Starting with the creation of the MED in 1942 and continuing through the early 1950s, there was a demand for large quantities of purified uranium that would be turned eventually into uranium metal and rolled into rods. To meet this early demand, the MED [subsequently the Atomic Energy Commission (AEC) beginning in 1946] contracted with a number of commercial facilities to develop processes for separating uranium from ore, converting the purified compounds of uranium metal, and shaping the metal into rods.

There are a number of processes involved in the refinement of uranium, including ore handling, chemical extraction, denitration, oxide reduction, and reduction to metal. While there were several commercial facilities involved in the refinement process, the Mallinckrodt Chemical Company serves as a good example of the types of exposures encountered at these early facilities. The MED asked the Mallinckrodt Chemical Works in April 1942 to begin research on uranium refining and processing operations that could lead to large-scale uranium production operations. The work began immediately, and by July 1942, Mallinckrodt was producing almost a ton of UO₂ per day (ORAU 2010). In addition to Mallinckrodt, some of the other major early facilities involved in the refining of uranium from ore included the Linde Ceramics Plant in Tonawanda, New York, and the Harshaw Chemical Company in Cleveland, Ohio.

The exposures at these early AWE facilities that refined uranium were characterized by high concentrations of airborne uranium, with airborne levels exceeding hundreds of times the then maximum allowable concentration of 70 dpm m⁻³.^† In the very early years of uranium refining, prior to the establishment of the AEC in 1946, monitoring data are sparse. Even when data are available, the monitoring techniques employed are not well documented, making the results difficult to interpret. With the creation of the AEC, however, the Health and Safety Laboratory was formed to provide radiation monitoring support to the sites involved in uranium processing. This laboratory, located in New York City, developed and standardized workplace exposure assessment techniques for the existing AEC facilities. For an 8-y period between 1948 and 1956, the Health and Safety Laboratory completed 60 workplace evaluations at seven uranium refining facilities. Based on their published data, a graph depicting air concentration data at these facilities as a function of time has been constructed (Strom 2007). This graph, provided in Fig. 3, shows the decreasing trend of geometric mean exposures over an 8-y period, along with estimates of the associated uncertainties. From inspection of Fig. 3, it can be seen that there is about a 30-fold drop in the geometric mean value over the decade.

On a number of occasions, the uranium extraction operations during this period involved the processing of high-grade uranium ore, which contained equilibrium quantities of uranium progeny. Because of this, the external exposure environment had levels of gamma radiation that could easily produce annual exposures of 20 R y⁻¹. As an example, Table 3 provides a summary of the annual external exposures received by Mallinckrodt workers between 1947 and 1956. Although there is some component of this exposure that can be attributed to the gamma emissions associated with the purified uranium product, most of this is due to the gamma radiation from the short-lived gamma-emitting progeny of ²²⁶Ra (i.e., ²¹⁴Pb and ²¹⁴Bi).

The ore refining process separated the progeny that was originally in secular equilibrium into several waste streams, creating isolated pathways where one or more of the progeny could become concentrated. Notably, ²³⁰Th was known to concentrate in the filter cake in one portion of the refining process. Thus, the potential for unmonitored exposure to this source term was created. In addition, ²²²Rn was also present at any facility that handled uranium ore in equilibrium with its progeny. Radon, being a chemically unreactive gas, would readily accumulate in areas where ore was processed and stored, particularly in locations with minimal ventilation. An example of the levels of radon that existed at early ore processing facilities is provided in Table 4. Here it can be seen that the upper radon levels at the Mallinckrodt Facility between 1949 and 1957 were well into the hundreds of pCi L⁻¹. Assuming an average progeny equilibrium ratio of 50% for indoor air, this equates to a work environment of several working levels.

Subsequent to the production of purified uranium compounds, much of the material was converted to metal and eventually shaped into rods for use as targets in reactors. As with the chemical processing operations, the AEC relied on existing commercial or research facilities to perform this conversion. Early in 1942, faculty members in the Chemistry Department at Iowa State College in Ames, Iowa, with expertise in rare earth metallurgy, were called on to develop a method to produce uranium metal. By November 1942, successful methods had been developed, and approximately one-third of the uranium used in the Chicago pile was supplied by the Ames Project. Between mid-1942 and August 1945, >1,000 tons of pure uranium metal was supplied to the Manhattan Project. Eventually, uranium production was established at a number of commercial facilities, including Harshaw Chemical Company, Mallinckrodt, and Electromet. Given that the uranium had been separated from its progeny during the refining process, radiation exposures during the production of uranium metal were almost entirely due to the three naturally occurring isotopes of uranium, ²³⁴U, ²³⁵U, and ²³⁸U.

Once the immediate need for uranium metal was fulfilled, the Ames Project began to develop methods for purifying thorium in 1943. By late 1944, a large-scale process for thorium metal production was developed. Between 1950 and 1953, when thorium production was turned over to commercial operations, >65 tons of pure thorium metal and thorium compounds were produced by the Ames Laboratory (Ames 1960). The production of thorium was not limited to Ames, and in fact, a number of the early AWE facilities that processed uranium also processed thorium, although on a more limited basis. In the MED (prior to 1946) and early AEC years, air samples were not collected with sufficient frequency to quantify worker intakes, but urine samples were collected occasionally. Unfortunately, the high detection limit for the urinalysis method employed, in conjunction with the low urinary excretion of insoluble thorium, rendered these data unusable for reconstruction of workers exposure to thorium. Because of the limited sampling that was conducted and the technology shortfall associated with thorium bioassay, NIOSH has determined that thorium doses cannot be reconstructed with sufficient accuracy at several of the early processing facilities.^‡

Several steel mills contributed to the production of uranium metal rods used by Hanford as targets for the production of plutonium. Rolling of uranium metal rods was investigated at Joslyn Manufacturing and Supply during and after the war effort to evaluate methods to improve product quality and reduce losses of product during the manufacturing process. Another development that promised improvements in the production of uranium metal rods was the successful rolling of lead-dipped uranium billets by Joslyn in 1948, which, according to the early AEC reports, were far superior to the Hanford^§ materials in terms of blistering (USDOE 1997). Blistering of the uranium surface not only led to loss of product but also created the potential for the generation of high concentrations of airborne uranium in the workplace.

During the late 1940s, uranium metal was delivered as billets to Simonds Saw and Steel Company, Lockport, New York, and Vulcan Crucible Steel Company, Aliquippa, Pennsylvania, where they rolled the billets into rods that were shipped to Hanford. Although it is known that other rolling mills also participated in rolling operations during this early time period, Simonds Saw and Steel became the principal manufacturer of rods as Vulcan was unable to roll the larger billets coming from Mallinckrodt. The levels of airborne uranium during these early years reached >500 times the then maximum allowable concentration of 70 dpm m⁻³. The distribution of air samples taken at the Simonds Saw and Steel facility is provided in Fig. 4. It can be seen that in 1948, the measured air concentrations are characterized by a log-normal distribution with a median value of approximately 1,200 dpm m⁻³ and a geometric standard deviation of eight (ORAU 2011). These levels were associated with the rolling of uranium that had been heated in an open furnace. As new techniques were developed that heated the uranium billets in molten salt baths, the levels of uranium in air were reduced substantially.

The Feed Materials Production Center located near Cincinnati, Ohio, started operation as a uranium production facility in 1952. As such, the Fernald facility in Ohio took on the role of uranium refinement that was performed previously by commercial AWE facilities. In 1952, construction also began on the gaseous diffusion plant in Piketon, Ohio, which later became known as the Portsmouth Gaseous Diffusion Plant. Uranium enrichment, which was the primary activity at Portsmouth, Ohio, began in 1954. Fig. 5 provides an example of the magnitude of the 50th percentile annual external exposures at the Fernald and Portsmouth facilities between their startup in the early 1950s through the 1990s. Although both facilities processed primarily uranium, the Fernald site mostly handled natural or depleted uranium, while the Portsmouth site is a gaseous diffusion plant that was engaged in the enrichment of uranium. Given the differences in the uranium source terms, it is interesting that the initial magnitude of the annual external exposure at these facilities was in the 10 mSv range with exposures dropping to nondetectable levels starting around the end of the 1980s. It should be noted that the annual exposures plotted in these graphs include “missed dose.” That is, for every badge result that was reported to be below the limit of detection, it was considered to have received a dose that is equal to one-half the limit of detection.

As with most occupational exposure datasets, the annual distribution of external doses was found to be log-normally distributed. Based on the geometric standard deviation that was calculated for each year, Fig. 6 provides a plot of the time-dependent 95th percentile external doses. While the 95th percentile exposures at Portsmouth tend to follow the general decline of the 50th percentile exposures over time, the 95th percentile exposures at Fernald increase dramatically in the early 1960s and stay elevated at around 10 mSv through the early 1980s. A review of the activities at Fernald revealed that the elevated exposures correspond to the receipt and processing of thorium at the site. Although exposures for most workers were reduced over this time period, as indicated by the decreasing 50th percentile exposure, those workers associated with thorium handling continued to receive elevated exposures.

External exposures at the Hanford and Oak Ridge X-10^** Facilities are examples of large facilities with a variety of external exposure source terms. As shown in Fig. 7, exposures decrease over time from their initial annual exposures in the mid-1940s of (~14 and 8 mSv) at Hanford and X-10, respectively. While X-10 exposures continue to decrease through the middle of 1960, exposures at Hanford decrease through the end of 1950, then start rising to a peak in 1965. This trend corresponds to the years of peak plutonium production at the facility. The 95th percentile doses do not decline as dramatically at Hanford and X-10. As seen by comparing Fig. 7 with Fig. 8, the 50th percentile doses drop over time by more than an order of magnitude, while the 95th percentile typically stay within a band of about a factor of three.

Using the bioassay monitoring information that NIOSH has collected from a number of DOE facilities, the urinary excretion of various radionuclides has been characterized as a function of time. As they predominate the internal exposure potential at DOE facilities, the radionuclides with the most monitoring data are uranium and plutonium. As with the external monitoring data, the bioassay samples for a given time period were found to be log-normally distributed, which allows the data to be described by 50th and 84th percentile excretion rates, where the 84th percentile corresponds to the excretion at one geometric standard deviation. To date, NIOSH has used these data to develop coworker exposure models at a dozen facilities. While it is not possible to discuss all these facilities during this publication, an example of a site with uranium and plutonium excretion curves is provided.

Fig. 9 provides a graph depicting the 24 h urinary excretion of ²³⁹Pu at the Hanford facility between 1946 and 1988. As shown, the 50th percentile for the 24 h urinary excretion decreased by more than an order of magnitude over this time period, decreasing from ~0.3 dpm d⁻¹ to levels at or near the detection limit around the mid-1950s. The geometric standard deviation tracks closely with the 50th percentiles and was ~2.6. The sharp drop in the mid-1980s is associated with an improvement in the analytical sensitivity of the measurement technique and not a reduction in intakes. As will be discussed in a subsequent section of this presentation, the sensitivity of the measurement technique has a direct effect on the level of internal dose that could have been received by a worker yet undetected by the bioassay sample.

Fig. 10 provides a graph depicting urinary excretion of uranium for workers at the Fernald site between 1952 and 1990. After a fairly steep rise in excretion at the onset of production, there is a peak median output of 35 μg L⁻¹ in 1955 followed by a gradual decline. Starting in about the mid-1960s, the 50th percentile values are at about the detection limit of the fluorometric fusion technique employed. As was the case in the early years at AWE facilities, the reduction in internal exposure was mostly the result of improvements in administrative and engineering controls. In fact, some of the technical staff from the AEC’s Health and Safety Laboratory who were responsible for recommending improvements at AWE facilities eventually joined the health and safety staff at the Fernald site.