NIOSH Response to SC&A’s Review of 

Internal Exposures to Thorium and its 

Progeny at the KCP during Mg-Th 

Machining

Response Paper 

Rev. 0 

National Institute for Occupational 

Safety and Health 

July 7, 2015 

Pat McCloskey

Oak Ridge Associated Universities Team

- 

Reviewed by Peter Darnell and Jim Neton

Division of Compensation Analysis and Support 

This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

Page 2 of 9 

This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

PURPOSE

In January 2015, NIOSH provided a response paper titled, Internal Exposures to Thorium and its 

Progeny at the Kansas City Plant during Mg-Th Machining, addressing SC&A’s issues 

contained within their August 2014 paper titled, Review of Internal Exposures to Thorium and Its 

Progeny at the Kansas City Plant during Mg-Th Machining Operations. SC&A reviewed 

NIOSH’s response paper and provided another response on May 12, 2015, titled Review of 

NIOSH’s White Paper: Internal Exposures to Thorium and its Progeny at the Kansas City Plant 

during Mg-Th Machining. 

In their latest May 12th response, SC&A agreed with NIOSH that Mg-Th machining operations 

began at KCP in August 1961 and those exposures from operations can be bounded with the 

Evaluation Reports (ER) methodology through March 31, 1963. 

For the remaining period, 1963–1979, SC&A agrees the ER’s bounding method (3E-11 μCi/ml) 

is claimant favorable. However, SC&A requests monitoring data and validation showing that 

controls and working conditions did not worsen during those years with incomplete monitoring 

data. The issue centers on the need for a better understanding of operational parameters such as 

(1) machining workload (i.e., Was Mg-Th even being machined?), (2) location (Was it colocated with DU operations?), and (3) dates of operations. The following sections discuss 

NIOSH’s understanding of the processing and control of Mg-Th alloy at KCP from 1963 through 

1979. 

Clarifications regarding April 1963–August 28, 1970 

Prior to the January 2015 Work Group meeting, while reviewing NIOSH’s January 2015 paper,

SC&A noticed what appeared to be “contradictory information about the period April 1963– 

August 1970” and asked for some clarification to enable a clearer exchange at the subsequent 

meeting. 

In response to SC&A’s request, NIOSH provided the following clarification to SC&A and the 

Work Group: 

The wording here could have been better. First off, we have not reviewed 

information regarding Mg-Th machining during the period April 1963–August 

1970. It remains obvious that these Mg-Th operations were small-scale and my 

point was that while the “more important” DU operations were in full swing, it 

makes sense that KCP’s focus would have shifted to DU, and Mg-Th 

documentation would dwindle. 
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

We have kept April 1963–August 1970 as a Mg-Th operational period for our 

bounding method (3E-11 μCi/ml) since we do not have records of D&D work at 

the start of this period, and believe this work would have continued in Department 

20 until it was moved into the Model Shop in 1970. 

Since January, NIOSH has continued to obtain and review documents (e.g., medical records, 

inventory) and perform interviews. Based on a review of the information available from the 

1963–1970 period, NIOSH has determined that Mg-Th operations were suspended beginning 

April 1, 1963, and did not begin again until after receiving approval from Health Services on 

August 28, 1970 (SRDB 128433, PDF p. 2). This suspension of activities during this time period 

is corroborated by inventory information that does not support a start date any earlier for the 

second campaign. One data source with inventory information beginning in 1969, documents the 

presence of Mg-Th inventory starting in 1971 (SRDB 135987). Another source, the Nuclear 

Materials Management and Safeguards System (NMMS), corroborates the inventory information 

and also documents the presence of Mg-Th beginning in 1971 (SRDB 137786). 

The Weekly Activities reports also corroborate the suspension of Mg-Th operations. They 

document a very small staff (consisting of five personnel) working one shift, and D&D in 

Department 22 (the area where this work would have been done) beginning in May 1964, and by 

August 1964, half of the machines were removed from the area, and the staff were reduced to 

two, part-time personnel (SRDB 137860). 

Second Campaign, August 1970–December 1977

NIOSH has reviewed a memorandum that helps to better define the dates of operations for the 

second operational campaign starting in 1970 and ending in 1977 (SRDB 128420). In this 

memorandum, a 1970 start date was identified by Model Shop management, which corroborates 

the operational information discussed above. The 1977 ending date agrees with the Source and 

Special Nuclear Material inventory information, which shows the last receipt in March 1977 

(SRDB 135987), and NMMS information does not indicate a later date of operations other than 

waste management (SRDB 137786). 

SC&A has requested air-monitoring results applicable to work during this period. NIOSH has 

not located additional data other than results from three weeks of air-monitoring previously 

discussed in NIOSH’s ER, a January 2015 Response Paper, and SC&A’s August 2014 and May 

2015 Response Papers where KCP industrial hygienists followed development and production 

through its various machining operations, and reported that the “air-borne concentrations of 
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

thorium and its daughter-decay products were within the limits specified by the AEC’s Standards 

for Protection Against Radiation” (SRDB 108264, PDF p. 16). Although NIOSH has not located 

additional air monitoring data, there are other factors considered in establishing the basis for the 

ER’s bounding method. 

Pyrophoricity

One factor considered by NIOSH is the pyrophoric nature of Mg-Th and the controls KCP 

implemented to prevent fires. From the beginning of these operations in 1961, KCP was sensitive 

to this hazard and required onsite fire department involvement prior to any Mg-Th work (SRDB 

108264, PDF p. 44). KCP was more explicit about this hazard in 1963, “This alloy (Mg-Th) is a 

potential problem primarily as a result of the pyrophoricity of the magnesium” (97% of the alloy) 

(SRDB 128181, PDF p. 5). Health and Safety (H&S) guidelines followed throughout the second 

campaign in the 1970s continued to address pyrophoricity, and included the statement “the Fire 

Protection Department shall be contacted before initiation of project and regarding any 

alterations in processing” (SRDB 128433, PDF p. 4).

Interviews in March 2015 corroborate this respect for the hazard, and adherence on the floor was 

confirmed with statements such as the following: “We were told Mg-Th was extremely 

flammable.” 

Mg-Th’s pyrophoric nature was the driver for many of the other industrial hygiene (IH) controls 

implemented including: “Good Housekeeping” to prevent waste and dust accumulation; a very 

important fire suppression technique for pyrophoric materials; and “wetting controls” which 

provided collateral health physics’ benefits by reducing airborne dust generation.

Wetting Controls 

Another factor NIOSH considers relevant in our determination of the ER’s bounding method is 

that all reports indicate Mg-Th work was a wet process. Some machining operations such as 

those utilizing the Tape Lathe at Post #F-25.5 were performed completely submerged in coolant 

(SRDB 108264, PDF p. 18).

KCP implemented IH Best Practices with H&S guidelines and management procedures. These 

practices included employing all of the controls considered state-of-the-art for the 1970s, and 

most (e.g., medical surveillance and respiratory protection) continue unchanged to this day. 

Section D.1 of the H&S Guide issued at the beginning of this campaign in 1970 (SRDB 128433, 

PDF p. 4) states, “In all machining operations, the material shall be machined “wet,” using the 
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

mineral oil base coolant, Cadet Z.” A year later in 1971, this Guide was revised to reflect a 

change in the issuing department from H&S to Industrial Hygiene and Health Physics (SRDB 

128155). However, all of the IH best practices remained in tact, including the Section D.1 

requirement to perform machining operations “wet” (SRDB 128155). The 1975 revision of this 

guide changed Section D.1 to add requirements for establishing a restricted area within the 

Model Shop where this work was to be performed, and the wetting controls requirement was 

moved to Section D.2 (SRDB 108264, PDF p. 4). KCP’s implementation and adherence to these 

wetting controls was also corroborated by interviewees conducted during the March 2015 site 

visit with statements such as “Chips were kept wet.”

Given that the material was wet, it is not plausible that KCP machinists would generate a 

significant amount of dust. After 1970, the Mg-Th was 2% Th by weight, and it would not seem 

plausible to reach concentrations as great as 3E-11 on a consistent basis. NIOSH estimates that 

breathing 3E-11 air, one would inhale ~33 grams of alloy in a year—a high number for a wet 

process.

NIOSH also considered data found in SC&A’s 2007 report, A Focused Review of Operations 

and Thorium Exposures at the Dow Chemical Company Madison Plant (SRDB 53614), which

describes dosimetric implications of working with the same Mg-Th Dow supplied to KCP. One 

worker’s affidavit stated that on mill #1, there was “no airborne dust.” He opined, “Any 

generated dust would have been smothered by the mill coolant” (PDF p. 22).

SC&A’s report provides data from air samples collected in the breathing zone of workers that 

were performing aggressive activities with Mg-Th that would have likely exceeded the airborne 

generating capacity of KCP’s machining work. These activities included open-wheel surface 

grinding, air-operated vibration sanding, buffing, and “drumming” Mg-Th powder. For 

comparison, the highest observed air concentration in the workers’ breathing zone while 

performing these airborne generating activities at Dow was 3.9 E-12 μCi/ml. Using information 

SC&A provided in Section 3.5 (PDF p. 32), this concentration would yield a dose of less than 4 

mrem/hr or 8 rem/year (CEDE).

SC&A offered the following description of the available data at Dow: “With respect to 

radiological data pertinent to reconstructing internal exposures, it is clear that the data are limited 

to a few air samples; however, these data are informative” (PDF p. 29). NIOSH believes that 

these operations at Dow represent a worst-case exposure scenario, and it is not likely that KCP 

machinists were exposed to a higher concentration on a 2000-hour time-weighted-average 

(TWA) basis.
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

One additional observation from the SC&A report regarding Dow that NIOSH finds informative 

is the following: “In spite of the fact that only limited processing of uranium was done in 

Building 6, while Mg-Th alloys had been processed there for more than 30 years, the uranium 

contamination was far greater. This observation is consistent with other evidence presented in 

this report indicating that thorium mobility is low” (PDF p. 25). 

Source Term

SC&A has requested corroborating data, and in the absence of such data recommends sourceterm-based exposure modeling. The following is taken from the conclusion of the SC&A May 

2015 white paper: 

SC&A recommends that in the absence of measurement data, NIOSH should 

validate its proposed 3E-11 μCi/ml air concentration limit through source termbased exposure modeling, followed by suitable sample dose reconstructions to 

demonstrate the feasibility of applying this limit for the various operational time 

periods in question (i.e., 1963-1966, 1966-1970, 1970-1979). 

Following SC&A’s recommendation, NIOSH has reviewed the available source-term 

information. NIOSH has not located extensive source term information and recognizes that

inventory information is limited. However, this apparent lack of information could be explained 

by the small-scale nature of these operations, and/or that there simply was not much inventory 

after KCP started tracking it in 1969. For example, the Source and Special Nuclear Material 

inventory information has no data from October 1973 until December 1975, which is explained 

by the Waste Management plan that indicates a break in these Operations by virtue of no waste 

generated in 1974 (SRDB 134675, PDF p. 315). 

NIOSH was able to locate the largest inventory for 1973 (SRDB 135987). Quantities were 

identified as being received during eight separate months during the year and totaled 42 kg of 

thorium. If we apply Equation 1.2 from NUREG 1400 to this largest (worst-case) annual amount, 

we can calculate an intake amount (I).

I = Q × 10-6 × R × C × D 

Where:

• Q = The total quantity of unencapsulated thorium processed for the year, or 42,000 g is used. 
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

Page 7 of 9 

This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

• R = The release fraction of thorium likely to be released into the workplace, taken from Table 

1.1. Although the dominant material forms were solids, exposure potential increased as 

material was machined; therefore, “nonvolatile powders” or 0.01 is used. 

• C = The Confinement fraction. Although all machining operations used worksite local 

ventilation (400 CFM with absolute filters) and material was maintained wet, we take no credit 

for it and use a factor of 1. 

• D = The Dispersibillity factor. Although KCP’s machining operations were likely lessenergetic than the examples given in the NUREG, NIOSH will conservatively use their 

suggested factor of 10. 

Yields: I = 4.2E-3 g/year

Using the specific activity of Th-232 (1.1E-7 Ci/g), we can convert this to 4.62E-10 Ci/year or 

17.1 Bq/year. We can compare the amount 17.1 Bq/year to the intake amount based on the ER’s 

bounding concentration (3E-11 μCi/ml) calculated by SC&A (Finding 7, PDF p.13) as 2664 

Bq/year. Therefore, this source-term calculation shows an intake rate much smaller (156 times) 

than that based on the ER’s method. 

Although a sufficient amount of conservatism has been used in this calculation, NIOSH suggests 

the assumption of 1973 as a worst-case and the throughput for that year (42 kg) could be 

validated by reviewing the classified NMMS data in Germantown. 

NIOSH provides this NUREG 1400 source-term calculation to satisfy SC&A’s request. It is only 

meant to provide an additional layer of assurance, and add to a preponderance of evidence 

justifying that the ER’s method is bounding. 

Conclusion 

NIOSH, along with SC&A and the Advisory Board Work Group, has been reviewing KCP 

documents and interviewing personnel since 2004 regarding radiological work at KCP. For the 

last several years, NIOSH has specifically been searching for Mg-Th information. NIOSH 

continues to seek and review additional information. 

Based on a review of the information available at this time, NIOSH believes the weight of 

evidence supports the ER’s bounding method, as modified with the Advisory Board’s and 

SC&A’s assistance, as plausible and claimant favorable.
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

Page 8 of 9 

This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

REFERENCES

SRDB 53614, A Focused Review of Operations and Thorium Exposures at the Dow Chemical 

Company Madison Plant (Draft)

SRDB 108264, PDF p. 4, Revised Health and Safety Guide for Handling Magnesium-Thorium in 

Department 851 (August 28, 1970; Revised July 16, 1975); Document spans PDF pp. 3-6 

SRDB 108264, PDF p. 16, Correspondence and Results-Summary of Air Samples of MagnesiumThorium Project in D/851; Document spans PDF pp. 16-20 

SRDB 108264, PDF p. 18, Correspondence and Results-Summary of Air Samples of MagnesiumThorium Project in D/851; Document spans PDF pp. 16-20 

SRDB 108264, PDF p. 44, Recommendations for Handling Magnesium-Thorium Alloy

SRDB 128155, Handling Magnesium-Thorium in Dept. 851 (select pages from “Industrial 

Hygiene and Health Physics Guide”)

SRDB 128181, PDF p. 5, Magnesium-Thorium (3.5% MAX.) Alloy Suggested Care and 

Handling Methods; Document spans PDF pp. 5-7 

SRDB 128420, D/823 Radioactive Materials Usage

SRDB 128433, PDF p. 2, Development Support Project Using Magnesium-Thorium Alloy 

SRDB 128433, PDF p. 4, Revised Health and Safety Guide for Handling Magnesium-Thorium in 

Department 851 (August 28, 1970); Document spans PDF pp. 3-5 

SRDB 134675, PDF p. 315, Radioactive Waste Management Site Plan (1974 Submission); 

Document spans PDF pp. 313-322 

SRDB 135987, Statements of Measurement Methods/Balance Report/Inventory

SRDB 137786, Handwritten Inventory Notes
NIOSH Response to SC&A’s Review of Internal Exposures to Thorium and its Progeny at the KCP during MgTh Machining

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) 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. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

SRDB 137860, Weekly Activities Reports