A Review of the Radiological Assessments Corporation's Fernald Dose Reconstruction Report (1997)

Environmental radionuclide releases can lead to the contamination of air, soil, surface water and groundwater. Each of those pathways must be evaluated if the health consequences of releases are to be reliably assessed. Of primary concern at FMPC are the releases of uranium and radon.

The RAC report offers 310,000 kg as the best estimate of the release of uranium to the atmosphere over the period of operation of the FMPC. This estimate is based on data from dust collectors, plants 2 and 3, plant 8 scrubbers, and miscellaneous other sources. That release to the atmosphere is substantially larger than uranium releases in liquid effluents and the releases of thorium and other radionuclides. Moreover, the atmospheric uranium releases are of greater importance because the material is more available for exposure of off site populations. Particle-size analysis is important in terms of eventual radiation dose calculations for the lung since particles of certain sizes are quickly propelled out of the pulmonary tree, swallowed, and no longer available to expose the lung.

Overall, the uranium-release estimates are well done and reflect the committee's earlier comments on the source-term modeling. However, the sums of the median quantities in tables 2-5 are inconsistent, and the total is closer to 290,000 than to 310,000 kg.

Radon released from the waste stored in the K-65 silos, which were partially sunk below ground level, constitutes the principal source of radiation exposure off site, so it is important to estimate it as reliably as possible. The RAC team has chosen to derive its source term from 1986 gamma-ray measurements of the radon accumulated under the silo dome. The release of radon from this source to the environment is assumed to have been unlimited and driven by thermal pumping before the dome was sealed in 1979 and by diffusion through cracks in the dome since then. Only one measurement of radon concentration in the head

space, made in 1987, has been reported to provide direct confirmation of the radon concentration (see figure 14 in volume I of the RAC report). The model derived from the gamma-ray measurements has been referred to by RAC as the “preferred method.”

The committee has proposed the use of what has been termed the “conventional method” both to simplify assumptions and to validate the assumed source term by deriving the radon inventory in the head space from the reported radium inventory in the waste mass. The detailed calculation involved is outlined in the appendix “Additional Comments,” beginning on page 36 of the present report. The main result of the conventional method is to indicate a substantially lower radon release rate than was obtained with the RAC's preferred method, based on the principle that radon cannot be released from the dome faster than it emanates from the waste mass. The lower release rate is consistent with the few available air measurements off site which are below the concentrations predicted with the preferred method.

The practical implications of the conventional method are to lower the amount of radon released by a factor of 2-3 and increase the range of uncertainties by a factor of 2 or 3. A qualitative assessment of the input data and parameters used in the preferred method indicates that the uncertainty depicted in figure 17 of volume I of the RAC report is unrealistically small and should be increased substantially. In contrast, figure 17 assigns too large an uncertainty to the conventional method, in the committee's view.

It should be noted that the releases estimated by RAC are larger than the Mound Laboratory measurements as shown in figure N-3 of volume II. Those estimates suggest that the stated radium inventories in the K-65 silo residues are too low and the estimated release rates are too high. The only available measurements, at the crosswind station BS-6, while validating the model in the usual sense, also seem to support a much lower release rate than estimated. Overall, the suggested release rates of about 6,200 Ci/yr before 1979 and about 950 Ci/yr for 1980-1987 seem to be grossly overestimated. Such estimation would result in an overestimate of the radon-progeny exposure of off site populations.