Draft

Review of NIOSH’s White Paper: 

Assessment of Sealed Radioactive Sources, and Fission and Activation 

Products as Radiological Exposure Sources in the Rocky Flats Plant 

Critical Mass Laboratory (Building 886 Cluster) 

Contract Number 211-2014-58081

Prepared by 

Ron Buchanan, PhD 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, Virginia 22182 

Saliant, Inc. 

5579 Catholic Church Road 

Jefferson, Maryland 21755 

July 2015 

Disclaimer 

This document is made available in accordance with the unanimous desire of the Advisory Board 

on Radiation and Worker Health (ABRWH) to maintain all possible openness in its deliberations. 

However, the ABRWH and its contractor, SC&A, caution the reader that at the time of its release, 

this report is pre-decisional and has not been reviewed by the Board for factual accuracy or 

applicability within the requirements of 42 CFR 82. This implies that once reviewed by the 

ABRWH, the Board’s position may differ from the report’s conclusions. Thus, the reader should 

be cautioned that this report is for information only and that premature interpretations regarding 

its conclusions are unwarranted.
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NOTICE: This memo has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

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S. COHEN & ASSOCIATES:

Technical Support for the Advisory Board on 

Radiation & Worker Health Review of

NIOSH Dose Reconstruction Program

Document No.: Review of NIOSH’s

White Paper Regarding Radioactive 

Sources at the RFP CML

Effective Date:

Draft – July 8, 2015

Revision No. 

0 (Draft)

Review of NIOSH’s White Paper: Assessment of 

Sealed Radioactive Sources, and Fission and 

Activation Products as Radiological Exposure 

Sources in the Rocky Flats Plant Critical Mass 

Laboratory (Building 886 Cluster)

Page 2 of 8

Task Manager:

__________________________Date:____________

Joseph Fitzgerald

Supersedes:

N/A

Project Manager:

__________________________Date:____________

John Stiver, MS, CHP

Reviewer(s):

 Joseph Fitzgerald

 John Stiver

Record of Revisions

Revision 

Number

Effective 

Date Description of Revision

0 (Draft) 07/8/2015 Initial issue
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NOTICE: This memo has been reviewed for Privacy Act information and has been cleared for distribution.

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

TABLE OF CONTENTS

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

1. Introduction..........................................................................................................................5

2. Radiation Exposure Potential...............................................................................................5

3. SC&A’s Evaluation of NIOSH’s White Paper....................................................................6

4. References............................................................................................................................8
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However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

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ABBREVIATIONS AND ACRONYMS

Bq Becquerel

CML Critical Mass Laboratory

DCAS Division of Compensation Analysis and Support (formerly OCAS)

DOE (U.S.) Department of Energy

FAP fission and activation product

g U/L grams of uranium per liter

IMBA Integrated Modules for Bioassay Analysis

mrem millirem

mW milliwatt

μCi microcuries

NIOSH National Institute for Occupational Safety and Health

NOCTS NIOSH/OCAS Claims Tracking System

OCAS Office of Compensation Analysis and Support (now DCAS)

pCi picocuries

RFP Rocky Flats Plant

SC&A S. Cohen and Associates

SRDB Site Research Database
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However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

1. INTRODUCTION

NIOSH provided a white paper on June 9, 2015, titled: Assessment of Sealed Radioactive 

Sources, and Fission and Activation Products as Radiological Exposure Sources in the Rocky 

Flats Plant Critical Mass Laboratory (Building 886 Cluster). It stated:

This white paper describes the Critical Mass Laboratory (CML) at the Rocky 

Flats Plant [RFP] and the programs and experiments conducted there. It 

discusses the radioactive materials present and their amounts and assesses these 

materials as radiological sources.

After the introduction, the report is divided into the following sections:

 History of the critical mass laboratory

 Radioactive materials used in the CML

 Generation of fission and activation products

 Decommissioning and demolition

 Assessment of unmonitored radiation dose at the CML

 Summary and conclusions

2. RADIATION EXPOSURE POTENTIAL

NIOSH’s white paper provided detailed information concerning the exposure potentials at the 

CML, which operated from 1964–1987, was in standby 1987–1997 (at which time all the fissile 

materials were removed), and was demolished in 2002. The major exposure potentials were 

from:

 Sealed Sources – Sealed radioactive check sources and neutron start-up sources

 FAP – Fission and activation products (FAP) generated in the fuels, test apparatuses, and 

building materials as a result of the nuclear criticality experiments conducted

 Fuels – Plutonium and uranium fuels (solids and liquids) used in the experiments

Sealed Sources

A summary of these sources is listed in Table 2 on page 4 of NIOSH’s white paper. 

FAP

Fission and activation products (FAPs) could be generated in the fuels, in the test apparatuses, 

and building materials as a result of the nuclear criticality experiments conducted. NIOSH’s 

white paper (NIOSH 2015, pp. 4–11) provides an analysis of the potential FAP inventory in the 

fuel generated under the operating conditions at the CML; Table 5 on page 10 summarizes the 

dosimetrically important radionuclides generated from a series of 1-hour runs at 10 mW using

highly enriched uranium concentration of 145.68 g U/L in each experiment. The largest amount of 

FAPs generated is in the neighborhood of 5E5 Bq, which corresponds to 14 μCi of activity. The 

FAPs generated in the fuel would be the controlling factor in this situation, since the activation 
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However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

of the test apparatuses and building structures would be far less than in the fuel, because the

neutron flux is concentrated in the fuels and decreases rapidly with distance and through 

material. 

Fuels

Plutonium and uranium fuels (liquids and solids) were used in the experiments; much of the solid 

fuels were not contained in a metal cladding. These fuels presented a potential for internal 

uptake during both normal operations and during incidents, such as spills.

3. SC&A’S EVALUATION OF NIOSH’S WHITE PAPER

Sealed Sources

External exposure – Sealed sources presented only potential external exposures. Workers were 

monitored for external exposures as required by the RFP badging policy when working in a 

radiation area containing radioactive sources; therefore, external doses should be accounted for 

in the worker’s records.

Internal Exposure – Sealed sources, by definition, do not present an internal exposure hazard.

FAP

External Exposure – Workers were monitored for external exposures as required by the RFP 

badging policy when working in a radiation area containing FAPs; therefore, external doses 

should be accounted for in the workers’ records.

Internal Exposure – Generally, subcritical and near-critical assemblies operating in the mW 

range do not produce significant FAPs; this is demonstrated in the NIOSH white paper. The 

potential intakes from the re-suspension of FAP contaminants is summarized in Table 6, page 19, 

of the NIOSH white paper. The maximum intakes over a 2-year period would in the 

neighborhood of 5 Bq, which corresponds to 135 pCi. The maximum committed organ doses 

from inhalation of FAPs for re-suspended contamination are summarized in Table 7 on page 20 

of NIOSH’s white paper. The doses range from 0.04 mrem to 0.1 mrem; these would not be 

significant doses for dose reconstruction purposes.

Fuels

External Exposure – Workers were monitored for external exposures as required by the RFP 

badging policy when working in a radiation area containing uranium and plutonium; therefore, 

external doses should be accounted for in the workers’ records.

Internal Exposure – Plutonium and uranium fuels (liquids and solids) were used in the 

experiments; much of the solid fuels were not contained in sealed metal cladding. These fuels 

presented a potential for internal uptake during both normal operations and during incidents, 

such as spills. As can be seen from the current evaluation, the largest exposure potential at the 

CML was from the fuels, especially liquid fuels during incidents. 
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NOTICE: This memo has been reviewed for Privacy Act information and has been cleared for distribution.

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

NIOSH’s white paper (NIOSH 2015), Table 1, page 3, lists the uranium and plutonium fuels 

used at the CML; Table 3, page 5, provides some experimental uses of these fuels; and Table 8, 

page 17, lists the major spills over the operating history of the CML. NIOSH addressed potential 

internal uptakes from CML fuel on pages 15–17 of the white paper; in summary, it states: 

...intakes of uranium or plutonium are accounted for by the RFP radiation 

dosimetry monitoring programs. Sufficient detail exists about both routine 

operations and incidents, as well as about the ambient radiation dose 

environment prior to decommissioning, that external doses to, and intakes of 

radioactive materials by, individual claimants could be reconstructed with 

sufficient accuracy if dosimetry records are unavailable.

And on page 21:

The personnel dosimetry program also included periodic bioassay (urinalysis and 

body counts) that focused primarily on identifying uranium and plutonium 

intakes. The in-vivo bioassay, using gamma spectrometry, would be expected to 

easily detect most fission and activation products present in any significant 

amount except for radioisotopes like Sr-90, which emit beta radiation not 

detectable in a routine body count or in a urinalysis evaluated for alpha-emitters.

Estimates of the total activity in fission and activation products from irradiation 

of high-enriched uranyl nitrate solution fuel (the fuel contributing most to 

contamination within the facility) indicate that only inconsequential amounts of 

these products [such as Sr-90] were available to contribute to radiation doses 

from re-suspension of residual contamination.

To evaluate NIOSH’s conclusions, SC&A searched some of the RFP claims on the 

NIOSH/OCAS Claims Tracking System (NOCTS) and found documentation for a few incidents 

in the claimants’ files, but not a large number. Many of the incidents at the CML are recorded in 

a publication by Rothe (Rothe 2005). However, it is very difficult to quantitatively correlate the 

various incidents to potential intakes. Therefore, SC&A approached this issue by analyzing the 

CML workers’ claim files for bioassay data. 

Analyses of CML Claimants

SC&A obtained 25 names of CML workers from Rothe’s publication (Rothe 2005) that had the 

most potential for exposure/uptakes, such as experimenters, technicians, and nuclear engineers. 

Four of these workers were claimants listed in NOCTS. SC&A analyzed these four workers’

Department of Energy (DOE) files for bioassays that were performed while working at the CML; 

the employment years fell in the range of 1964–1982, which were the most active years for 

experimentation and incidents, according to NIOSH’s white paper (Figure 2, page 9, and 

Table 8, page 17, respectively). SC&A looked for the frequency of bioassays (i.e., did the 

bioassays appear to be conducted on approximately an annual basis, or were they hit and miss), 

and also if there was an indication that bioassays were conducted more frequently than once per 

year, which would indicate special bioassays for an incident. SC&A noted that many of the 
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Regarding Radioactive Sources at the RFP CML

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NOTICE: This memo has been reviewed for Privacy Act information and has been cleared for distribution.

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82.

original recorded bioassay results did not indicate the purpose of the bioassay; i.e., routine, 

special, etc.

Bioassay Results

A summary of the bioassay results is as follows:

Table 1. Summary of Four Claimant Bioassays 1964–1982

Case Years at CML

No. of Bioassays

1964–1982

Average No.

Bioassays per Year

Range of No. 

Bioassays/Year

No. years with

>3 Bioassays/Year

A 17 38 2.2 0 to 9 5

B 4 10 2.5 1 to 3 2

C 5 7 1.4 0 to 2 0

D 18 58 3.2 1 to 11 8

Total: 44 113 2.6 - 15

While not conclusive, these results indicate that generally the workers at the CML received 

bioassays on an annual basis, and that there were years when the workers received above the 

normal number of bioassays (i.e., for 15 years out of 44 years, there were >3 bioassays 

performed per year on a given worker, which would be approximately 1/3 of the time),

indicating possible monitoring for non-routine intakes.

SC&A performed some genetic Integrated Modules for Bioassay Analysis (IMBA) runs using 

urinalyses for Pu-239 and U-235 to determine if an acute intake occurring between routine 

bioassays might be missed (in case special bioassays were not taken after an incident). The 

results of these IMBA runs indicate that if the acute intake occurred midway between annual 

bioassays, the remaining radionuclide concentration in the urine bioassays would range from 

approximately 45% to 85% of the concentration just after the acute intake, depending on the 

radionuclide and solubility type. Therefore, significant acute or chronic intakes would most 

likely not go undetected if only routine annual bioassays were conducted.

SC&A Conclusion

The results of this analysis suggest that the external doses and internal intakes from potential 

exposures at the CML were monitored in a manner that would mostly likely not result in 

significant exposures going undetected for dose reconstruction purposes. 

4. REFERENCES

NIOSH 2015. White Paper: Assessment of Sealed Radioactive Sources, and Fission and 

Activation Products as Radiological Exposure Sources in the Rocky Flats Plant Critical Mass 

Laboratory (Building 886 Cluster), Rev. 0. National Institute for Occupational Safety and 

Health, Division of Compensation Analysis and Support (DCAS), Cincinnati, Ohio. June 9, 

2015.

Rothe, R.E., 2005. A Technically Useful History of the Critical Mass Laboratory, LA-UR-05-

3247, Los Alamos National Laboratory, Los Alamos, New Mexico. May 2005. SRDB Ref. ID

#21358.