ORAU TEAM

Dose Reconstruction

Project for NIOSH

Oak Ridge Associated Universities I Dade Moeller I MJW Technical Services

Page 1 of 24

DOE Review Release 05/27/2010

Document Title:

Analysis of Savannah River Site Thoria and 

Thoron Air Sampling Data as a Method of Intake 

Bounding between 1965 and 1971

Document Number: ORAUT-RPRT-0046

Revision: 00

Effective Date: 05/18/2010

Type of Document: Report

Supersedes: None

Subject Expert(s): Billy P. Smith, Robert L. Morris, Melton H. Chew, Samuel T. Chu, Lori Arent, and 

James M. Mahathy

Site Expert(s): Melton H. Chew 

Approval: Signature on File Approval Date: 05/12/2010

James M. Mahathy, Document Owner

Concurrence: Signature on File Concurrence Date: 05/13/2010

Matthew C. McFee, Objective 4 Manager

Concurrence: Signature on File Concurrence Date: 05/17/2010

Kate Kimpan, Project Director

Approval: Grady Calhoun Signature on File for Approval Date: 05/18/2010

James W. Neton, Associate Director for Science

New Total Rewrite Revision Page Change

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

REVISION AND DISCARD / DESTROY ALL COPIES OF THE PRIOR REVISION.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 2 of 24

PUBLICATION RECORD

EFFECTIVE 

DATE

REVISION 

NUMBER DESCRIPTION

05/18/2010 00 This report is a working document prepared by NIOSH or its 

contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees and is now being incorporated as an 

ORAU Team controlled document. Draft, preliminary, interim, and 

White Paper documents are not final NIOSH or ABRWH (or their 

technical support and review contractors) positions unless 

specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its 

contractor. Not subject to FOIA. Incorporates formal NIOSH review 

comments. Training required: As determined by the Objective 

Manager. Initiated by James M. Mahathy.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 3 of 24

TABLE OF CONTENTS

SECTION TITLE

1.0 Executive Summary .................................................................................................................

PAGE

5

2.0 Introduction and Facility Description.........................................................................................5

3.0 Thorium Air Sample Data and Analysis ....................................................................................5

3.1 Air Sample Data ...........................................................................................................5

3.2 Air Sample Data Analysis ...........................................................................................10

4.0 Contamination Survey Data and Analysis .............................................................................. 12

4.1 Contamination Survey Data........................................................................................12

4.2 Contamination Survey Data Analysis..........................................................................12

5.0 Thoron Air Concentrations ..................................................................................................... 13

5.1 Data Sources..............................................................................................................14

5.2 Methodology...............................................................................................................14

5.3 Results and Conclusions ............................................................................................16

6.0 Interview Highlights................................................................................................................ 16

7.0 Daily Thorium Intake Rate...................................................................................................... 18

8.0 Summary ...............................................................................................................................19

References ........................................................................................................................................20

LIST OF TABLES

TABLE TITLE

1 Beginning and ending dates of thoria production campaigns in the SRS 300 area 

based on monthly reports..........................................................................................................

PAGE

10

2 Transferable alpha contamination survey results by year for 313-M thorium area..................... 13

3 Number of thorium chest and whole body counts reported in 1965 ........................................... 18

4 Intake rates of Th-232 and Th-228 expressed in units of activity for two different 

levels of chronic airborne activity .............................................................................................. 18
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 4 of 24

LIST OF FIGURES

FIGURE TITLE

1 Architectural sketch of Building 313-M emphasizing the modular design of the facility................

PAGE

6

2 1980s aerial view of the 300-M area, looking southeast.............................................................. 6

3 View of Building 313-M looking to the northeast.......................................................................... 7

4 Building 313-M layout showing where thorium operations were performed ................................. 7

5 Thorium glovebox during construction.........................................................................................8

6 Thorium glovebox during operations ...........................................................................................8

7 Thorium glovebox during operations ...........................................................................................8

8 Containment ventilation equipment with HEPA filtration.............................................................. 8

9 HEPA filter and ventilation blower for the enclosure.................................................................... 9

10 Worker at thorium glovebox containment ....................................................................................9

11 Welding equipment .....................................................................................................................9

12 Welding enclosure with radiation chain in the foreground............................................................ 9

13 Welding enclosure – side view.................................................................................................. 10

14 Thorium slug .............................................................................................................................10

15 Probability plot for thorium air samples results .......................................................................... 11

16 Airborne thorium concentration and thorium production rate ..................................................... 12

17 Transferable alpha contamination survey results by year for 313-M thorium area..................... 13

18 Kinetics of thoron decay illustrating time intervals used in Equations 1 through 8 ..................... 15
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 5 of 24

1.0

This report describes the facilities, processes and equipment used to prepare and encapsulate 

thorium at Savannah River Site (SRS) 300-M area in the 1960s. Although whole body counting 

equipment was used to quantify thorium exposure at SRS (DuPont 1965a) in this time frame, the site 

in vivo records are located within the personal radiological records. In order to locate these records, a 

complete review of all personnel files would be necessary to develop a co-worker model. Since such 

a review is considered infeasible, an alternative method was developed to bound intake rates of 

thorium for unmonitored workers during the 1965 to 1971 time period. This report analyzes air 

sample data as a method to bound thorium intakes. Exposures to thorium are feasible even after the 

thorium production period ended. Bounding intake rates during this period are based on routine 

contamination survey measurements in the area.

EXECUTIVE SUMMARY

2.0

Whole body counting equipment was used to quantify thorium exposure at the Savannah River Site 

(SRS) (DuPont 1965a), however, due to difficulties in recovery of these records as noted above, an 

alternative method was developed to bound intake rates of thorium during that period when thoria 

(thorium oxide) fuel components were assembled in the 300-M area. This report analyzes air sample 

data as a method to bound thorium intakes. 

INTRODUCTION AND FACILITY DESCRIPTION

The 300-M area facilities, equipment and processes are described in Reed and Swanson (2006). 

Figures 1 through 14 illustrate Building 313-M and the thorium processing facilities and equipment. 

The major thoria processing campaigns which occurred in the 300-M area for the period of interest 

(1964 through 1971) are shown in Table 1 (Morris 2008). With minor exceptions, for example helium 

leak testing on encapsulated slugs in Building 320-M, this work occurred in Building 313-M in a room 

specially built for the purpose. The Building 313-M thorium processing room was nominally 20 feet by 

30 feet. A single glovebox cabinet was centered in the room (Chew 2009). 

Air samples were collected and analyzed to determine the internal exposure potential to SRS workers 

assigned to this area. Data from 1964 were included in the analysis because it was reasonable to 

assume that there would have been a delay between chronic thorium exposure and routine in vivo

counting. Routine contamination surveys of the area after processing were completed and evaluated 

to assess the exposure potential after the operational periods.

3.0

3.1 AIR SAMPLE DATA

THORIUM AIR SAMPLE DATA AND ANALYSIS

Two hundred seventy (270) individual air sample results were used in this analysis. The air sample 

data sheets used in this analysis are in the Site Research Database (DuPont 1964, 1965b, 1965c, 

and 1967a). Two hundred sixty two (262) of the samples were identified as "routine" and were 

collected using portable air pumps operating nominally at an airflow rate of 3.5 cubic feet per minute. 

The remaining eleven samples were special impactor samples discussed below. The routine 

samplers were placed to represent the location of the operator and the sampling head was extended 

up to face-level. The intent was that the routine sample would represent a breathing zone sample. 

This was confirmed in an interview with a former health physics technician who collected the majority 

of the samples (Chew 2009). Routine samples may have been counted several times. Initial counts 

may have been made to estimate the order of magnitude of the air concentration. Inevitably, the 

short-lived radon progeny decay and the counts made 24 or 72 hours after sampling provide a better 

estimate of the long-lived radioactive material concentration, which is the quantity of interest. For this 

analysis the data representing the longest counting delay were used. 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 6 of 24

Figure 1. Architectural sketch of Building 313-M emphasizing the modular 

design of the facility. The east side of the building is facing the viewer.

Figure 2. 1980s aerial view of the 300-M area, looking 

southeast. Building 313-M is the large building at the center.

Eight (8) samples were collected with special "impactor" sampling equipment. Impactor samples were 

high volume [1] samples taken for a short duration, typically 10 minutes, to characterize short-term 

operations with elevated exposure potential. Notes associated with the sampling show that 

respiratory protection was worn when the sampled operations were occurring. The former health 

physics technician who conducted the sampling confirmed that these operations were rare (Chew 

2009). The impactor was designed to eliminate most the sub-micrometer-sized radon progeny. 

These small particles would follow the air stream and exit the back of the impactor while larger 

particles associated with mechanical processes would be collected on a greased planchette. The 

impactor air sample counts were promptly counted. 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 7 of 24

Figure 3. View of Building 313-M looking to the northeast.

Figure 4. Building 313-M layout showing where thorium operations were performed. Location 

labeling provided by SRS classification analyst T. Coughenour.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 8 of 24

Figure 5. Thorium glovebox during construction.

Note the highbay setting and partial height walls. 

Source: Coughenour (2009). The photograph is 

unclassified and does not contain UCNI.

Figure 6. Thorium glovebox during operations. 

Source: Coughenour (2009). The photograph is 

unclassified and does not contain UCNI.

Figure 7. Thorium glovebox during operations.

Note the thorium slugs lying horizontal in the 

far right of the glovebox. Source: Coughenour 

(2009). The photograph is unclassified and 

does not contain UCNI.

Figure 8. Containment ventilation equipment with 

HEPA filtration. Source: Coughenour (2009). 

The photograph is unclassified and does not 

contain UCNI.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 9 of 24

Figure 9. HEPA filter and ventilation blower for the enclosure. 

Also note the bagged cans of material in foreground behind 

the radiological chain. Source: Coughenour (2009). The 

photograph is unclassified and does not contain UCNI.

Figure 10. Worker at thorium 

glovebox containment. Source: 

Coughenour (2009). The 

photograph is unclassified and does 

not contain UCNI. 

Figure 11. Welding equipment. Source: 

Coughenour (2009). The photograph is 

unclassified and does not contain UCNI.

Figure 12. Welding enclosure with radiation 

chain in the foreground. Source: Coughenour 

(2009). The photograph is unclassified and 

does not contain UCNI.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 10 of 24

Figure 13. Welding enclosure – side view.

Source: Coughenour (2009). The photograph is 

unclassified and does not contain UCNI.

Figure 14. Thorium slug. Source: 

Coughenour (2009). The photograph is 

unclassified and does not contain UCNI.

Table 1. Beginning and ending dates (to the nearest month) of thoria

production campaigns in the SRS 300 area based on monthly reports.

Beginning Ending

Amount of thoria 

processed, kg

Average thoria 

production rate, kg/day

8/1964 12/1964 49,000 322

1/1965 8/1965 101,400 419

4/1967 8/1967 114,156 751

11/1967 12/1967 3,570 60

1/1968 2/1968 6239 108

One impactor sample yielded results that were 680% of the Maximum Permissible Concentration 

(MPC) [2]. The other 7 impactor samples were below the MPC value and near zero concentration. 

3.2 AIR SAMPLE DATA ANALYSIS

To provide convenient reporting context, the air samples results are converted to units of MPC. The 

MPC value for thorium is 2 x 10-12 μCi/cm3

. This is the same value that was used to control airborne 

plutonium and was the thorium MPC value in use at SRS during the period in question (Stoddard 

1964). A convention of assigning the value 0.01 when a measured value is reported as zero has 

been applied to prevent illogical data from interfering with lognormal statistical analysis which 

demands that all values exceed zero.

The air concentrations were calculated using the raw data available in the data sheets and 

assumptions regarding alpha counting efficiency (30% under 2 π geometry for a calibration source 

[3]), particle collection efficiency of the sampling media (95% for both routine [3] and impactor [4]), 

and sample self-absorption (50% for routine [3] and 55% for impactor [4]). This has the advantage of 

separately accounting for the important variable factors. The overall counting efficiency for routine 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 11 of 24

samples (29%) and impactor samples (25%) is similar to the reciprocal of the "counter factor" value 

shown on some of the contemporary air sample counting sheets. 

The routine and impactor air sample data are combined for analysis. The data appear to be 

lognormally distributed; the hypothesis that the data are lognormally distributed cannot be rejected at 

the 95% confidence level. Figure 15 is a probability plot of the sample data. It is compared on the 

same graph to the cumulative probability function of the fitted lognormal distribution. 

Figure 15. Probability plot for thorium air samples results.

When the routine thorium air sample data are fitted to a lognormal distribution the resulting geometric 

mean (GM) is 8.1% of the MPC with a geometric standard deviation (GSD) of 3.73. (The median and 

GM are numerically identical in a lognormal distribution.) The arithmetic mean is 20% and the 95th 

percentile of this distribution is 71% of the MPC. Ninety-six percent of the routine samples were 

below 100% of MPC value. The maximum observed sample was 678% of the MPC. This sample 

was taken using the impactor during “chopper operations.” It represented a 10 minute period when 

respiratory protection was in use. 

Figure 16 illustrates the impactor and routine air sample data referenced to the semi-log scale on the 

left side of the graph. Superimposed on the same graph is an illustration of the daily average thoria 

production rate (Morris 2008). To enable semi-log graphing and lognormal distribution fitting, samples 

with air concentration values of zero are set equal to 0.01. In some cases multiple impactor samples 

were taken on the same day with identical results. In these cases the zero data are shown as 0.011 

and 0.012 to separately represent the samples on the graph. The figure demonstrates that routine 

thorium air sampling results were consistently below half of the MPC and that the sampling was done 

during the thoria production campaigns.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 12 of 24

Figure 16. Airborne thorium concentration and thorium production rate.

4.0

4.1 CONTAMINATION SURVEY DATA

CONTAMINATION SURVEY DATA AND ANALYSIS

Available Radiation Survey Log Sheets (Form OSR 4-17) include transferable alpha contamination 

sample results and were reviewed for the 300-M area for the period of 1965 through 1972. “Disc 

smear” (swipe) samples to evaluate transferable alpha contamination were collected and analyzed 

following standard procedures. Routine room survey (e.g., floor and work surface) results were 

included in this review; contamination survey results for production equipment, product, waste drums, 

etc., were not evaluated. A total of 449 routine room survey results were complied for the 300-M 

area, 130 of which were specifically identified with thorium-related operations in 313-M during the 

period 1966 through 1972. The survey logs for the sample results used in this analysis are in the Site 

Research Database (DuPont 1966a-e, 1967b-c, 1968a-b, 1969b-f, 1970c-d, and 1972). 

4.2 CONTAMINATION SURVEY DATA ANALYSIS

Alpha contamination survey results for routine room surveys in the thorium area of 313-M are 

depicted in Table 2 and Figure 17. In general, the highest contamination results are observed during 

the 1967-1968 production campaigns. Elevated results in the post-production period are generally 

due to decontamination and decommissioning operations when previously inaccessible contamination 

was exposed. Data are provided as background information to assist in better characterizing the 

thorium operations.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 13 of 24

Table 2. Transferable alpha contamination survey results by year for 313-M thorium area.

Year Location

Transferable contamination (alpha) in dpm

<100 100–1000 >1000 Total

1966 313-M Thorium Room 3 2 1 6

1967 313-M Slugs Final Inspection 3 1 4

313-M Thorium Room 13 11 2 26

1968 313-M Thorium Room 1 2 4 7

1969 313-M Autoclave 14 14

313-M Coke & Smoking 10 10

313-M Plating Line 6 8 14

313-M Recovery 4 9 13

313-M Regulated Areas 2 2

313-M Slugs Final Inspection 14 14

1970 313-M Autoclave 2 1 3

313-M Coke & Smoking 2 2

313-M Plating Line 2 2

313-M Recovery 2 2

313-M Slugs Final Inspection 2 1 3

1972 313-M Thorium Room 5 3 8

Total 78 43 9 130

Figure 17. Transferable alpha contamination survey results by year for

313-M thorium area.

5.0

Various radiological air concentrations were measured at SRS during the thorium operations period. 

Although air samples were collected and measured for thorium during these operations, there is no 

evidence that thoron and its daughters were measured. 

THORON AIR CONCENTRATIONS

SRS measured thorium air samples by performing several alpha counts (an initial count shortly after 

the completion of sampling and time decayed counts, usually at 24 hours and 72 hours). The results 

of these measurements are documented on three different formats of data sheets, i.e., single Air 

Sample Log Sheets or Thoria Compaction Facility data sheets attached to the Air Sample Log Sheets 

or Weekly Routine Air Sample Results data sheets (see Attachment A-1 for examples of the data 

sheets). 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 14 of 24

Air Sample Log Sheets documented single and multiple sample collections with the types of 

instrument used to collect the sample, the location of the sample, the work processes underway, in 

some cases (single samples) the date and time of the beginning of the initial count, the flow rate, 

volume sampled, and initial alpha count (usually between one minute and a few hours) after sample 

collection and a second alpha count about 24 hours after the initial count. Data sheets attached to the 

Log Sheets take two formats; the first is the Thoria Compaction Facility data sheet which contains 

sampling date, start and stop date and time, the date of the initial count, the initial count without a 

time, 24-hour count and a 72-hour count. The second format is the Weekly Routine Air Sample 

Result sheet which contains building number, sampling location, sampling date, sampler type, flow 

rate, net alpha count rate, and a time basis for the calculations (usually 16 hours). From these data, 

air concentrations could be calculated for thorium. 

The analysis of SRS air samples for thorium was performed by collecting a known volume of sample 

and performing multiple alpha counts (initial-a short time after sampling, 24 hours and 72 hours). 

These delayed counts were made in order to allow the relatively short-lived daughters of radon and 

thoron to decay, thereby leaving the long-lived alpha activities for the evaluation of airborne thorium 

concentrations. 

5.1 DATA SOURCES

The air sample data sheets used for the thoron analysis are in the same collection of documents used 

for the thorium analysis in Section 3.0 of this report (Site Research Database (DuPont 1964, 1965b, 

1967). Eighty-seven (87) air sample results from the original 270 thorium air sample results were 

used for this thoron analysis. The samples in the thorium operations study that represented 

background or samples collected with impactor sampler were not used in this thoron analysis since 

they were not used to evaluate the general work environment nor were the air sample results from 

data sheets other than the Thoria Compaction Facility data (see attached examples of various data 

sheets). This data were chosen as a result of the multiple counts available on these data sheets and 

was conducive to the model described below to determine thoron concentrations. 

5.2 METHODOLOGY

This section presents a methodology for estimating the bounding thoron activities which could have 

been present in the workplace from airborne thoron and its daughters. Thoron air concentrations 

were determined using the previously described air sample counting data results as follows:

Thoron Air Samples

Normally when an air sample is collected for the analysis of a long lived isotope, the activity on the 

filter increases linearly during the sample time and does not decay after. The activity on the filter can 

then be measured at any time after the sample ended and it is a direct measurement of the filter 

activity when the sample collection period ended. That activity on the filter is divided by the volume of 

air sampled to get the air concentration. Inherent with this is the assumption that the isotope does not 

decay appreciably during the sampling or analysis.

When the isotope being sampled has a half-life along the magnitude as the sample collection period 

and delay times, the above assumption is not valid. Figure 18 illustrates both situations.

To complicate the issue further, one must assume there is both long-lived activity and short-lived 

activity on the filter paper when it is being counted. The short lived activity can be determined with 

two counts but it is not the same as a typical long-lived calculation.
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 15 of 24

Figure 18. Kinetics of thoron decay illustrating time intervals used in Equations 1 

through 8.

To determine the thoron concentration from air sample data from SRS the following model is 

presented.

Let A1 and A2 be the filter activity during the first and second count.

Let AL be the long-lived activity on the filter paper

Let AT1 and AT2 be the thoron activity on the filter during the first and second count

Let ATF be the thoron activity on the filter paper at the end of the sampling

Let TS be the sample time (time the sample ran)

Let T1 be the time between the end of sampling and the first count

Let T2 be the time between the first and second counts.

λ is the Pd-212 decay constant (ln(2)/10.64 hours)

Eq. 1

Eq. 2

Eq. 3

Solve the three equations for AT1

Eq. 4

Correct for decay between the end of the sampling and the first count.

Eq. 5
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 16 of 24

Correct for the buildup of thoron on the filter during the sampling interval.

) Eq. 6

Where:

CT = the thoron concentration in the air

R = the sampling rate (volume per time)

Equation 6 can be rearranged to

Eq. 7

Combine Equation 7 with the identities developed in previous equations.

Eq. 8

The concentration calculated using equation 8 is in units of counts/minute/volume (cpm/cm3

). 

Parameters specific to the sampling and counting system may be used to convert the count rate 

concentration to activity/volume (μCi/cm3

). 

• After the activities are determined for all of the terms in the above equations they are used in 

the spreadsheet to convert these “thoron” CPM values, using the air sample parameters in the 

original thorium analysis spreadsheet to activity concentrations (μCi/cc) and thoron intake 

rates in pCi/day. 

• The intake rates in pCi/d were calculated initially based on a worker breathing rate of 1.2 m3

/hr 

(ICRP 1994) for 8 hours. Since the breathing intake rate is based on an 8-hour period and the 

samples were collected over a 24-hour period, the 8-hour concentration is increased by a 

factor of 3 to assume that this increased activity concentration was inhaled over the 8-hour 

work shift. 

5.3 RESULTS AND CONCLUSIONS

Figure 19 shows a probability plot of the estimated thoron concentration in units of uCi/cc. A 

lognormal distribution representing the fitted data is shown on the same plot. The geometric mean of 

the fitted lognormal distribution is 4.34E-12 uC/cc with a GSD of 5.27. The observed 95th percentile 

of this distribution is approximately 6.67E-11 uCi/cc. The maximum observed concentration from this 

distribution is 1.51E-10 uCi/cc. Calculations of thoron inhalation rate at the 95th percentile is 1,900

pCi/day or 0.11 WLM per year. The result of this analysis suggests that the thoron air concentrations 

are small and may be negligible.

6.0

An interview with the former health physics technician (Chew 2009) provided insight into the thoria 

operations. The interviewee was asked to provide specific process, air sampling, and respiratory 

protection details. The following bullets include excerpts of this interview:

INTERVIEW HIGHLIGHTS

• The routine samples were taken using this [Bell and Gossett] machine. [Routine samples]

were representative of the breathing zone of the worker. They were located where the 

operators were standing and were not mounted on a wall. 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 17 of 24

0.5 2 10 30 50 70 90 98 99.5

Probability

1E-014

1E-013

1E-012

1E-011

1E-010

1E-009

T

h

oro

n

C

o

n

c

e

ntratio

n in

μ

Ci p

er c

m3

Estimated Thoron Air Concentration - Cumulative Probability Plot

Lognormal Distribution

n=87

GM=4.34x10-12

GSD=5.26

Figure 19. Probability plot for thoron air samples results.

• There were no opportunities for workers to be highly exposed without monitoring. 

• The impactor was used when sampling near the chopper. The chopper was rarely used and 

when it was assault masks were worn. 

• Respiratory protection was used prior to establishing the contamination level of a slug as it left 

the airlock, and during chopper operations. 

• Respiratory protection was used during recovery from an incident in when a slug melted in a 

furnace in Building 320. 

• Contamination was well controlled. 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 18 of 24

• The processing line was a closed system and it was effective as designed. 

• Thoria operations never involved more than ten people at a time. There was one area 

supervisor, four or five foremen, and five or six operators assigned to thoria operations.

The relatively low number of operators is somewhat confirmed through the monthly technical reports 

for the 300-M area. Table 3 presents the number of workers monitored for thorium in the whole body 

counter in 1965 (DuPont 1965d-h). 

Table 3. Number of thorium chest and whole body counts reported in 1965.

Month 

in 1965

No. thorium 

chest counts 

reported

Total no. of WBCs 

reported (including 

Th chest counts) Summary of results

Jan 10 32 The monthly reports state there were no positive results for 

gamma emitting radionuclides other than normal amounts of 

Cs-137 and K-40. The one exception is a positive WBC 

result (10.3 nCi of Zn-65) for a new employee counted in Mar 

1965 due to assimilation at another site.

Feb 11 39

Mar 6 29

Apr 2 28

Totals 29 128 The Aug 1965 progress report, 3/700 M area section states 

chests counts using the whole body counting equipment 

showed no significant assimilation of thoria (DuPont 1965h).

7.0

Dose reconstruction requires that air concentration be converted to daily intake rate in units of activity 

or mass for each thorium isotope. Assuming a breathing rate of 1.2 m3 per hour (ICRP 1994) and an 

8-hour work day, the intake rate can be calculated. A multiplicative correction factor of 3 is applied to 

the air sample concentration parameters to account for the likelihood that essentially all of the 

exposure potential represented by 24-hour routine air samples was generated during the 8-hour shift 

when workers were present. To ensure an assumption that is favorable to the claimant, the measured 

airborne thorium activity is assumed to be fully equilibrated between thorium-232 and thorium-228. 

This maximizes the activity per unit mass and as a consequence the isotopes exist in a 1-to-1 ratio. 

Due to the extremely long half life (and therefore low specific activity) of thorium-232, the mass is 

essentially all associated with thorium-232. Conversely the mass of thorium-228 is immeasurably 

small and is set to zero. 

DAILY THORIUM INTAKE RATE

The chronic annual thorium intake rates are of interest for simplified dose calculations and for 

comparison to in vivo counting data. The only additional assumption needed to convert daily intake 

rate to annual intake rate is the exposure duration, which is set at 260 thorium work days per year. 

Table 4 presents daily and annual intake rates for thorium isotopes based on an 8-hour daily 

exposure at the 50th and 95th percentile concentrations modified to include the factor of 3 account for 

the likelihood that the airborne activity may have been generated during one 8-hour work shift per day 

instead of the 24-hour shift represented by the routine air samples.

Table 4. Intake rates of Th-232 and Th-228 expressed in units of 

activity for two different levels of chronic airborne activity.

Intake rates 50th percentile 95th percentile

Daily intake rate Th-232, Bq/d 0.087 0.76

Daily intake rate Th-228, Bq/d 0.087 0.64

Annual intake rate Th-232, Bq/y 23 200

Annual intake rate Th-228, Bq/y 23 200

The theoretical Minimum Detectable Activity (MDA) for thorium was calculated from whole body count 

data collected in the years just before 1965. This calculation assumed the highest observed 
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 19 of 24

background rate in the region of interest where photons from Pb-212, a thorium progeny, would be 

found. The counting efficiency in this region was extrapolated from calibration data. Other 

assumptions included a 20-minute counting time and a 42% equilibrium between thorium-228 and 

thorium-232. The resulting upper-bound MDA for thorium-232 is approximately 1700 Bq (45 nCi). 

This is almost a factor of 9 greater than the annualized intake rate shown in Table 4 and shows that 

the bounding intake rate would not be detectable using whole body counting equipment in use at that 

time. 

8.0

The air samples taken during thoria production operations in the period when in vivo counting records 

have not been located and an interview with the former health physics technician who monitored the 

operations indicate that the thoria production operations were well controlled and the intake potential 

was low. The data and interviews show that during the periods when thoria processing campaigns 

were being conducted, a bounding thorium intake rate can be established based on the log-normally 

fitted air sample data and assumptions regarding breathing rate. The interview with the former health 

physics technician suggested that exposure potential was equally low during periods of maintenance 

and decommissioning. This was confirmed by review of routine contamination survey data for that 

period. No data specific to the radon-220 (thoron plus progeny) exposure potential was located, but a 

conservative interpretation of air sample data allowed a bounding assessment to be conducted. The 

potential exposure from radon-220 and progeny is bounded at 6.3E-4 WLM per day.

SUMMARY

9.0

[1] Billy Smith. A technical error during the interview with the former health physics technician is 

noted. He stated that the impactor ran at 10 cfm. Based on the sample times and total 

volume sampled the actual flow rate had to be approximately 35 cfm. This error is judged to 

be of no consequence.

ATTRIBUTIONS AND ANNOTATIONS

[2] Robert Morris. Two impactor samples were taken on the same day, at the same location, 

sampling the same volume of air and reporting the same count rate data. They were reported 

on two different data sheets. We assume this is one sample reported twice. 

[3] Robert Morris. These assumptions are based on extensive experience with alpha counter and 

air sampling equipment. Several of them can be substantiated in a memo prepared by the Air 

Force Institute for Operational Health Physics (Nichelson 2005) 

[4] Billy Smith. Page 46 in DuPont (1965b) shows that the impactor used was a Staplex brand. 

From professional experience we know that the Staplex Annual Kinetic Impactor (AKI) has 

been in use for decades without significant modification. Performance data on the AKI is 

available on line (Staplex 2009).
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 20 of 24

REFERENCES

Chew M. H., 2009, Interview with Former Health Physics Technician, August 13. [SRDB Ref ID 

73087]

Coughenour, T., 2009, Thoria Process Pictures and Classification Analyst Statement, October 13. 

[SRDB Ref ID 75980]

DuPont, 1964, Air Sample Log Sheets, 300-M, August 17, 1964 - February 22, 1965. [SRDB Ref ID 

70157]

DuPont, 1965a, Works Technical Department Progress Report of August 1965, DPSP-65-1-8, p. 12. 

[SRDB Ref ID 68180] 

DuPont, 1965b, Air Sample Log Sheets, 300-M, February 22, 1965 - December 24, 1965. [SRDB Ref 

ID 70196]

DuPont, 1965c, Air Sample Log Sheets, 300-M, December 1965 - August 1967. [SRDB Ref ID 

70122]

DuPont, 1965d, Progress Report, Works Technical Department, DPSP 65-1-1, January 1965. (SRDB 

68138]

DuPont, 1965e, Progress Report, Works Technical Department, DPSP 65-1-2, February 1965. 

(SRDB 68104]

DuPont, 1965f, Progress Report, Works Technical Department, DPSP 65-1-3, March 1965. (SRDB 

68367]

DuPont, 1965g, Progress Report, Works Technical Department, DPSP 65-1-4, April 1965. (SRDB 

68360]

DuPont, 1965h, Progress Report, Works Technical Department, DPSP 65-1-8, August 1965. (SRDB 

11875]

DuPont, 1966a, Radiation Safety Log Sheets, 300-M, March - April 1966. (SDRB Ref ID 70243]

DuPont, 1966b, Radiation Safety Log Sheets, 300-M, April - May 1966. (SDRB Ref ID 70120]

DuPont, 1966c, Radiation Safety Log Sheets, 300-M, June - August 1966. (SDRB Ref ID 70118]

DuPont, 1966d, Radiation Safety Log Sheets, 300-M, August - October 1966. (SDRB Ref ID 70115]

DuPont, 1966e, Radiation Safety Log Sheets, 300-M, October 1966 - February 1967. (SDRB Ref ID 

70114]

DuPont, 1967a, Air Sample Log Sheets, 300-M, September 1967 - March 1969. [SRDB Ref ID 

70091]

DuPont, 1967b, Radiation Safety Log Sheets, 300-M, February - July 1967. (SDRB Ref ID 70882]

DuPont, 1967c, Radiation Safety Log Sheets, 300-M, October 1967 - February 1968. (SDRB Ref ID 

70123]
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 21 of 24

DuPont, 1968a, Radiation Safety Log Sheets, 300-M, February - April 1968. (SDRB Ref ID 70119]

DuPont, 1968b, Radiation Safety Log Sheets, 300-M, May - July 1968. (SDRB Ref ID 70117]

DuPont, 1969a, Air Sample Log Sheets, 300-M, March 1969 - February 1970. [SRDB Ref ID 72669]

DuPont, 1969b, Radiation Safety Log Sheets, 300-M, March - May 1969. (SDRB Ref ID 70098]

DuPont, 1969c, Radiation Safety Log Sheets, 300-M, May - June 1969. (SDRB Ref ID 72687]

DuPont, 1969d, Radiation Safety Log Sheets, 300-M, June - August 1969. (SDRB Ref ID 72686]

DuPont, 1969e, Radiation Safety Log Sheets, 300-M, August - October 1969. (SDRB Ref ID 72684]

DuPont, 1969f, Radiation Safety Log Sheets, 300-M, December 1969 - March 1970. (SDRB Ref ID 

72680]

DuPont, 1970a, Air Sample Log Sheets, 300-M, February - April 1970. [SRDB Ref ID 72669]

DuPont, 1970b, Air Sample Log Sheets, 300-M, April - July 1970. [SRDB Ref ID 72669]

DuPont, 1970c, Radiation Safety Log Sheets, 300-M, March - May 1970. (SDRB Ref ID 72679]

DuPont, 1970d, Radiation Safety Log Sheets, 300-M, September - November 1970. (SDRB Ref ID 

72674]

DuPont, 1972, Radiation Safety Log Sheets, 300-M, August - November 1972. (SDRB Ref ID 70882]

ICRP, 1994. International Commission on Radiological Protection (ICRP), Human Respiratory Tract 

Model for Radiological Protection, Publication 66, Pergamon Press, New York, N.Y.

Morris, R. L., 2008, SRS Thorium Processing Timeline Support SRS SEC Evaluation Report

September 26. [SRDB Ref ID 50589]

Nichelson, S. M. 2005, Consultative Letter, IOH-SD-BR-CL-2005-0081, Alpha Correction Factor for 

New Bioenvironmental Engineering Radiological Air Sampling Equipment - RADeCO Model H809VII, www.airforcemedicine.afms.mil/idc/groups/public/documents/afms/ctb_039469.pdf. 

Accessed August 20, 2009. [SRDB Ref ID 71951]

NIOSH 2006, Lung Dose Conversion Factor for Thoron WLM, OCAS-TIB-0011, Rev. 2, Office of 

Compensation Analysis and Support, Cincinnati, Ohio, January 13, 2006.

Reed, M.B and Swanson, M.T., 2006, Savannah River Site Cold War Historic Property 

Documentation, 300/M Area Fuel and Target Fabrication, New South Associates, Stone 

Mountain, GA, Technical Report 1189, November 27. [SRDB Ref ID 71618]

Staplex 2009, Annular Kinetic Impactors and High Volume Cascade Impactors, 4, 5, & 6 Stage 

Models, http://www.staplex.com/airsamplers/TFIA/aki.htm Accessed August 21, 2009. 

Stoddard, D. H., 1964, Radiation Control and Handling of Natural Thorium. [SRDB Ref ID 49528]
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 22 of 24

ATTACHMENT A

EXAMPLES OF VARIOUS AIR SAMPLE DATA SHEETS

Page 1 of 3
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 23 of 24

ATTACHMENT A

EXAMPLES OF VARIOUS AIR SAMPLE DATA SHEETS

Page 2 of 3
Document No. ORAUT-RPRT-0046 Revision No. 00 Effective Date: 05/18/2010 Page 24 of 24

ATTACHMENT A

EXAMPLES OF VARIOUS AIR SAMPLE DATA SHEETS

Page 3 of 3