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PubMed2005

Sensitivity analyses of the standards for the proposed Yucca Mountain repository--a review, evaluation, and commentary.

The standards and regulations for the proposed Yucca Mountain high level radioactive waste repository, which were developed and promulgated by the U.S. Environmental Protection Agency and the U.S. Nuclear Regulatory Commission, respectively, are complex and challenging. A major reason is that they are divided into three parts, an Individual Protection Standard, a Human Intrusion Standard, and multiple Ground Water Protection Standards. Because the individual parts are not fully integrated, the one that controls under a specific set of circumstances depends on the radionuclide being evaluated, its mechanisms of transport, its avenues of intake, and differences in the specified limits. Although the coefficients in Federal Guidance Report (FGR) No. 11 are being used to estimate the doses, other sources (for example, Title 10, CFR, Part 20, and/or FGR No. 13) may deserve consideration. Since the regulations specify that the reasonably maximally exposed individual is an adult, this leaves unanswered the estimated doses to other age groups, such as infants and adolescents. Summarized in this paper are comparisons of the dose coefficients for different age groups, as well as evaluations of the sensitivity of effective and organ dose estimates for adults, depending on the source of the coefficients. All the latter analyses were based only on the consumption of ground water. While the dose estimates are different, depending on the sources of the coefficients, this was not unexpected. What these evaluations demonstrate is the caution that must be exercised to ensure that a full range of considerations is taken into account in interpreting the outcome of the dose assessments being made with respect to the proposed repository.

Body Burden↗

PubMedDate not supplied

Uncertainties in coupled thermal-hydrological processes associated with the Drift Scale Test at Yucca Mountain, Nevada.

Understanding thermally driven coupled hydrological, mechanical, and chemical processes in unsaturated fractured tuff is essential for evaluating the performance of the potential radioactive waste repository at Yucca Mountain, Nevada. The Drift Scale Test (DST), intended for acquiring such an understanding of these processes, has generated a huge volume of temperature and moisture redistribution data. Sophisticated thermal-hydrological (TH) conceptual models have yielded a good fit between simulation results and those measured data. However, some uncertainties in understanding the TH processes associated with the DST still exist. This paper evaluates these uncertainties and provides quantitative estimates of the range of these uncertainties. Of particular interest for the DST are the uncertainties resulting from the unmonitored loss of vapor through an open bulkhead of the test. There was concern that the outcome from the test might have been significantly altered by these losses. Using alternative conceptual models, we illustrate that predicted mean temperatures from the DST are within 1 degrees C of the measured mean temperatures through the first 2 years of heating. The simulated spatial and temporal evolution of drying and condensation fronts is found to be qualitatively consistent with measured saturation data. Energy and mass balance computation shows that no more than 13% of the input energy is lost because of vapor leaving the test domain through the bulkhead. The change in average saturation in fractures is also relatively small. For a hypothetical situation in which no vapor is allowed to exit through the bulkhead, the simulated average fracture saturation is not qualitatively different enough to be discerned by measured moisture redistribution data. This leads us to conclude that the DST, despite the uncertainties associated with open field testing, has provided an excellent understanding of the TH processes.

Forecasting↗

PubMed1998

FISH-based analysis of stable translocations in a Techa River population.

PURPOSE: Measurements of symmetrical translocations by fluorescence in situ hybridization (FISH) were performed for retrospective biodosimetry in a Techa River population exposed to external (gamma-rays) and internal (90Sr, 137Cs) irradiation. MATERIALS AND METHODS: Chromosome analyses were carried out on peripheral lymphocytes from 73 radiation-exposed residents from settlements along the Techa River (Southern Urals, Russia) located 7-148 km downstream from the site of release of liquid radioactive waste from the plutonium production facility Mayak. Thirty-nine unexposed persons from non-contaminated areas were used as controls. Whole-chromosome painting probes for chromosomes 1, 4 and 12 were used simultaneously with a pancentromeric probe. RESULTS: A significantly elevated mean translocation frequency compared with controls was found for the total study group and for either of two subgroups of inhabitants residing in villages of the upper regions of the Techa River (7-60 km) during 1950 to 1951, or in villages of the lower regions (78-148 km) until the time of blood sampling. Within the first subgroup, subjects born between 1937 and 1949 showed higher translocation frequencies than those born between 1914 and 1936. Collective biodosimetry estimates for the various groups were between 0.24 and 0.54 Gy. Individual dose estimates for seven subjects with at least five translocations ranged between 0.77 and 1.80 Gy and compared well with doses reconstructed on the basis of 90Sr whole-body counts (WBC) and electronic paramagnetic resonance (EPR) measurements. Individual translocation frequencies from 40 subjects with existing WBC data and calculated cumulative red bone marrow doses below 0.6 Gy fall within the 95% prediction limits of the calibration curve. CONCLUSIONS: FISH-based translocation measurements can provide useful information for a retrospective biodosimetric interpretation. However, with the analysed number of cells, individual estimates required for a reliable evaluation of this highly variable exposure situation can be obtained only for a minority of subjects. In addition, influence of a temporal decline on the yield of stable translocations cannot be fully excluded.

Body Burden↗

PubMed1989

Technetium-99 cycling in maple trees: characterization of changes in chemical form.

Prior field studies near an old radioactive waste disposal site at Oak Ridge, TN, indicated that following root uptake, metabolism by deciduous trees rendered 99Tc less biogeochemically mobile than expected, based on chemistry of the pertechnetate (TcO-4) anion. Subsequently, the form of technetium (Tc) in maple tree (Acer sp.) sap, leaves, wood and forest leaf litter was characterized using one or more of the following methods: dialysis, physical fractionation, chemical extraction, gel permeation chromatography, enzymatic extraction, or thin layer chromatography (TLC) on silica gel. Chromatography (Sephadex G-25) of TcO-4 incubated in vitro with tree sap showed it to behave similar to TcO-4 anion. When labeled wood and leaf tissues were processed using a tissue homogenizer, 15% and 40%, respectively, of the Tc was solubilized into phosphate buffer. Most (65% to 80%) of the solubilized Tc passing a 0.45-micron filter also passed through an ultrafiltration membrane with a nominal molecular weight cutoff of 10,000 atomic mass units (amu). A majority (72% to 80%) of the Tc in wood could be chemically removed by successive extractions with ethanol, water and weak mineral acid. These same extractants removed only 23% to 31% of the Tc from maple leaves or forest floor leaf litter. Most of the Tc in leaves and leaf litter was removed only by strongly alkaline reagents typically used to release structural polysaccharides (hemicelluloses) from plant tissues. Chromatography (Sephadex G-25) of the ethanol-water extract from wood and the alkaline extract from leaves demonstrated that Tc in these extracts was not principally TcO-4 but was complexed with molecules greater than 1000 amu. Incubations of leaf and wood homogenates with protease approximately doubled the amount of Tc released from contaminated tissues. Ultrafiltration of protease-solubilized Tc from leaves and wood showed that 40% and 93%, respectively, of the Tc was less than 10,000 amu. TLC of the less than 10,000 amu fraction indicated the presence of TcO-4 in wood but not in leaves. In the leaf, TcO-4 is converted to less soluble forms apparently associated with structural components of leaf cell walls. This conversion explains why 99Tc is not easily leached by rainfall from tree foliage and why 99Tc appears to accumulate in forest floor leaf litter layers at the Oak Ridge study site.

Chromatography, Gel↗

PubMed2007

Factors affecting dose estimates for long-term performance assessments: case study-Armagosa Valley.

The purpose of this study was to review and evaluate some of the factors that influence the dose estimates to members of the public due to chronic, long-term releases of radioactive materials into the environment. Although the examples discussed are based on data from the Amargosa Valley located near the proposed Yucca Mountain high-level radioactive waste repository, the factors evaluated are common to any such assessment. While it is recognized that such factors include those related to both the environmental transport of radionuclides from the point of release through the environmental media to the receptor, and the influence of his/her location and living habits, the assessments that follow are primarily limited to the latter. The specific goal in all cases was to illustrate how the assumptions and input values relative to certain factors influence the dose estimates and to quantify, to the extent possible, their relative significance. At the same time, it must be recognized that the assessments presented here were limited to doses due to the ingestion of food and water; those due to the inhalation of airborne radionuclides and external exposures were not considered. The factor that proved most important from the standpoint of the overestimation of doses (i.e., conservatism) was the implementation of the regulatory requirements pertaining to the withdrawal of groundwater from the local aquifer. Another significant source of conservatism was the dose estimates provided by the U.S. Department of Energy, assuming that the Reasonably Maximally Exposed Individual (RMEI) resided at the U.S. EPA designated site, 18 km from the repository, vs. the Amargosa Valley, about 35 km away. Also important, for selected radionuclides, was the impact of their effective half-lives on the committed doses, as estimated, in comparison to those that would actually be received. Having lesser impacts were the status of a local aquaculture farm on the intake of C, and the intake of stable iodine on dose estimates for I. On the basis of these evaluations, one can reasonably conclude that the overall conservatism in the dose assessments, based on these sources that were identified, approaches an order of magnitude. The sole factor that led to an underestimation of the doses (i.e., non-conservatism) was the regulatory requirement that the concept of the RMEI, as defined by U.S. EPA, in contrast to that of the Critical Group (CG), as recommended by the International Commission on Radiological Protection, be applied in estimating the doses to potentially affected population groups. Rather than dwell on the differences in the impacts of the application of each of these two concepts, the next step should be to subject each concept to a systematic and rigorous analysis, the goal being to gain an understanding of the range of dose estimates that would be yielded, the underlying reasons for the differences that are observed, and the lessons to be learned in terms of improving the methodologies for estimating doses due to environmental radionuclide releases.

Artifacts↗

PubMed1960