Mountain High-level Radioactive Waste Research Articles

LAURISTON S. TAYLOR LECTURE: YUCCA MOUNTAIN RADIATION STANDARDS, DOSE/RISK ASSESSMENTS, THINKING OUTSIDE THE BOX, EVALUATIONS, AND RECOMMENDATIONS

The Yucca Mountain high-level radioactive waste repository is designed to contain spent nuclear fuel and vitrified fission products. Due to the fact that it will be the first such facility constructed anywhere in the world, it has proved to be one in which multiple organizations, most prominently the U.S. Congress, are exercising a role. In addition to selecting a site for the facility, Congress specified that the U.S. Environmental Protection Agency (U.S. EPA) promulgate the associated Standards, the U.S. Nuclear Regulatory Commission establish applicable Regulations to implement the Standards, and the U.S. Department of Energy (U.S. DOE) design, construct, and operate the repository. Congress also specified that U.S. EPA request that the National Academy of Sciences (NAS) provide them guidance on the form and nature of the Standards. In so doing, Congress also stipulated that the Standards be expressed in terms of an "equivalent dose rate." As will be noted, this subsequently introduced serious complications. Due to the inputs of so many groups, and the fact that the NAS recommendations conflicted with the Congressional stipulation that the limits be expressed in terms of a dose rate, the outcome is a set of Standards that not only does not comply with the NAS recommendations, but also is neither integrated, nor consistent. The initial goals of this paper are to provide an independent risk/dose analysis for each of the eight radionuclides that are to be regulated, and to evaluate them in terms of the Standards. These efforts reveal that the Standards are neither workable nor capable of being implemented. The concluding portions of the paper provide guidance that, if successfully implemented, would enable U.S. DOE to complete the construction of the repository and operate it in accordance with the recommendations of NAS while, at the same time, provide a better, more accurate, understanding of its potential risks to the public. This facility is too important to the U.S. nuclear energy program to be impeded by inappropriate Standards and unnecessary regulatory restrictions. As will be noted, the sources of essentially all of the recommendations suggested in this paper were derived through applications of the principles of good science, and the benefits of "thinking outside the box."

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.

COMPARISON OF NATURAL BACKGROUND DOSE RATES FOR RESIDENTS OF THE AMARGOSA VALLEY, NV, TO THOSE IN LEADVILLE, CO, AND THE STATES OF COLORADO AND NEVADA

In the latter half of 2005, the U.S. Environmental Protection Agency (U.S. EPA) published a Proposed Rule (40 CFR Part 197) for establishing a dose rate standard for limiting radionuclide releases from the proposed Yucca Mountain high-level radioactive waste repository during the time period from 10 to 10 years after closure. The proposed standard was based on the difference in the estimated dose rate from natural background in the Amargosa Valley and the "average annual background radiation" for the State of Colorado. As defined by the U.S. EPA, "natural background radiation consists of external exposures from cosmic and terrestrial sources, and internal exposures from indoor exposures to naturally-occurring radon." On the basis of its assessments, the U.S. EPA estimated that the difference in the dose rate in the two identified areas was 3.5 mSv y. The purpose of this paper is to provide an independent evaluation and review of this estimate. One of the first observations was that, because site-specific dose rate measurements for the Amargosa Valley "were not available," the dose rates for various sources of natural background in that area, used by the U.S. EPA in its assessment, were based on modifications of the average values for the State of Nevada. A second observation was that the conversion coefficient applied in estimating the dose rates due to exposures to indoor radon and its decay products was a factor of >2 higher than the currently internationally accepted value. Further review revealed that site-specific data for many natural background sources in the Amargosa Valley were available. One particularly important observation was that about 91% of the residents of that area live in mobile homes which, due to their construction and design, have indoor radon concentrations comparable to, or less than, those outdoors. For that reason, alone, the U.S. EPA estimate of the average dose rate for residents of the Amargosa Valley, due to exposures to indoor radon, was not valid. For purposes of the comparisons in this paper, site-specific dose rates were estimated for all major natural background sources of exposure to residents of the Amargosa Valley, and those in Leadville, CO. The latter community was selected for comparison because of its altitude (3,200 m) and accompanying relatively high cosmic radiation dose rate, and the fact the size of its population is comparable to that of the Amargosa Valley. Another reason for this selection was that a comparison of the average natural background dose rate in the Amargosa Valley to that for the State of Colorado is not suitable because it fails to consider those locations within the State that have dose rates that are higher than the average. Nonetheless, for completeness, and to provide a number that could be compared to the U.S. EPA estimated difference, similar comparisons of the estimated dose rate in the Amargosa Valley to those for average residents of the States of Colorado and Nevada were included in the assessments that follow. The outcome showed that the estimated dose rates in Leadville, the State of Colorado, and the State of Nevada, were higher than those in the Amargosa Valley by 3.94 +/- 1.09, 2.54 +/- 2.18, and 0.95 +/- 0.82 mSv y, respectively. Associated uncertainties were highest for the estimated dose rates due to exposures to radon and its decay products. Had the systematic errors in the radon dose conversion coefficient and the random distribution in radon concentrations been included, the overall uncertainty in the total dose rate estimates could have been as high as 150%.

SIGNIFICANCE OF 14C AND 228Ra IN TERMS OF THE PROPOSED YUCCA MOUNTAIN HIGH-LEVEL RADIOACTIVE WASTE REPOSITORY

C and Ra are two of the radionuclides that have either been identified as being potentially significant in terms of releases from the proposed Yucca Mountain high-level radioactive waste repository, or are specifically cited for consideration and evaluation in the regulations promulgated by the U.S. Nuclear Regulatory Commission. The purpose of this study was to estimate the concentrations and associated doses for these two radionuclides, if released under conditions of a scenario assumed to apply to a repository containing some of the features of the one proposed at Yucca Mountain, NV, and to compare these estimates to the regulatory limits for that facility. For C, the postulated condition was that an annual fractional release of 10 of its total remaining inventory occurs beginning at 10,000 y after repository closure. For Ra, the same fractional release rate was assumed, but in this case it was presumed to occur when the Ra inventory was projected to reach a maximum at more than 10 y after repository closure. The estimated concentrations and doses were, in turn, compared to the concentration limit, specified in the Ground Water Protection Standards (GWPSs) in the case of Ra, or derived, in the case of C, on the basis of the regulatory dose rate limit. Due to the small inventory of C in the waste, and its short half-life relative to the performance period evaluated, its estimated concentration in the ground water would be slightly more than 4% of the derived GWPS. Due to the relatively small initial inventory of Th, the precursor of Ra, and the correspondingly small quantities of higher atomic number actinides that could, through decay, produce additional quantities of Th, its estimated concentration in the ground water would be less than 3% of the GWPS, leaving the remaining portion of the limit for potential contributions from Ra. At the same time, however, it must be recognized that, in this case, the regulations require that any contributions of naturally occurring Ra and Ra already present in the ground water must be included in the determination of compliance. If this is done, the total concentration of Ra, combined with the naturally occurring concentration of Ra, would be about 10.5% of the limit. In a similar manner, the committed doses due to the annual consumption of each of these two radionuclides in ground water and food, produced in the local biosphere, were evaluated in terms of the Individual Protection Standard (IPS). Based on these analyses, the estimated effective dose for C, using the coefficients in Federal Guidance Report (FGR) No. 13, was 4.15 muSv y, less than 3% of the IPS. For Ra, the comparable estimate at the time of maximum inventory, excluding in this case the contributions from naturally occurring Ra and Ra, was 7.39 muSv y, representing about 5% of the IPS. Based on the value assumed for the fractional release rate (10 y), it was concluded that neither C nor Ra will be significant in terms of either the applicable GWPS or the IPS. While it was recognized that, due to the time spans involved, these analyses were primarily an academic exercise, it is believed that the perspectives and accompanying insights are useful.

Extracting Risk Insights from Performance Assessments for High-Level Radioactive Waste Repositories

Performance assessments (PAs) are important sources of information for societal decisions in high-level radioactive waste (HLW) management, particularly in evaluating safety cases for proposed HLW repository development. Assessing risk from geologic repositories for HLW poses a significant challenge due to the uncertainties in modeling complex systems of such large temporal and spatial scales. Because of the extensive uncertainties, a typical safety case for a proposed HLW repository is comprised of PA results coupled with various defense-in-depth elements, such as the multibarrier requirement for repository design, and insights from supplementary analyses. This paper proposes an additional supplementary analysis, the Strategic Partitioning of Assumption Ranges and Consequences (SPARC), that could be used (a) in a safety case to help build confidence in a repository system and (b) to provide risk information for decisions on how to allocate resources for future research. The SPARC method extracts risk information from existing PAs and supporting databases by uncovering new information—namely, what sets of model parameter values taken together could produce substantially increased doses (SIDs) from the repository—and displays the results in SPARC trees. These sets of parameter values correspond to the failure scenarios of reactor probabilistic risk assessments. The SPARC method is applied to the proposed Yucca Mountain HLW repository, as a demonstrative example, and the results indicate that just one or a couple of the repository features working alone could “save” the repository from SIDs even in extremely challenging conditions. Such insights produced with the SPARC method could help significantly in focusing resources on future research to build confidence in the repository.

IMPACTS OF STABLE ELEMENT INTAKE ON 14C AND 129I DOSE ESTIMATES

The purpose of this study was to evaluate and provide insights related to the influence of the intake of stable isotopes of carbon and iodine on the committed doses due to the ingestion of (14)C and (129)I. This was accomplished through the application of two different computational approaches. The first was based on the assumption that ground (drinking) water was the only source of intake of (14)C and (129)I, as well as stable carbon and stable iodine. In the second, the intake of (14)C and (129)I was still assumed to be restricted to that in the ground (drinking) water, but the intake of stable carbon and stable iodine was expanded to include that in other components of the diet. The doses were estimated using either a conversion formula or the applicable dose coefficients in Federal Guidance Reports No. 11 and No. 13. Serving as input for the analyses was the estimated maximum concentrations of (14)C or (129)I that would be present in the ground water due to potential releases from the proposed Yucca Mountain high-level radioactive waste repository during the first 10,000 y after closure. The estimated contributions of stable carbon and iodine through the consumption of ground water were based on analyses of samples collected in the Amargosa Valley, NV. The contributions through dietary intake were based on surveys conducted in the United States. Based on the accompanying analyses, it was noted that stable isotope intake has a significant effect on the estimated doses due to the intake of radioactive isotopes of the same element. While this is a well-known fact, this observation has international implications in terms of dose estimates for key radionuclides, such as (14)C and (129)I, a primary reason being the wide variations in the intakes of stable carbon and iodine in various countries. For this reason, analysts planning to apply the dose coefficients developed by the International Commission on Radiological Protection (ICRP) should either confirm that the average total intake in their country of stable isotope(s) of the radioactive isotope being evaluated is in reasonable agreement with the value assumed by the ICRP or suitably modify the ICRP dose coefficients to account for any differences. If such a procedure is to be implemented, there is a need for periodic updates of the dietary intakes of various stable elements in countries throughout the world. The importance of this is documented by recent surveys in Asia that revealed that their average total daily intake of stable iodine was less than half of the ICRP value for Reference Man. In this case, application of the ICRP dose coefficients, without modification, would underestimate the dose due to ingested (129)I by a factor of more than two. A related situation exists in the United States where the latest surveys indicate that the daily intake of stable iodine is 75% of the ICRP value.

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.