[en] The greater confinement disposal (GCD) project is an ongoing project examining the disposal of orphan wastes in Area 5 of the Nevada Test Site. One of the major tasks for the project is performance assessment. With regard to performance assessment, a preliminary conceptual model for ground-water flow and radionuclide transport to the accessible environment at the GCD facilities has been developed. One of the transport pathways that has been postulated is diffusion of radionuclides in the liquid phase upward to the land surface. This pathway is not usually considered in a performance assessment, but is included in the GCD conceptual model because of relatively low recharge estimates at the GCD site and the proximity of the waste to the land surface. These low recharge estimates indicate that convective flow downward to the water table may be negligible; thus, diffusion upward to the land surface may then become important. As part of a preliminary performance assessment which considered a basecase scenario and a climate-change scenario, a first approximation for modeling the liquid-diffusion pathway was formulated. The model includes an analytical solution that incorporates both diffusion and radioactivity decay. Overall, these results indicate that, despite the configuration of the GCD facilities that establishes the need for considering the liquid-diffusion pathway, the GCD disposal concept appears to be a technically feasible method for disposing of orphan wastes. Future analyses will consist of investigating the underlying assumptions of the liquid-diffusion model, refining the model is necessary, and reducing uncertainty in the input parameters. 11 refs., 6 figs

[en] Site characterization is an integral component of any environmental restoration project; however, it is often difficult to know how to prioritize site characterization activities. In the absence of a preliminary analysis, site characterization decisions are sometime guided by little more than intuition. The objective of this paper is to show that a performance assessment methodology used very early in a project can be a useful tool for guiding site characterization activities. As an example, a preliminary performance assessment for the greater confinement disposal (GCD) project is used to demonstrate implementation of the methodology. The GCD site is a waste disposal site in Frenchman Flat on the Nevada Test Site. The GCD site is so named because the disposal strategy (in which the waste is placed at the bottom of 120-ft holes) provides greater confinement of wastes from the accessible environment than the shallow land burial pits that are also used at this waste management site. At this site, transuranic wastes and low-level wastes unsuitable for shallow land burial are stored in unsaturated alluvium at ∼650 ft above the water table

[en] The performance of the Nevada Test Site Greater Confinement Disposal (GCD) site was assessed against the Environmental Protection Agency's requirements for disposal of high-level waste, spent fuel and transuranic waste, 40 CFR Part 191. No new data were collected for this assessment. This preliminary assessment indicates that the GCD concept is a viable method of disposing of transuranic wastes. The results are being used to direct future site characterization activities

[en] The U.S. Department of Energy (DOE) is responsible for disposing of a variety of radioactive and mixed wastes, some of which are considered special-case waste because they do not currently have a clear disposal option. It may be possible to dispose of some of the DOE's special-case waste using greater confinement disposal techniques at the Nevada Test Site (NTS). The DOE asked Sandia National Laboratories to investigate this possibility by performing system configuration analyses. The first step in performing system configuration analyses is to estimate the characteristics of special-case waste that might be destined for disposal at the NTS. The objective of this report is to characterize this special-case waste based upon information available in the literature. No waste was sampled and analyzed specifically for this report. The waste compositions given are not highly detailed, consisting of grains and curies of specific radionuclides per cubic meter. However, such vague waste characterization is adequate for the purposes of the system configuration task. In some previous work done on this subject, Kudera et al. [1990] identified nine categories of special-case radioactive waste and estimated volumes and activities for these categories. It would have been difficult to develop waste compositions based on the categories proposed by Kudera et al. [1990], so we created five groups of waste on which to base the waste compositions. These groups are (1) transuranic waste, (2) fission product waste, (3) activation product waste, (4) mobile/volatile waste, and (5) sealed sources. The radionuclides within a given group share common characteristics (e.g., alpha-emitters, heat generators), and we believe that these groups adequately represent the DOE's special-case waste potentially destined for greater confinement disposal at the NTS

[en] The current management system in the United States for commercial spent nuclear fuel does not emphasize integration among storage, transportation, and disposal. The large containers in use for dry-storage remain at high temperatures for decades and, thereby, delay removal from decommissioned reactors. The large containers also have no easy path to disposal unless (1) disposal is delayed (up to 150 years for some geologic media); (2) the contents are repackaged into smaller, cooler packages; or (3) the high temperatures are used as de facto site-selection criteria for a repository. Implementing consolidated interim storage could address many issues that exist because of this lack of integration. A consolidated interim storage facility that includes appropriate capabilities acts as a universal coupler that allows existing disparate parts to integrate as a system. Previous agencies and commissions have noted this theme before as a way to provide flexibility in the waste management system. This rationale is elaborated upon here. (author)

[en] This report culminates work performed by Sandia National Laboratories (SNL) for the US Nuclear Regulatory Commission (NRC) under FIN A1165 (Technical Assistance for Performance Assessment) on uncertainties associated with performance assessment of HLW repositories. The purpose of this report is to summarize the work in the topical area of uncertainty conducted under FIN A1165. Many different types of uncertainty can affect the performance of an HLW repository. In a performance assessment, these uncertainties should be identified and considered, and to the extent practicable, should be quantified and reduced. Conventionally, the different types of uncertainty are classified in three major categories: uncertainty in the future state of the disposal system; uncertainty in models needed to simulate the behavior of the disposal system; and uncertainty in data, parameters, and coefficients needed for the analysis of the system. All three major categories of uncertainty are covered in this report. Only a short overview is presented with numerous references to SNL reports where different uncertainty topics are discussed in detail; as such, this report is not a stand-alone report. The report can be used by managers to familiarize themselves with the issues regarding uncertainty in HLW repository performance and technical staff as a review of SNL's work for NRC in this area. 52 refs

[en] The Department of Energy's Nevada Operations Office (DOE/NV) has disposed of a small quantity of transuranic waste at the Greater Confinement Disposal facility in Area 5 of the Nevada Test Site. In 1989, DOE/NV contracted with Sandia National Laboratories to perform a preliminary performance assessment of this disposal facility. This preliminary performance assessment consisted of analyses designed to assess the likelihood of complying with Environmental Protection Agency standards for the disposal of transuranic waste, high level waste, and spent fuel. The preliminary nature of this study meant that no other regulatory standards were considered and the analyses were conducted with specific limitations. The procedure for the preliminary performance assessment consisted of (1) collecting information about the site, (2) developing models based on this information, (3) implementing these models in computer codes, (4) performing the analyses using the computer codes, and (5) performing sensitivity analyses to determine the more important variables. Based on the results of the analyses, it appears that the Greater Confinement Disposal facility will most likely comply with the Environmental Protection Agency's standards for the disposal of transuranic waste. The results of the sensitivity analyses are being used to guide site characterization activities related to the next iteration of performance assessment analyses for the Greater Confinement Disposal facility

[en] A small amount of transuranic (TRU) waste has been disposed of at the Greater Confinement Disposal (GCD) site located on the Nevada Test Site's (NTS) Radioactive Waste Management Site (RWMS). The waste has been buried in several deep (37 m) boreholes dug into the floor of an alluvial basin. For the waste to remain in its current configuration, the DOE must demonstrate compliance of the site with the TRU disposal requirements, 40 CFR 191. Sandia's approach to process modelling in performance assessment is to use demonstrably conservative models of the site. Choosing the most conservative model, however, can be uncertain. As an example, diffusion of contaminants upward from the buried waste in the vadose zone water is the primary mechanism of release. This process can be modelled as straight upward planar diffusion or as spherical diffusion in all directions. The former has high fluxes but low release areas, the latter has lower fluxes but is spread over a greater area. We have developed analytic solutions to a simple test problem for both models and compared the total integrated discharges. The spherical diffusion conceptual model results in at least five times greater release to the accessible environment than the planar model at all diffusivities. Modifying the planar model to allow for a larger release, however, compensated for the smaller original planar discharge and resulted in a new planar model that was more conservative that the spherical model except at low diffusivities

[en] The relative apparent electron-excitation cross sections sigma (v)/sigma (0) for vibrational levels 1 less than or equal to v less than or equal to 4 of the N₂⁺ B ²Σ/subu/⁺ state were measured over the temperature range of 300--900 K using an electron beam of 10-keV energy. Enhancement was observed for all temperatures investigated for the values of sigma (v)/sigma (0) for v = 2, 3, and 4 relative to the values predicted using emission band strengths and excitation Franck-Condon factors. The values of sigma (1)/sigma (0) agreed with calculated values to within experimental uncertainty. The apparent cross sections were determined for the N (II)levels 2p (²P/supo/)3p, 2p (²P/supo/)3d, and 2p (²P/supo/)4f and the 2p²(³P)4p level of N (I). To the accuracy of the measurement no temperature dependence of the atomic cross sections was observed

[en] In the United States, radioactive wastes are conventionally classified as high-level wastes, transuranic wastes, or low-level wastes. Each of these types of wastes, by law, has a ''home'' for their final disposal; i.e., high-level wastes are destined for disposal at the proposed repository at Yucca Mountain, transuranic waste for the proposed Waste Isolation Pilot Plant, and low-level waste for shallow-land disposal sites. However, there are some radioactive wastes within the United States Department of Energy (DOE) complex that do not meet the criteria established for disposal of either high-level waste, transuranic waste, or low-level waste. The former are called ''special-case'' or ''orphan'' wastes. This paper describes an ongoing project sponsored by the DOE's Nevada Operations Office for the disposal of orphan wastes at the Radioactive Waste Management Site at Area 5 of the Nevada Test Site using the greater confinement disposal (GCD) concept. The objectives of the GCD project are to evaluate the safety of the site for disposal of orphan wastes by assessing compliance with pertinent regulations through performance assessment, and to examine the feasibility of this disposal concept as a cost-effective, safe alternative for management of orphan wastes within the DOE complex. Decisions on the use of GCD or other alternate disposal concepts for orphan wastes be expected to be addressed in a Programmatic Environmental Impact Statement being prepared by DOE. The ultimate decision to use GCD will require a Record of Decision through the National Environmental Policy Act (NEPA) process. 20 refs., 3 figs., 2 tabs