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DOE Review Release 04/17/2017

Appropriateness of using 1997 Gross Alpha 

Air Sampling Data as a Method of Bounding 

Thoron Intakes at the SRS H-Area Tank Farm 

Between 1972 and 1994

ORAUT-RPRT-0081 Rev. 00

Effective Date: 04/07/2017

Supersedes: None

Subject Expert(s): James M. Mahathy

Document Owner 

Approval: Signature on File

James M. Mahathy, Document Owner

Approval Date: 04/04/2017

Concurrence: Signature on File

Daniel H. Stempfley, Objective 4 Representative

Concurrence Date: 04/04/2017

Concurrence: Vickie S. Short Signature on File for

Kate Kimpan, Project Director

Concurrence Date: 04/05/2017

Approval: Signature on File

James W. Neton, Associate Director for Science

Approval Date: 04/07/2017

FOR DOCUMENTS MARKED AS A TOTAL REWRITE, REVISION, OR PAGE CHANGE, REPLACE THE PRIOR 

REVISION AND DISCARD / DESTROY ALL COPIES OF THE PRIOR REVISION.

New Total Rewrite Revision Page Change
Document No. ORAUT-RPRT-0081 Revision No. 00 Effective Date: 04/07/2017 Page 2 of 14

PUBLICATION RECORD

EFFECTIVE 

DATE

REVISION

NUMBER DESCRIPTION

04/07/2017 00 New report for the analysis of Savannah River gross alpha air 

sampling data as a method of thoron intake bounding between 1972 

and 1994. Incorporates formal internal and NIOSH review 

comments. Training required: As determined by the Objective

Manager. Initiated by James M. Mahathy.
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TABLE OF CONTENTS

SECTION TITLE PAGE

Acronyms and Abbreviations ............................................................................................................... 4

1.0 Introduction .............................................................................................................................. 5

1.1 Background .................................................................................................................. 5

2.0 Methodology ............................................................................................................................ 6

3.0 Representativeness ................................................................................................................. 7

3.1 Data and Analysis......................................................................................................... 7

4.0 Conclusions ........................................................................................................................... 11

References ........................................................................................................................................ 13

LIST OF TABLES

TABLE TITLE PAGE

3-1 Airborne thoron progeny concentrations by period ................................................................... 7

LIST OF FIGURES

FIGURE TITLE PAGE

3-1 Lognormal probability fit of 1982–1983 air sample data ........................................................... 8

3-2 Lognormal probability fit of 1984–1985 air sample data ........................................................... 9

3-3 Lognormal probability fit of 1990 air sample data ................................................................... 10

3-4 Lognormal probability fit of 1997 air sample data ................................................................... 11
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ACRONYMS AND ABBREVIATIONS

CFR Code of Federal Regulations

Ci curie

cm3 cubic centimeter

DOE U.S. Department of Energy

dpm disintegrations per minute

ft feet

FTF F-Area Tank Farm

hr hour

HTF H-Area Tank Farm

L liter

NIOSH National Institute for Occupational Safety and Health

ORAU Oak Ridge Associated Universities

SRDB Ref ID Site Research Database Reference Identification (number)

SRS Savannah River Site

WL working level

μCi microcurie
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1.0 INTRODUCTION

During a February 2014 meeting of the Advisory Board on Radiation and Worker Health‘s Work Group 

on the Savannah River Site (SRS), discussion was held regarding whether air sampling 

measurements at the H-Area Tank Farms would bound thoron exposures for thorium storage areas. 

The discussion centered on whether measurements made during the mid- to-late 1990s could be 

used to bound thoron exposures for the prior two decades, or whether there were modifications made 

to the tanks that would impact whether these later measurements would be bounding for workers in 

the Tank Farm area. 

As an outcome of this discussion, NIOSH committed to collect a representative selection of air 

samples from the tank farm area over the time period under consideration. Initial counts, six-hour 

counts, and 24-hour counts would then be evaluated to determine the thoron component of the total 

alpha activity on the filter. Measurements from the 1970s and 1980s would then be compared to the 

measurements made by the site during the 1990s. The calculated airborne concentration would then 

be used to assign a dose to workers in the area of the tank farm. 

1.1 BACKGROUND

Since the early 1950s, the primary mission of the SRS has been to produce nuclear materials 

primarily for national defense and deep space missions. A legacy of the SRS mission was the 

generation of liquid waste from chemical separations processes in both F and H Areas. Since the 

beginning of SRS operations, an integrated liquid waste system consisting of several facilities 

designed for the overall processing of liquid waste has evolved. Two of the major components of this 

system are the H-Area Tank Farm (HTF) and F-Area Tank Farm (FTF), which are near the center of 

the site. In H Area, neptunium, uranium, and other radionuclides were separated from irradiated fuel 

and target assemblies using chemical separations processes. The tank farms, which store and 

process the chemical separations waste, include waste tanks, evaporators, transfer line systems, and 

other ancillary structures (SRR 2015).

There are 29 waste tanks in HTF. The waste tanks are built of carbon steel and reinforced concrete, 

but the designs vary. There are four principal types of waste tanks in the HTF designated as Types I, 

II, III, and IV. Type III tanks are sub-categorized as Type III and IIIA, based on the cooling 

mechanisms installed. The waste tanks were constructed at different times, with design features 

improving over time. The HTF waste tank numbering along with their design type is as follows (SRR 

2012):

• Type I. Tanks 9 through 12.

• Type II. Tanks 13 through 16.

• Type IV. Tanks 21 through 24.

• Type III. Tanks 29 through 32.

• Type IIIA. Tanks 35 through 43 and 48 through 51.

During the 1960s, the H-modified chemical separations process in 221-H was used to recover 233U 

from irradiated 232Th targets. Quantities of 232Th are known to exist in Tanks 11, 12, 13, 14, 15, 40, 42 

and 51 (Thames 2000). A consequence of the storage of thorium waste is an increase in the levels of 

thorium progeny, including 220Rn (thoron) (Epperson 1995a).

In 1995, SRS noted that the radon/thoron emissions from Tank 15 were higher than normal because 

of a decrease in the cooling capacity of the in-tank condenser (Epperson 1995b). This observation 

prompted an in-depth study, conducted by SRS in 1997, that evaluated exposure potentials due to 

periodic venting releases from Tanks 12 and 15 by collecting samples from several areas around the 
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two tanks. These samples were analyzed via gamma spectroscopy, making mathematical derivation 

of the thoron progeny concentration unnecessary (Sigg et al. 1997).

A series of results from air samples taken at the tank farm during 1974 to 1979 was retrieved from the 

site (DuPont 1974–1979). However, many of these records did not include an entry for a count taken 

6 hours after sampling. Of 20 results specific to the tank farm, 9 had both the 6-hour counts and the 

24-hour counts. This limited quantity of data was not sufficient to draw any statistical conclusions 

from its analysis.

Historically, the dose contribution from thoron and its progeny has been considered negligible in 

comparison with other sources of dose. However, recent research has suggested that the deposition 

of thoron progeny within the respiratory tract may contribute to a non-negligible portion of the total 

dose (Haanes et al. 2016). There is no indication that an attempt was made to sample for thoron or 

thoron progeny at SRS before 1995 (Epperson 1995a; Epperson 1995b; Sigg, Ross, and Weber 

1997). However, there are methods available for estimating airborne thoron concentrations based on 

routine air samples (Epperson 1995a).

2.0 METHODOLOGY

Because radon is a noble gas, it has no significant interaction with air sampling filter papers. 

However, the thoron progeny are solid matter that can aggregate to form larger particles, and can 

attach to airborne dust particles in the atmosphere. It is these particles that are collected during air 

sampling and analyzed through either spectroscopy or gross counting.

Air sample results available to NIOSH were analyzed for gross alpha activity. A challenge associated 

with the use of gross alpha results for estimating airborne thoron concentrations is the presence of 

222Rn and long-lived alpha-emitting radionuclides such as plutonium that could lead to overestimating 

thoron concentrations. At SRS air samples were counted for the first time 6 hours after the sample 

collection period, allowing for the alpha-emitting 222Rn progeny to decay away. A second count was 

completed 24 hours after the filter was removed from the sampler. Decay and build-up of thoron

daughters must be accounted for when calculating the concentration of thoron daughters on the filter 

at the end of collection period. Concentrations (μCi/cc) of 212Pb on the filter at the end of collection 

were derived using the methodology given in ORAUT-RPRT-0084 (ORAUT 2017) for each set of 

gross alpha 6 and 24 hour gross alpha count results for the years 1982 to 1983, 1984 to 1985 and 

1990. Only those data points that corresponded to Tanks 11 to 15, 21, 40, 42, and 51 were evaluated 

because these specific tanks were the ones that were identified as receiving significant quantities of 

thorium wastes (Thames 2000; O’Bryant and Weiss 2003). 

Note the count data recorded on the air monitoring results forms are measured count rates. From the 

measured count rates, activities were calculated using an alpha scaling factor reported for each count 

by SRS and then converted to microcuries. The alpha scaling factor included the collection efficiency, 

including self-absorption on the filter, and geometry of the counter (Patterson 1957, DuPont 1986). 

Concentrations of thoron daughters were then derived using the methodology and equations given in 

ORAUT-RPRT-0084 (ORAUT 2017). 
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3.0 REPRESENTATIVENESS

3.1 DATA AND ANALYSIS

The position of workers relative to the location of sample collection must be considered when using air 

sampling to bound dose. However, the position of workers or the heights of catwalks is rendered 

inconsequential by basing the concentration to be assigned on data from the primary exhaust and 

annulus vents. It is unlikely that a configuration could exist that would place workers in an area with a 

higher thoron concentration than that found in the tank exhaust. 

Derived concentrations for 1982-1983, 1984-1985, and 1990, as well as published thoron 

concentrations for 1997, were statistically analyzed. Derived values of 0.00 μCi/cc were censored at 

0.2 μCi/cc, which correlates to the detection level that all samples were being assessed against. 

Summary statistics for each year are given in Table 3-1. Summary statistics for each year include the 

number of data points, geometric mean, 95th and 99th percentile values.

Table 3-1. Airborne thoron progeny concentrations (μCi/cm3

) by period.

Period N n GM 95th 

percentile 99th percentile Reference

1982–1983 184 183 1.24 x 10-12 9.03 x 10-12 2.05 x 10-11 DuPont 1982–1983

1984–1985 184 174 3.97 x 10-12 2.89 x 10-11 6.58 x 10-11 DuPont 1982–1983

1990 134 113 8.98 x 10-13 6.95 x 10-12 1.62 x 10-11 WSRC 1990a, 1990b

1997 22 22 2.84 x 10-11 2.67 x 10-09 1.76 x 10-8 Sigg et al. 1997

A significant source of variation in the datasets is not readily obvious from values given in the table: 

all of the available data points from 1984 to 1985 are direct measurements from either the primary 

vent of the tank, the annulus vent of the tank, or the tank purge exhaust stream itself. Of the 174 data 

points for that year, 110 were taken from the exhaust stream and annulus vent of Tanks 12 and 15, 

the two tanks holding the highest total thorium activities. Similarly, available data from 1982 to 1983 

contain 90 data points from the annulus vent and exhaust stream. None of the available data points 

from 1990 indicate that they were sampled directly from the exhaust stream. Rather, the data points 

from 1990 are identified as having been taken from the annulus vent and the general air surrounding 

the tank, resulting in lower airborne concentrations as compare to the 1982-1983 and 1984-1985 

values. Data for 1997 were recorded from the study conducted by SRS. Samples were collected 

from several areas around Tanks 12 and 15 during periodic venting of tank gases and are higher 

because of a decrease in the cooling capacity of the in-tank condenser (Sigg et al. 1997).

Probability plots of the tabulated data for each year are presented in Figures 3-1 through 3-4, below.
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Figure 3-1. Lognormal probability fit of 1982–1983 air sample data. 
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Figure 3-2. Lognormal probability fit of 1984–1985 air sample data. 
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Figure 3-3. Lognormal probability fit of 1990 air sample data. 
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Figure 3-4. Lognormal probability fit of 1997 air sample data. 

4.0 CONCLUSIONS

Air concentrations of thoron measured at H Area tanks in the 99th-percentile for the periods 1982-83, 

1984 to 1985 and 1990 are less than 1% of the derived air concentration for thoron when its progeny 

are present (9.0 × 10-9 μCi/cm3

). The majority of the data used to arrive at these values are from 

measurements taken at the point of exhaust, maximizing the concentrations compared to open air. 

Both tanks with the greatest concentrations of thorium (Tanks 12 and 15) had exhaust stacks that 

rose 10 ft above ground level. In addition, Tank 15 had a catwalk at a height of 15 ft above ground 

level (Sigg et al. 1997). Such configuration makes it unlikely that a worker would be positioned in 

such a way as to be exposed to thoron progeny concentrations approaching those at the tank 

exhaust. However, the exhaust values represent a conservative upper bound of potential thoron 

progeny exposures. Further, measurements taken at the emission point ensure that assigned upper 

bound doses are independent of the worker location within the tank farm. 
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As previously discussed, the radon/thoron emissions from Tank 15 measured in the later 1990s were 

higher than normal because of a decrease in the cooling capacity of the in-tank condenser (Epperson 

1995b). This can be seen in the concentrations of thoron daughters derived during the 1997 study, 

which are two to three orders of magnitude higher than concentrations derived from air monitoring 

conducted during the previous decades. NIOSH concludes that concentrations of thoron daughters 

reported in the 1997 study are sufficient to bound thoron exposures of workers in thorium storage 

areas. 
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REFERENCES

DuPont (E. I. du Pont de Nemours and Company), 1974–1979, collection of Process Air Sample 

Results, Savannah River Plant, Aiken, South Carolina. [SRDB Ref ID: 159267]

DuPont (E. I. du Pont de Nemours and Company), 1982–1983, collection of Process Air Sample 

Results, Savannah River Plant, Aiken, South Carolina. [SRDB Ref ID: 159268]

DuPont (E. I. du Pont de Nemours and Company), 1984–1985, collection of Process Air Sample 

Results, Savannah River Plant, Aiken, South Carolina. [SRDB Ref ID: 159270]

DuPont (E. I. du Pont de Nemours and Company), 1986, Air Sample Calculations, DPSOL 193-306, 

Rev. 4, Savannah River Site, Aiken, South Carolina, November 17. [SRDB Ref ID: 86221, 

pp. 106-107]

Epperson, S. A., 1995a, “Evaluation of Elevated Thoron Progeny at 221-H,” memorandum to G. G. 

Tunno, Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, 

October 9. [SRDB Ref ID: 105023]

Epperson, S. A., 1995b, “Tank 15 Radon/Thoron Evaluation,” memorandum to J. R. Cantrell, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, 

August 9. [SRDB Ref ID: 105024]

Haanes, H., I. E. Finne, T. Kolstad, A. Mauring, S. Dahlgren, and A. L. Rudjord, 2016, “Outdoor 

Thoron and Progeny in a Thorium Rich Area with Old Decommissioned Mines and Waste 

Rock,” Journal of Environmental Radioactivity, volumes 162–163, pp. 23–32.

O’Bryant, R. F., and W. R. Weiss, 2003, Characterization of Radionuclides in H-Modified and Purex 

Waste Sludges from H-Area High Level Waste Tanks, WSRC-TR-2000-00249, Rev. 2, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, March. 

[SRDB Ref ID: 84040]

ORAUT (Oak Ridge Associated Universities Team), 2017, Two-Count Filter Method for Measurement 

of Thoron Progeny in Air, ORAUT-RPRT-0084, Rev 00, Oak Ridge, Tennessee, March 27.

Patterson, C. M., 1957, Health Physics Area Survey Bulletin Number 22, April 9, 1957. [SRDB Ref 

ID: 116669]

SRR (Savannah River Remediation), 2012, Performance Assessment for the H-Area Tank Farm at 

the Savannah River Site, SRR-CWDA-2010-00128, Rev. 1, Savannah River Site, Aiken, South 

Carolina, November. [SRDB Ref ID: 161262]

SRR (Savannah River Remediation), 2015, Industrial Wastewater Closure Module for Liquid Waste 

Tank 16H, H-Area Tank Farm, Savannah River Site, SRR-CWDA-2013-00091, Rev. 0, 

Savannah River Site, Aiken, South Carolina, February. [SRDB Ref ID: 161263]

Sigg, R. A., R. H. Ross, and A. H. Weber, 1997, Personnel Exposures from Periodic Venting of Radon 

Gas from High-Thorium-Content Tanks at H-Tank Farm, WSRC-TR-97-0033, Rev. 0, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, April 8. 

[SRDB Ref ID: 49677]

Thames, R. D., 2000, “HLW Stack Samples and Thoron,” memorandum to E. M. Perett and J. D. Gall, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, 

November 20. [SRDB Ref ID: 105017]
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WSRC (Westinghouse Savannah River Company), 1990a, collection of Special Air Sample Results, 

Savannah River Site, Aiken, South Carolina. [SRDB Ref ID: 153886]

WSRC (Westinghouse Savannah River Company), 1990b, collection of Special Air Sample Results, 

Savannah River Site, Aiken, South Carolina. [SRDB Ref ID: 153887]