[en] Scenario development and screening is a fundamental part of performance assessment, but its importance in satisfying a standard is sometimes underestimated. The first step in scenario development in support of performance assessment for the standard's containment requirements is to identify a set of potentially disruptive events and processes. This set must be broad enough to allow the identification, as required by the standard, of those processes and events that might affect the disposal system; data can then be collected on the scenarios identified in this step. The standard also requires that releases be estimated for all significant processes and events; thus the final step in scenario development is systematically screening the scenarios, on the basis of their probabilities and consequences, to select those that are important enough to be modeled in detail. In general, a few hundred scenarios for the release of radionuclides from a nuclear-waste repository can be identified, but only a few of these can or should be modeled in detail. Without a description of the broad suite of scenarios that were originally considered, the regulator and the public would have difficulty in determining whether consequence modeling has been carried out for the appropriate processes and events. 17 refs

[en] Nine release phenomena - normal flow of water, tectonic disturbance of the fracture network, intersection of a new fault with the repository, glaciation, fluvia erosion, thermomechanical disturbances, subsidence, seal failure, and drilling - give rise to 318 preliminary scenarios for the release of waste from a hypothetical high-level-waste repository in basalt. The scenarios have relative probabilities that range over several orders of magnitude. The relative probabilities provide a means of screening the scenarios for the more limited set to be subjected to consequence analysis. Lack of data and of preliminary modeling, however, lead to great uncertainties in the highly conservative probabilities assigned here. As a result, the recommendations in this report are directed at resolution of the major uncertainties in the relative probabilities of the preliminary scenarios. The resolution of some of the uncertainties should help in the selection of the suite of scenarios for final consequence analysis. 38 references, 22 figures, 18 tables

[en] The WIPP water-balance study area defined here comprises approx.2000 mi² in Eddy and Lea Counties, southeastern New Mexico. Inflows to the study area are precipitation (roughly 1.47 x 10⁶ ac-ft/y), surface water (roughly 1.1 x 10⁵ ac-ft/y), water imported by municipalities and industries (roughly 3 x 10⁴ ac-ft/y), and ground water (volume not estimated). Outflows from the area are evapotranspiration (roughly 1.5 x 10⁶ ac-ft/y), surface water (roughly 1.2 x 10⁵ ac-ft/y), and possibly some ground water. The volume of surface and ground water in storage in Nash Draw has increased since the beginning of potash refining. Regional ground-water flow in aquifers above the Salado Formation is from the northeast to the southwest, although this pattern is interrupted by Clayton Basin, Nash Draw, and San Simon Swale. The Pecos River is the only important perennial stream. Most of the area has no integrated surface-water drainage. The available data suggest that approx.1600 mi² of the study area are hydrologically separate from Nash Draw and the WIPP site. Ground water north of Highway 180 apparently discharges into Clayton Basin and evaporates. Water in San Simon Swale apparently percolates downward and flows to the southeast. Data are inadequate to create a water budget for the Nash Draw-WIPP site hydrologic system alone, although an attempt to do so can provide guidance for further study

[en] Many scenarios of interest for a repository in the Pasco Basin begin with glaciation. Loading and unloading of joints and fractures due to the weight of ice sheets could affect the hydrologic properties of the host rock and surrounding units. Scoping calculations performed using two-dimensional numerical models with simplifying assumptions predict stress changes and uplift or subsidence caused by an advancing glacier. The magnitudes of surface uplift and subsidence predicted by the study agree well with previous independent predictions. Peak stress unloading near the repository horizon is a small fraction of the ambient stress. Any resultant aperture increase is likewise small. Based on the results of this study, mechanical loading caused by a glacier is expected to have a minimal effect on rock permeability, assuming that the excess compressive loads do not crush the rock. 13 refs., 3 figs., 1 tab

[en] Commercial nuclear-power plants have generated large volumes of high-level radioactive waste (HLW) since the late 1950s. These wastes are highly toxic and will remain so for thousands of years, necessitating a safe and permanent method of isolating the wastes from the biosphere. The Nuclear Waste Policy Act of 1982 requires that all HLW be permanently isolated in deep geologic repositories. While the Department of Energy (DOE) will design and construct the repositories, disposal will be governed by the regulatory criteria and standards of the Nuclear Regulatory Commission and the Environmental Protection Agency. This paper presents an overview of the disposal of nuclear waste in deep geological repositories

[en] The Department of Energy (DOE) is actively pursuing the disposal of transuranic (TRU) waste in a geologic environment at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico. The accepted Standard is divided into two subparts. Supart A limits annual radiation doses to members of the public from waste management and storage operations at disposal facilities, and Subpart B limits cumulative releases of radioactive materials for 10,000 years, radiation doses to members of the public for 1000 years, and radioactive contamination of certain sources of ground water for 1000 years, as a result of disposal of wastes. The WIPP Project will assess compliance with Subpart A primarily by means of an extensive monitoring program. The Project will assess compliance with Subpart B primarily using performance-assessment techniques developed for the evaluation of the performance of high-level-waste repositories. Compliance with the Assurance Requirements and the Ground Water Protection Requirements will be assessed using other techniques. The WIPP Project will be the first to evaluate the compliance of a mined geologic repository with the EPA Standard. Although Subpart B of the Standard was remanded to EPA by the United States Court of Appeals for the First Circuit, this paper discusses the Standard as first promulgated. Compliance plans for the WIPP will be revised as necessary in response to any changes in the Standard. 47 refs., 7 figs

[en] The US Environmental Protection Agency (EPA) has set a probabilistic standard for the performance of geologic repositories for the disposal of radioactive waste. This report treats not only geologic events and processes like fault movement, but also events and processes that arise from the relationship between human actions and geology, like drilling for resources, and some that arise from nongeologic processes that in turn affect a geologic process, like climatic change. It reviews the literature in several fields to determine whether existing probabilistic methods for predicting events and processes are adequate for implementation of the standard. Techniques exist for qualitatively estimating the potential for endowment of portions of earth's crust with mineral resources, but such techniques cannot easily predict whether or not human intrusion will occur. The EPA standard offers explicit guidance for the treatment of human intrusion, however. A complete method for climatic prediction could be assembled from existing techniques, although such a combination has not been tested. Existing techniques to support a probabilistic assessment of tectonic activity and seismic hazard at a repository site should be combined with expert judgment in performance assessments. Depending on the regional setting, either analytic techniques or expert judgment may be appropriate in assigning probabilities to volcanic activity. The individual chapters of this report have been cataloged separately

[en] This report describes the construction and preliminary analysis of nearly 4000 scenarios for the release of radioactive waste from a hypothetical repository at Yucca Mountain, Nevada Test Site. Preliminary analyses were carried out for four rock units: the Topopah Spring Member of the Paintbrush Formation, the bedded tuffs of the Calico Hills, and the Bullfrog and Tram Members of the Crater Flat Formation. Only a few of the scenarios were found to have appreciable probabilities of occurrence. Preliminary modeling of certain possible release mechanisms shows that convective cells can form in saturated tuff. The scenarios can be used to guide future consequence analyses and exploratory programs

[en] This report describes the construction and preliminary analysis of 430 scenarios for the release of radioactive waste from a hypothetical repository located in or near the Nevada Test Site. These scenarios will be used to guide future consequence analyses. The preliminary analysis of each scenario was carried out separately for each of ten different rock types. It suggests that the flow of ground water is of great importance to the long-term performance of the repository and that many of the scenarios are more likely in some of the rock types than in others

[en] The US Environmental Protection Agency has set a standard for the performance of geologic repositories for the disposal of high-level radioactive waste. The standard is divided into several sections, including a section on containment requirements. The containment requirement is probabilistic, in that it allows certain small amounts of radioactive waste to be released at high probabilities and larger amounts to be released at lower probabilities. The US Nuclear Regulatory Commission is responsible for implementing the standard. Implementation of the standard will probably involve development and screening of scenarios, assignment of probabilities to the scenarios, determination of consequences of the scenarios, and analysis of uncertainties. Scenario development consists of first, identifying events and processes that could initiate waste releases or affect waste transport, and second, combining the events and processes in physically reasonable ways. Scenarios can be screened on the basis of low probabilities or consequences. Consequences of scenarios are estimated using a series of models that simulate the movement of radionuclides out of the waste package and underground facility and the transport of the radionuclides by ground water or other means to the accessible environment. Sensitivity and uncertainty analysis examines the sources and effects of uncertainties on the calculations. This document uses a simple example to illustrate techniques for the implementation of the standard