10-12 μCi/cm3). For example, if there were 10 monitoring periods with one 

monitoring result reported at the CL, the weighted average would be calculated as: 

[(1 × CL) + (9 × 1⁄2CL)]/10 = 0.55 × CL

On average, the weighted air concentrations for a given area by year were all less than the CL. 

However, some areas had weighted air concentrations that were greater than the CL, with the 

maximum weighted air concentration approximately 4.65 times the CL. Evaluation of the air 

monitoring data in 1981 and 1987 showed that approximately 10% of the areas with positive air 

samples had weighted averages that exceeded the CL. In 1984, the percentage was closer to 20% 

(10 of 57 locations had weighted averages above the control limit). Table 3 gives additional 

details about the areas in which weighted air concentrations exceeded the CL. 

Table 2. Summary of Positive Alpha Air Monitoring Results Observed for the Years 1981, 

1984, and 1987 

Time period 1981 1984 1987 All 

samples

Total air monitoring periods sampled 127 108 60 295

# of positive alpha air samples 469 210 215 894

% of positive results noting respiratory 

protection used 100.0% 62.8% 73.7% 84.8%

# unique locations reporting positive results 115 56 76 174
Memo – RPRT-0070 Review 5 SC&A – October 11, 2018 

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Time period 1981 1984 1987 All 

samples

Weighted average concentration for areas with 

positive results (× 10-12 μCi/cm3) 0.80 0.94 0.75 0.66

Maximum weighted concentration for areas with 

positive results (× 10-12 μCi/cm3) 4.65 4.58 2.61 3.81

# of areas with weighted concentrations greater 

than censoring limit (% of total)

11

(9.6%)

10

(17.9%)

7

(9.2%)

11

(6.3%)

Table 3 shows that the maximum weighted average air concentration was less than a factor of 5 

compared to the CL. Notably, the areas where the higher weighted average air concentrations 

occurred were generally in the high-level cave (HLC) areas, where respiratory protection was at 

or near 100%. Given that any small-scale handling of thorium during general storage or material 

transfer activities would likely have been of short duration, SC&A agrees that the use of the CL

over the course of an entire year represents a reasonably bounding estimate of thorium exposure 

under normal conditions. 

Table 3. Overview of Locations Displaying Weighted Air Concentrations Greater than the 

Censoring Level (1×10-12 μCi/cm3

) 

Year (air 

sampler 

location 

number)

Air sampler location 

description

# positive 

samples

Weighted 

average × CL 

(1×10-12) μCi/cm3

% positive results 

indicating respiratory 

protection was used

1981 (55, HLC) 773-A (C-118) 30 4.65 100.0%

1984 (55, E 

Area) 773-A (C-118) 15 4.00 93.3%

1984 (28, E 

Area) 773-A (B-134) 18 3.56 77.8%

1981 (42, HLC) 773-A (B-155) 30 3.49 100.0%

1981 (29, HLC) 773-A (B-138) 31 3.21 100.0%

1981 (27, HLC) 773-A (B-131) 32 3.20 100.0%

1981 (28, HLC) 773-A (B-134) 31 3.11 100.0%

1981 (41, HLC) 773-A (B-154) 28 3.06 100.0%

1981 (54, HLC) 773-A (C-114) 22 2.86 100.0%

1984 (42, E 

Area) 773-A (B-155) 15 2.79 86.7%

1984 (29, E 

Area) 773-A (B-138) 17 2.78 94.1%

1981 (43, HLC) 773-A (B-196) 31 2.77 100.0%

1984 (54, E 

Area) 773-A (C-114) 14 2.40 85.7%

1984 (43, E 

Area) 773-A (B-196) 14 2.17 85.7%

1981 (22, HLC) 773-A (B-126) 17 1.93 100.0%

1984 (27, E 

Area) 773-A (B-131) 17 1.84 82.4%

1984 (41, E 

Area) 773-A (B-154) 14 1.57 85.7%
Memo – RPRT-0070 Review 6 SC&A – October 11, 2018 

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Year (air 

sampler 

location 

number)

Air sampler location 

description

# positive 

samples

Weighted 

average × CL 

(1×10-12) μCi/cm3

% positive results 

indicating respiratory 

protection was used

1987 (9-5) Lab Area (B-005 NW 

Center) 16 1.52 93.8%

1987 (9-12) Lab Area (B-003) 26 1.47 92.3%

1987 (9-1) Lab Area (B-005 NE) 21 1.27 95.2%

1984 (9-46) 776-4-A (After 1st 

Stage Filter) 9 1.21 22.2%

1987 (9-2) Lab Area (B-005 NW) 13 1.19 92.3%

1987 (9-14) Lab Area * 13 1.19 92.3%

1984 (9-14) Lab Area * 14 1.12 78.6%

1981 (12, HLC) 773-A (B-114) 2 1.06 100.0%

1981 (9-12) Lab Area (B-003) 10 1.05 100.0%

* Specific lab area not designated.

Observation 4: Based on SC&A’s analysis of the relative magnitude and location of positive air

sampling results, SC&A agrees that the use of the CL of 1×10-12 μCi/cm3 over the course of an 

entire year represents a reasonable upper bound on the exposure potential to thorium under 

normal conditions associated with storage and material transfer activities.

Item 2: Use of NUREG-1400 Methodology in Dose Reconstruction 

In Section 3.1, NIOSH presents an alternate method for estimating intakes of radioactive 

material that utilizes various correction factors multiplied against known inventory of material; 

this methodology is outlined in NUREG-1400 (NRC 1993). The fractional intake method takes 

the known amount of a material in process and uses various correction factors3 to arrive at an 

approximate intake over the course of a sufficiently long period of time (nominally 1 year). 

NIOSH states in Section 4 (pp. 11–12): 

3 The correction factors themselves are presented in Section 3.1 of NIOSH 2017a and so are not discussed here.

Based on a source term analysis, NUREG-1400, the average air concentration 

that a worker could have been exposed to would likely not exceed 1 × 10-13

μCi/cm3

.... 

The NUREG-1400 approach likely gives the most realistic exposure estimate for 

thorium from 1981 to 1989. 

SC&A does not believe the approach or application in this instance is appropriate for dose 

reconstruction. The methodology presented in NUREG-1400 was meant to help facilities 

develop an appropriate air monitoring program to protect workers from potential intakes. It was 

not intended to be used in lieu of an already established air monitoring program or, further, a 

sufficiently accurate internal monitoring program to evaluate internal dose potential. 

In cases where such internal monitoring programs do not exist, the application of NUREG-1400 

may be considered acceptable with appropriate conservatism considered. Most recently, NIOSH 

 
Memo – RPRT-0070 Review 7 SC&A – October 11, 2018 

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has used NUREG-1400, and SC&A has recommended its use be accepted (SC&A 2018), in the 

evaluation of exposure potential to 23 unmonitored radionuclides at the X-10 site (NIOSH 2018). 

However, in that case, more viable methods of bounding doses were unavailable, and significant 

adjustments were made to assure a bounding scenario. Specifically, NIOSH 2018 uses the basic 

fractional intake multiplier (10-6) and multiplies it by 10 to assure conservatism. Essentially, the 

NIOSH 2018 application multiples the known amount of material in process by a fractional 

intake multiplier of 10-5. 

In the application of the NUREG-1400 methodology presented in Section 3.1 of RPRT-0070, 

NIOSH develops a fractional intake factor equal to 10-8 (three orders of magnitude lower than 

that used for the X-10 site). Nonetheless, it should be noted that the NUREG-1400 approach is 

not being proposed for actual dose reconstruction in this instance. Rather, the method is used as a 

comparative analysis to justify the use of the air monitoring CL. Therefore, any findings, 

observations, or further discussion is likely not warranted here. 

Item 3: Comparison of Thorium Exposures Using Air Sampling Data versus Derived 

Coworker Intakes

In the concluding section of RPRT-0070 (Section 4.0), NIOSH states the following: 

The coworker intakes presented in ORAUT (2016) are considered bounding but 

result in estimated exposures that are not supported by the air data during the 

same time period. If routine air concentrations were at the levels associated with 

the coworker intake rates, there should have been a systemic air concentration 6 

to 7 times the reporting level. There is no indication of a systemic air 

concentration above the reporting level; therefore, after 1980, the intake rate 

based on the reporting level of the air data is considered the best estimate of 

routine thorium exposure and represents a sufficiently accurate bounding 

scenario. [pp. 12–13; emphasis added]

As stated in Observation 4, SC&A agrees that the use of the CL likely represents a sufficiently 

bounding scenario for exposure to thorium under normal conditions during the period for which 

no major thorium campaigns were undertaken (1981–1989). However, RPRT-0070 does not 

address situations in which nonroutine or “off-normal” conditions may have existed. These 

activities might include direct maintenance, decontamination and decommissioning (D&D), or 

modification of facility equipment that had previously been used for significant work with 

thorium. These facilities would also expand beyond the 773-A Laboratory Areas to other areas of 

potential thorium exposure during nonroutine work conditions. 

Finding 1: Per the hierarchy of data criteria, bioassay sampling is considered scientifically 

preferable to other data and methods, such as the use of air sampling and/or modeling 

assumptions based on source term inventories. While alternate methods other than the use of 

bioassay monitoring may be useful in situations where exposure potential is essentially routine or 

ambient in the facility, such methods may not be applicable to workers involved in off-normal 

conditions, such as direct maintenance, modification of previously contaminated equipment, or 

other D&D activities. 
Memo – RPRT-0070 Review 8 SC&A – October 11, 2018 

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Conclusion 

While the proposed approach using the CL for alpha air sampling (1×10-12 μCi/cm3) may 

generally reflect the routine ambient exposure conditions experienced at the Savannah River Site 

during the period 1981–1989 when little to no thorium was in process, it does not address offnormal conditions such as D&D activities in former thorium work areas. Such activities may 

create localized exposure potential that may not be captured through routine air sampling or sitewide source term analysis. NIOSH should consider continuing its use of the derived coworker 

intakes for thorium as presented in ORAUT-OTIB-0081 (NIOSH 2016) for workers potentially 

involved in off-normal radiological work that may have brought them into contact with thorium. 

References

DuPont 1981a. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 159272] 

DuPont 1981b. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 159285] 

DuPont 1981c. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 159291] 

DuPont 1981d. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 161029] 

DuPont 1981e. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 161030] 

DuPont 1981f. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 161031] 

DuPont 1984a. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108678] 

DuPont 1984b. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108685] 

DuPont 1984c. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108687] 

DuPont 1984d. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108691] 

DuPont 1984e. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108692] 
Memo – RPRT-0070 Review 9 SC&A – October 11, 2018

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

DuPont 1987a. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108585]

DuPont 1987b. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108592]

DuPont 1987c. Collection of air sampling information and results, E.I. du Pont de Nemours and 

Company, Savannah River Site, Aiken, SC. [SRDB Ref. ID 108651]

NIOSH 2016. Internal Coworker Dosimetry Data for the Savannah River Site, ORAUT-OTIB0081, Revision 03, National Institute for Occupational Safety and Health, Cincinnati, OH. 

November 22, 2016.

NIOSH 2017a. Evaluation of Method for Assessment of Thorium-232 Exposures at the Savannah 

River Site from 1972 to 1989, ORAUT-RPRT-0070, Revision 00, National Institute for 

Occupational Safety and Health, Cincinnati, OH. May 15, 2017. 

NIOSH 2017b. Potential Neptunium Exposure to Plutonium Fuel Facility Construction Workers 

in Building 235-F at the Savannah River Site, ORAUT-RPRT-0080, Revision 00, National 

Institute for Occupational Safety and Health. Cincinnati, OH. February 2, 2017.

NIOSH 2018. Monitoring Feasibility for Exotic Radionuclides Produced by the Oak Ridge 

National Laboratory Isotopes Division, ORAUT-RPRT-0090, Revision 00, National Institute for 

Occupational Safety and Health, Cincinnati, OH. March 29, 2018.

NRC 1993. Air Sampling in the Workplace, Final Report, NUREG-1400, U.S. Nuclear

Regulatory Commission, Washington, DC. September 1993. [SRDB Ref. ID 20129]

SC&A 2018. SC&A’S Evaluation of RPRT-0090, “Monitoring Feasibility Evaluation for Exotic 

Radionuclides Produced by the Oak Ridge National Laboratory Isotopes Division,” Revision 00,

SCA-TR-2018-SEC004, Revision 0, SC&A, Inc., Arlington, VA. October 9, 2018.