Safety of the low-level waste disposal facilities, as well as al US DOE facilities, is a primary criterion in their design and operation. Safety of low-level waste disposal facilities is evaluated from two perspectives. Operational safety is evaluated based on the perceived level of hazard of the operation. The safety evaluations vary from simple safety assessments to very complex safety analysis reports, depending on the degree of hazard associated with the facility operation. Operational requirements for the Department's low-level waste disposal facilities, including long-term safety are contained in DOE Order 5820.2A, Radioactive Waste Management (1). This paper will focus on the process of conducting long-term performance analyses rather than on operational safety analysis.

Department of Energy (DOE) facilities, located at sites across the nation, generate large quantities and a wide variety of low-level radioactive waste (LLRW) from nuclear defense production and research and development activities. All DOE-generated LLRW is disposed of at DOE disposal sites. Most DOE waste generating sites do not have disposal facilities on site and so must ship their LLRW to one of six currently active DOE disposal locations. Four disposal sites are located in generally arid regions: the Hanford Reservation (HANF) in the state of Washington, the Nevada Test Site (NTS), the Idaho National Engineering Laboratory (INEL), and the Los Alamos National Laboratory (LANL) in New Mexico. The other two disposal sites are located in the humid southeast: The Savannah River Plant (SRP) in South Carolina and the Oak Ridge National Laboratory (ORNL) in Tennessee.

The United State Department of Energy (US DOE) disposes of low-level radioactive waste in near-surface disposal facilities. Safety of the disposal operations is evaluated for operational safety as well as long-term safety. Operational safety is evaluated based on the perceived level of hazard of the operation and may vary from a simple safety assessment to a safety analysis report. Long-term safety of all low-level waste disposal systems is evaluated through the conduct of a radiological performance assessment. The US DOE has established radiological performance objectives for disposal of low-level waste. They are to protect a member of the general public from receiving over 25 mrem/y, and an inadvertent intruder into the waste from receiving over 100 mrem/y continuous exposure or 400 mrem from a single exposure. For a disposal system to be acceptable, a performance assessment (PA) must be prepared which must be technically accurate and provide reasonable assurance that these performance objectives are met. Technical quality of the performance assessments is reviewed by a panel of experts. A number of lessons have been learned from conducting several preliminary reviews of performance assessments. These lessons are shared among the various US DOE sites to improve the process of evaluating low-level waste disposal sites for long-term performance.

The Department of Energy's waste management Order, DOE O 435.1, requires that low--level waste disposal facilities develop and maintain a radiological performance assessment to ensure that disposal operations are within a performance envelope defined by performance objectives for long-term protection of the public and the environment. The Order also requires that a radiological composite analysis be developed and maintained to ensure that the disposal facility, in combination with other sources of radioactive material that may remain when all DOE activities have ceased, will not compromise future radiological protection of the public and the environment. The Order further requires that a Disposal Authorization Statement (DAS) be obtained from DOE Headquarters and that the disposal facility be operated within the performance assessment, composite analysis, and DAS. Maintenance of the performance assessment and composite analysis includes conducting test, research, and monitoring activit ies to increase confidence in the results of the analyses. It also includes updating the analyses as changes are proposed in the disposal operations, or other information requiring an update, becomes available. Personnel at the Savannah River Site have developed and implemented an innovative process for reviewing proposed or discovered changes in low-level radioactive waste disposal operations. The process is a graded approach to determine, in a disciplined manner, whether changes are within the existing performance envelope, as defined by the performance assessment, composite analysis, and DAS, or whether additional analysis is required to authorize the change. This process is called the Unreviewed Disposal Question (UDQ) process. It has been developed to be analogous to the Unreviewed Safety Question (UDQ) process that has been in use within DOE for many years. This is the first formalized system implemented in the DOE complex to examine low-level waste disposal changes the way the U nreviewed Safety Question process examines changes in nuclear facility operations. The process, which ensures that proposed or discovered changes receive the appropriate level of review, is now being used whenever changes such as new waste streams or changes in the design of a waste disposal unit are proposed at SRS. The process involves going through a series of questions to ensure that the change is within the existing performance envelope. Two series of questions are used. The first is a simple screening process. If the change is obviously within the performance envelope, it will be screened from further evaluation. If it cannot be screened, technical personnel involved in the performance assessment, composite analysis, and DAS processes, perform a more detailed evaluation using the second set of questions. If the evaluation shows that the change is within the performance envelope, it can be approved within the contractor's organization. If the evaluation does not clearly conclude t hat the change is within the performance envelope, then a Special Analysis or other more extensive study is triggered. This is a disciplined way to be sure that one knows which changes are significant and which are not, so that the proper attention can be given to the changes that are significant.

The US Department of Energy (DOE) and its contractors have long been pioneers in the field of radiological performance assessment (PA). Much effort has been expended in developing technology and acquiring data to facilitate the assessment process. This is reflected in DOE Order 5820.2A, Radioactive Waste Management Chapter III of the Order lists policy and requirements to manage the DOEs low-level waste; performance objectives for low-level waste management are stated to ensure the protection of public health and the environment. A radiological PA is also required to demonstrate compliance with the performance objectives. DOE Order 5820.2A further requires that an Oversight and Peer Review Panel be established to ensure consistency and technical quality around the DOE complex in the development and application of PA models that include site-specific geohydrology and waste composition. The DOE has also established a Performance Assessment Task Team (PATT) to integrate the activities of sites that are preparing PAs. The PATT's purpose is to recommend policy and guidance to DOE on issues that impact PAs so that the approaches taken are as consistent as possible across the DOE complex.

Information on performance assessment capabilities and activities was collected from eight DOE sites. All eight sites either currently dispose of low-level radioactive waste (LLW) or plan to dispose of LLW in the near future. A survey questionnaire was developed and sent to key individuals involved in DOE Order 5820.2A performance assessment activities at each site. The sites surveyed included: Hanford Site (Hanford), Idaho National Engineering Laboratory (INEL), Los Alamos National Laboratory (LANL), Nevada Test Site (NTS), Oak Ridge National Laboratory (ORNL), Paducah Gaseous Diffusion Plant (Paducah), Portsmouth Gaseous Diffusion Plant (Portsmouth), and Savannah River Site (SRS). The questionnaire addressed all aspects of the performance assessment process; from waste source term to dose conversion factors. This report presents the information developed from the site questionnaire and provides a comparison of site-specific performance assessment approaches, data needs, and ongoing and planned activities. All sites are engaged in completing the radioactive waste disposal facility performance assessment required by DOE Order 5820.2A. Each site has achieved various degrees of progress and have identified a set of critical needs. Within several areas, however, the sites identified common needs and questions.

A performance evaluation (PE) is an analysis that estimates radionuclide concentration limits for 16 potential Department of Energy (DOE) mixed low-level waste (ULLW) disposal sites based on the analysis of two environmental exposure pathways (air and water) to an off-site individual and an inadvertent-intruder exposure pathway. Sites are analyzed for their ability to attenuate concentrations of specific radionuclides that could be released from wastes in a hypothetical ULLW disposal facility. Site-specific data and knowledge are used within a generic framework that is consistent across all sites being evaluated. After estimates of waste concentrations for the three pathways are calculated, the minimum of the waste concentration values is selected as the permissible waste concentration for each radionuclide. The PE results will be used as input to the process for DOE's ULLW disposal configuration. Preliminary comparisons of results from the PE and site-specific performance assessments indicate that the simple PE results generally agree with results of the performance assessments, even when site conditions are complex. This agreement with performance-assessment results increases confidence that similar results can be obtained at other sites that have good characterization data. In addition, the simple analyses contained in the PE illustrate a potential method to satisfy the needs of many regulators and the general public for a simple, conservative, defensible, and easily understandable analysis that provides results similar to those of more complex analyses.