IV. ATMOSPH0ERIC PATHWAYS

The greatest contributors to doses associated with the atmospheric pathway were the chemical separation plants that were active between 1944 and 1972. PNWD-2228 HEDR therefore focuses on estimating the doses for that period, and doses published in the Hanford annual environmental reports are used to complete the dose history for 1973-1992. The former estimates were made for representative individuals in a 75,000-square-mile area surrounding the Hanford site. For iodine-131 dose estimates, 12 categories of representative individuals are distinguished by age, sex, and lifestyle determinants for the years of maximum release, 1944-1951, over 1,102 locations within the HEDR study area. Doses from strontium-90, ruthenium-103, ruthenium-106, cerium-144, and plutonium-239 are also estimated for a maximum representative adult in nine locations for 1944-1972. To do that, the authors estimated the discharge of radionuclides to the atmosphere from the separation plants at the Hanford site. Radionuclide transport in the atmosphere was simulated with computer models to estimate the concentrations of key radionuclides in the air and deposited on the soil. Environmental accumulation was determined, and doses were then estimated with lifestyle information in representative individuals.

The authors of the report state (paragraph 4, page 5.3, PNWD-2228 HEDR) that “the factors determining the uncertainty are more related to the individual than to the environment” but do not explain what that means. It could mean that variation in thyroid uptakes and retention times vary greatly from person to person, that individual intakes of milk and vegetables differ substantially, or both. If, however, it means that milk and vegetable intakes vary, then this source of uncertainty is being used inappropriately. Person-to-person variation in milk intake would be a source of uncertainty in assessment of doses received by representative or “average” persons. But in assessment of doses received by actual persons, it would not be a source of uncertainty; only the degree to which a person’s report of milk intake was unreliable or biased would contribute to uncertainty. Hence, there is a possibility that the authors are overstating the uncertainty.



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OCR for page 23
A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY IV. ATMOSPH0ERIC PATHWAYS The greatest contributors to doses associated with the atmospheric pathway were the chemical separation plants that were active between 1944 and 1972. PNWD-2228 HEDR therefore focuses on estimating the doses for that period, and doses published in the Hanford annual environmental reports are used to complete the dose history for 1973-1992. The former estimates were made for representative individuals in a 75,000-square-mile area surrounding the Hanford site. For iodine-131 dose estimates, 12 categories of representative individuals are distinguished by age, sex, and lifestyle determinants for the years of maximum release, 1944-1951, over 1,102 locations within the HEDR study area. Doses from strontium-90, ruthenium-103, ruthenium-106, cerium-144, and plutonium-239 are also estimated for a maximum representative adult in nine locations for 1944-1972. To do that, the authors estimated the discharge of radionuclides to the atmosphere from the separation plants at the Hanford site. Radionuclide transport in the atmosphere was simulated with computer models to estimate the concentrations of key radionuclides in the air and deposited on the soil. Environmental accumulation was determined, and doses were then estimated with lifestyle information in representative individuals. The authors of the report state (paragraph 4, page 5.3, PNWD-2228 HEDR) that “the factors determining the uncertainty are more related to the individual than to the environment” but do not explain what that means. It could mean that variation in thyroid uptakes and retention times vary greatly from person to person, that individual intakes of milk and vegetables differ substantially, or both. If, however, it means that milk and vegetable intakes vary, then this source of uncertainty is being used inappropriately. Person-to-person variation in milk intake would be a source of uncertainty in assessment of doses received by representative or “average” persons. But in assessment of doses received by actual persons, it would not be a source of uncertainty; only the degree to which a person’s report of milk intake was unreliable or biased would contribute to uncertainty. Hence, there is a possibility that the authors are overstating the uncertainty.

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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY It is noted that average meteorologic data from 1944-1949 were used for assessing all releases through 1972. Whereas use of average data is appropriate for assessing exposure of representative or average persons, it introduces uncertainty in assessment of exposure of real persons, who are the ultimate objectives of dose calculations. Efforts need to be made to use as much real-time meteorologic information as possible, or the justification for using only 1944-1949 data should be provided in the report. The committee is not certain whether the final model used for iodine-131 dose calculations allows inputs of meteorologic parameters related to the locations of the real persons being considered, or whether it includes appropriate radiation- or tissue-weighting factors. The committee saw no evidence that the authors made any attempt to quantify the degree of unreliability of anamnestic (or even contemporaneous) reporting of milk or vegetable intakes. This is an important oversight because such unreliability could be a major contributor to total uncertainty. The most important observed doses from the air pathway were doses of iodine-131 received by the thyroid. These doses could have occurred through food consumption, particle inhalation, and external exposure. Although the relative contributions of these types of exposure varied somewhat with location and age, most locations and age groups showed food consumption as a predominant source of exposure (contributing from 60% to over 95% of the total absorbed dose to the thyroid). The committee recommends that the authors make a special effort to present these dose estimates in a convincing manner. The latest estimates are about half the doses set forth in the 1991 feasibility report, and it will be important for the authors to explain the large discrepancy. Credibility would probably increase if the reason for the reduction from 2,091 locations in RATCHET to 1,102 locations in DESCARTES were explained (see page 3.6, PNWD-2228 HEDR). The use of the term “maximum representative individual” is unfortunate because it will be the one that will receive the attention of the public.

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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY The committee was surprised to find no mention of the use of powdered milk during 1945; some members recall that it was not uncommon. If such use was abundant, it should be discussed. A word about the prevalence of breastfeeding at this period would also be relevant (page 4.40, PNWD-2228 HEDR). The authors are to be commended for Section 3.2 (page 3.3, PNWD-2228 HEDR), in which they detail more thoroughly source term, atmospheric transport, environmental accumulation, and individual dose and note that these four items are represented by their own computer codes, ultimately tied together. Figure 4.23 and Table 4.5 (pages 4.52 and 4.53, PNWD-2228 HEDR) clearly illustrate the magnitude of the effective dose equivalents; perhaps these should be put into perspective by comparing them with annual background radiation doses from external sources. The hierarchic approach to determining the importance of each parameter in the calculation is a good approach and was a good way to determine that the ingestion conversion parameter was the most important. According to the authors, of all the foods consumed, milk was found to be the predominant contributor (from less than 20% to over 90% of dose), followed by fruit (5 to 50%), and beef (less than 1 to 20%). The exact contributions of the different foodstuffs depend on whether they were homegrown or commercially produced; the largest exposures came from fresh milk of pasture-fed cows and was 3-5 times more than exposure from milk of cows fed solely on stored feed. The contributions of dominant foodstuffs to the median daily consumption rates for 1945 varied substantially with the age (See Table 1, page 4.2 PNWD-2228 HEDR) and location of individuals. Iodine-131 contributed nearly 99% of the cumulative effective dose equivalent (EDE) potentially received by an adult in Richland, six other nuclides from the separation plants contributed less than 1%. Age-dependent dose-conversion factors from ICRP Publication 56 (International Commission on Radiological Protection, 1989) were used to convert iodine-131 intake to absorbed dose. The EDE for radionuclides other than iodine-131 was estimated using methods described in ICRP Publication 26 (International Commission on Radiological Protection, 1977.)

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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY Over 99% of the radiation exposure occurred during the first 7 years of operation, and about 75% in 1945. The report adequately covers the remaining period, although attention should have been directed to the changing quality of the available data. The exposure area considered covered most of central Washington State, and the calculated doses changed several orders of magnitude for the different areas on the basis of their location in relation to the Hanford site. Even within a given area, exposure ranges included 3 to 4 fold differences; this indicates the importance of exact location and movement within the area. The committee has some specific concerns including the following. Breastfeeding patterns, which have changed over time, were not considered. The authors assumed a geographically stable population in the modeling; however, population movement in and out of the study area and within the area would have a substantial influence on dose. It is the committee ’s understanding that the CIDER code takes residential history into account, but it is not clear how this is done. Table 4.1 (page 4.2, PNWD-2228 HEDR) seems strange: a total food consumption by an infant (sum of all columns within a row, 1,250 g/day) or a child 5-9 years old (1,410-1,503 g/day) far exceeds that of a woman (909 g/day). The authors should verify that and perhaps provide additional information on how the backcasting ratios were used to obtain the numbers in Table 4.1. Much careful work and elaborate modeling effort have gone into the estimation of doses. However, the effort is uneven: detailed attention was paid to source estimation and modeling but much less to other uncertainties. For example, the pattern of food consumption is based on one report of the U.S. Department of Agriculture. The applicability of this information to the population and period of interest is questionable and was left unexplored.

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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY Another example of large uncertainty acknowledged in the report is the amount of iodine transferred through cow’s milk. If that is, in fact, the major contributor to the total uncertainty, the extent of the variability in milk transfer rate could have been determined experimentally, and it is also extensively covered in the literature. More effort could have been spent on collecting additional information to validate other assumptions. For example, a random (or stratified random) sample of individuals could have been chosen and interviewed for a variety of information, including movement and food consumption. Such information could have been used to estimate the dose distribution for the “typical” individual instead of or in addition to the “maximally” exposed individual used throughout the report. As previously mentioned, estimates to the maximally exposed individual are misleading when a few people live in high-dose areas. A major advantage of estimating a dose to a “critical group” is that this approach avoids having dose estimates being prejudiced by a small number of individuals with unusual habits. Bias, or systematic error, has not been adequately considered. For example, much effort has gone into ensuring that all the available information is incorporated into source-term estimates. However, the available information might be biased in some way (e.g., large releases could have been more or less carefully documented). Incorporation of biased information without appropriate correction could produce systematic overestimation or underestimation of exposures and doses. The report is short on details. For instance, a list or table of variables included in the different models, with the mathematical form used, would have been useful. Similarly, DESCARTES estimates radionuclide concentrations in 20 media; a list of the media would have been useful. There are substantial differences by age and sex in the reduced doses from commercial milk (see page 4.33, PNWD-2228 HEDR). The reason for those differences is not explained.

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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY Comparison of doses from this study with doses from the feasibility study indicates that some dose estimates have increased and others have decreased; but the total impact on the dose distribution is unclear. That information is needed both for the power calculations of the Hanford Thyroid Disease Study and to develop procedures for scoping and epidemiologic studies at other locations.