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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY II. OVERALL IMPRESSIONS The HEDR project is a massive and complex effort that will likely stand as an example and model for future dose-assessment projects of the same scope. It is imperative, therefore, that the final documents present fully and clearly the methods used, the parameters involved, the validation of models, the nature and demographic characteristics of the exposed population, and the uncertainties inherent in each of these. It is important to remember that the primary purpose of this project is to answer questions about exposures of the populations living in the vicinity of the plants definitively and credibly. Hence, the project should make every possible effort to arrive at final determinations of doses received by both individuals and population groups in a fashion that leaves few gaps and is as defensible as such historical reconstructions can be. The two reports under review present a detailed summary of the HEDR dose assessment process for the atmospheric (PNWD-2228 HEDR) and Columbia River (PNWD-2227, HEDR) pathways and provide dose-distribution tables and maps that constitute the final outcome of the HEDR project for the period 1944-1992. This Research Council review is required to judge the completeness of the work, its accuracy, the justifications for assumptions that are underlying the inputs to the various models and the thoroughness and quality of the calculations, and thus the overall credibility of the results. However, critical portions of the HEDR project, such as the definition of source terms, model validation, and demographic details are not included in these two reports in summary form. Instead they refer the reader to other reports. In the absence of any discussion of these crucial topics, the committee is required to accept a great deal on trust. The committee views it as essential that the model calculations clearly demonstrate not only consistency and an appropriate choice of parameters, but also satisfactory agreement with past measurements and observations, a reasonable representation of actual exposure conditions of “real” people, and a capacity to serve a suitable basis for epidemiologic studies including the planned Hanford Thyroid Disease Study.
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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY For example, the source-term description reviewed by the committee previously (National Research Council, 1994) was confined to releases in the period 1944-1947. Although that period apparently contained the largest fraction of iodine-131 releases, any extension of the source-term description to the whole period, that is to 1992, obviously required additional justification and a thorough discussion of any new sources and pathways and of how gaps in the document trail and the release records were bridged or extrapolated. As they stand, the reports give the impression that the models were used to generate a vast quantity of data, once operational parameters were selected, with little regard as to whether the output data were meaningful or useful. It will be important to compare whether mapped areas correspond to the presence of substantial population. The modeling process itself is described in several reports using hourly release data for its input. That description is of questionable credibility for the whole period and of doubtful value, considering the lower rates of atmospheric and aquatic transport and movement in the food chain. The absence of detailed discussion of model validation for the various pathway descriptions raises the question of agreement between calculation and measurement, the method of selection of the many parameters involved, and the reality of the ranges of uncertainty for each dose descriptor and the graphically presented dose values. Median dose values are shown in various graphs and maps, but public releases on this work have emphasized doses received by the “maximally exposed individual” (MEI). That is unfortunate because it tends to result in misleading and unnecessarily alarming information, which it is almost impossible to correct effectively. There is no clear definition of the maximally exposed individual, who in any case is different for different pathways, and it is questionable whether the corresponding maps match any demographic reality. It is apparent that very few people may have lived in the high-dose areas and using an estimate of the dose to the “critical group,” as recommended by the ICRP (International Commission on Radiological Protection, 1985), would have an advantage in that it would avoid having dose estimates be prejudiced by a small number of individuals with unusual habits. For the
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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY atmospheric pathway, the MEI presumably is an infant consuming solely milk from a backyard goat or cow. Even if one such person can be found, for the war years, it does not make the whole map presentation any more sensible, nor can it form the basis for any valid epidemiologic study and it hardly justifies the focus of attention. For the public, it would be useful to present median-dose maps overlying population centers with some indication of a realistic dose range. Most of the diagrams suggest a range of uncertainty equivalent to a factor of 100. In a report intended for public consumption, the relevance of such a wide range requires some explanation to provide perspective. The committee recommends that the diagrams include bars to show the 5th to 95th percentile results rather than the entire range. The length of the entire range is often greatly affected by one or two aberrant values. Cutting off outliers tends to give a more realistic picture. One can then make the statement that 95% of the values are expected to be less than a certain value. Although there are some passing references to implemented quality-assurance procedures, with details in some other report, it is important for credibility in the final results to outline how quality assurance and quality control were applied and what, if any, modifications resulted. Except for a graph (see page 3.11, PNWD-2227 HEDR) of chronium-51 concentrations in river water, the authors seem deliberately to reject comparison of predicted or calculated environmental concentrations with measured values. That puts the whole model computation into a stance of isolation from reality and apparent avoidance of explanation or justification of discrepancies that would undoubtedly occur. A more positive approach to validation by comparison with measurement, where available, would add appreciably to the project ’s credibility. Similar considerations apply to the drinking-water pathway and to consumption of local fish, for which measured concentrations were available; a discussion of the affected population would add a helpful dimension to any conclusions. The work that went into these reports is impressive, but some serious questions
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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY remain. The reports should be edited to show which portions of the calculations have been validated to an acceptable degree of accuracy, to indicate which segments represent projections (and projections should be justified), and to define dose ranges received by representative, well-described individuals. The graphs should be reviewed for consistency of symbols and made self-explanatory by showing units and distance scales. A consistent use of SI units is advocated—for example, becquerels rather than curies and sieverts rather than rem, particularly to facilitate comparison with other dose assessments, such as the Sellafield study. The ICRP in its Publication 60 (International Commisssion on Radiological Protection, 1991) recommends the use of “equivalent dose” instead of “dose equivalent” and “effective dose” instead of “effective dose equivalent.” The latter is not a trivial change because the tissue-weighting factors have changed significantly from the ones used in ICRP Publication 26 (International Commission on Radiological Protection, 1977). A more thorough discussion is also needed to explain the meaning of the various periods selected for different exposure pathways. Presumably, a scoping analysis was done to evaluate the relative importance of the various pathways at different periods, to focus the work, and to select the most important impacts for more detailed assessment. This process and the evaluation procedure used should be fully described and justified to avoid any appearance that any pathways or exposure conditions were eliminated from detailed consideration arbitrarily or capriciously. A tabulation of pathways considered and reasons for rejection might be useful. Similarly, it is important to indicate the outcome of any studies on particular pathways or exposure conditions that were undertaken in response to public inquiries, political priorities, or concerns regarding selected target groups, such as American Indian tribes or migratory workers. This dose-reconstruction study constitutes an effort that is likely to have many applications and uses in studies of both populations around Hanford and other exposed populations. Therefore, it is important to collect further data to evaluate the effort. The committee recommends that it be done through collection of the dose estimates calculated
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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY for specific individuals included in epidemiologic studies and for the members of the public who request such estimates. The comparison of these data with the distribution of dose estimates presented in the dose-reconstruction effort can be used to evaluate the reliability and utility of the effort. To make the evaluation valid, one must keep track of the source of requests for the estimates because the requests from the public are likely to come from a nonrepresentative group. The HEDR reports stop short of estimating the collective doses that have occurred. By concentrating on doses to individuals, the reports imply that each individual can, through his or her personal dose estimate, predict their chances of developing a cancer as a result of their exposure. It would be beneficial to provide estimates of the total number of excess cancers and cancer deaths that might be expected in order to provide an estimate of the societal impact of the exposure doses. For the planning and conduct of future dose-reconstruction studies, it would be useful to attempt to gauge the relative precision gained by the various dose calculations, such as preliminary, more-detailed, and full-blown modeling of doses. In particular, in the future one could estimate how much time and effort was spent to achieve the last 20%, 10%, and 1% improvement in dose precision. Briefly summarized, the committee’s overall impression is that the dose-reconstruction effort is thorough, uses up-to-date methods, and is creative. However, an enormous emphasis was placed on modeling, sometimes at the expense of other, more-empirical approaches. The stated intention of the authors of the reports was to use historical measurements supplemented by modeling. However, the extent of the historical measurements that are available is never made clear. It would be highly desirable to indicate, for each of the 12 segments of the Columbia River that are considered and for each calendar year, the number of environmental samples of each type (untreated water, treated water, sediment, omnivorous fish, etc.) and the nature of the radionuclides measured. It is important that the readers of the HEDR reports understand the difference
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A REVIEW OF TWO HANFORD ENVIRONMENTAL DOSE RECONSTRUCTION PROJECT(HEDR) DOSIMETRY REPORTS: COLUMBIA RIVER PATHWAY AND ATMOSPHERICPATHWAY between modeling and dose calculation for an environmental-impact assessment and for a retrospective dose assessment and that there are uncertainties in the factors used in the dose estimates as well as in the modeling. In an environmental-impact assessment, as has been done at Hanford in the past, a model is developed with a hypothetical target individual and a hypothetical population in mind, and a “ representative” source term is developed as the input to the model to calculate projected exposures. In contrast, for a dose-reassessment study, exposure to actual individuals or population groups must be estimated by using documented releases and specific environmental transport conditions at the time of releases. The HEDR modelers have not always appreciated that distinction, nor the fact that dose-assessment estimates for a nonexistent individual or population group are meaningless and can be misleading to the public. Dose-distribution estimates must be suitable as a basis for epidemiologic assessments, so appropriate demographic data should be developed in parallel. Accordingly, the committee recommends that the HEDR study focus more on the dose to real people and the use of realistic, not worst-case, exposure pathways. In this regard, it would be helpful if the reports indicated clearly what constitutes a minimal set of observations and as the ideal set for calculating an individual dose with the CIDER code and what the default values are in case any of these observations are absent.
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