Division of Compensation Analysis and Support

Blockson Chemical Company

Special Exposure Cohort 

James W. Neton, Ph.D., CHP

Associate Director for Science

National Institute for Occupational Safety and Health

November 2015

Oakland, CA
Division of Compensation Analysis and Support

Petition Overview

 Special Exposure Cohort petition received 

February 25, 2015 

 Petitioner requested class definition:

All maintenance and operations personnel who 

worked in any area at Blockson Chemical Co. in 

Joliet, Illinois, during the period from July 1, 1960 

through December 31, 1991.
Division of Compensation Analysis and Support

Petition Overview

 Qualified for evaluation on May 5, 2015

 Basis for qualification:

Radiation exposures incurred by 

members of the proposed class were not 

monitored either through personal or area 

monitoring

 Some area monitoring results are available
Division of Compensation Analysis and Support

Petition Overview

 The class evaluated by NIOSH:

All employees who worked in any area at the 

Blockson Chemical Co. site in Joliet, Illinois, 

during the period from July 1, 1960 through 

December 31, 1991
Division of Compensation Analysis and Support

Previous Evaluation

 SEC petition for 1951 through 1962

 The Board deemed the methods to 

estimate radon dose from Building 40 

during the Atomic Weapons Employer 

(AWE) operational period not sufficiently 

accurate

 Class added to the SEC for all workers at 

Blockson from March 1, 1951 through June 

30, 1960
Division of Compensation Analysis and Support

Data Sources

 DCAS-TKBS-0002, Technical Basis 

Document for Atomic Energy Operations at 

Blockson Chemical, Joliet, Illinois

 Technical Information Bulletins

 Information from petitioners and former 

workers

 NIOSH Site Research Database (SRDB)

 Previous dose reconstructions
Division of Compensation Analysis and Support

Previous Dose Reconstructions

 Total claims submitted for dose reconstruction 143

 Cases submitted for energy employees who 

worked during the period under evaluation 

(7/1/1960 – 12/31/1991) 130

 Number of dose reconstructions completed 

for energy employees who worked during the 

period under evaluation 127

 Cases for which internal dosimetry records 

were obtained 0

 Claims for which external dosimetry records 

were obtained 0
Division of Compensation Analysis and Support

Facility Background

 Blockson processed Florida phosphate rock into 

phosphoric acid and other chemicals

 Processed about 6,000 tons phosphate rock per 

week

 Phosphate rock contained about 0.012% uranium

 Contracted in 1951 with the Atomic Energy 

Commission to develop process to recover 

uranium as byproduct

 Blockson modified their process and built 

Building 55 for uranium recovery operations
Division of Compensation Analysis and Support

Blockson Phosphate Process

 Used the wet process to produce 

phosphoric acid

 Phosphate rock was calcined and transferred 

to Building 40

 Rock was pulverized and digested in sulfuric 

acid to produce phosphoric acid and 

precipitated phosphogypsum waste

 Waste contained radium which was deposited 

into large outdoor piles known as 

phosphogypsum stacks
Division of Compensation Analysis and Support

Uranium Recovery Process

 The phosphoric acid product stream 

contained most of the uranium

 The acid was partially neutralized to form 

monosodium phosphate and then diverted 

to Building 55

 Used a multistep process to precipitate 

and then concentrate the uranium

 Phosphate stream returned to other plants 

for processing into commercial chemicals
Division of Compensation Analysis and Support

Uranium Product

 Uranium concentrates dried and packaged 

in drums for shipment to the AEC 

 Final product contained 40-50% uranium

 Production ended in 1960

 Blockson recovered 118.3 tons of uranium 

from 1952 through June 1960
Division of Compensation Analysis and Support

Sources of Residual Contamination

 Internal and external doses from uranium 

contamination

 Dose from progeny and associated 

radionuclides

 Exposure to radon

 Building 55 is used to bound dose from 

residual AEC-related contamination

 Phosphogypsum stacks bound radon 

exposures from AEC-related waste
Division of Compensation Analysis and Support

Radiological Data

 Bioassay data from uranium recovery workers

• Samples collected from 25 workers between 

1954 and 1958

 Air sampling performed in 1978 and 1983

 Contamination and radiation surveys 

performed in 1978

 Radon monitoring data

• Measurements in 1978 and 1983

• Flux data from phosphogypsum piles in 1993
Division of Compensation Analysis and Support

Internal Dose

 TBD provides intakes of uranium during 

operations—used for start of residual 

period

• Bounding value of 13 pCi/day

 Contamination data from Building 55 in 

1978 used to estimate intakes in 1978

• Intake from resuspension equals 0.28 pCi/day

 Values used to determine exponential 

depletion rate and annual intake estimates
Division of Compensation Analysis and Support

Internal Dose

 TBD methods and values favorable in 

comparison with methods described in 

ORAUT-OTIB-0070

 Both inhalation and ingestion pathways 

considered

 Intakes of associated radionuclides based 

on intakes of uranium
Division of Compensation Analysis and Support

External Dose

 External dose rate measurements in 1978

• Surveys taken in Building 55

 Dose rates from hot spots were used to 

estimate annual dose rates

 Hot spots were from normally inaccessible 

locations

 Assigned as a lognormal distribution

• Median value is 0.03 mR/hr with 95th percentile 

of 0.2 mR/hr
Division of Compensation Analysis and Support

External Dose

 Annual photon dose of 0.060 rem

 Allows for dose from beta radiation

 Favorable in comparison with dose 

estimates based on general contamination 

survey data

 Includes dose from contamination that 

accrued from commercial operations 

during the residual period
Division of Compensation Analysis and Support

Radon Exposures

 Radon measured in 1978 and 1983

• Building 55 values in 1978 ranged from 0.14 to 0.61 

pCi/Liter

• Single measurement of phosphogypsum area was 

<0.0042 WL

 Radon from active phosphate work not applicable

 Phosphogypsum stacks are presumed to bound 

radon exposures from residual AEC-related waste

 Radon flux measurements taken of inactive stacks 

in 1993
Division of Compensation Analysis and Support

Radon Exposures

 10.1 pCi/m2-s highest mean value reported for 

sides of Blockson stacks in 1993

 Radon air concentrations not provided

 Texas City Chemicals inactive phosphogypsum 

stack had similar radon flux and provided 

associated radon concentrations

• Evaluated against IG-004 surrogate data 

criteria

 Maximum 0.42 pCi/L concentration from Texas 

City also presumed for Blockson in 1993
Division of Compensation Analysis and Support

Radon Exposures

 Factor of 5 adjustment made to allow for 

concentration from an active stack

 2.1 pCi/L estimated for Blockson in 1960

 Exponential depletion presumed to 

connect the 1960 and 1993 values

 Annual exposures bound all available 

radon data for Blockson
Division of Compensation Analysis and Support

Feasibility Summary

July 1, 1960 through December 31, 1991

Source of Exposure Feasible Not Feasible

Internal Dose X

Radon Exposure X

External Dose X