The Hanford Environmental Dose Reconstruction Project: A Review of Four Documents (1994)

Plutonium production began at Hanford in September 1944 and continued until 1987. As knowledge of the Hartford operations, especially in the 1940s and 1950s, became available, public concerns were raised about the potential impact of the activities at Hanford on the surrounding population. Continuing declassification of historical Hanford documents, including the release of additional information about iodine-131 releases, has made the public more concerned about the possibility of health effects on residents in the area surrounding the Hanford site.

The plutonium-production process created iodine-131 (in addition to many other nuclides), which was released to the atmosphere principally during the reprocessing of fuel from the reactors. This relatively short-lived radionuclide (half-life, 8.04 days) has received considerable attention during the dose-reconstruction studies that followed because it was judged to be the largest contributor (over 90% of the effective dose) to the historical radiation dose attributable to the Hanford Nuclear Facility.

The four documents reviewed by this National Research Council committee detail the iodine-131 released by the plutonium-production reactors and the separations-plant operations. In calculations of radiation doses to people, the major environmental transport pathways include the atmosphere, food pathways, surface water, and groundwater. A different sequence of modeling techniques is adopted for each of the transport pathways. Different modeling methods are described for each pathway and depend on the available dose data, the degree of confidence justified by available data, and the level of sophistication deemed appropriate for the particular pathway and period being considered. The documents also cover the environmental accumulation and dose-assessment characteristics used to estimate the impacts of airborne releases of iodine-131 from the Hanford site.

In the first few years of Hanford operations, the urgency of recovering plutonium caused the fuel to be reprocessed after only a few days of cooling and at a time when it still contained substantial amounts of iodine-131. That is the main reason why most of the iodine-131 releases from Hanford took place in 1944-1947; cooling times were later extended to about 6 months to allow most of the iodine-131 to decay before processing was begun.

The release of 685 kCi of iodine-131 in 1944-1947 is the most important airborne source term. The operational measurements from the daily logs can be used reliably to determine iodine-131 inventories in the reactor fuel, and cooling times, power levels, and residual amounts are carefully considered. Those inventories and a conservative release scenario were used to establish the amounts released and the periods when the releases occurred. The amount of iodine-131 released to the environment after dissolution is the fuel inventory minus the amount retained (and unavailable for release) in the processing chemicals, the amounts removed because of plateout by the processing systems and off-gas systems, and the amounts removed from the off-gas by scrubbers and filters. Those determinations are conservative (i.e., overestimated) in that there was no reduction for off-gas losses of iodine-131. It was assumed conservatively that the only form released was elemental iodine, I₂; this volatile form would yield the highest calculated offsite doses.

In a few experiments conducted at the Hanford site, inventories of iodine-131 in small batches of fuel were released to determine migration, deposition, and behavior of iodine-131 in the environment. Those releases appear to be well documented; even though they were important, they were considerably smaller (because smaller amounts of fuel were involved) than those associated with the fuel reprocessing of total reactor-fuel inventories. In later years (1950s and 1960s), releases due to experiments could have dominated the annual releases of iodine-131 because in these periods fuel was cooled to allow iodine-131 to decay before processing.

Although iodine-129 was not a major source of human exposure, its long half-life should afford the opportunity for measurement of environmental iodine-129 and so provide a way to validate the iodine-131 modeling. Releases after about 1950 contained minimal amounts of iodine-131 because fuel was cooled before processing, but substantial amounts of iodine-129 would have been released. For completeness, the source-term analysis should address total radioactive-iodine releases during reactor operations and iodine-129 releases from fuel reprocessing.

The pathways of exposure considered for the people onsite are inhalation, air submersion, and groundshine (radiation emitted by radionuclides on the ground). Those pathways are also considered for people offsite, in addition to ingestion of milk, meat, eggs, poultry, leafy and other vegetables, fruits, and grains, all of which contain radionuclides. However, the committee is concerned about omissions in the documents that should have been addressed; for example:

• Pathways other than those mentioned above are to be included if specific criteria are met. The nature of those pathways and criteria that are not covered in the code DESCARTES were not described.

• Onsite groundwater pathways are not addressed; the committee recommends that this decision be discussed and justified.

• The code DESCARTES itself, which is used to calculate the environmental concentrations of the radionuclides of interest, is not adequately described. The only information available is a list of equations in PNWD 2023.

• Although up to 12 radionuclides are considered in the atmospheric pathways, the radionuclide-specific parameter values used in DESCARTES are given only for iodine-131.

Quality control is important but is barely discussed in the documents. The following are examples of committee concerns regarding quality control:

• During 1944-1949, very few monitoring data were available. How was DESCARTES validated for all the radionuclides that are considered?

• For the other two periods, 1950-1972 and 1973-1991, it seems that the available monitoring data are accepted without question. The amount of effort expended to review the validity of those data should be described.

• The uncertainty analysis carried out for iodine-131 is impressive. However, biases that tend to increase the dose estimates have been accepted (for example, in the case of the chemical form to be considered), triangular distributions have often been preferred to lognormal distributions, and obvious errors of a factor of 10 in the choice of some parameter values (for example, the deposition velocity or the immersion-dose rate factor) need to be corrected. The committee hopes that the uncertainty analysis for other radionuclides considered will be as thorough as that conducted for iodine-131 for the atmospheric pathways.

• There is no indication that the models are to be validated by comparison with available measured values of soil, food, and milk concentrations. Such validations should be made if possible.

• The exclusion of migrant farm workers from consideration might be contentious, although it is recognized that it will be hard to collect reliable data. Migrant farm laborers should probably be treated similarly to other short-term residents.

  With respect to the dose calculations, the committee has the following concerns:

• The radionuclides that will be considered in the dose calculations are not identified.

• It is not indicated whether the persons whose doses are to be calculated will be specified (i.e., actual exposed persons) or unspecified (i.e., represented by hypothetical persons).

• The type of annual dose that will be calculated should be made clear (are these effective doses or doses to particular organs or tissues?), as well as the dose coefficients that will be used.

• How the individual doses are to be validated and the expected uncertainty estimates associated with them should be specified.

• The decision to exclude the ingestion pathway in the doses received by the populations onsite should be justified.

• Some areas in American Indian reservations are outside the current grid of major focus, and concern has been expressed for more information on the exposures and potential doses received by the American Indian communities. Americans Indians living in the area surrounding the Hanford site have been concerned about Hartford's impact on their historical lands and on the health of their people.

• An additional pathway is to be included if it is determined that it has “the potential to add more than 5% of the total annual dose for any individual at a time when the dose exceeds the TSP guidelines. ” However, those TSP guidelines are not specified in the documents provided to the committee. For such pathways, the documents should indicate whether only the effective dose or organ-specific or tissue-specific dose is considered.

• The distribution systems of agricultural products are taken into account for the 1944 to 1949 period but not for the periods that follow. That difference should be justified, especially because the dairies are not uniformly distributed around the site.

• Report PNWD 1983 (on the modeling approach) is difficult to read without detailed knowledge of the entire project, and it has gaps that should be filled.

The Technical Steering Panel has made every effort possible to ensure that the public is involved in the HEDR project. The committee recommends that such efforts be mentioned in the documents. The committee also strongly recommends that the documents be made “self-standing” so that any scientist or administrator interested in the HEDR project can understand and use them.

The TSP recognized early that it would have to assume responsibility for the HEDR project's communications with the public if the TSP were to meet its mandates for independence and credibility. Demographic and food-consumption data are to be collected and used as the basis for estimating preliminary iodine-131 doses for a range of life-style patterns characteristic of each of eight participating American Indian tribes. However, HEDR's effort that will go into modeling the aquatic pathways is not yet determined. Those pathways seem to be of particular concern to some of the tribes and a high level of public concern was expressed for these pathways at the committee's meeting in Richland. Even if the dose contribution of these pathways is relatively low, that should be established clearly. The committee applauds the collaborative efforts to address the distinctive cultural and life-style patterns of Americans Indians and the TSP's commitment to a publicly open scientific study.

The committee's overall impressions of the four documents at their stage of evolution are favorable. The approach used for determining the source term is comprehensive and sound in requiring and using all relevant data, whether classified or not, and will establish the state of the art of the source-term analysis in dose reconstruction not only at Hanford but also elsewhere. Although less well developed at this stage, the dose-modeling approach is reasonable and thorough; however, more justification needs to be provided in selecting parameters before the dose models are used. Finally the committee recommends that all doses and quantities of radioactive materials be expressed in SI units followed by English

units in parentheses. The HEDR project is still in transition, and the eventual summary of the HEDR project should be clearer and must carefully address the public's concern if it is to be accepted. The committee would like to see the final HEDR report before making a definitive judgment. Other specific comments and recommendations are in the appendix of this report.