A. E. Hechanova, B. T. Mattingly, P. S. Morris, and L. J. O'Neill
Harry Reid Center for Environmental Studies
University of Nevada, Las Vegas.

Results of the present study indicate that current risks to the public at the perimeter of the Nevada Test Site (NTS) from radioactive waste disposal at the Area 3 and 5 Radioactive Waste Management Sites (RWMSs) are negligible because there is no significant pathway by which contamination can migrate to the NTS border. Results for risks 100 years in the future from public land use of the RWMS locations are dependent on land-use scenarios. Low or negligible risks are predicted for land-use scenarios that do not disturb the waste, such as industrial surface, cultural, and recreational land-use scenarios. Medium risks (and one case of high risk for the Greater Confinement Disposal Test borehole) are predicted for the Maximally Exposed Hypothetical Individual (MEHI) if some waste locations are intruded upon through inadvertent drilling (part of the Ranching and Residential land-use scenario). The MEHI concept is a type of worst-case scenario and used as a bounding case to identify potential significant hazards.

The results presented in this paper are interim in light of the following considerations: (1) complete inventories of NTS RWMS contaminants are not yet publicly available and source term characterization is based upon currently available information from the U. S. Department of Energy, its contractors, and other sources; (2) land-use scenarios are based upon stakeholder interests and are still evolving; and (3) refinements to technical assumptions (e.g., site-specific transport and exposure parameters) and techniques may produce different results. However, because of the iterative nature of risk assessments and the interactive nature of risk communication, it is important to convey interim approaches and results in order to address broad stakeholder concerns.

The Nevada Risk Assessment/Management Program (NRAMP) is a joint effort of the Harry Reid Center for Environmental Studies at the University of Nevada, Las Vegas (UNLV), and the firm of E. J. Bentz and Associates of Springfield, Virginia, funded by a U. S. Department of Energy grant (No. DE-FG01-96EW56093) to conduct independent stakeholder-based risk assessment and risk management evaluation of DOE environmental management activities in Nevada. Andrews et al.⁽¹⁾ presented the basis for preliminary results at WM97. The present paper gives the results from one of the milestones of that study, namely, for the Radioactive Waste Management Sites source category.

The concept for NRAMP results from National Research Council (NRC) recommendations to the DOE that the credibility of site-wide risk assessments would be greatly enhanced if the evaluator were other than the DOE and its contractors⁽²⁾. The NRC identified six criteria which an institution should satisfy in order to establish credibility:

• It should be perceived as being neutral;
• It should have management capability;
• It should have the ability to conduct scientifically valid and responsible risk assessments;
• Its assessments should be subject to independent, external review by technical experts;
• It should have the ability to plan, organize, manage, and facilitate public participation; and,
• It should have the ability to effectively communicate complicated scientific information on potential risks and uncertainties.

The Harry Reid Center for Environmental Studies is an interdisciplinary research center at UNLV that satisfies the above criteria and administers NRAMP. Potential hazards from DOE radioactive waste management at the Nevada Test Site (NTS) is one of several sources of risk under the purview of NRAMP.

Since the inception of NRAMP in March 1995, various mechanisms have been implemented to gather a broad-based understanding of stakeholder concerns including monthly stakeholder working group meetings (held May 1995 through December 1996), radioactive waste management focus group meetings (ongoing since October 1997), a Nevada public opinion survey⁽³⁾, interactions with the Community Advisory Board for NTS Programs and the DOE Nevada Operations Office (DOE/NV), UNLV Continuing Education courses, and special lectures and seminars. Short- and long-term risks to the public and environment from radioactive waste management at the NTS was a clearly identifiable issue of concern to public stakeholders.

There are two operational Radioactive Waste Management Sites (RWMSs) located on the NTS, one in Area 3 and another in Area 5. The Area 3 RWMS, see Fig. 1, is located in south-central Yucca Flat which is an alluvium-filled valley. The waste cells are seven nuclear subsidence craters (subsidence craters are formed by the collapse of the ground into a nuclear test cavity) which were chosen because (1) of the density of the alluvium chimney, (2) their nuclear detonation points were located at greater than 500 feet above the groundwater table, and (3) the groundwater table is approximately 1500 feet below the surface in Area 3. It has been disposing of mostly bulk low-level radioactive waste from the cleanup of NTS sites previously contaminated by atmospheric testing since the disposal site opened in the mid-1960s. Several years later, Area 3 began receiving waste from off-site Defense Waste generators. Wastes are mainly in the form of contaminated soil, concrete, and scrap metal debris from nuclear test operations.

The Area 5 RWMS, see Fig. 2, is located on Frenchman Flat which is a closed alluvium-filled basin in the southeastern corner of the NTS with a minimum depth to the groundwater table of 770 feet. This RWMS has accepted non-bulk packaged low-level radioactive waste (LLRW) from on-site since 1961 and began accepting off-site DOE waste in 1976. The Area 5 RWMS also houses the Greater Confinement Disposal (GCD) boreholes which contain primarily high activity wastes (such as tritium and ⁹⁰Sr) and transuranic (TRU) wastes which are not suitable for disposal in the Waste Isolation Pilot Plant near Carlsbad, New Mexico.

While stakeholder interactions were progressing, the NRAMP staff was developing risk assessment methodologies that could answer some of the questions (i.e., those of a technical nature) being posed. Since the components of a scientific risk assessment (such as source term characterization; contaminant transport, exposure, and dose modeling; and risk evaluation) are still under development, refinement, and peer review, the preliminary risk assessment presented in this paper utilized a Maximally Exposed Hypothetical Individual (MEHI) risk concept. This concept is a type of worst-case analysis in which (1) individuals (defined by Reference Man⁽⁴⁾) conduct typical lifestyle activities at contaminated sites without regard to the contamination and (2) these hypothetical activities occur at a location of maximum exposure to the contamination source under the prescribed scenarios (e.g., 70-year lifetime chronic exposure). The MEHI risk concept is used for the following reasons:

• The DOE sites of concern in Nevada are controlled to restrict public and private use and are not currently available for general public use. Therefore, risk assessments are hypothetical and a MEHI concept allows a consistent methodology to relate or rank hazard potentials based on potential public future land uses.
• If a present danger to a public group were to be identified, that issue would take precedence over the MEHI issue. The MEHI evaluation could facilitate the identification of such dangers if they exist and have not yet been identified.
• Analyzing risks based on population is unacceptable to the Nevada public since an understanding of the dangers would be clouded by the small absolute risk values produced by a sparse population.
• A preliminary MEHI risk assessment provides a first step in the iterative process of risk assessment in light of insufficient analytical data to perform a detailed risk assessment (an ongoing NRAMP task).
• The analyses of worst-case scenarios provide insight on the location, timing, and severity of hazards.
• A MEHI evaluation provides insight for risk management discussion via a relative risk comparison with other radiologic source term categories (e.g., groundwater and surface soil contaminated by nuclear testing).

Some precautions in using the MEHI risk concept are:

• MEHI scenarios are analyzed under the maximal exposure assumption without regard to the probability of the scenario actually occurring.

• Risk evaluations are not applicable to the public at large, but are designed to be conservative (protective) estimations of the extent of the dangers.
• Values obtained are not all maximal values for affected receptors because incidence of risk falls differently among different population subgroups, such as high-sensitive groups like pregnant women and children.
• The MEHI risk differs greatly from the definition of risk used in many other risk assessments.
• Land-use scenarios do not consider final closures that must be approved by appropriate federal and state agencies.

Public land use scenarios formulated by stakeholder discussions consider occasional (e.g., cultural and recreational) use and continual (e.g., industrial, ranching, and residential) use. MEHI exposures were assumed at the NTS site boundary at the present time and on the RWMS locations in 100 years. The latter date is a result of an arbitrary but generally accepted assumption for the date of site release for public access since the assurance of federal institutional control beyond 100 years was not acceptable to stakeholders. Examples of these public land use scenarios are given in Table I. Input parameters to the Multimedia Environmental Pollutant Assessment System (MEPAS) transport and exposure code representing these scenarios are given in Table II. The detailed methodology used in this study is contained in Volume II of the NRAMP Preliminary Risk Assessment⁽⁷⁾.

The term risk has many technically precise as well as colloquial meanings. The risk terminology used in this assessment is defined in Table III and reflects discussions with a variety of stakeholder groups. The nominal probability risk factor for a fatal cancer⁽⁸⁾ is used in calculating risks because this is the detrimental effect expected to generate the highest risk value and the linear no-threshold assumption for radiation protection provides an easy method to evaluate relative risk. Other detrimental effects, such as genetic mutation, will have a lower incremental lifetime risk value. The dividing lines listed in Table III (e.g., between Medium and High risk) have relation to regulatory and protection standards such as those recommended by the International Commission on Radiological Protection⁽⁸⁾.

The above components of a radiological risk assessment have undergone scientific peer review⁽⁹⁾ and were part of a standardized methodology used by NRAMP staff for preliminary risk assessments⁽⁷⁾. Where possible, radiological exposure parameters were taken directly from available DOE Performance Assessments ^{(6, 10, 11)}. Most of the RWMS contamination source data available for this analysis are from performance assessments and inventory reports published by DOE. Tables IV and V list the inventories from these reports considering decay to the 100-year future time frame. Supplemental information such as waste volume and configuration was provided by personal conversations with DOE/NV personnel. However, much information concerning the complete RWMS radionuclide inventory is still unavailable because of inadequate record-keeping practices in the past and the fact that wastes disposed of before 1988 were not required to be part of DOE Performance Assessments. The inventories and doses used in this study are based on as much information as could be reliably obtained. For some cases, the actual exposures were not calculated, but were inferred based on the known results of other calculations and a few additional assumptions. For example, the cultural, recreational, and industrial land use scenarios were considered to be equivalent because they do not disturb (infiltrate into) the waste zone.

Another difficulty in determining the source term is that the RWMSs are still actively accepting new waste from cleanup projects in and outside of Nevada, and the source term is constantly changing. Therefore, in this study, in addition to existing contamination, a worst-case disposal site filled to capacity is also analyzed in order to perform risk assessments for the future time frame.