Publications

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An in-package chemistry model is presented to calculate pH in the pore space of degradation products inside a breached waste package in the unsaturated environment of the Yucca Mountain repository. The pH is calculated as a function of liquid influx rate, partial pressure of carbon dioxide, solid-water volume ratio in the porous degradation products (provided by a coupled water balance model), and the relative rate of steel and waste form degradation. The EQ3/6 code is used to calculate pH at high liquid influx rates and zero liquid influx rates (vapor influx only). For mid-range liquid influx rates, a Damkohler ratio is defined and used to interpolate between the pH values calculated at the two extremes. This approach allows the in-package pH to be calculated over broad ranges of key parameters in a total system performance assessment.

The U.S. is currently re-evaluating the policy on high-level radioactive waste (HLW) management and has been studying generic disposal system environment (GDSE) concepts to support the development of a long-term strategy for geologic disposal of HLW. The GDSE study focuses on the analysis of different GDSE options, and a salt repository is one of the options currently under study. The immediate goal of the generic salt repository study is to develop the necessary modeling tools to evaluate and improve understanding on the repository system response and processes relevant to long-term HLW disposal in salt. An initial version of the salt GDSE performance assessment model and the preliminary analysis results are discussed, emphasizing key attributes of a salt repository that are potentially important to the long-term safe disposal of HLW. Also discussed are the preliminary results on the repository response to the effects of different waste types (commercial UNF, existing DOE HLW, and hypothetical reprocessing HLW), and radionuclide release scenarios (undisturbed and human intrusion). Soluble, non- to weakly sorbing fission products, particularly 129I, 79Se, and 26Ra are the major dose contributors. However, the conservative assumptions made about their geochemical behaviors contribute to their calculated dose. The paper elaborates on the identified knowledge gaps and path forwards for future R&D efforts to advance understanding of salt repository system performance for HLW disposal.

Deep boreholes have been proposed for many decades as an option for permanent disposal of high-level radioactive waste and spent nuclear fuel. Disposal concepts are straightforward, and generally call for drilling boreholes to a depth of three to five kilometers into crystalline basement rocks. Waste is placed in the lower portion of the hole, and the upper several kilometers of the hole are sealed to provide effective isolation from the biosphere. The potential for excellent long-term performance has been recognized in many previous studies. This paper reports updated results of what is believed to be the first quantitative analysis of releases from a hypothetical disposal borehole repository using the same performance assessment methodology applied to mined geologic repositories for high-level radioactive waste. Analyses begin with a preliminary consideration of a comprehensive list of potentially relevant features, events, and processes (FEPs) and the identification of those FEPs that appear to be most likely to affect long-term performance in deep boreholes. Performance assessment model estimates of releases from deep boreholes, and the annual radiation doses to hypothetical future humans associated with those releases, are extremely small, indicating that deep boreholes may be a viable alternative to mined repositories for disposal of both high-level radioactive waste and spent nuclear fuel.

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