___________________________________________________________________________________ 

 

 

 

 

 

 

DEPARTMENT OF HEALTH & HUMAN SERVICES
 

Memorandum 

Date January 10, 2007 

From Lewis Wade, DFO 

Subject Draft SC&A Report on Blockson Chemical 

To The File 

 "" The attached Draft Letter Report, Blockson SEC Petition Review Issues, was issued on 

January 10, 2007. As stated in the Executive Summary, "This report presents SC&A's initial 

review of the Blockson Chemical Company Special Exposure Cohort (SEC) petition, Technical 

Basis Document (TBD), Evaluation Report, and selected supporting documentation....". 

On January 8, 2007 Larry Elliott of NIOSH informed the Advisory Board that , "...NIOSH 

OCAS has decided to withdraw the SEC Petition Evaluation Report and the Blockson TBD for 

further evaluation and possible revision based on a determination of what radiation exposures 

must be included in dose reconstructions for former Blockson Chemical Workers." It is 

anticipated that once NIOSH releases an updated Petition Evaluation Report and TBD, the 

Advisory Board will ask SC&A to update its Draft Letter Report based on a review of the 

updated documents. At that time the SC&A updated report will be made public. 

The Draft Letter Report dated January 10, 2007 is being made available to ensure complete 

public disclosure of the Advisory Board's activities. "" 
 

 

 

Draft Letter Report 

REPORT TO THE ADVISORY BOARD 

ON RADIATION AND WORKER HEALTH
 

National Institute for Occupational Safety and Health 

FOCUSED REVIEW OF THE NIOSH EVALUATION 

REPORT FOR THE BLOCKSON CHEMICAL COMPANY SEC 

PETITION
 

Contract No. 200-2004-03805 

Task Order No. 5 

Prepared by 

S. Cohen & Associates 

1608 Spring Hill Road 

Suite 400 

Vienna, Virginia 22182 

January 10, 2007 

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 and 42 CFR 83. 

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. 
TABLE OF CONTENTS 

Executive Summary .........................................................................................................................3 

1.0 Introduction and Scope ........................................................................................................5 

2.0	 Background Information ......................................................................................................6 

3.0	 Overview of Blockson AEC Operations, Sources of Exposure to Radioactive 

Materials, and Exposure Scenarios......................................................................................7 

4.0	 Internal Dose from Uranium ..............................................................................................11 

5.0 Inhalation of Thorium........................................................................................................19 

5.1	 Thorium-232 and Thorium-228 .............................................................................19 

5.2	 Thorium-230 ..........................................................................................................19 

5.3	 Possible Thorium-230 Raffinates ..........................................................................20 

6.0 Radon Exposures ...............................................................................................................21 

7.0 External Exposures ............................................................................................................25 

References......................................................................................................................................28 

Attachment 1: Presence and Disposition of Thorium-230 in Uranium Production from
 

Phosphoric Acid.................................................................................................................30 

Attachment 2: Radon Exposures at Blockson Chemical Company ..............................................33 

Draft Letter Report	 2 SC&A – January 10, 2007 
 

EXECUTIVE SUMMARY 

This report presents SC&A’s initial review of the Blockson Chemical Company Special 

Exposure Cohort (SEC) petition, Technical Basis Document (TBD), Evaluation Report, and 

selected supporting documentation, including ORAUT-OTIB-0043 (ORAU 2006), which 

describes National Institute for Occupational Safety and Health’s (NIOSH’s) generic default 

methods for reconstructing exposures to radon and its short-lived progeny for Atomic Energy 

Commission (AEC) contract workers involved in the recovery of uranium from phosphate 

materials. This report is limited to the identification of potential issues, especially those that 

might be considered SEC-related issues. It does not contain a detailed analysis of the issues, and 

is intended solely for use by the designated working group for Blockson Chemical Company. 

SC&A has identified the following seven findings. 

Finding No. 1: The default upper-bound uranium inhalation rate of 82 pCi/day of Type M 

uranium, which was derived based on bioassay data and which is used in the TBD as the upperbound intake rate for all production workers at Blockson, would result in a large underestimate 

of the doses to some organs if the uranium at the site behaved more as Type S than as Type M. 

It is not apparent from our review of the TBD that exposures to uranium at Blockson consisted 

entirely of Absorption Type M. If there are uncertainties regarding this matter, NIOSH should 

consider adopting the strategy used in the exposure matrix for Chapman Valve, where the dose 

reconstructor would use the form of uranium (i.e., Type M or Type S) that results in the highest 

dose to the organ of concern. 

Finding No. 2: Since the ratio of the concentration of U-238 to Th-232 in phosphoric acid is 

reported to be as high as about 10:1, NIOSH should consider adopting a high-end default intake 

rate for Th-232 and Th-228 of about 8.2 pCi/day instead of 1.1 pCi/day. NIOSH adopted an 

intake rate of 1.1 pCi/day for Th-232 and Th-228 based on the ratio of U-238 to Th-232 observed 

in phosphate ore concentrate, which, on face value, would appear to be an appropriate approach. 

However, based on a report by Guimond et al. (1977), it appears that Th-232 might be more 

efficiently partitioned to the phosphoric acid stream than uranium, resulting in an enrichment of 

Th-232 relative to that of U-238. 

Finding No. 3: The TBD should explicitly include Th-230 in the exposure matrix. The TBD 

seems to have overlooked the possibility that Th-230 also partitioned to the phosphoric acid 

stream along with uranium. If this occurred, Th-230 would not have been detected by the 

bioassay program, but could have contributed significantly to the internal doses; hence, the 

possibility of an important missed dose. 

Finding No. 4: The TBD should address whether a separate waste stream containing thorium 

raffinates might have been produced during the processing of the uranium and, if such a stream 

was produced, there is a need to develop a method for reconstructing the doses associated with 

exposures to the raffinates. 

Finding No. 5: It is not apparent that 0.036 WLM/yr, which is adopted in the TBD for deriving 

upper-bound internal doses to Blockson workers from radon and its short-lived progeny, is a 

Draft Letter Report 3 SC&A – January 10, 2007 
 

plausible and bounding exposure rate as applied to Blockson. The TBD based its default value 

by selecting the best estimate of a vast amount of data gathered at phosphate mining and 

processing facilities in Florida, as reported in ORAU 2006. ORAU 2006 makes an effort to 

select data that might be applicable to facilities such as Blockson (i.e., facilities contracted by the 

AEC to separate uranium from phosphate ores). However, additional information is needed 

regarding the location, size, and characteristics of the tailings piles, which contain the radium, at 

Blockson and at the Florida facilities in order to support the default value adopted in the TBD. 

Finding No. 6: If the tailings produced during routine phosphate ore processing operations at 

Blockson were used as construction fill or in the actual construction of Building 55, the levels of 

radon and its short-lived progeny in Building 55 might have been higher than the values adopted 

in the TBD. SC&A’s review of data gathered during the radiological survey performed at the 

site in 1978 as part of the Formerly Utilized Site Remedial Action Program (FUSRAP) (DOE 

1978) seems to indicate that tailings were not used in such a manner. Nevertheless, a review of 

historical records regarding the construction of Building 55 and a review of the radiological 

surveys performed in support of the FUSRAP investigations might help to answer this question. 

Finding No. 7: The TBD should explicitly address the amounts of Ra-226 and its short-lived 

progeny that may have partitioned into the phosphoric acid stream, the various byproducts 

associated with uranium separation and purification, and in the final uranium product. Even if 

only a small fraction of the Ra-226 in ore partitioned to the phosphoric acid stream, it has the 

potential to contribute significantly to the external exposures experienced by workers, due to the 

relatively high-photon flux from its short-lived progeny, especially Bi-214. 

In the opinion of SC&A, among these findings, Finding 4 has the greatest potential to represent 

an SEC issue because, if determined to be valid, it is not immediately apparent how this issue 

can be resolved. 

Draft Letter Report 4 SC&A – January 10, 2007 
1.0 INTRODUCTION AND SCOPE 

During the meeting of the Advisory Board on Radiation and Worker Health (the Board) held in 

Naperville, Illinois, on December 11–13, 2006, SC&A was directed to perform a focused review 

of the Special Exposure Cohort (SEC) petition and the National Institute for Occupational Safety 

and Health’s (NIOSH’s) evaluation of the petition for Blockson Chemical Company. This report 

presents the results of SC&A’s review. This report is designed to be used by the Board as part of 

the basis for determining whether radiation doses can be estimated with sufficient accuracy 

based on the following criteria: 

Radiation doses can be estimated with sufficient accuracy if NIOSH has 

established that it has access to sufficient information to estimate the maximum 

radiation dose, for every type of cancer for which radiation doses are 

reconstructed, that could have been incurred in plausible circumstances by any 

member of the class, or if NIOSH has established that it has access to sufficient 

information to estimate the radiation doses of members of the class more 

precisely than an estimate of the maximum radiation dose [42 CFR 83.13(c)(1)]. 

The scope of this report consists of a technical review of the latest version of the site profile 

(OCAS 2006) (also referred to as the Technical Basis Document (TBD)), the petition (Petition 

SEC-00058 and -00045), the evaluation report (NIOSH 2006), and applicable technical 

information bulletins (specifically ORAU 2006), and selected other references as judged to be 

pertinent to the identification of potential SEC issues. Specific attention is given to the radon 

model proposed by NIOSH and related radon exposure issues. To the extent feasible, this report 

identifies those issues that, in the opinion of SC&A, might rise to the level of an SEC issue, 

including a brief description of our rationale for raising these issues. SC&A was specifically 

directed not to perform a full SEC petition review at this time, as described in our draft SEC 

petition review procedures (SC&A 2006). As such, this report does not present an in-depth 

analysis of any of the issues, nor does the work performed by SC&A in support of this report 

include SC&A interviews with any claimants or petitioners. 

This report includes 2 attachments that address two issues in some detail that are especially 

important to this review. Attachment 1 addresses Th-230, which is a radionuclide that is not 

addressed in the TBD but could be an important contributor to the radiation exposures 

experienced by AEC contract workers at Blockson. Attachment 2 addresses in some detail 

potential exposures of AEC contract workers to radon and its progeny. Attachment 2 includes an 

aerial photograph of the facility, which helps in understanding the proximity of Building 55 to 

the phosphogypsum storage pile (i.e., the principal source of exposure to radon and its progeny). 

Draft Letter Report 5 SC&A – January 10, 2007 
2.0 BACKGROUND INFORMATION 

On August 9, 2006, SEC-00058 was qualified by the Department of Health and Human Services 

(Federal Register Vol. 71, No.159, August 17, 2006). On September 9, 2006, NIOSH issued its 

SEC Evaluation Report for Petition SEC-00058. The petitioner’s class definition is as follows: 

All Atomic Weapons Employer [AEC] employees, contractors, and 

subcontractors, who worked in Building 55 at the Blockson Chemical Company 

(also known as Olin Matheson) from January 1, 1951 to December 21, 1962. 

NIOSH’s proposed class definition is slightly different than the petitioner’s definition, as 

follows: 

All Atomic Weapons Employer personnel who worked on uranium recovery pilot 

studies and/or in Building 55 of the Blockson Chemical Company, Joliet, Illinois, 

from January 1, 1951 through December 31, 1962. 

The main differences between the two definitions are that NIOSH excluded subcontractors from 

the definition, as required by Statute for AWE facilities, but expanded the scope of the class by 

including workers that were involved in the pilot studies, which preceded AEC operations in 

Building 55 under contract to the Atomic Energy Commission (AEC). 

In the evaluation report, NIOSH concludes that “it has access to sufficient information to 

(1) estimate the maximum radiation dose incurred by any member of the class; or (2) estimate 

radiation doses more precisely than a maximum dose estimate.” NIOSH states that the basis for 

this conclusion is information available from the site profile and additional resources. Section 

7.4 of the evaluation report explicitly addresses each of the issues raised in the petition, and 

provides NIOSH’s rationale for its determination that, notwithstanding the issues raised in the 

petition, doses can be reconstructed with sufficient accuracy. In the sections that follow, SC&A 

presents a brief description of the various possible exposure scenarios for AEC contract workers 

at Blockson and the data, methods, and assumptions used by NIOSH in the site profile and 

evaluation report for reconstructing worker doses. Each description is followed by a review of 

the methods adopted by NIOSH for reconstructing doses, including potential SEC-related issues. 

An issue is considered “SEC-related” if it raises a question that doses may not be able to be 

reconstructed with sufficient accuracy. 

Draft Letter Report 6 SC&A – January 10, 2007 
 

3.0 OVERVIEW OF BLOCKSON AEC OPERATIONS, SOURCES OF 

EXPOSURE TO RADIOACTIVE MATERIALS, AND EXPOSURE 

SCENARIOS 

A letter from E.B Lopker, Director of Engineering at Blockson Chemical Company, to Sheldon 

Wimpfen of the Atomic Energy Commission, dated July 31, 1951, describes the process for 

recovering uranium from phosphate rock.1 In addition, a report by Emil M. Stoltz (Stoltz 1953) 

presents a detailed description of the chemical processes used to recover uranium from 

phosphate rock. These documents present the various research activities and pilot studies 

performed by Blockson, and the methods adopted by Blockson to efficiently and economically 

recover uranium from phosphate rock on a commercial scale. These reports explain that 

Blockson Chemical Company was one of several companies actively engaged in the commercial 

processing of phosphate rock. 

Blockson received phosphate ore concentrates from Florida mining operations and produced 

technical grades of sodium phosphate, which met strict purity specifications. Blockson differed 

from many other phosphate ore processing companies because most other companies produced 

triple phosphates, for which the purity requirements were not nearly as stringent. This 

distinction is being pointed out because Blockson had to carefully research a number of different 

strategies in order to continue to produce technical grades of sodium phosphate, and also costeffectively separate the uranium contained in the ore. Based on our review of the literature, it 

appears that, in order to produce a high level of purity in its phosphate product and also meet 

AEC requirements to extract uranium from the phosphate ore, some changes had to be made to 

the overall chemical operations at Blockson. For example, it appears that the calcining process 

and perhaps some of the steps in the treatment of phosphoric acid to produce sodium phosphate 

were also modified. 

This point is being made because the evaluation report and site profile describe the AEC 

operations as a simple add-on to ongoing operations, where the phosphoric acid was processed 

using the procedures described in Figure 2 of the TBD to produce sodium phosphate, and the 

filtrate was then processed to recover the uranium contained therein. It appears that many of the 

steps may have been modified in order to accommodate the AEC portion of the operations. This 

brings us to our first observation. In addition to the exposures associated with the AEC 

operations, NIOSH should describe the changes in the overall operations in order to fully 

disclose and characterize how these changes may have impacted the radiation exposures 

experienced by workers involved in both AEC and non-AEC operations at the facility. 

Without going into the details, as provided in Stoltz (1953), the overall process used by Blockson 

to produce their primary product, technical grade sodium phosphate, required that, prior to being 

shipped from the mine in Florida, the ore was processed to ore concentrates, which were then 

shipped to Blockson. Table 1 summarizes the radiological characteristics of typical Florida 

phosphate mine products. 

1

 On October 10, 1951, a contract was executed between the Atomic Energy Commission and Blockson 

Chemical Company for the scope of services described in the letter from E.B. Lopker (Contract No. AT-(49-1)-611). 

Draft Letter Report 7 SC&A – January 10, 2007 
 

 

 

Table 1: Natural Radioactivity Concentrations in Florida Phosphate Mine Products 

and Wastes 

(pCi/gm) (from Guimond et al. 1977) 

Material Ra-226 U-238 Th-230 Th-232 

Marketable Rock 42 41 42.3 0.44 

Slimes 45 44 48 1.4 

Sand Tailings 7.5 5.3 4.2 .89 

As may be noted, the marketable rock contained U-238 series radionuclides in equilibrium, and 

typical background concentrations of Th-232. 

At Blockson, the ore concentrates were heated (calcined) to destroy organic matter. In order to 

optimize the oxidation of the uranium in the ore concentrates in a manner that was able to 

increase the efficiency in the recovery of uranium, special calcining procedures were developed 

(Stoltz 1953). This is one of the fundamental changes made to its ore processing procedures in 

order to accommodate uranium recovery, and which could have impacted non-AEC contract 

workers. 

The calcined ore concentrates were then pulverized to the required degree of fineness. Sulfuric 

acid was then added to the pulverized ore concentrates, creating calcium sulfate and phosphoric 

acid. The calcium sulfate (gypsum or phosphogypsum) produced in this reaction was separated 

from the phosphoric acid using conventional methods. The phosphogypsum was then sent to the 

phosphogypsum storage pile (tailings) as a slurry, creating a large phosphogypsum pile (see 

Figure 2-1 in Attachment 2). Because of the chemical similarity of radium and calcium, the 

majority of the radium partitioned with the gypsum and was transported to the phosphogypsum 

pile. However, about 1% of the Ra-226 (by activity) remained with the phosphoric acid 

(Guimond et al. 1977; Roessler et al. 1979). 

The tailings, which were stored on site, represented a source of exposure of all workers to 

Ra-226 and radon and its progeny, and also to lower concentrations of uranium and thorium. 

These tailings and their associated exposures to radioactive material occurred as a byproduct of 

the production of sodium phosphate, and not specifically as a result of AEC contract operations. 

Hence, exposures that were part of the standard phosphate production operations at such 

facilities are not of concern to this report. However, exposures to the radium and radon in the 

tailings of workers that were onsite specifically in support of AEC contract operations are 

appropriately included in the dose reconstruction. NIOSH addresses these sources of worker 

exposure in its site profile and evaluation report, and these exposure scenarios are included in our 

review of those documents. 

The phosphoric acid produced in the reaction of sulfuric acid with the ore concentrate contained 

numerous impurities, including calcium, iron, aluminum, sulfate, fluorine, and fluorosilicate. 

Among these impurities are uranium, chromium, vanadium, magnesium, and manganese. In 

addition, the Th-232 and Th-230 contained in, or associated with, the original ore stayed 

primarily with the phosphoric acid (Guimond et al. 1977; DOE 1978). The various chemical and 

physical operations performed on the phosphoric acid and the resulting production of sodium 

Draft Letter Report 8 SC&A – January 10, 2007 
phosphate, and also the production and storage of the uranium product, constitute the AEC 

contract operations that took place at Blockson. Figure 2 of the TBD presents the flow diagram 

for the production of sodium phosphate and uranium from the phosphoric acid. The steps in this 

process were developed by Blockson to produce both high quality sodium phosphate and 

purified uranium. It is our understanding, based on Blockson (1951), that this process was 

tailored to meet both needs, and was different than the original process employed by Blockson 

when its primary product was sodium phosphate. Hence, this is another example where it 

appears that the overall process was modified to accommodate the AEC contract. 

The processing of phosphoric acid for the removal and purification of sodium phosphate and 

uranium involved the following steps: 

(1) Chlorination of the phosphoric acid in order to oxidize the uranium 

(2) Neutralization of the solution with soda ash (NaOH), which produced monosodium 

phosphate (that contained the uranium) and the precipitation of iron and aluminum salts 

for removal 

(3) The addition of sodium hydrosulfite to the monosodium phosphate liquor to precipitate 

out the uranium for filtration (the monosodium phosphate solution was the input stream 

to sodium phosphate production) 

(4) The uranium precipitate was purified by a series of steps involving redisolution and 

reprecipitation and filtration 

The final product was 50% to 60% U3O8. The purified uranium was dewatered, dried, and then 

deposited into 55-gallon storage drums, either by shovel or through the use of a hopper. 

The entire process involved processing about 6,000 net tons of phosphate rock per week, which 

contained an average of 0.014% U3O8, and the production of about 50,000 pounds of uranium 

oxide per year in the form of a dry powder, containing 50% to 60% U3O8. The uranium oxide 

product was deposited into 55-gallon drums, each containing about 1,000 pounds of uranium 

product. Hence, about 1 drum was produced per week. Exposures experienced by the workers 

involved in these operations, and also workers that were onsite while these operations took place, 

are of concern to the petitioners and are the subject of the SEC petition, the site profile, the 

petition evaluation report, and this report. 

In the sections that follow, a description is provided of the data, models, and assumptions used 

by NIOSH to reconstruct the doses to workers from processing the phosphoric acid, under 

contract to the AEC, for the purpose of producing a high-quality uranium product for use in the 

weapons production program. The sections are organized according to the various exposure 

scenarios. As part of these descriptions, SC&A provides commentary with regard to issues that 

need to be addressed that may be pertinent to the completeness, scientific robustness, and 

claimant-favorability of the dose reconstruction methods adopted by NIOSH. 

Draft Letter Report 9 SC&A – January 10, 2007 
In this report, a distinction is made between “site profile issues” and “SEC-related issues.” A 

site profile issue is one where SC&A raises a concern regarding the completeness, scientific 

robustness, and/or claimant-favorability of the dose reconstruction methods, and, based on our 

limited review of the issue, it appears that NIOSH could develop methods to address the issue. 

An SEC-related issue is one where it is not immediately apparent that sufficient data and/or 

process knowledge are available to correct the deficiency and, if left uncorrected, could result in 

the inability to reconstruct doses with sufficient accuracy. 

As an aid to the reader, we would like to point out at this time that among the various issues 

identified in the following sections, it appears that at least one issue, if found to be valid by 

NIOSH and the Board, might rise to the level of an SEC issue. This issue has to do with internal 

exposures to Th-230. Our concern regarding Th-230 relates to the fact that NIOSH models do 

not address exposure to Th-230 and the possibility that Th-230 was produced as a raffinate in the 

uranium production process. If Th-230 was produced as a raffinate, it could have dried out, 

become airborne, and resulted in an undetected source of internal exposures. 

In our initial review of the TBD, we also had concerns regarding radon exposures, which we 

believed might be an SEC-related issue. Our concerns had to do with uncertainties regarding the 

proximity of the phosphogypsum tailings to Building 55, where the AEC contract operations 

took place, or whether tailings were used as construction fill or construction material in Building 

55. If the tailings were used in this manner, the methods adopted by NIOSH to reconstruct doses 

from the inhalation of radon and its progeny might not be valid, and it was not immediately 

apparent how such doses can be reconstructed. In order to further explore this potential issue, we 

reviewed other selected reports, especially the FUSRAP report prepared by the Department of 

Energy in 1978 (DOE 1978). As discussed in Attachment 2, we determined that the 

characterization of the site and radiological measurements made in and around Building 55 at 

that time suggest that the tailings were several hundred yards away from Building 55, and that 

the radiological condition, at least in 1978, indicate that tailings were not used as construction 

material. However, this matter requires further investigation, as described below. 

Our limited investigations into the remaining issues discussed in the following sections indicate 

that NIOSH should be able to develop strategies to address these issues using available data and 

process knowledge. Hence, at this point in the review process, these other issues do not appear 

to be SEC issues. 

Draft Letter Report 10 SC&A – January 10, 2007 
4.0 INTERNAL DOSE FROM URANIUM
 

Section 3.1 of the TBD presents the data, models, and assumptions used by NIOSH to develop 

the exposure matrix for deriving internal doses to AEC contract workers from the intake of 

uranium. The TBD explains that, among the various steps used to extract and produce uranium 

from phosphoric acid at Blockson, the handling of dried uranium compounds in the packaging 

area in Building 55 created the greatest potential for internal exposures to uranium. SC&A 

agrees with this conclusion, as long as a separate dry thorium raffinate stream was not produced 

during the purification of the uranium (this topic is discussed below in Section 5.3). 

The TBD explains that no air-sampling data were collected at Blockson; however, a review of 

the literature is provided in the TBD that describes the various steps in the production of U3O8 

(also referred to as yellowcake) at uranium mills, and the data characterizing the concentration 

of airborne and deposited uranium oxide dust at various locations throughout uranium mills. The 

TBD summarizes the air-sampling data collected before, during, and after yellowcake packaging 

at these facilities, and found that the median airborne concentrations of uranium ranged from 

40 to 340 μg/m3 (27 to 498 pCi/m3). 

As explained in the TBD, the uranium mill air-sampling data cited in the TBD are considered by 

NIOSH to be of limited applicability to Blockson because of significant differences in the 

concentration of uranium in uranium ore as compared to phosphate rock, differences in the 

production rate of uranium, and differences in the operational practices and design of uranium 

mills as compared to phosphate production facilities. The TBD explains that Blockson produced 

about 5 drums of yellowcake per month, each containing about 1,000 pounds of yellowcake, 

while the production rate of uranium at uranium mills is much greater. 

SC&A does not necessarily agree with this conclusion, because, though the total number of 

exposed workers would differ due to differences in uranium production rates, the high-end 

exposures to individual workers at both types of facilities may be similar since the uranium 

packaging operations were likely similar. One could postulate that, at both facilities, the limiting 

exposure was to those individuals that worked full time at uranium packaging operations, and 

that the local dust loadings associated with uranium packaging operations were similar at both 

types of facilities. Under these conditions, the size of the operation wouldn’t affect the high-end 

exposures. However, we acknowledge that, if the uranium packaging operations at Blockson did 

not require at least 1 full-time operator, while packaging operations at uranium mills required at 

least 1 full-time operator, then exposures to uranium packaging personnel at uranium mills could 

have been higher than at the Blockson facility. 

The TBD also explains that air-sampling data are not essential in the case of Blockson, because a 

considerable amount of data are available characterizing the uranium concentrations in urine 

samples collected from AEC contract workers at Blockson during the time period covered by the 

petition. SC&A agrees that bioassay samples are preferable to air-sampling data if the bioassay 

data are adequate to reconstruct doses with sufficient accuracy. This means that sufficient data 

need to be available to either accurately reconstruct internal doses or place a plausible upper 

bound on the internal doses experienced by all workers. This section explores the degree to 

which the bioassay data satisfy this criterion. 

Draft Letter Report 11 SC&A – January 10, 2007 
The evaluation report states that, “NIOSH obtained results from 122 urine samples collected 

from 25 different workers between 1954 and 1958.” This is as compared to the worker 

population cited in Blockson (1951) that states that the annual labor costs required to produce 

50,000 pounds of yellowcake per year is estimated to be about $84,000. This cost estimate was 

based on 2 operators per shift, 1 chemist per shift, 2 daymen, 2 mechanics, 1 clerk, 

1 developmental chemist, and 1 foreman. Hence, staffing of the operation was estimated by 

Blockson Chemical Company (1951) to be about 10 workers on site at any given time. 

Understanding the worker population is important, because it provides the baseline information 

needed to determine if the bioassay data provide the information needed to judge whether 

internal doses could be reconstructed with sufficient accuracy. 

The TBD states that the first bioassay sample was reported as collected in April 1954 and on 

nine other occasions, as reported by the AEC Health and Safety Laboratory in February 1958. 

The TBD also states that 122 sample results are available, with results ranging from 0 to 

17 μg U/L of urine. Fluorometric analyses were performed on each sample, yielding the 

uranium concentration in urine on the date the sample was collected. Using these data, the TBD 

estimates that the upper 95% uranium inhalation rate for production workers (i.e., the workers 

with the highest potential for exposure) is 82 pCi/day. The TBD also determined that the most 

likely form of uranium was Type M. 

As an independent check, we downloaded the hand-written bioassay records for Blockson 

provided on the NIOSH Site Query Database and found the following: 

Table 2: Bioassay Results 

Person Date Urine Sent or Received Urine Bioassay Result (mg U/L) 

1 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.001 

0.013 

0.004 

0.001 

0.002 

None reported 

2 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.001 

0.002 

0.002 

0.001 

None reported 

0.000 

3 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.002 

0.008 

0.004 

0.005 

0.003 

None reported 

Draft Letter Report 12 SC&A – January 10, 2007 
 

 

Table 2: Bioassay Results 

Person Date Urine Sent or Received Urine Bioassay Result (mg U/L) 

4 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.001 

0.002 

None reported 

None reported 

None reported 

None reported 

5 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.007 

None reported 

None reported 

None reported 

None reported 

None reported 

6 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.004 

0.006 

0.002 

0.002 

0.002 

0.002 

7 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.002 

0.004 

0.003 

None reported 

None reported 

None reported 

8 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.001 

0.000 

0.002 

0.001 

0.004 

None reported 

9 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

2/20/1958 

0.001 

0.006 

0.000 

0.004 

0.001 

0.002 

10 1/18/1956 

5/31/1956 

10/18/1956 

3/28/1957 

8/15/1957 

0.003 

0.014 

0.002 

0.002 

0.000 

Draft Letter Report 13 SC&A – January 10, 2007 
 

 

 

 

 

Table 2: Bioassay Results 

Person Date Urine Sent or Received Urine Bioassay Result (mg U/L) 

2/20/1958 0.000 

11 1/18/1956 0.003 

5/31/1956 0.004 

10/18/1956 0.001 

3/28/1957 0.005 

8/15/1957 0.003 

2/20/1958 None reported 

12 1/18/1956 0.002 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 None reported 

2/20/1958 None reported 

13 1/18/1956 None reported 

5/31/1956 0.002 

10/18/1956 0.002 

3/28/1957 0.002 

8/15/1957 0.001 

2/20/1958 0.000 

14 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 0.001 

8/15/1957 0.001 

2/20/1958 0.000 

15 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 0.001 

8/15/1957 None reported 

2/20/1958 None reported 

16 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 0.001 

8/15/1957 0.005 

2/20/1958 0.000 

17 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

Draft Letter Report 14 SC&A – January 10, 2007 
 

 

 

 

 

Table 2: Bioassay Results 

Person Date Urine Sent or Received Urine Bioassay Result (mg U/L) 

8/15/1957 0.002 

2/20/1958 None reported 

18 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 0.001 

2/20/1958 0.002 

19 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 None reported 

2/20/1958 0.006 

20 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 None reported 

2/20/1958 0.000 

21 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 None reported 

2/20/1958 0.002 

22 1/18/1956 None reported 

5/31/1956 None reported 

10/18/1956 None reported 

3/28/1957 None reported 

8/15/1957 None reported 

2/20/1958 0.004 

As shown, the records available to SC&A for review at this time indicate that 69 bioassays were 

performed on 22 workers on one or more of the following dates: 

January 18, 1956 

May 31, 1956 

October 18, 1956 

March 28, 1957 

August 15, 1957 

February 20, 1958 

Draft Letter Report 15 SC&A – January 10, 2007 
Clearly there is some disparity between the time periods and numbers of urine analyses reported 

in the TBD and that found by SC&A to date. This disparity should be resolved. However, the 

range of concentrations of uranium in urine, as reported in the TBD and in the above table, is 

quite similar (i.e., 0–0.014 versus 0–0.017 mg U/L). 

Taking the TBD at face value regarding the number of samples, it appears that a relatively large 

fraction of the workers received multiple bioassays. Specifically, given that 122 urine samples 

were collected from 25 workers primarily over a 5-year period (1954–1958) as stated in the 

TBD, an average of 1 bioassay per year was collected from each worker over the 5-year period 

of AEC contract operations at Blockson when bioassay data were collected. In reality, as 

evidenced by Table 2, at least some workers received at least three bioassays in 1956 and 1957. 

Whether or not such a sample is deemed to be sufficiently representative of the exposures 

experienced by the workers depends on the range of differences in exposures among workers and 

categories of workers, the range of differences in exposures as a function of time, and whether 

the uranium inhaled or ingested by workers is rapidly cleared (i.e., whether the uranium was 

Absorption Type F, M, or S). The latter question is important, because, if the uranium were 

Type F or Type M, a worker could have experienced a large intake, but a urine analysis could 

have missed it. 

Based on our review of the literature, it is likely that the uranium to which workers were exposed 

was either Type M or Type S. In either case, it appears that, if a given worker experienced one 

or more large acute intakes, it is likely that at least one of the samples would have revealed such 

an intake. Nevertheless, the TBD would do well to explore this issue and demonstrate that 

either (1) the bioassay program would have detected a large intake or (2) that the exposure 

matrix adopted by NIOSH for dose reconstruction is sufficiently conservative to provide a 

high level of assurance that no workers likely experienced a cumulative intake in excess of 

the intakes adopted in the exposure matrix. 

SC&A believes that such an analysis can and should be performed, thereby demonstrating that a 

plausible upper bound can be, and has been, placed on the time-integrated uranium intake 

experienced by all workers. As a quick check on this issue, SC&A ran IMBA using the default 

upper-bound chronic uranium inhalation rate adopted in Table 1a of the TBD for use in dose 

reconstructions for production workers (i.e., 82 pCi/day). Assuming Type M uranium with 

5 micron AMAD, the chronic uranium excretion rate associated with a chronic inhalation rate of 

82 pCi/day is 5.7 pCi/L. Using a natural uranium specific activity of 683 pCi U/mg, the mass 

concentration in urine is 0.008 mg/L. Hence, the default uranium intake rate of 82 pCi/day 

results in a chronic concentration of uranium in urine that is toward the high end of the 

concentrations observed in workers. The assumption that all production workers have a chronic 

intake of 82 pCi/day, and therefore a chronic urine concentration of 0.008 mg/L, appears to be 

bounding, given the actual uranium concentrations observed in the workers. It is important to 

recognize that, though some workers at some points in time were observed to have uranium 

concentrations in urine greater than 0.008 mg/L, it appears to be highly conservative to assume 

that the same worker was chronically exposed for 8 hours per day, 7 days per week, 52 weeks 

per year for every year he or she worked at Blockson, such that the uranium concentration in 

urine was chronically at or above 0.008 mg/L for that entire time. For this reason, we believe the 

exposure matrix in the TBD is bounding if the uranium was Type M. 

Draft Letter Report 16 SC&A – January 10, 2007 
 

It is important to recognize that the exposure matrix adopted in the TBD assumes that all 

production workers were exposed to 82 pCi/day every calendar day per year, which translates to 

29,930 pCi/yr. The question becomes, is it possible that the bioassay program could have missed 

a large acute intake that one or more workers might have experienced, where an acute intake 

comparable to 29,930 pCi would be considered large, because it would double the annual intake? 

In order to evaluate the likelihood that such intakes could have occurred and be missed by the 

bioassay program, Figure 1 presents the uranium concentration in urine as a function of time 

following an acute intake of 29,930 pCi of Type M uranium. Given that the lower limit of 

detection is between 0.002 and 0.0038 mg U/L of urine (see Section 7.1.1 of the evaluation 

report), such an intake would have been detectable for about 150 days following intake. Given 

that virtually all workers appear to have had at least 1 urine sample per year, it is unlikely that 

such an exposure could have occurred without being detected. Another consideration related to 

this scenario is that such an acute exposure would be associated with an 8-hour dust loading of 

4.5 mg/m3 . Section 4.1.2 of ORAU 2006 cites a study that found airborne particulate 

concentrations at a phosphate plant ranged from 10 to 100 mg/m3

; hence, it appears that the 

potential existed for dust loadings to reach as high as several mg/m3 at phosphate facilities for 

short periods of time. The implications are that such an exposure scenario is not out of the 

question, but if it did occur, it would have been detected by the bioassay program and is, defacto, 

accommodated by the default uranium intake rates used in the TBD. 

0.0001 

0.001 

0.01 

0.1 

1 

1 40 82 122 163 203 244 284 326 365 

Day Following Acute Intake 

U conc in urine (mg/L) 

Figure 1: Concentration of Uranium in Urine Following Acute Intake of 2.933E4 pCi 

Type M 

The above conclusions are valid only if the inhaled uranium was Type M, as assumed in the 

TBD. However, if the inhaled uranium was Type S, a chronic inhalation rate of over 

4,000 pCi/day would be required in order to observe a chronic concentration of 0.017 mg U/L in 

urine (i.e., the upper-bound excretion rate observed among the urine samples collected at 

Draft Letter Report 17 SC&A – January 10, 2007 
Blockson). The implications of this scoping analysis are that, if the uranium inhaled by AEC 

contract workers at Blockson were Type S, the inhalation rate could have been much higher than 

that adopted by NIOSH in the exposure matrix. This would mean that the default assumptions 

adopted by NIOSH in the TBD for estimating the doses to workers from the inhalation of 

uranium could result in a significant underestimate of the doses to some organs, particularly the 

lungs. When one considers that (1) the uranium produced at Blockson was primarily U3O8, (2) 

that an inhalation rate of 4,000 pCi/day is associated with an airborne uranium dust loading of 

357 pCi/m3

, and (3) the observed dust loading at uranium mills ranged from 27 to 498 pCi/m3 

(see above discussion), it seems plausible that the uranium at Blockson could have been Type S. 

Finding No. 1: The default upper-bound uranium inhalation rate of 82 pCi/day of Type M 

uranium would result in a large underestimate of the doses to some organs if the uranium 

at the site behaved more as Type S than as Type M. 

In SC&A’s opinion, this finding does not appear to be an SEC issue, because if it is found to be a 

valid concern, NIOSH can modify its exposure matrix to take into consideration the possibility 

that the uranium was Type S. This strategy was adopted by NIOSH in the exposure matrix for 

Chapman Valve, and it seems appropriate to adopt a similar strategy here, unless it can be 

demonstrated with certainty that the chemical form of the uranium at Blockson was, in fact, 

Type M. 

Draft Letter Report 18 SC&A – January 10, 2007 
5.0 INHALATION OF THORIUM 

Our review of the TBD identified a number of issues associated with the methods adopted by 

NIOSH for addressing possible exposures to thorium, one of which we consider to be a possible 

SEC issue. Given its importance, this issue was independently evaluated by SC&A 

radiochemists, and the results of that evaluation are presented in Attachment 1. 

5.1 THORIUM-232 AND THORIUM-228 

Table 1a of the TBD assumes that the intake rate of Th-232 and Th-228 for production workers 

is 1.1 pCi/day. On first inspection, this appears to be reasonable, because the typical 

concentration of Th-232 in phosphate rock concentrate is reported as 0.44 pCi/g, while the 

average concentration of U-238 in phosphate rock concentrate is cited as 31.9 pCi/g (see 

Section 4.1.2 in the TBD). Hence, the ratio of U-238 to Th-232 in typical phosphate rock 

concentrate is about 72:1. Accordingly, given that the high-end intake rate of uranium is 

82 pCi/day, the high-end intake rate for Th-232 (and its progeny Th-228) would be expected to 

be about 1.1 pCi/day. However, inspection of the U-238 and Th-232 concentration in 

phosphoric acid, as reported in Guimond et al. 1977, reveals that the ratio is closer to 8:1 to 10:1. 

The implications are that the Th-232 in phosphate rock concentrate might be more available for 

partitioning to phosphoric acid than the uranium in phosphate rock concentrate. If this is, in fact, 

the case, then the default high-end intake rate of Th-232 and Th-228 for production workers at 

Blockson should be perhaps 8 to 10 times higher than the values adopted in the TBD. It is also 

noteworthy that Table 1 of DOE 1978 reveals that the ratio of U-238 to Th-232 in the sodium 

phosphate solution was closer to 40:1. The data, therefore, appear to be somewhat ambiguous 

regarding this matter and require further investigation. 

Finding 2: Since the ratio of the concentration of U-238 to Th-232 in phosphoric acid 

might be as high as 10:1, consideration should be given to assigning a high-end default 

intake rate for Th-232 and Th-228 of about 8.2 pCi/day instead of 1.1 pCi/day. 

We do not consider this to be an SEC issue, because, if this finding is determined to be correct, 

the TBD can be easily revised. 

5.2 THORIUM-230 

The TBD is silent regarding the contribution of Th-230 to the internal doses. According to 

Guimond et al. (1977), and Table 2 of FIPR 1998, Th-230 was found to be in equilibrium with 

U-238 in phosphoric acid. The implications are that the activity concentration of Th-230 in the 

uranium product should be the same as U-238. Hence, the intake rate of Th-230 should be 

explicitly considered in the TBD, and it should be comparable to that of U-238 if the Th-230 

remains with the uranium throughout the process. Th-230 could be an important contributor to 

internal dose, because the dose conversion factors for the various organs for Th-230 are larger 

than those for U-238. For example, the inhalation dose conversion factors (expressed in terms of 

50-year effective dose commitment) reported in the International Commission on Radiological 

Protection (ICRP) Publication No. 68 (ICRP 1994) for 5 micron Types M and S uranium are 

1.6E-6 and 5.7E-6 Sv/Bq inhaled, respectively. For Th-230, the respective inhalation dose 

Draft Letter Report 19 SC&A – January 10, 2007 
 

conversion factors are 2.8E-5 and 7.2E-6 Sv/Bq inhaled; i.e., by not considering Th-230, the 

effective dose commitment, if limited to only uranium inhalation, might be underestimated by 

17.5-fold for Type M or about 1.3-fold for Type S Th-230. 

Finding 3: The TBD should explicitly include Th-230 in the exposure matrix. 

We do not consider this to be an SEC issue, because, if this finding is determined to be correct 

and if the Th-230 remains with the uranium, the TBD can be easily revised to accommodate 

this source of exposure. 

5.3 POSSIBLE THORIUM-230 RAFFINATES 

As discussed above, the TBD is silent regarding the contribution of Th-230 to the internal doses 

to AEC contract workers; but this issue can be resolved if the Th-230 remains with the U-238 in 

the same proportion as in the ore. However, it is not apparent that the Th-230 (and the Th-232 

and Th-228) remains with the uranium through all the purification steps, up to and including the 

production of the uranium product. If thorium is separated from the uranium at one of the 

purification steps, a thorium raffinate might be produced. If this raffinate dries out and becomes 

airborne, it may not be feasible to reconstruct the internal doses associated with this exposure 

scenario, since there are no air samples or bioassay data addressing Th-230. 

Finding 4: The TBD should address whether a separate waste stream containing thorium 

raffinates might be produced during the processing of the uranium and, if such a stream 

was produced, there is a need to develop a method for reconstructing the doses associated 

with exposures to the raffinates. 

This finding might be an SEC issue, because, if thorium raffinates are produced during the 

purification of the uranium, it is not immediately apparent how the internal doses from exposure 

to such raffinates can be reconstructed. 

Draft Letter Report 20 SC&A – January 10, 2007 
 

 

 

 

 

 

6.0 RADON EXPOSURES 

Section 5.2 of the TBD provides the exposure matrix for reconstructing doses to AEC contract 

workers to radon and its progeny. The TBD acknowledges that, though radium and its progeny 

do not partition to the phosphoric acid solution (i.e., radium predominantly stays with the 

calcium sulfate and ends up in the phosphogypsum pile), AEC contract workers can still 

experience exposures to radon and its progeny because of the large quantities of phosphogypsum 

stored on site (see Figure 2-1 in Attachment 2). The approach used by NIOSH to develop the 

dose reconstruction protocol to reconstruct doses to AEC contract workers due to exposure to 

radon and its progeny employed a 2-step process. First, NIOSH used the vast amount of data 

characterizing the concentration of radon and its progeny at phosphate ore processing facilities in 

Florida to place a plausible upper bound on the airborne concentration of radon and its progeny 

that may have been experienced by workers in Building 55 at Blockson. Second, NIOSH 

derived the doses to respiratory and non-respiratory tissues using well-established methodologies 

that have been previously reviewed by SC&A and found to be scientifically valid. Hence, this 

section is limited to a review of the first step in the process; i.e., the data, models, and 

assumptions used in the TBD to determine the radon and radon progeny concentrations to which 

AEC contract workers might have been exposed. Specifically, Table 2 of the TBD adopts 0.036 

WLM per year as the default upper-bound exposures for all AEC contract workers at Blockson.2 

It is appropriate to point out that this exposure level is comparable to the naturally occurring 

background levels of radon and its progeny to which everyone is exposed. 

As explained in the TBD, NIOSH derived this value from data provided in ORAUT-OTIB-0043 

(ORAU 2006). ORAU 2006 presents a review of the vast literature compiled by various 

organizations and agencies (especially the EPA) characterizing the radiological conditions 

primarily at phosphate recovery facilities in Florida. As is the case at Blockson, the production 

of phosphoric acid from phosphate ore generates large volumes of tailings in the form of 

phosphogypsum. These tailings are stored onsite and contain most of the radium that was 

originally present in the phosphate ore. The concentration of radium in these tailings range from 

12.8 to 42.8 pCi/g, and radon exhalation rates from these piles were observed to be as high as 

8,070 pCi/m2

-min, with measured radon fluxes at selected Florida phosphate plants ranging from 

26.4 to 2,526 pCi/m2

-min.3 

As described in the TBD, the Florida Institute of Phosphate Research (FIPR) has evaluated and 

compiled an enormous amount of data characterizing the radiological conditions at these 

facilities in Florida, including personnel monitoring data, exposure rates, area monitoring, 

environmental monitoring, and radon measurements. Samples were collected in the mine area, 

rock-handling area, phosphoric acid production area, dry production area, shipping area, and 

services area. 

2

 For individuals not familiar with the concept of a WLM (working level month), it is approximately 

equivalent to being exposed to 100 pCi/L of radon (with short-lived progeny in full equilibrium) for 160 hours per 

month. 3

 For expediency, these values were taken directly from ORAUT-OTIB-0043 and accepted at face value at 

this time. 

Draft Letter Report 21 SC&A – January 10, 2007 
 

 

 

Clearly, there are abundant data characterizing the radiological conditions at Florida phosphate 

mines and processing facilities. Because Blockson did not make its own radon and radon 

progeny measurements, the TBD uses these data to place a plausible upper bound on the radon 

and radon progeny concentrations experienced by AEC contract workers at Blockson. The 

question is, can the experience at the Florida facilities be used as a surrogate for Blockson, and if 

so, has NIOSH used these data in a scientifically plausible and claimant-favorable manner? Of 

particular interest is the adoption of 0.036 WLM/yr as the default exposure rate to radon 

progeny. These questions are addressed in this section4 and in more detail in Attachment 2. 

ORAU 2006 explains that, although some data exist characterizing the radium and radon 

exposures at AWE phosphate facilities, it was determined that “the available documentation does 

not provide enough detail” to characterize the levels of exposures to radium, radon, and radon 

progeny at AWE facilities. Hence, FIPR data were selected by NIOSH as appropriate 

surrogates. 

Sections 3, 4, and 5, and Attachment B of ORAU 2006 present a detailed description of the 

sources of the data and summaries of the data characterizing the levels of radon and radon 

progeny at Florida phosphate mines and processing facilities. ORAU 2006 explains that the 

phosphate plant operations and activities with the potential for occupational exposure include 

mining and beneficiation, ore drying and grinding, the wet acid process, maintenance, work in 

the vicinity of phosphogypsum stacks, and product packaging and handling. As also explained 

in ORAU 2006, “only the work locations and activities related to the extraction of uranium from 

phosphate ores should be considered,” since the other activities would occur with or without the 

presence of AEC contract activities. 

By selecting those data from Appendix B of ORAU 2006 that were judged by NIOSH to be most 

applicable to AEC contract workers at Blockson, NIOSH selected the following radon and 

progeny concentrations and default levels of exposures: 

Table 3: Phosphate Plant Worker Radon WL Values 

(Taken directly from Table 4-4 of ORAUT-OTIB-0043) 

DR Approach Radon Concentration 

(pCi/L) F WL WLM/yr Distribution 

Best estimate 0.751 0.4 0.003 0.036 Lognormal, 

GSD=1.989 

Maximizing 2.33 0.4 0.0093 0.112 Constant 

In this table, 0.4 is the progeny fractional equilibrium as recommended by the United Nations 

Scientific Committee on the Effects of Atomic Radiation. Hence, the values of WLM are 

derived as follows: 

WLM = 0.751 pCi/L × 0.01 WL per pCi/L × 0.4 × 12 work months/yr = 0.036 WLM/yr 

4

 Exposure to airborne levels of Ra-226 is not of particular interest, since it is self-evident that exposure to 

airborne uranium dust (and perhaps Th-230) at Blockson is by far the limiting internal exposure scenario for longlived airborne particulates at Blockson. 

Draft Letter Report 22 SC&A – January 10, 2007 
 

One issue comes to mind with regard to this strategy for developing a surrogate model for 

Blockson workers. In order for the measurements of radon and radon progeny concentrations at 

Florida facilities to be used as a surrogate for radon exposures for workers in Building 55 at 

Blockson, there needs to be a level of assurance that the exposure settings were comparable or 

bounding; i.e., the size of the piles, their proximity to the radon concentration measurements, and 

the radon emanation rates at the Florida facilities result in radon concentrations that are high as 

applied to Building 55 and the location and characteristics of the tailings piles at Blockson. In 

addition, some consideration of similarities and differences in building ventilation rates and the 

use of localized ventilation would provide greater assurance of the applicability of the Florida 

data. 

One strategy SC&A used to assess the validity of the default radon progeny exposure level 

adopted in the TBD was to compare the TBD value to the radon progeny concentrations for 

Building 55 made in 1978 as part of the FUSRAP program (DOE 1978). The 5 values reported 

in DOE (1978) range from 0.0014 to 0.0061 WL. The value of 0.003 WL adopted in the TBD 

falls right in the middle of this range, and is actually comparable to natural background levels. 

The implication is that the default value is reasonable if the 1978 data are considered 

representative of the conditions at the site during uranium production. For example, we are 

not quite certain whether the Blockson tailings piles and also the Florida tailings piles were 

stabilized at the time that the various measurements were made at these sites. It is instructive to 

note that under EPA regulations (National Emission Standards for Hazardous Air Pollutants 

(NESHAPs) set forth in 40 CFR Part 61, phosphate facilities that produce fertilizer are required 

to stabilize their tailings piles to meet the standards, but Blockson, which did not produce 

fertilizer, was not required to meet the NESHAPs (see Attachment 2 for details). It is also 

noteworthy that the size of the tailings piles at the Florida facilities were likely much larger than 

those at Blockson, as discussed in Attachment 2. Hence, there appear to be factors that could 

argue both for and against the default value for radon progeny exposure adopted in the TBD. It 

would seem that this issue requires further documentation. 

Lacking assurances that the radon measurements made at Florida phosphate facilities represent 

bounding conditions for Blockson, the values used in the TBD may not be claimant favorable. It 

may be more appropriate to use the maximizing value of 0.112 WLM/yr as the bounding default 

value for Blockson, as opposed to the best estimate. However, even if this value were to be 

adopted, some assurance is needed that the value is bounding as applied to Blockson. 

Finding 5: It is not apparent that 0.036 WLM/yr is a plausible and bounding exposure rate 

for radon progeny as applied to Blockson, especially considering that this exposure rate is 

associated with naturally occurring levels of radon. 

This is not necessarily an SEC-related issue, because with appropriate analysis, NIOSH may be 

able to demonstrate whether the best estimate of 0.036 WLM/yr or the maximizing exposure rate 

of 0.112 WLM/yr is bounding as applied to Blockson. 

A related issue is the possibility that the phosphogypsum tailings might have been used as 

construction fill or construction material for Building 55. Under these conditions, it might be 

difficult to place an upper bound on the radon exposures experienced by the Blockson workers. 

Draft Letter Report 23 SC&A – January 10, 2007 
However, in light of the radon progeny concentrations reported in DOE 1978 for Building 55, it 

appears to be unlikely that tailings were used as construction material. In addition, surface soil 

samples collected in the vicinity of Building 55 in 1978 and reported in Table 5 of DOE 1978 

reveal that residual contamination had substantially higher concentrations of uranium as 

compared to radium. If tailings were used as construction fill, one would have expected at least 

some of the soil contamination to be higher in radium than uranium (see Attachment 2 for more 

details). 

Finding 6: The TBD should address the possibility that tailings might have been used in 

the construction of Building 55. 

Draft Letter Report 24 SC&A – January 10, 2007 
 

7.0 EXTERNAL EXPOSURES
 

Section 4.2 of the TBD explains that data are not known to exist that characterize the external 

exposures experienced by Blockson workers. As a result, the TBD adopts an approach that takes 

advantage of process knowledge as a means to place a plausible upper bound on the external 

doses experienced by AEC contract workers at Blockson. 

Based on process knowledge, NIOSH assumes that the highest external exposures were 

experienced by workers involved in the packaging of the finished uranium product. SC&A 

agrees with this assumption, because the purified uranium product had the greatest localized 

concentration and quantities of uranium and its short-lived progeny. Hence, the uranium 

packaging area appears to have had the potential to produce the highest external gamma radiation 

field among all the activities and locations associated with AEC contract activities at the site. 

NIOSH estimated the external photon and beta radiation fields in the vicinity of the yellowcake 

and 55-gallon drums filled with yellowcake. These calculations were performed assuming that 

short-lived progeny of U-238 (Th-234, Pa-234 m, Pa-234) were in equilibrium with U-238, and 

that U-234 and U-235, including its short-lived progeny (Th-231), were present at their natural 

abundance relative to U-238. NIOSH used conventional methods to derive the external beta and 

gamma exposure rates and organ dose rates as a function of density of the yellowcake and the 

workers’ distance from the yellowcake and 55-gallon drums. SC&A performed similar 

calculations in the past and concurs in the external beta and photon exposure rates reported in the 

TBD. 

Though we concur with the external beta and gamma exposure rates derived by NIOSH for 

uranium and its short-lived progeny, we have one concern related to the exclusion of any Ra-226 

and its short-lived progeny. As indicated in the TBD, a small fraction of the Ra-226 contained in 

the ore partitions to the phosphoric acid (about 1% in terms of activity (Roessler et al. 1979)). 

Hence, it is possible that for every curie of U-238 in a drum, there might be 0.01 Ci of Ra-226 

with its sort-lived progeny. On first inspection, one might consider this negligible; however, the 

external dose conversion factors for Ra-226 plus its short-lived progeny are much greater than 

that of natural uranium and its short-lived progeny. For example, Table 4 presents the external 

photon dose conversion factors as reported in Federal Guidance Report No. 12 (EPA 1993): 

Table 4: Comparison of External Dose Conversion Factors* 

Radionuclide Ext DCF 

(Sv/sec per Bq/m2

) Abundance 

Normalized Ext DCF 

(Sv/sec per Bq/m2

 of 

parent radionuclide) 

U Series 

U-238 5.51E-19 1 5.51E-19 

Th-234 8.32E-18 1 8.32E-18 

Pa234m 1.53E-17 1 1.53E-17 

Pa234 1.84E-15 .0016 2.94E-18 

U-234 7.48E-19 1 7.48E-19 

Th-230 7.50E-19 1 7.50E-19 

Total 2.87E-17 

Draft Letter Report 25 SC&A – January 10, 2007 
 

 

Table 4: Comparison of External Dose Conversion Factors* 

Radionuclide Ext DCF 

(Sv/sec per Bq/m2

) Abundance 

Normalized Ext DCF 

(Sv/sec per Bq/m2

 of 

parent radionuclide) 

Ra-226 Series 

Ra-226 6.44E-18 1 6.44E-18 

Rn-222 3.59E-19 1 3.59E-19 

Po-218 8.88E-21 0.9998 8.88E-21 

At-218 4.18E-18 0.0002 8.36E-22 

Bi-214 1.41E-15 1 1.41E-15 

Po-214 8.13E-20 0.9998 8.13E-20 

Tl-210 NA 0.0002 NA 

Pb-210 2.48E-18 1 2.48E-18 

Bi-210 1.05E-18 1 1.05E-18 

Po-210 8.29E-21 1 8.29E-21 

Tl-206 1.99E-18 .0000013 2.59E-24 

Total 1.42E-15 

* These values were taken from Table III.3 of EPA 1993. The values represent the external effective whole-body 

dose rate equivalent at 1 meter above an infinite plane. 

As may be noted, the external dose rate from Ra-226 in equilibrium with all its progeny is about 

50 times higher than that of natural uranium in equilibrium with all its short-lived progeny. 

Hence, even if only a small fraction of Ra-226 partitioned to the phosphate stream, it could have 

contributed significantly to the external dose rate of workers relative to that of natural uranium. 

The literature cited in the TBD indicates that about 1% of the Ra-226 might partition to 

phosphoric acid. However, a report by the National Academy of Sciences (NAS 1999) implies 

that as much as 20% of the Ra-226 might partition to phosphoric acid. This is based on page 64 

of that report, which states, “About 80% of the Ra-226 in the ore follows the PG” 

[phosphogypsum]. The implications of that statement are that about 20% of the Ra-226 might 

partition to phosphoric acid. This statement, though inconsistent with the data cited in the TBD, 

warrants some follow-up in light of its potential importance to reconstructing external doses 

associated with AEC contract activities at Blockson. Further, Table 2 of FIPR 1998 indicates 

that the concentration of Ra-226 in phosphoric acid is about 4% that of U-238. Given these 

statements, which we uncovered during our review of the partitioning of Ra-226 to phosphoric 

acid (i.e., 1% versus 4 % versus 20%), there appears to be a degree of uncertainty and variability 

in the amount of Ra-226 that partitions to phosphoric acid. Hence, NIOSH should evaluate the 

possible contribution of trace levels of Ra-226 and its short-lived progeny in the phosphoric acid 

stream to the external dose rate. 

Finding 7: The contribution of trace levels of Ra-226 and its short-lived progeny in the 

phosphoric acid stream and final uranium product to the external gamma field experienced 

by workers needs to be explicitly considered in the exposure matrix. 

SC&A does not consider this finding to be an SEC-related issue, because it appears to be feasible 

to make adjustments to the exposure matrix to account for this possible source of external 

exposure. 

Draft Letter Report 26 SC&A – January 10, 2007 
Once the beta and gamma exposure rates were derived, NIOSH made certain assumptions 

regarding the time spent by workers in the vicinity of the filled drums of uranium. The median 

exposure was determined by assuming all workers were working 8 hours per day, 1 day per week 

at a distance of 1 foot from the drum, which was normalized to 400 hours per work year. The 

upper 95th percentile exposure time was determined by assuming the worker spent a standard 

2,000 work hours a year at 1 foot from the drum. 

Intuitively, one could argue that some workers may have spent more than 400 hours per year 

1 foot from the drum, or that there may have been more than 1 drum in storage at a given time. 

However, these assumptions would appear to be reasonable upper bounds for most if not all 

workers, especially if it is assumed that such exposures occurred over the time period (up to 

12 years) that the workers worked at the facility and considering that an average of about only 

1 drum per week was produced. 

Draft Letter Report 27 SC&A – January 10, 2007 
REFERENCES 

40 CFR 61. “National Emission Standards for hazardous Air Pollutants” Part 61 of Title 40 of 

the Code of Federal Regulations. 

42 CFR 83. 2005. Procedures for Designating Classes of Employees as Members of the Special 

Exposure Cohort Under the Energy Employees Occupational Illness Compensation Program Act 

of 2000, Final Rule; May 28, 2005. 

Blockson Chemical Company, 1951. Letter from E.B. Lopker (approved by Louis Block, 

President of Blockson Chemical Company) to Sheldon Wimpfen, Atomic Energy Commission, 

July 31, 1951. 

DOE 1978. “Formerly Utilized MED/AEC Sites Remedial Action Program, Radiological 

Survey of Chemicals Group, Olin Corporation (Formerly Blockson Chemical Company), Joliet, 

Illinois, March 27-November 28, 1979, DOE/EV-0005/35. Prepared by the Occupational health 

and Safety Division, Health Physics Section of Argonne National Laboratory, Argonne, Illinois. 

EPA 1993. “External Exposure to Radionuclides in Air, Water, and Soil,” Federal Guidance 

Report No. 12, EPA/402-R-93-081, September 1993. 

FIPR 1998. “Evaluation of exposures to technologically Enhanced Naturally Occurring 

Radioactive Materials (TENORM) in the Phosphate Industry,” Prepared by Applied 

Environmental Consulting, Inc. with Environmental Radiation Group and Polk County Public 

Health Unit under a grant sponsored by the Florida Institute of Phosphate Research, FIPR, 

Publication No. 05-046-155, July 1998. 

Guimond, R. J., Mills, W. A., and Windham, S. T., 1977. "Radiation Exposures in the Florida 

Phosphate Industry," International Radiation Protection Association Meeting #4, Paris, France, 

Volume 3, pp.1049-1052. 

ICRP (International Commission on Radiological Protection) 1994. “Dose Coefficients for 

Intakes of Radionuclides by Workers, ICRP Publication 68, ISSN 0146-6453, July 1994. 

NAS (National Academy of Sciences) 1999. “Evaluation of Guidelines for Exposure to 

Technologically Enhanced Naturally Occurring Radioactive Material.” Committee on 

Evaluation of EPA Guidelines for Exposure to Naturally Occurring Radioactive Materials – 

Board on Radiation Effects Research – National Research Council. National Academy Press, 

Washington, DC. 1999. 

NIOSH (National Institute of Occupational Safety and Health) 2006. SEC Petition Evaluation 

Report – Petition SEC-00058, Report Rev # 0, 9/5/06. 

OCAS (Office of Compensation and Support) 2006. Technical Basis Document for Atomic 

Energy Operations at Blockson Chemical Company, Joliet, Illinois. OCAS-TKBS-0002, 

September 11, 2006 http://www.cdc.gov/niosh/ocas/pdfs/tbd/ocblocksnr0.pdf. 

Draft Letter Report 28 SC&A – January 10, 2007 
ORAU 2006. “Characterization of Occupational Exposure to Radium and Radon Progeny 

During Recovery of Uranium from Phosphate Materials,” ORAUT-OTIB-0043, Revision 00, 

January 6, 2006. 

Roessler, C.E., Z.A. Smith, W.E. Bolch, and R.J. Prince, et al. 1979. “Uranium and Radium-226 in 

Florida Phosphate Materials,” Health Physics, Vol. 37(3):269-277. 

SC&A 2006. Board Procedures for Review of Special Exposure Cohort Petitions and Petition 

Evaluation Reports, Revision 1 (SCA-TR-TASK5-0002, June 12, 2006) and the Report to the 

Working Group on Special Exposure Cohort Petition Review (Draft January 16, 2006), S. Cohen 

& Associates, Inc., Vienna, Virginia. 

Stoltz, E.M. Jr. 1953. “Uranium Recovery from Wet Process Phosphoric Acid,” Blockson 

Chemical Company, Joliet, Illinois, Research Contract No. AT[49-1]606, RMO-2850. February 

23, 1953. 

Draft Letter Report 29 SC&A – January 10, 2007 
 

 

 

 

 

 

ATTACHMENT 1: PRESENCE AND DISPOSITION OF THORIUM-230 IN
 

URANIUM PRODUCTION FROM PHOSPHORIC ACID 

Prepared by Charles Phillips (SC&A, Inc.) 

Document OCAS-TKBS-0002 (OCAS 2006) is the TBD for exposures resulting from nuclear 

weapons-related work at the Blockson Chemical Company. The document summarizes the known 

radiological processes and source terms, as well as the radiological controls and monitoring 

practices at Blockson. The document is intended to provide information on conditions and 

establish the basis for dose reconstruction related to the uranium removal from phosphoric acid that 

was routinely produced at the plant. 

According to OCAS 2006: 

In the Blockson process, the phosphate rock was calcined and then digested 

with sulfuric acid resulting in phosphogypsum and phosphoric acid. The 

phosphogypsum partitions most of the calcium and radium, and the phosphoric acid 

partitioned about 90% of the uranium. Very little uranium was lost to the 

phosphogypsum. The phosphoric acid was then converted into monosodium 

phosphate and other phosphorus derivatives. The uranium byproduct was 

precipitated from the monosodium phosphate stream. The monosodium phosphate 

liquor was heated and clarified. Sodium hydrosulfite (Na2S2O4) was added 

to precipitate the uranium. The liquor was filtered and the filtrate returned to 

the phosphate-processing plant. The precipitate, containing about 5% U3O8 

was slurried in water in an upgrading step in which the uranium was 

redissolved. The uranium was then reprecipitated as uranous phosphate. 

The slurry was filtered and the precipitate, known as yellowcake and containing 

40 to 60% U3O8, was dried for shipping (Clegg and Foley 1958, McGinley 

2002, Wimpfen 2002). 

For Blockson Chemical, only exposures relevant to uranium removal and processing are 

considered to be applicable to Energy Employees Occupational Illness Compensation Program 

Act (EEOICPA); therefore, the radionuclides contributing to the dose reconstruction are those 

available in the phosphoric acid stream serving as the feed material to the uranium recovery 

processes. Possible exceptions are radon and those radionuclides contained in materials from the 

phosphoric acid process incorporated into the construction of Building 55. 

In OCAS 2006, Section 3.0, it states the following: 

Workers in Building 55 and related activities were potentially exposed to airborne 

uranium. Thorium, assumed to follow the uranium in the Blockson process, and 

radon are also considered to be present in Building 55 and [are] included in this 

document. 

Draft Letter Report 30 SC&A – January 10, 2007 
 

 

 

 

 

The estimation of internal exposures for Blockson Chemical workers is based on the results of 

122 bioassay samples analyzed by the AEC New York Operations Office Health and Safety 

Laboratory (HASL) using fluorimetry. 

Fluorimetry methods only report uranium results and do not include the attendant radionuclides, 

namely radiothorium. According to OCAS-TKBS-0002, “The presence of associated 

radionuclides that could be present and contribute to significant internal dose is assumed (ORAU 

2006).” OCAS 2006, referring to ORAU 2006, uses the results of phosphate ore analyses 

reported in Guimond et al. 1977 for estimating Th-228 and Th-232 activity in urine to estimate 

thorium inhalation rates. 

The results are summarized in Table 1a below (OCAS 2006): 

Table 1a: Inhalation Rate for Operations 

Worker category Intake rate for Type M material1 Distribution 

Administrative 25 pCi/day total U Constant value 

Administrative 0.35 pCi/day Th-2282 Constant value 

Administrative 0.35 pCi/day Th-2322 Constant value 

Production workers 82 pCi/day M total U Constant value 

Production workers 1.1 pCi/day Th-2282 Constant value 

Production workers 1.1 pCi/day M Th-2322 Constant value 

1 – Intake rates are normalized to units of calendar days. The intake period for operations is March 1, 1951, 

through March 31, 1962. 

2 – Thorium intake rates are derived from ratios in ORAU 2006. Solubility types for thorium are based on 

recommendations in ICRP Report 68 (ICRP 1994b). 

Two significant concerns arise from this estimation of thorium inhalation rates. Since the doses to 

be considered are related to the uranium removal from phosphoric acid, would it not be more 

appropriate to use the relative concentrations of Th-232 to U-238 in the phosphoric acid, as 

reported in Guimond et al. 1977, rather than the concentrations in phosphate ore? Phosphoric 

acid is the feed material to the uranium removal process and not phosphate ore. The use of the 

ratio of Th-232 to U-238 in phosphoric acid (0.12) as opposed to that in phosphate pebble 

rock (0.014) for estimating Th-228 and Th-232 inhalation rates would result in over an 8-fold 

increase in these thorium intake values. This is clearly a more claimant-favorable assumption. 

Possibly of greater concern is the omission of Th-230 from the radionuclides, considered to be 

contributors to doses assigned to Blockson workers. Based on the analyses reported in 

Guimond et al. 1977, Th-230 is in radioactive equilibrium with U-238 in both phosphate ore 

and phosphoric acid. If Th-228 and Th-232 are deemed to be relevant radionuclides, then 

there is no apparent reason to exclude Th-230, particularly since it would be processed in 

much greater quantities than Th-228 and Th-232. It is not clear why Th-230 is not considered 

in the contributing radionuclides, but it seems to be a major omission from the Blockson 

assessment that could potentially result in significant unmonitored exposures. 

Draft Letter Report 31 SC&A – January 10, 2007 
To properly assign doses attributable to radiothorium to workers in the Blockson uranium 

processing operations (Building 55), it is necessary to identify the partitioning of the 

radiothorium into the various products and byproducts of the Blockson uranium recovery 

process. There are no firm data in the material reviewed by SC&A that identifies the 

concentrations of radiothorium in the final uranium product (yellowcake) or in process 

byproducts, such as waste streams, raffinates, and recycled materials. A review of the details 

of the uranium recovery process by an SC&A senior radiochemist concluded that it was not 

possible to assess the partitioning of the radiothorium with any degree of certainty using the 

process descriptive materials available. 

References 

Clegg, J.W., and Foley, D.D., “Uranium Ore Processing,” prepared under contract with the U.S. 

AEC, Addison-Wesley Publishing Company, Inc., 1958, pp. 375-378. 

Guimond, R. J., Mills, W. A., and Windham, S. T., 1977, “Radiation Exposures in the 

Florida Phosphate Industry,” International Radiation Protection Association Meeting #4, Paris, 

France, volume 3, pp. 1049–1052. 

ICRP (International Commission for Radiological Protection) 1994b. Dose Coefficients for 

Intakes of Radionuclides by Workers. ICRP Publication No. 68. 

McGinley, Frank, Personal Communication, December 2002. 

OCAS (Office of Compensation and Support) 2006. Technical Basis Document for Atomic 

Energy Operations at Blockson Chemical Company, Joliet, Illinois. OCAS-TKBS-0002, 

9/11/2006 http://www.cdc.gov/niosh/ocas/pdfs/tbd/ocblocksnr0.pdf. 

ORAU 2006, “Characterization of Occupational Exposures to Radium and Radon Progeny 

During Recovery of Uranium from Phosphate Materials, ORAUT-OTIB-0043, Rev 00, 

January 6, 2006. 

Roessler, C.E., Z.A. Smith, W.E. Bolch, and R.J. Prince, et al. 1979. “Uranium and Radium-226 in 

Florida Phosphate Materials,” Health Physics, Vol. 37(3):269-277. 

Wimpfen, Sheldon, Personal Communication, November 2002. 

Draft Letter Report 32 SC&A – January 10, 2007 
ATTACHMENT 2: RADON EXPOSURES AT BLOCKSON CHEMICAL
 

COMPANY 

Prepared by William Thurber (SC&A, Inc.) 

This attachment considers two issues related to possible radon exposures of uranium recovery 

workers at Blockson Chemical Company: 

• 	 Whether the model developed by NIOSH for uranium extraction workers at phosphate 

plants (ORAU 2006) is reasonable for use at Blockson Chemical Company 

• 	 Whether workers in Building 55 at Blockson might have received incremental radon 

exposures if phosphogypsum tailings were used in construction of that building 

Each of these issues is addressed in the following sections. 

Appropriateness of the NIOSH Model for Uranium Extraction Workers 

NIOSH notes in ORAU 2006 that very few data were available at the time when Atomic 

Weapons Employers (AWE) operations for uranium extraction were conducted at phosphate 

plants. Consequently, NIOSH used data published by the Florida Institute of Phosphate 

Research (FIPR 1998) to develop worker radon exposures. The relevant FIPR data are from a 

variety of anonymous sources, including the following: 

• 	 Chemical plant radon readings – summary statistics, 1989 to 1994 

• 	 Radon measurements summary (chemical plant readings), 1995 to 1996 

• 	 Terredex radon measurements – summary statistics, 1982 to 1996 

• 	 Chemical plant track-etch radon results, 1993 to 1996 

• 	 Phosphate facility radon measurements (electret ion chambers), 1997 

The ORAU authors state the following: 

This was possible because the factors that affect radon concentrations have 

changed very little over time. No significant changes in the construction of wet 

process acid plants had occurred since the time of AWE operations (Birky 

2005b). While environmental regulations led to decreased overall emissions, the 

controls had little or no effect on occupational radon levels. In addition, the rate 

of ore processing has increased over time (Birky 2005b). 

It is easy to understand that, since wet acid plant designs have not changed significantly, radon 

exposures from plant operations, per se, should be similar today to what they were in the 1950s 

and 1960s. However, occupational exposures resulting from radon emissions from 

phosphogypsum stacks could have changed due to new regulations. In 1989, EPA issued 

Subpart R of 40 CFR Part 61 limiting radon emissions from inactive phosphogypsum stacks to 

20 Ci/m2

-sec of Rn-222. Any stacks that were inactive at the time or have since then become 

inactive might need to be modified to meet the standard. An EPA study conducted prior to 

Draft Letter Report	 33 SC&A – January 10, 2007 
promulgating the standard indicates that, based on surveys of four active stacks in central 

Florida, the average annual radon flux was at the standard level (Horton et al. 1988). Thus, little 

remediation should be required for regulatory compliance of these stacks. In contrast, another 

author has described extensive modifications made to several phosphogypsum stacks in Florida 

(Morris 2004). Although many of the modifications were made to meet State of Florida 

requirements rather than the requirements of 40 CFR Part 61, Subpart R, the changes have likely 

reduced radon emissions. An industry expert from FIPR has commented that even though some 

phosphogypsum stacks have been closed, including lining and covering, there is always an active 

stack at each chemical plant (Birky 2005b). 

The NIOSH radon exposure modeling of uranium recovery workers, as described in Section 3.2 

of OCAS 2006 and Section 4.2 of ORAU 2006, was based on eliminating those operations that 

were not generally related to phosphoric acid production from the full dataset in FIPR 1998. 

These included rock tunnel operations, float plant operations, heavy equipment operations, wet 

rock loading, etc. Peripheral operations involving the burn area, the auto shop, and the main 

office building were not excluded from the dataset. Using this reduced data set of 130 

measurements, ORAU calculated that the mean radon exposure was 0.036 WLM/yr and the 95th 

percentile exposure was 0.112 WLM/yr. While the use of a mean exposure of 0.036 WLM/yr 

should provide a reasonable estimate for all workers at a phosphoric acid plant, it is not clear that 

it is appropriate for the more limited cohort of uranium recovery workers in Building 55. For 

example, that cohort would not have included low-exposure jobs, such as office workers or 

trailer occupants, which tend to lower the mean dose. While some portion of the dataset in FIPR 

1998 could be used to estimate radon exposures at Blockson, it would need to be adjusted to 

include only those types of jobs relevant to the Building 55 operations. 

As described in the Technical Basis Document for Blockson Chemical Company (OCAS 2006), 

the Company provided, in a pre-operational letter to AEC, cost estimates based on processing 

6,000 tons of phosphate rock concentrate per week (or about 300,000 tons per year). Assuming 

that 6,000 tons per week was the operating capacity of the Blockson wet chemical phosphate 

process (OCAS 2006, p. 6), and the relevant reaction for the production of phosphoric acid and 

phosphogypsum from phosphate rock is given by the following equation, Blockson would have 

produced about 520,000 tons of phosphogypsum annually: 

3 H2SO4(l) + Ca3(PO4)2(s) + 6 H2O(l) 2 H3PO4(s) + 3 CaSO4·2H2O(s) 

By comparison, the phosphate producers in Florida generate about 30 million tons of 

phosphogypsum per year, and a total of nearly 1 billion tons is stored in 25 phosphogypsum 

stacks (http://www.fipr.state.fl.us/research-area-chem.htm). Clearly, the scale of operations at 

Blockson was substantially smaller than the operations in Florida. Therefore, use of an 

appropriate subset of the data from FIPR 1998, as discussed above, should provide a reasonably 

conservative basis for modeling radon exposures. 

An aerial photograph of the Blockson Chemical Company site is presented in Figure 2-1. 

Building 55 is easily identifiable in this figure, based on its unique floor plan with a truncated 

left side (see also Figure 1 in OCAS 2006). From this figure, it can be seen that the tailings 

ponds are at least 300 yards east or northeast of Building 55. Conceivably, some tailings could 

Draft Letter Report 34 SC&A – January 10, 2007 
have become airborne and accumulated in and around Building 55, resulting in increased radon 

exposures. However, no evidence supporting this conjecture has been found. As discussed in 

the next section, soil samples taken not only around Building 55, but also across the property, do 

not exhibit significant increases in Ra/U ratios over those expected from the feedstock to the 

process. Since Ra is concentrated in the tailings along with the Ca from the phosphate rock 

concentrate, high Ra/U ratios might indicate mobilization and transport of tailings from the 

storage areas. No such evidence was found by ANL (DOE 1983). For example, two samples 

were taken from the roof gravels on Building 55—a likely place for any accumulations from 

airborne mobilization. Samples 4-S1 and 4-S2 had Ra/U ratios of 0.11 and 0.32, respectively. 

These ratios are less than those for the rock concentrate fed to the process, indicating that tailings 

mobilization and transport would not be a measurable contributor to the dose of Building 55 

workers. 

Figure 2-1: Aerial Photo of Blockson Chemical Company Site, Building 55 marked with 

Arrow
 

Draft Letter Report 35 SC&A – January 10, 2007 
Use of Phophogypsum as Fill 

Conceptually, phosphogypsum tailings could have been used as fill or in other construction 

materials during the construction of Building 55. Based on the limited available evidence, this 

possibility appears to be remote. According to Wissa (undated), “Phosphogypsum, because of its 

relatively high solubility, high compressibility and radioactivity is not recommended as structural 

fill for land reclamation, especially if the land will be used for housing.” 

In addition, review of soil and building samples taken by ANL in 1978 do not indicate high 

radium or radon concentrations (DOE 1983). Soil samples 4-S6, 4-S7, and 4-S8 were taken 

immediately adjacent to Building 55 (see DOE 1983, Figure 10). The 30-cm long sample cores 

were analyzed for uranium and radium. Based on analysis of the phosphate rock concentrate 

received at Olin (formerly Blockson Chemical) at the time of the ANL survey, the ratio of 

radium to uranium in the concentrate was 0.45 (DOE 1983, Table 5, sample 4-S5). If the soil 

samples were enriched in tailings, the ratio of Ra/U would be substantially greater than 1, since 

radium is concentrated in the tailings. The Ra/U ratio for soil samples 4-S6, 4-S7, and 4-S8 were 

1, 2, and 0.09, respectively, indicating that use of tailings around Building 55 was unlikely. In 

each case, the calculated ratios are based on the 5-cm surface increment of the 30-cm long cores. 

For sample 4-S6, the ratio was calculated using the one-sigma values rather than the mean, since 

use of a zero mean resulted in a useless ratio. 

ANL also obtained air samples from Building 55 and used these to determine Rn-222 

concentrations and Working Levels. The Working Levels ranged from 0.0014 to 0.0061, which 

according to ANL, were within the expected background range. These observations provide 

further support to the contention that use of tailings around Building 55 at the time of its 

construction is unlikely. Even if tailings were used, the ANL measurements suggest that they 

would have provided no significant incremental contribution to doses. 

References 

Birky, B., 2005b. “Phosphate Plant Radon Levels,” electronic mail to R. Gard (Oak Ridge 

Associated Universities Team), September 1, Florida Institute of Phosphate Research, Bartow, 

Florida. 

DOE (U. S. Department of Energy) 1983. FUSRAP, Radiological Survey of Chemicals Group, 

Olin Corporation (formerly Blockson Chemical Company, Joliet, Illinois, March 27 – November 

28, 1978. Argonne National Laboratory; DOE/EV-0005/35; ANL-OHS/HP-83-103, May 1983 

[SRDB Ref ID 23615]. 

FIPR (Florida Institute of Phosphate Research) 1998. Evaluation of Exposure to Technologically 

Enhanced Naturally Occurring Radioactive Materials (TENORM) in the Phosphate Industry, 

Publication 05-046-155, Bartow, Florida. 

http://www1.fipr.state.fl.us/FIPR/FIPR1.nsf/9bb2fe8f45c4945e85256b58005abaec/deb248481f9 

8842e85256b2f005d7524/$FILE/05-046-155Final.pdf. 

Draft Letter Report 36 SC&A – January 10, 2007 
Horton, T.R., R.L. Blanchard, and S.T. Windham 1988. A Long-Term Study of Radon and 

Airborne Particulates at Phosphogypsum Stacks In Central Florida. EPA/520/5-88-021. U.S. 

Environmental Protection Agency, Montgomery, Alabama. 

Morris, O. 2004. Phosphogypsum Stack Systems, Closed and Lined Replacements at Cargill 

Crop Nutrition. Presented at 2004 IFA Technical Conference, Beijing, China. 

http://www.fertilizer.org/ifa/publicat/PDF/2004_tech_beijing_morris.pdf. 

OCAS (Office of Compensation and Support) 2006. Technical Basis Document for Atomic 

Energy Operations at Blockson Chemical Company, Joliet, Illinois. OCAS-TKBS-0002, 

9/11/2006 http://www.cdc.gov/niosh/ocas/pdfs/tbd/ocblocksnr0.pdf. 

ORAU (Oak Ridge Associated Universities) 2006. Characterization of Occupational Exposure 

to Radium and Radon Progeny During Recovery of Uranium from Phosphate Materials. 

ORAUT-OTIB-0043. 1/06/2006. http://www.cdc.gov/Niosh/ocas/pdfs/tbd/tib/tib43.pdf 

Wissa, Anwar E.Z. (undated). Phosphogypsum and the Environment. Ardaman & Associates. 

Inc. http://www.fipr.state.fl.us/pondwatercd/phosphogypsum_disposal.htm 

Draft Letter Report 37 SC&A – January 10, 2007