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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 III. EXPOSURE PATHWAYS AND DOSE ASSESSMENT Radiation doses will be estimated for exposure pathways resulting from atmospheric releases and releases to the surface and groundwater. In addition, doses from external irradiation will result from γ radiation emitted from the storage silos. Exposure pathways resulting from atmospheric releases are discussed in Appendixes O and Q, (RAC, 1993), those from releases to the surface water are dealt with in Appendix R, and those from releases to the groundwater are treated in Appendix S; γ radiation from the storage silos is considered in Appendix P. Various aspects of the dose assessment are discussed in Appendixes A, B, C, T, and U. The conclusions of a screening exercise, described in Appendix C, are that the uranium isotopes are the most important contributors to the dose resulting from atmospheric releases and that inhalation is the most important pathway. With respect to surface-water releases, the most important radionuclides are 228Ra, 226Ra, 99Tc, 238U, and 234U. No screening exercise was carried out for the groundwater releases. The purpose of the dose assessment is to provide unbiased estimates of dose, together with uncertainties, that can be used in epidemiologic studies under consideration. To this end, attention can be restricted to the populations that lived within the 8-km radius of the site at any time between 1951 and 1988. A. MEASUREMENTS AND METEOROLOGIC DATA The report makes an impressive effort to use measurements to validate model estimates. For example, RAC selected meteorologic parameters for years for which no site data were available. Although the authors are clearly reluctant to use climate data from either the Cincinnati or the Dayton airport, they made a reasonable decision and presented a compelling analysis of why the Cincinnati data, adjusted for local patterns, are used. The Monte Carlo sampling method for using the Cincinnati data is clever; a better method probably does not exist.
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 The near-field deposition of particles and building wake effects should continue to be reviewed in the context of near-field dispersion and runoff because these mechanisms could have a considerable influence on the pattern of the introduction of uranium and other materials into the environment. The endpoint objectives are appropriate, and the use of a concise summary followed by detailed appendixes is a good choice. Although the report assesses effects out to 50 mi, this appears to be unnecessary in view of the clear significance of near-field transport and deposition. The general impression of the NRC committee is that the conceptual approach being considered for the modeling of environmental pathways is sound and that much work has been done in preparation for the actual dose calculations. However, it is difficult to judge the study as a whole because many decisions–such as determining the dosimetric endpoints of the study and the pathways and sources that are insignificant, selecting the size of the assessment domain, and deciding what foodstuffs need special attention–will not be made before the first comprehensive results are obtained, and many loose ends are left pending. It is the committee's opinion that a comprehensive screening exercise should have been conducted first to clarify the scope and spirit of the study. The screening exercise described in Appendix C is a step in the right direction, but it is not as useful as it could have been for several reasons: Direct γ exposure from radiation is not considered The doses that result from inhalation of radon decay products are not included among the consequences of atmospheric releases Drinking water is considered in estimating the doses from surface-water releases, even though the Great Miami River is not used as a drinking-water source The doses from groundwater releases are not considered The doses from the various sources are not compared on the same absolute scale.
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 A comprehensive screening analysis also will indicate whether an epidemiologic analysis is warranted. The executive summary should include an uncertainty analysis to prepare the reader for “reasonable” dose estimates that vary by orders of magnitude. It should indicate the authors' expectations about the range of reasonable estimates and whether error is primarily the result of the choice of the environmental model or whether it comes from uncertainties in the source-term estimates, the selection of lifestyle and dietary habits, or individual variabilities in the dose-conversion factors. The authors should identify the potential sources and magnitude of error so that they can concentrate their effort on the areas of the greatest uncertainty. B. EXPOSURE PATHWAYS 1. Atmospheric Releases The pathways of exposure considered are inhalation, ingestion, and external irradiation from deposited radionuclides. Radionuclide concentrations will be estimated for outdoor conditions only for the inhalation estimates. However, since people spend much of their time indoors, it is important to estimate indoor concentrations as well. The report does not include any discussion of methodology or the parameter values needed to estimate indoor air concentrations. The committee had several concerns about the ingestion estimates. According to NCRP Report 77 (National Council on Radiation Protection and Measurements, 1984a), the five food categories that contribute most to uranium intake from natural background in the United States are, in order of decreasing importance: potatoes, meat, fresh fruit, bakery products, and dairy products. It is understood that the origin and physicochemical form of uranium is different for natural background and for atmospheric releases. However, it appears that estimates of uranium concentrations in potatoes, fresh fruit, and bakery
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 products (which are either ignored or not incorporated in the RAGTIME model described by Pleasant and colleagues (Pleasant et al., 1980) that will be used in the RAC study) are needed. Alternatively, the report should explain why these are not credible pathways for Fernald. Radionuclide concentrations will be estimated for raw agricultural products. However, people usually consume food after some amount of handling and preparation, such as washing and removal of the outer leaves of green vegetables, peeling and cooking of potatoes, and cooking of meat products. The report does not discuss methodology or parameter values needed to estimate radionuclide concentrations in prepared foodstuffs. The influence of the physicochemical forms of the radionuclide releases on the resulting concentrations in agricultural products is not discussed. It also would have been useful to show the variation with time of the concentration of several radionuclides in important agricultural products for a unit release of activity. As is done in Appendix R, the relative importance of several foodstuffs with respect to dose could have been assessed. The main text (p. 8) indicates that external irradiation from deposited radionuclides is considered, and dose-conversion factors for external irradiation are provided in Appendix T. However, there is no information about how external irradiation will be treated in the model (variation of the radionuclide concentration profile in soil with time, calculation of indoor exposures, etc.). In view of the gaps in the environmental pathway analysis, much work is needed on quality control, especially in model validation and uncertainty analysis. For example, it is not clear whether there is any information from environmental monitoring reports that could be used to validate the models. With respect to the criteria of clarity and detail, the main text is appropriately concise and not overdetailed. The appendixes offer a great amount of detail about the material that is discussed, but, unfortunately, they do not deal with a lot of important information, as indicated above.
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 2. Gamma Radiation from the Storage Silos Large inventories of natural radionuclides are present in the K-65 silos and in the metal oxide silo. These radionuclides emit γ radiation which is a source of potential exposure. All available data on the source term seem to have been considered, and there is no obvious gap in this aspect of the pathway analysis. The preliminary results of the model validation are encouraging. Additional environmental exposure rate data will be evaluated in the Task 5 report. There is no description of the computer software package, MicroShield, version 4.00. The characteristics and limitations of the software should be discussed. In particular, it should be shown that it is appropriate to use this package for the conditions that are considered (estimates of exposure rates for distances of up to 5 km from the source; influence of relief, trees, and buildings.). 3. Surface-Water Releases (Appendixes C and R) Pathways of exposure to contaminated river water include direct consumption, swimming, boating, shoreline exposure, consumption of fish, and consumption of irrigated crops. A screening exercise is carried out using two approaches: the NCRP Screening Method and the GENII model, using source term estimates for 1960-1962. The results of this screening analysis are used to justify the use of the authors' monthly dilution model. Appendix C states that the most important radionuclides for dose estimates from surface-water releases are 228Ra, 99Tc, and 226Ra, followed by uranium. It is therefore astounding that the more detailed analysis carried out in Appendix R considers uranium alone. Clearly, the doses from all important radionuclides should be estimated before any conclusions can be known about which pathways should be considered in the dose assessment.
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 The committee has two concerns about the report's estimates of exposure by ingestion: First, if it is known that the Great Miami River is not used as a drinking-water source and because there is no water treatment facility along the river between FMPC and the Ohio River, there is no reason to include the consumption of water from the river in the exposure pathways. The only exception is water consumed while swimming. The decision to disregard the irrigation of crops and garden produce seems arbitrary. Second, radionuclide concentrations are to be estimated for freshwater fish and raw agricultural products. As previously mentioned, foodstuffs are usually consumed by people after some culinary preparation, and the report does not discuss the methodology and parameter values needed to estimate the radionuclide concentrations in prepared foodstuffs. The committee assumes that external irradiation ought to be considered. However, there is no information in the report about how this mode of exposure will be treated. For example, what will be done regarding the variation of the radionuclide concentration profile in sediments and on the river banks with time and will there be calculations of indoor exposures? The quality control methodology is conceptually sound, but the fact that the most important radionuclides were not considered shows that more work should be done. Assuming that irrigation of crops and garden produce needs to be considered, it will be important to use models that are consistent with those used for atmospheric releases and to consider the same agricultural products. It is not clear whether there is any information from environmental monitoring reports about radionuclide concentrations in fish that can be used to validate models. 4. Groundwater Releases (Appendix S) Exposure pathways due to groundwater contamination are similar to those of the surface-water contamination, discussed in Appendix R. In the report, the authors conclude that the important pathways for groundwater are ingestion of drinking water and ingestion
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 of produce from irrigated gardens. It is implicitly assumed that uranium is the only radioelement that will be found in well water, but the validity of this assumption must be confirmed. The methodology described is conceptually sound. The committee expects that the same parameter values will be used for irrigation water from the river and from wells. The comment made previously with regard to the influence of food processing and preparation also applies here. C. DOSE ASSESSMENTS Doses will be assessed on the basis of the environmental concentrations of radionuclides in air, soil, and agricultural foodstuffs for the atmospheric pathways, and in water, sediments, and aquatic foodstuffs for the surface water and groundwater pathways. The estimated doses are intended for use in epidemiologic studies. Comments that are common to several exposure pathways are considered first. 1. Population at Risk The authors compiled several data bases that help define the historic size and demographic characteristics of the population within 8 km of the FMPC site. These data bases were of varying usefulness, but even in the composite they still require considerable interpolation and extrapolation to estimate population sizes over time. Several approaches to modeling population growth were attempted based on information from these data bases. The approach chosen, one that uses U.S. census tract data from 1990 and U.S. Geological Survey lists of buildings from 1950 and 1980, appears to be the most reasonable approach of those considered. The estimates resulting from the use of the model appear to be adequate for the intended use, which is to estimate the size of the population at risk within different dose
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 zones around the site. This information is needed for an assessment of statistical power for possible epidemiologic studies. Nevertheless, several improvements can be made in the procedures and conclusions: It is unclear why U.S. census tract data from 1980 (and earlier censuses, if available) were not explored for their utility in making estimates. It is estimated that 15% of the structures in the U.S. Geological Survey list are nonresidential. No justification for this estimate is given. The authors provide an estimate of population percentages by age for ages 0-9, 10-20, and >20 yr. Although this crude breakdown might be sufficient for dose-estimating purposes, it is inadequate for drawing conclusions about statistical power if consideration of a future epidemiology study is warranted. Disease rates are highly age dependent, so age groupings should be refined. The authors conclude that they have provided “estimates of the number of individuals that may have been exposed to environmental releases from the FMPC between 1952 and 1988.” Actually, they estimate the number of residents at various points in time. To derive “the number of individuals that may have been exposed,” they also would need to add population turnover rates to their model. The committee suggests that the conclusion be changed so that RAC need not claim to have done more than was the case. The purpose of the demographic modeling process was to provide a matrix of population size by calendar time, which is a fundamental building block in estimating the person-dose distribution and the person-years at risk for use in possible epidemiologic studies. The demographic estimates provided are credible and basically adequate. The authors of the report were careful to check for internal consistency and validity. It is recommended, however, that more detailed age categories be provided and that the conclusions be couched appropriately.
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DOSE RECONSTRUCTION FOR THE FERNALD NUCLEAR FACILITY: A REVIEW OFTASK 4 2. Type of Doses Because the dose estimates are intended for use in epidemiologic studies, consideration should be given to estimate absorbed doses instead of equivalent doses and to separating alpha from beta and gamma radiation. Also, the effective dose need not be calculated because the epidemiologists are interested only in annual doses to specific tissues or organs. The influence of the chemical form on the dose coefficient must be considered. Dose coefficients are listed in Appendix T for selected radionuclides. It is expected that this list will be complemented later with values for the missing radionuclides. 3. Dietary and Lifestyle Habits Very little information is given in the report about the dietary and lifestyle habits of the population under consideration. 4. Uncertainties Uncertainties are not treated thoroughly for lifestyle and dietary habits or for the dose-conversion factors. There is no indication of what will be done to identify the uncertainties in the dose-conversion factors. Because the factors present a large variability, the associated uncertainties should be considered carefully, especially for the various physical and chemical forms of uranium that were released into the environment.
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