White Paper:

NIOSH’s Method and Supporting Basis for Bounding

Uranium Internal Exposure at Pantex

by

Stuart Hinnefeld

Based on material by

Jason Davis

Executive Summary

The Secretary of Health and Human Services has designated a single class of employees at the Pantex 

Plant for addition to the Special Exposure Cohort (SEC) authorized under EEOICPA. On January 18, 

2012, NIOSH designated a class that includes all employees of the Department of Energy, its 

predecessor agencies, and their contractors and subcontractors who worked at the Pantex Plant in 

Amarillo, Texas, during the period from January 1, 1958 through December 31, 1983, for a number of 

work days aggregating at least 250 work days, occurring either solely under this employment or in 

combination with work days within the parameters established for one or more other classes of 

employees included in the SEC. This report presents a method and supporting basis for bounding 

uranium internal dose associated with the maintenance, assembly, and disassembly of weapons at the 

Pantex Plant beginning on January 1, 1984. 

Natural uranium in rock (by weight, 99.27% U-238, 0.72% U-235, and 0.006% U-234) has 

approximately equal activity concentrations of U-238 and U-234. Studies summarized by the U.S. 

Environmental Protection Agency (EPA, 1991) have indicated that U-234 (via Th-234) has a greater 

environmental mobility than U-238 and may well have a higher concentration in ground water. To 

establish the Pantex environmental baseline against which employee urine samples could be 

compared, 75 urine samples were obtained over a three-year period from Pantex site personnel not 

occupationally exposed to uranium. These samples were radiochemically analyzed for their U-234 

and U-238 content (Long, 1994).

Data from this study led to the conclusion that the U-238 activity in a sample and its ratio relative to 

U-234 were independent variables. The 95th percentile of the lognormal distributions of U-238 and 

U-234 in a sample were 0.16 dpm/d and 1.2 dpm/d, respectively (Long, 1994). The Long report 

concluded that, for the Pantex site, elevated concentrations of U-234 with respect to U-238 in a urine 

sample are more indicative of an individual’s consumption of well water in the area than occupational 

exposure to enriched material. 

The NIOSH white paper, Analysis of Pantex 1990 Uranium Bioassay Results for Workers Identified as 

Being Production Technicians on the W28 Program (Bihl, 2012), revealed that the exposure scenario 

yielding the highest intake consistent with the measured bioassay data is one in which a chronic intake 

occurred over a single year in 1984. Based on results of this paper, NIOSH intends to apply the 

single-year (1984) exposure scenario to dose reconstructions for energy employees who worked at 

Pantex during any portion of the 1984-1990 time period. For employees who worked during this time 

period who may have been hired after January 1, 1984, the single-year intake scenario will be applied 

to the first year of employment.

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 2 of 8

It is possible that some of the uranium detected in the 1990 urine samples was due to intakes prior to 

1984. However, intake and dose for the 1984-to-1990 period are maximized by assuming that the 

intake occurred during 1984. A later end to the intake period yields a smaller intake rate. An earlier 

assumed intake period would yield a larger intake rate, but would fall entirely within the SEC period, 

during which it has been determined that dose reconstruction is infeasible.

Pantex provided routine urinalysis of uranium in 1991 and 1992. Since 1993, monitoring of uranium 

exposures has been event-driven and is identified by air-monitoring data. Since the mid-1990s, Pantex 

has used lapel air samplers to monitor for intakes and trigger bioassay measurements. NIOSH intends 

to use any available monitoring data for reconstruction of internal doses that may have occurred 

during or after 1991. When bioassay data are not available to assess the intake of a worker whose job 

had a lower potential for intake, but might have had incidental exposure to contamination from 

disassembly activities, NIOSH has suggested that dose reconstructors should assign 10% of the 

average worker intake for that year (ORAUT-TKBS-0013-5, Section 5.2.2.3.2, pdf p. 26).

Method and Supporting Basis

The following sections present the method and supporting basis for bounding uranium internal dose 

associated with the maintenance, assembly, and disassembly of weapons at the Pantex Plant. 

Bounding Method 1984-1990

NIOSH intends to apply the single-year (1984) exposure scenario to dose reconstructions for energy 

employees who worked at Pantex during any portion of the 1984-1990 time period. For employees 

who worked during this time period who may have been hired after January 1, 1984, the single-year 

intake scenario will be applied to the first year of employment. For reconstruction of internal doses 

that may have occurred during or after 1991, NIOSH intends to use any available monitoring data 

from the routine urinalysis performed during 1991-1992 and event-driven sample data from 1993 

onward. 

Background

A large set of urine samples (305) was collected from February 10 to April 2, 1990 and analyzed by 

the Y-12 Plant for uranium. Isotopic analyses were performed on the samples with results reported in 

units of disintegrations per minute (dpm). These bioassay samples were taken to assess potential 

intakes of DU-oxide by technicians working on the weapons disassembly program in the 1980s that 

resulted in observations of visible quantities of depleted uranium contamination. A statistical analysis 

was performed by Dan Strom at PNNL. Dr. Strom removed four results that had radiochemical 

recoveries reported as zero. Using the sum of the isotopic results for a given sample, there were 299 

results greater than zero. Those were fit with the lognormal analysis, producing a GM of 0.188 dpm/d

(Strom, 2004a; Strom 2004b). The results of the lognormal statistical analysis on this dataset were 

reported in the Pantex Occupational Internal Dose technical basis document (ORAUT-TKBS-0013-5, 
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 3 of 8

pdf p. 25). These data also formed the basis for the default median chronic DU intakes of 1.3 pCi/d for 

Type M inhalation and 19 pCi/d for Type S inhalation (ORAUT-TKBS-0013-5, 2007).

Because there was a need to determine a bounding dose, and because the research and interviews 

conducted from 2004 until recently indicated that the highest risk of U contamination and possible 

intakes came from the W28 disassembly project, ORAUT was requested to redo the analysis of the 

1990 U bioassay using only the results from the production technicians (PTs) associated with the W28 

disassembly. The thought process was that the PTs were at the highest risk of chronic intakes and,

therefore, represented the bounding group. A Pantex Health Protection memo was used to identify the 

PTs on the W28 project (Monitoring Results, 1989-1990, pdf p. 47). Excretion results from those 

workers were extracted from the original 305 results.

In a 2012 white paper, Analysis of Pantex 1990 Uranium Bioassay Results for Workers Identified as 

Being Production Technicians on the W28 Program, NIOSH explored various intake scenarios that 

could have resulted in the bioassay measurements recorded in 1990 (Bihl, 2012). As explained in the 

W28 paper, the results from the production technicians (PTs) were analyzed twice: once for all PTs 

who had worked on the W28 project and once for only PTs who were still active on the project in 

1989 when the project was suspended. The names for both groups came from the Pantex Health 

Protection memo (Monitoring Results, 1989-1990, pdf p. 47). These groups were analyzed separately 

because of the effect on the urine excretion rate of the time between last intake and sample collection. 

The same chronic intake rate will produce different excretion rates if samples are collected on 

different days after the end of the intake period, with daily excretion decreasing over time. When the 

analysis goes from excretion back to intake, and the time from end of intake to collection date is not 

known, different intakes would be calculated from the different excretion rates, even if the intakes 

were in fact the same. The actual intake scenarios for each of the workers in the “all PTs” dataset is 

not known and potentially could vary considerably. It was assumed that the time from end of intake to 

collection date had less variability for the small dataset for PTs still active on the W28 project in 1989. 

However, because the dataset is smaller it is less statistically robust.

Potential Intake Scenarios

In the first scenario, it is assumed that the workers continued to be exposed to DU through other work 

activities at the plant after the cessation of the W28 program. The next two scenarios look at short 

periods of work in the project (one year of exposure) and gaps between the end of chronic intake and 

the sample date in which no exposure occurred. A fourth set of scenarios all start in January 1984 and 

involve intake periods that increase in one-year increments. A fifth set of scenarios ends in February 

1989, but varies the intake start dates.
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 4 of 8

The scenarios are listed below:

• Intake started January 2, 1984 and was continuous through to the sample date (2257 days). This 

assumption used the geometric mean (GM) for the workers active in February 1989.

• Intake occurred in 1984 for one year only. This used the GM for all W28 workers.

• Intake occurred in 1987 for one year only. This used the GM for all W28 workers.

• A series of intakes all starting January 2, 1984 and occurring for 2, 3, 4, 5, and 6 years, 

respectively. These used the GM for all W28 workers.

• A series of intakes all ending on February 28, 1989 but with start dates of January 2, 1988; January 

2, 1987; January 2, 1986; January 2, 1985; and January 2, 1984. These used the GM for the 

workers active in February 1989.

The analysis presented in the W28 Program white paper revealed that the exposure scenario yielding 

the highest intake consistent with the measured bioassay data is one in which a chronic intake 

occurred over a single year in 1984.

For the purpose of maximizing dose, the single-year exposure scenario is still assumed to be bounding 

if the employee had the potential to have received an intake prior to 1984. Material exposures prior to 

1984 would contribute to the uranium detected in the urine. Assuming all of the material was taken in 

during the single year maximizes internal dose as compared to assuming that the exposure occurred 

over a protracted period of timr.

Post-1990 Method

Pantex dose records in the 1990s specifically state whether internal emitters were monitored (and 

provide a dose) or not monitored (N/M). Pantex provided routine urinalysis of uranium in 1991 and 

1992. Since 1993, monitoring of uranium exposures has been event-driven and is identified by 

air-monitoring data. Since the mid-1990s, Pantex has used lapel air samplers to monitor for intakes 

and trigger bioassay measurements. Analysis of Biological Samples for Uranium, Thorium, and 

Plutonium (Kerr, 1991) provided the following workplace indicators that would trigger bioassay: 

• All personnel ... not wearing ... respiratory protection whose tracked internal annual exposure is 

equal to 40 DAC-hours 

• All personnel whose breathing zone monitor indicates that they have been exposed to 40 DAChours [also lists the DAC for U-238 as 6 × 10-11 μCi/mL] 
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 5 of 8

• All personnel found to have skin contamination equal to or greater than ... 1000 dpm/100 cm2

238U.

Table 1 presents the total numbers of available bioassay records. 

Table 1: Pantex Urinalysis Records

Isotope Number of Individuals Providing Samples

1990 1991 1992 1993 1994

Uranium 385 671 463 538 455

Plutonium --- --- 9 81 105

Thorium --- --- 4 87 88

The site’s technical basis manual at the time (BWXT Pantex, 1992) stated that the uranium urinalysis 

method was isotopic analysis using alpha spectrometry that “can detect 0.03 pCi/isotope/sample.” 

The document reported an environmental background urinary excretion rate of 0.15 dpm/d of U-238

based on studies of potentially-exposed and unexposed Pantex workers. This environmental screening 

level was carried over to the internal dose assessment procedure (MHSMC, 1991), which indicated 

that dose assessment was to occur for any uranium result with a net activity greater than or equal to 

0.15 dpm. Average, non-zero urine concentrations of uranium isotopes are presented in Table 2.

Table 2: Pantex Uranium Concentrations in Urine

Isotope Mean Uranium Concentrations in Urine (dpm/ml)

1990 1991 1992 1993 1994

U-234 0.1423 0.1347 0.1436 0.1300 0.1528

U-235 0.0233 0.0143 0.0111 0.0418 0.0175

U-236 0.0086 0.0059 0.0073 0.0267 0.0115

U-238 0.0622 0.0673 0.0935 00.1093 0.0884

When bioassay data are not available to assess the intake of a worker whose job had a lower potential 

for intake, but might have had incidental exposure to contamination from disassembly activities, 

NIOSH has suggested that dose reconstructors should assign 10% of the average worker intake for 

that year (ORAUT-TKBS-0013-5).
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 6 of 8

Position Basis

The evaluation documented in the 2012 white paper, Analysis of Pantex 1990 Uranium Bioassay 

Results for Workers Identified as Being Production Technicians on the W28 Program (based on the

urinalyses taken after the W28 incident), is a reasonable basis for a bounding estimate of potential 

exposures to claimants. The W28 units had the highest potential for exposure based on information 

received during interviews with former Pantex workers and documentation from the SRDB (Personal 

Communication, 2011a; Personal Communication, 2011b).

The bases for this position are:

1. The W28 weapon was the weapon system with the largest potential uranium exposure for workers 

at Pantex.

2. The large-scale dismantlement of the W28 weapon during the 1984-1989 time period presents 

greater exposure potential to workers than earlier assembly operations or disassemblies for the 

Stockpile Surveillance Program or modification.

3. Because the above exposures resulted from the highest potential uranium exposure operations at 

Pantex, the exposure rate for this period is a bounding estimate for the exposure rates to workers at 

Pantex from 1984 to 1989. 

The reasons for concluding that the W28 weapon represented the largest potential uranium exposure to 

Pantex workers are listed below.

• The U metal was not alloyed in the W28 unit; uranium metal associated with W28 had a much 

greater potential for corrosion than the material from weapons with a uranium alloy material.

• Corrosion was time-dependent; therefore, internal exposure potential was a greater issue during 

disassembly, not during assembly when the components were new.

• The “28” warhead weapons were stored in various temperature and humidity conditions

throughout the world until returned for disassembly, including conditions known to result in 

greater corrosion.

• Mason & Hanger identified the “28” program as one in which uranium oxide problems were 

anticipated (others included the W43, W50, W53, W55, W56, W69, and B83 programs) (Incident 

Report, 1990).

• Mason & Hanger workers disassembled W28 units during period of 1984-1989 (Incident, 1989; 

Monitoring Results, 1989-1990).
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 7 of 8

• The W28 had one of the largest depleted uranium surface areas of all the various weapons; hence,

more metal was exposed to potentially-corrosive environments.

• The W28 weapons were returned for disassembly after many years in the stockpile or storage

(approaching 30 years) which tends to maximize corrosion.

Conclusions

Although the personnel exposure consequences were relatively minor, the 1989 W28 incident became 

a notable event, ultimately being classified as an unusual occurrence based upon employee concerns. 

When asked about assembly and disassembly of other weapon systems (i.e., W55, W56), no other 

contamination event or uranium uptakes stood out compared to the W28 event occurring in February 

1989 (Personal Communication, 2011a; Personal Communication, 2011b). Consequently, for this 

reason and those discussed above, the potential exposure from W28 disassembly represents a 

bounding assessment of uranium exposures to Pantex workers. The analysis of data from that event 

and from air sampling data over a period of many years indicates that the potential calculated intakes 

are less than the intakes based on bioassay data recommended by the Pantex Site Profile (ORAUTTKBS-0013-5).

References

Bihl, 2012, Analysis of Pantex 1990 Uranium Bioassay Results for Workers Identified as Being 

Production Technicians on the W28 Program, Rev. 2, D. E. Bihl and T. LaBone; January 17, 2012; 

ORAUT white papers available on the ORAUT Intranet

BWXT Pantex, 1992, Technical Basis for the Internal Dosimetry Program and the DOE Pantex 

Facility, Rev. 1; Radiation Safety Department, Pantex Plant; February 14, 1992; SRDB Ref ID: 13312

EPA, 1991, National Primary Drinking Water Regulations; Radionuclides; Proposed Rule; 

Environmental Protection Agency; Federal Register Vol 56 No 138, p. 33068; July 18, 1991; publicly 

available; https://www.federalregister.gov/articles/2000/04/21/00-9654/national-primary-drinkingwater-regulations-radionuclides-notice-of-data-availability

Incident Report, 1990, Incident Records 1990 – Investigation of 12-44 Cell 4 Off-normal Incident; 

Pantex Plant; 1990; SRDB Ref ID: 14206

Kerr, 1991, Internal Operating Procedure: Analysis of Biological Samples for Uranium, Thorium 
NIOSH’s Method and Supporting Basis for Bounding Uranium Internal Exposure at Pantex

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working 

Groups or Subcommittees. 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.

Page 8 of 8

and/or Plutonium, J. C. Kerr; Mason & Hanger-Silas Mason Co., Inc.; January 2, 1991; SRDB Ref 

ID: 12319

Long, 1994, Practical Issues in Discriminating Between Environmental and Occupational Sources in 

a Uranium Urinalysis Bioassay Program, M. P. Long, E. H. Carbaugh, N. L. Fairrow; PNL-SA24340; September 1994; SRDB Ref ID: 11089

MHSMC, 1991, Internal Dose Assessment, Pantex Plant Procedure IOP D5491, Issue No. 2, Mason & 

Hanger-Silas Mason Company, Pantex Plant; May 24, 1991; SRDB Ref ID: 12549

Monitoring Results, 1989-1990, Personnel Bioassay Information Along with Incident Information and 

Some Radiological Surveys; various documents with dates ranging from 1989 through 1990; Pantex 

Plant; SRDB Ref ID: 80203 

ORAUT-TKBS-0013-5, Pantex Plant-Occupational Internal Dose, Rev. 01; ORAU Team Dose 

Reconstruction Project for NIOSH; June 22, 2007; SRDB Ref ID: 32528 

Personal Communication, 2011a, Personal Communication with Long-Time, Former Pantex 

Employee; Telephone Interview by ORAU Team; May 26, 2011; SRDB Ref ID: 100560

Personal Communication, 2011b, Personal Communication with Former Pantex Employee; Telephone 

Interview by ORAU Team; May 26, 2011; SRDB Ref ID: 100522

Strom, 2004a, Pantex 1990 Uranium Bioassay, D. J. Strom; Oak Ridge Associated Universities;

2004; SRDB Ref. ID: 106253

Strom, 2004b, Analysis of 1990 Uranium Urinalysis Data, D. J. Strom; Oak Ridge Associated

Universities; June 2004; SRDB Ref. ID: 14187