Draft 

REVIEW OF INTERNAL EXPOSURES TO THORIUM AND ITS 

PROGENY AT THE KANSAS CITY PLANT DURING MG-TH 

MACHINING OPERATIONS 

Contract No. 211-2014-58081

Prepared by 

John Mauro, PhD, CHP 

Arjun Makhijani, PhD 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, Virginia 22182 

Saliant, Inc. 

5579 Catholic Church Road 

Jefferson, Maryland 21755 

August 2014 

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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S. Cohen & Associates:

Technical Support for the Advisory Board on

Radiation & Worker Health Review of

NIOSH Dose Reconstruction Program

Document No.:

Exposure to Thorium and its 

Progeny at KCP

Effective Date: 

Draft – August 15, 2014

Revision No.:

0 (Draft)

REVIEW OF INTERNAL EXPOSURES TO 

THORIUM AND ITS PROGENY AT THE KANSAS 

CITY PLANT DURING MG-TH MACHINING 

OPERATIONS

Page 2 of 14

Task Manager:

_____________________________ Date: ___________

Joseph Fitzgerald

Supersedes:

N/A

Project Manager:

_____________________________ Date: ___________

John Stiver, MS, CHP

Reviewers:

 Joseph Fitzgerald

 Joyce Lipsztein

 John Stiver

Record of Revisions

Revision 

Number

Effective 

Date Description of Revision

0 (Draft) 08/15/2014 Initial issue
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TABLE OF CONTENTS 

List of Abbreviations and Acronyms .............................................................................................. 4

Introduction ..................................................................................................................................... 5

Internal Exposures to Th-232 .......................................................................................................... 5

The May 1, 1957, to October 31, 1959 Period.................................................................... 5

The November 1, 1959, to April 30, 1979 Period .............................................................. 8

Thorium Concentration in the Mg-Th Alloy ................................................................................ 10

Internal Exposures to the Progeny of Th-232 and Th-228 ........................................................... 10

Conclusions ................................................................................................................................... 12

References ..................................................................................................................................... 14
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LIST OF ABBREVIATIONS AND ACRONYMS 

AEC Atomic Energy Commission 

Bq Becquerel 

Ci/g Curie per gram 

cpm count per minute 

DU depleted uranium 

g gram 

KCP Kansas City Plant 

MPC maximum permissible concentration 

Mg magnesium 

m

3 cubic meter 

mg/cc milligram per cubic centimeter 

mg/m3 milligram per cubic meter 

mr/hr millirem per hour 

μCi/cc microcurie per cubic centimeter 

μCi/m3 microcurie per cubic meter 

μCi/ml microcurie per milliliter 

NBS National Bureau of Standards 

NIOSH National Institute for Occupational Safety and Health 

OCAS Office of Compensation Analysis and Support 

ORAUT Oak Ridge Associated Universities Team 

PER Petition Evaluation Report 

rem Roentgen equivalent man 

SC&A S. Cohen and Associates (SC&A, Inc.) 

SEC Special Exposure Cohort

Sv Sievert 

Th thorium
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INTRODUCTION 

This report specifically addresses the methods that NIOSH plans to use to reconstruct internal 

exposures associated with Mg-Th operations at the Kansas City Plant (KCP) from May 1, 1957,

through April 30, 1979, and then separately from May 1, 1979, through May 31, 1984. 

Section 7.2.3.1 of the Special Exposure Cohort (SEC) Petition Evaluation Report (PER Petition 

SEC-00219, January 7, 2014) addresses this subject. 

INTERNAL EXPOSURES TO TH-232 

As explained in the SEC PER, there are limited air sampling data that might be used to estimate 

the airborne dust loading of alpha emitters during Mg-Th operations, and there are no bioassay 

data that can be used to reconstruct the internal exposures to thorium. The basic strategy adopted 

by NIOSH in the SEC PER for estimating the airborne dust loading of thorium and associated 

internal exposures during the Mg-Th operations is to use a combination of process knowledge 

and limits established and enforced on the maximum allowable airborne concentration of 

thorium. This report presents a review of the source documents cited in the SEC PER in order to 

assess the degree to which the protocols adopted by NIOSH for reconstructing internal exposures 

to thorium are scientifically sound and claimant favorable. 

NIOSH proposes two different approaches to thorium dose reconstruction depending on the 

period. The first period would be from May 1, 1957, to October 31, 1959, and the second would 

extend from November 1, 1959, to April 30, 1979, the end of the Th-Mg alloy processing period. 

We will consider each of these in turn. 

The May 1, 1957, to October 31, 1959 Period 

NIOSH proposes to use a constant value of Th-232 in air of 9E-11 μCi/ml of air for this period. 

As stated on page 42 of the SEC PER: 

In 1957 at the beginning of Mg-Th operations, KCP established engineered 

airborne limits and controlled these operations so as to not exceed 9E-11 μCi/ml 

(Th-232). KCP also instituted an industrial hygiene and fire protection limit of 

0.1 mg/m3(thorium) (Thorium, unknown date). 

SC&A checked the reference (Thorium, unknown date) and confirmed that the KCP established 

strict controls for exposure to airborne toxins, primarily due to concern with respect to exposure 

to beryllium, and established a maximum permissible concentration (MPC) of 9E-11 μCi/ml of 

air. It certainly appears, therefore, that strict procedures were in place for maintaining exposures 

to Th-232 below this limit. 

The SEC PER further argues that, since the nominal content of the Mg-Th alloy was 3% of 

thorium by weight, if the Th-232 were present airborne at the MPC, the dust loading of Mg-Th 

would be 27.3 mg/m3, an extremely high dust loading that might create breathing difficulties for 

persons exposed to this airborne dust loadings for prolonged time periods. The specific activity 

of Th-232 is 1.1E-7 Ci/g. Hence, the MPC is associated with 9E-11 μCi/ml × 1/1.1E-7 Ci/g × 

1E-6 Ci/μCi × 1E3 mg/g = 0.818 E-6 mg/cc of Th-232. Assuming that the Th-232 is only 3% of 
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the Mg-Th alloy, and that all of the radioactivity in the air is due to Th-232, the dust loading of 

the alloy would be 27.3 mg/m3. This would mean that the chronic concentration of Th-232 in air 

could not exceed 9E-11 μCi/ml of air if the limit were maintained in practice. 

We have several concerns that need to be addressed by NIOSH before we can conclude that the 

9E-11 μCi/ml limit and associated 27.3 mg/m3argument are well founded: 

1. The document to which NIOSH refers is undated. Actually, the date cannot be 

determined from the available version because the first page is missing (Thorium, 

unknown date). As a result, it is not possible to establish the date on which the 

9E-11 μCi/cc limit was established. Specifically, one cannot say if the starting date was 

set “at the beginning of Mg-Th operations” in 1957, as claimed by NIOSH (SEC PER 

2014, p. 42).

2. It is unclear from the document whether the 9E-11 μCi/cc was applicable at all. The 

document does indeed give a limit for “[a]irborne contamination thorium-232 – 9 × 

10-11 microcuries per cubic centimeter of air;” however, the phrase “232 – 9E-11” has a 

manual strike out across it, so that the phrase as it stands is, “[a]irborne contamination 

thorium-232 – 9 x 10-11 microcuries per cubic centimeter of air.” A handwritten note that 

is rather faded and rather difficult to read appears next to the stricken phrase. So far as it 

is legible, SC&A believes it reads as follows: “For [illegible] – [illegible] × 10-12

[exponent unclear] μCi/cc for soluble material and [illegible] for insoluble. For soluble 

natural thorium (MPC) [illegible] is 2 × 10-12 μCi/cc. For insoluble natural thorium 6.9 × 

10-12 μCi/cc” (Thorium unknown date, pdf p. 2). It is not clear when the handwritten 

edits to the document were made. A change in the external dose limits similarly made in 

longhand indicates that the edits might have been made in 1959, sometime after the 

issuance of the National Bureau of Standards Handbook 69 (NBS 1959).1 However, even 

if true, this does not solve the problem of when the document was written and adopted at 

the KCP. 

3. While the deleted phrase refers to thorium-232, the handwritten note refers to natural 

thorium, which would include thorium-228 and possibly other decay products. Though 

the original phrase referred to thorium-232, no measurement protocol or assumptions 

about equilibrium of thorium-232 with its decay products are described. Therefore, it is 

not clear if the limit of 9E-11 μCi/cc was applied to gross alpha or to natural thorium,

including thorium-228, or to natural thorium with other decay products besides Th-228. 

If the MPC was applied to thorium-232 alone, it is not apparent what assumptions about 

equilibrium or lack thereof were made. 

4. The document “Thorium, unknown date” discussed strict air pollution controls for 

beryllium, which it noted was much more hazardous than thorium. In that context, it 

concluded that, “...any potential hazard from thorium could be adequately controlled” 

(Thorium unknown date, pdf p.2). However, there are no air monitoring data for thorium 

in the 1957–1959 period. Rather, there are depleted uranium air monitoring data for the 

1958–1970 period (ORAUT 2006, p. 19). These data are not relevant for addressing the 

 

1

 The change in the document reduces external dose to 3 rem per quarter, 12 rem per year, and 5 rem per 

year long-term average over several years. These limits were introduced in NBS 69 (NBS 1959, pp. 4–5). 
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issue of actual Mg-Th alloy dust concentrations. We know that maximum permissible air 

concentrations were established in Atomic Energy Commission (AEC) facilities, but it 

was also known that they were often exceeded, especially in the 1950s and 1960s. 

5. If the MPC was anything other than for thorium-232 alone, the presence of decay 

products, notably thorium-228, would need to be taken into account. Freshly separated 

thorium will include equal activities of Th-232 and Th-228; furthermore, Ra-224 

3.66 day half-life and its progeny will grow in quickly. Hence, a gross alpha air sample 

will contain other alpha emitters, all with much higher specific activities than Th-232. 

Therefore, the mass of Th-232 in a gross alpha sample would be considerably lower than 

0.818E-6 mg/cc. In turn, this means that the mass loading of Mg-Th alloy dust including 

Th-228 associated with 9E-11 μCi/ml of gross alpha would be much lower than 

27.3 mg/m3. Of course, this would also mean that the concentration of Th-232 in air 

would be lower than the 9E-11 μCi/ml limit. NIOSH’s conclusion that the 9E-11 μCi/ml 

is bounding is based on the inference of total mass loading of 27.3 mg/m3and the notion 

that continuous exposure to dust at this level is the maximum tolerable concentration. If 

other alphas, along with Th-232 were being counted, the mass loading would be much 

lower and an inference that the air dust concentration was at the tolerable upper limit for 

continuous exposure would no longer be valid. 

For workers involved in machining operations, the SEC PER states that exposure duration will 

be assumed to be 2,000 hours per year. If 9E-11 μCi/m3 were a Th-232 limit, it would be very 

claimant favorable to assume that workers involved in Mg-Th machining operations were 

continuously exposed to airborne Th-232 at that level, but it is premature to make that 

assumption at the present time. There are simply too many questions around the value of 

9E-11 μCi/cc for it to be used in any dose reconstruction, much less to show that it is a bounding 

value. 

The SEC PER further states the following: 

NIOSH will assume that the air concentration for general laborers was half the 

concentration of that of the operators. The supervisors of the operators and 

general laborers had half the concentration of the general laborers’ 

concentration. All other worker types, such as those performing primarily 

administrative and clerical duties with no reason to enter the restricted, 

radiological areas, had 10% of the supervisors’ concentration. 

As discussed with respect to uranium exposures, a certain degree of judgment will be required by 

the dose reconstructor when determining exposure duration for a given worker. This is an 

implementation issue that is best evaluated as part of dose reconstruction reviews, and is not 

considered an SEC issue. In addition, as with uranium exposures, the SEC PER states that 

internal exposures associated with the inadvertent ingestion of Th-232 will be derived using the 

guidance in OCAS-OTIB-009 (OCAS 2004). SC&A concurs with this approach for 

reconstructing internal exposures associated with the ingestion pathway based on our previous 

review of OTIB-009. 
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The November 1, 1959, to April 30, 1979 Period 

NIOSH proposes to use a constant value of 3E-11 μCi/ml of Th-232 for this period (SEC PER 

2014, p. 43). This is based on a document that specifies this value as an air concentration limit 

(Mg-Th 1957–1970, pdf pp. 13 and 16). 

However, an examination of the document reveals the matter to be more complex. The 

document in question states the following: 

Personnel exposure limits have been established for thorium related to its 

chemical toxicity and radioactivity as follows: 

(1) 0.1 milligrams of thorium per cubic meter of air. 

(2) 3 × 10

-11 microcuries per milliliter of air. [Mg-Th 1957–1970, pdf p. 16]

One issue is that if both values relate to Th-232, they are incompatible. The first, 0.1 mg/m3, 

translates into 1.1E-11 μCi/ml, which is much lower than the radioactivity limit specified. It is 

possible that the mass limit was different and stricter than the radioactivity limit. However, we 

should note that the maximum permissible air concentration established in the National Bureau 

of Standards Handbook 69 for thorium-232 was 1E-11 μCi/cc (NBS 1959, p. 82), which is close 

to the value of 0.1 mg/m3. It appears, therefore, that 3E-11 μCi/ml may have been a gross alpha 

limit, though the quote indicates it may well have been a thorium limit. In the latter case, it is 

unclear if thorium-228 was included or if the limit was for thorium-232 only. 

NIOSH cites gross alpha air concentration measurements in the Main Manufacturing Building 

between 1958 and 1971 and a thorium-related test measurement in 1970 as its basis for the 

choice of 3E-11 μCi/ml: 

KCP performed gross alpha fixed-filter air monitoring from 1958 through 1971 in 

the Main Manufacturing Building and maintained operations at: 2.85E-12 μCi/

ml (average measured level for the period) and <8.55E-11 μCi/ml (maximum 

measured level for the period) (ORAUT-TKBS-0031, pdf p. 20). [ORAUT 2006] 

In 1970, KCP performed an evaluation of Mg-Th machining operations in the 

Model Shop and validated that their process did not generate airborne 

radioactivity. As part of this test, each machining station was evaluated with 

breathing-zone air samplers running while performing the machining operation. 

KCP machinists maintained airborne levels during this test at background (0 cpm 

recorded) for long-lived activity, and <3.22E-9 μCi/ml for short-lived activity. 

Direct-probe surveys of the Mg-Th are shown as: 0.3-1.2 mr/hr for beta-gamma 

radiation, and 250–700 cpm alpha (Mg-Th, 1962–1975). [SEC PER 2014, p. 43] 

SC&A has some concerns about this reasoning. First, the air concentration measurements in the 

Site Profile (ORAUT 2006, p. 19) that NIOSH cited do not refer to thorium or gross alpha, but to 
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depleted uranium (DU).2 Presumably, gross alpha measurements were made in DU processing 

areas and hence the radioactivity was attributed to DU. In that case, they would have no 

relevance for making any conclusions about thorium air concentrations. Specifically, NIOSH 

has not provided any evidence that any of these air samples were taken at times and in areas of 

Mg-Th alloy processing. 

NIOSH’s second rationale concerns a single test done in 1970. SC&A evaluated the document 

in which this test is described (Mg-Th 1962–1975, pdf pp. 16–20). While the test results do 

indeed show that the counts of alpha activity after several days were zero, the sensitivity of the 

test was not high. Specifically, samples were taken over brief periods (5 minutes) and counting 

was also done over brief periods (2 to 5 minutes). Actually, these counting times were for the 

count 1 hour after the sample was taken. The counting times several days after the samples were 

taken are not provided. Only a result “0 cpm” is stated in each case.

It is evident from the 1-hour results that a considerable amount of thorium-related short-lived 

activity was present in the air. In most cases, the sample volume was 14 liters. The counting 

efficiency was estimated at 0.3. With these parameters, the test would register a single count in 

5 minutes at long-lived alpha air concentrations of 2E-11 μCi/ml. Background counts are 

provided; they are between 0 and 3 cpm. The latter would correspond to a thorium-232 plus 

thorium-228 count of about 6E-11μCi/ml, which is well above the value that NIOSH proposes to 

use as a constant for the 1959–1979 period. 

Finally, using a constant value of 3E-11 μCi/ml assumes that the regulatory limits were generally 

met at KCP. There are many of examples in the nuclear weapons complex when that was not the 

case in the 1950s and 1960s, and even into the 1970s. NIOSH’s assumption is at present without 

substantial empirical foundation, since there appear to be no thorium air samples for 1959–1969 

and 1971–1979. The one test during a short period in the 1970s is lacking in sensitivity to 

determine what value would be suitable even for that year, let alone any other year. 

We may conclude that: 

 The short-lived radionuclide counts show a definite presence of thorium-related decay 

products. 

 The counting times for long-lived radionuclide counts are not provided. If they were 

similar to the counting times for short-lived radionuclides, the method was quite 

insensitive and may not have registered amounts that are of the same order of magnitude 

as the constant value that NIOSH proposes to use. 

There appear to be no thorium air monitoring data from 1971 onward. At least NIOSH cites 

none.

In sum, the 1959–1979 period value for thorium-232 air concentration is based on a single test 

measurement in 1970. This test was inadequate at best for indicating conditions, even during the 

 

2

 The quote cites page 20 of the Site Profile (ORAUT 2006). However, that page does not contain air 

concentration measurements, which are on p. 19. SC&A assumes this is a typo in the PER. 
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time of the test, in so far as the constant value that NIOSH proposes to use is concerned. No data 

have been provided for the 1971–1979 period. The data for the 1959–1970 period that NIOSH 

has cited do not refer to thorium, but to DU and are, therefore, not relevant to thorium dose 

reconstruction. 

THORIUM CONCENTRATION IN THE MG-TH ALLOY 

NIOSH assumes that machining of Mg-Th alloy was limited to 3% Mg-Th alloy. During the 

Work Group meeting held on June 10, 2014, SC&A raised the question whether the KCP might 

have actually fabricated Mg-Th alloy in addition to simply machining the alloy. SC&A raised 

the question because, if Mg-Th alloy was fabricated at the KCP, it would likely have involved

using a master alloy that had much higher concentrations of thorium. During the meeting, 

NIOSH indicated that they have documentation indicating that only 3% Mg-Th alloy (e.g., HK31 

alloy) was handled at KCP, and that they would modify the SEC PER to that effect. 

However, a compilation of Mg-Th documents (Mg-Th 1957–1970) indicates that several 

different alloy compositions were processed: 

 A 3.5% Th concentration is given in an attachment to an October 1959 memorandum 

(mg-Th 1957–1970, pdf p. 14) 

 A 4% concentration is given in a 1960 document (Mg-Th 1957–1970, pdf p. 9) 

 A 2% concentration is mentioned for 1970 (Mg-Th 1957–1970, pdf p. 2) 

It is important for NIOSH to determine the proportions of thorium-232 in the Mg-Th alloy, 

especially for the 1957–1959 period, but also in the latter period. 

INTERNAL EXPOSURES TO THE PROGENY OF TH-232 AND TH-228 

The above discussion appropriately addresses exposures to Th-232, but we also raise possible 

issues associated with the progeny of Th-232. The SEC PER is silent regarding possible 

exposures to the progeny of Th-232. In general, all of the Th-232 progeny are present in 

equilibrium in thorium ore, as shown in Figure 1: 
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Figure 1. Thorium-232 Decay Series 

When thorium is chemically separated from its ore, the product includes equal activities of 

Th-232 and Th-228, and Mg-Th alloy prepared from freshly separated thorium will not contain 

any of the non-thorium progeny. As time passes, the Th-228 begins to decay with its 1.9 year 

half- life and its progeny grows in. In addition, as Th-232 decays, all of the progeny begin to 

grow in. Hence, the mix of radionuclides in the alloy depends very much on the age of the 

thorium in the alloy. 

The issue of the decay products of Th-232 present in air is also important from the point of view 

of dose estimation. Th-228 would likely contribute significantly to dose. In the process of 

separating thorium from its ore, the product will include equal activities of Th-232 and Th-228. 

As evident from Figure 1, the ratio of Th-232 to Th-238 will change in a complex manner as 

both the Th-232 and Th-228 decay. As it turns out, the inhalation dose conversion factor for the 

lung for Type M Th-228 is 1.81E-4 Sv/Bq, while for Th-232 it is 2.71E-5 Sv/Bq. Hence, if the 

contributions to internal organ dose from Th-228 are not included, the result could underestimate 

organ dose significantly, most importantly in the case of lung dose. Doses from other decay 

products may also contribute to dose, depending on the disequilibrium state of those products in 
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the air sample in question. In any case, ignoring decay products is not claimant favorable; they 

must be included whatever the approach chosen for reconstructing internal exposures to thorium 

associated with machining Mg-Th alloy. This subject was discussed during the June 10, 2014,

Work Group meeting, and NIOSH agreed to address this issue in a future revision of the SEC 

PER.

Finally, page 39 of the SEC PER addresses exposure to thoron, stating that the airborne 

concentration of thoron (and the associated exposures) will be calculated based on the airborne 

concentration of Th-232. Page 44 of the SEC PER states that NIOSH will assume that the thoron 

concentration is 3.2E-9 μCi/ml, which basically assumes that thoron is at about 30% equilibrium 

with Th-232. The following explains why we believe this is a reasonable assumption. 

One of the progeny, thoron, grows in quickly and can become airborne, because it is a noble gas. 

However, for it to become airborne as a separate radionuclide, it must escape from the alloy 

before it decays with its 55-second half-life. Hence, it is unlikely that much of the thoron will 

escape from the alloy before it decays (i.e., it is trapped in the relatively large mass of the alloy 

and decays before it has an opportunity to escape). However, the airborne particles of alloy dust 

generated during machining are very fine, and it is possible that some of the thoron could escape. 

Taking guidance from the behavior of radon (see Yu 1993), it is reasonable to assume that the 

emanation coefficient of thoron is about 30%. This means that when thoron’s parent (Ra-224) 

decays, its recoil energy could result in thoron escaping from the small particle and becoming 

airborne. Hence, SC&A concurs with NIOSH’s approach to addressing thoron exposure at the 

KCP. 

CONCLUSIONS 

NIOSH’s approach to reconstructing internal doses associated with the machining of Mg-Th 

alloy at the KCP is based on assumptions that need to be clarified. Data and evidence are also 

lacking in certain cases. 

Our overall conclusion is that the available data are insufficient to assign thorium doses to 

workers in the 1957–1979 period. Specifically: 

1. For the 1957–1959 period, it is unclear whether the 9E-11 μCi/ml was a Th-232 limit, 

a natural thorium limit, or a gross alpha limit. This issue is central for determining 

whether a mass loading limit can be used for this period. Furthermore, there are no 

data to enable a determination of whether this limit was enforced and actual air 

concentrations of Th-232 remained generally below this limit. 

2. The 1958–1970 air concentration data that NIOSH referred to are for DU (according 

to ORAUT 2006), not thorium. NIOSH has provided no evidence that any of these 

air samples related to Mg-Th processing. 

3. For the period after 1959, it is unclear whether the limit of 3E-11 μCi/cc includes 

Th-228 and possibly other decay products of Th-232. The limit for Th-232 based on 

the lung as the critical organ set in NBS 69 was 1E-11 μCi/cc. NBS 69 was 

published in 1959. 
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4. NIOSH has not provided any air monitoring data for 1971–1979.

5. NIOSH refers to one thorium machining air concentration test. This test is inadequate 

to determine the value that should be used even for the year of the test, much less for 

any other year. 

6. The issue of doses from the progeny of thorium needs to be addressed. 

7. NIOSH needs to determine the various alloy compositions that were machined, and 

whether variation in thorium content may have made a difference to particulate 

generation during machining. 
Effective Date:

August 15, 2014

Revision No.

0 – Draft

Document Description: White Paper:

Exposures to Thorium and its Progeny at KCP

Page No.

Page 14 of 14

NOTICE: This report 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.

REFERENCES

Mg-Th, 1957–1970. Various Documents/Correspondence concerning magnesium-thorium alloys; 

multiple authors; dates range from September 5, 1957 through August 24, 1970; SRDB Ref ID: 

128181

Mg-Th, 1962–1975. Various Documents/Correspondence concerning magnesium-thorium alloys; 

multiple authors; dates range from September 5, 1957 through August 24, 1970; SRDB Ref ID: 

108264.

NBS 1959. Maximum Permissible Body Burdens and Maximum Permissible Concentrations of 

Radionuclides in Air and in Water for Occupational Exposure. U.S. Department of Commerce 

National Bureau of Standards Handbook 69. Washington, DC. U.S. Government Printing 

Office, June 5, 1959.

OCAS 2004. Estimation of Ingestion Intakes, OCAS-TIB-009, Rev. 0; Office of Compensation 

Analysis and Support; effective April 13, 2004; SRDB Ref ID: 22397

ORAUT 2006. Site Profile for the Kansas City Plant, ORAUT-TKBS-0031, Rev. 00 PC-1, Oak 

Ridge Associated Universities Team, Cincinnati, Ohio. January 6, 2006. SRDB Ref ID: 20217.

SEC PER 2014. Special Exposure Cohort Petition Evaluation Report, Petition SEC-00210, 

Rev. 0, January 7, 2014.

Thorium, unknown date. Thorium Exposure Levels and Handling Instructions, First page is 

missing; unknown author; unknown date; SRDB Ref ID: 128148

Yu, C., 1993. Data Collection Handbook to Support Modeling the Impacts of Radioactive 

Materials in Soil, Argonne National Laboratory, ANL/EAIS-8 (April 1993).