Nevada System of Higher Education
Previous studies by scientists at Los Alamos National Laboratory (LANL) found elevated ratios of chlorine-36 to total chloride (36C1/C1) in samples of rock collected from the Exploratory Studies Facility (ESF) and the Enhanced Characterization of the Repository Block (ECRB) at Yucca Mountain as the tunnels were excavated. The data were interpreted as an indication that fluids containing "bomb-pulse" 36C1 reached the repository horizon in the -50 years since the peak period of above-ground nuclear testing. Moreover, the data support the concept that so-called fast pathways for infiltration not only exist but are active, possibly through a combination of porous media, faults and/or other geologic features. Due to the significance of 36C1 data to conceptual models of unsaturated zone flow and transport, the United States Geological Survey (USGS) was requested by the Department of Energy (DOE) to design and implement a study to validate the LANL findings. The USGS chose to drill new boreholes at select locations across zones where bomb-pulse ratios had previously been identified. The drill cores were analyzed at Lawrence Livermore National Laboratory (LLNL) for 36C1/C1 using both active and passive leaches, with the USGS/LLNL concluding that the active leach extracted too much rock-Cl and the passive leach did not show bomb-pulse ratios. Because consensus was not reached between the USGS/LLNL and LANL on several fundamental points, including the conceptual strategy for sampling, interpretation and use of tritium (3H) data, and the importance and interpretation of blanks, in addition to the presence or absence of bomb-pulse 36C1, an evaluation by an independent entity, the University of Nevada, Las Vegas (UNLV), using new samples was initiated. This report is the result of that study. The overall objectives of the UNLV study were to investigate the source or sources of the conflicting results from the previous validation study, and to obtain additional data to determine whether or not there are bomb-pulse isotopes at the repository horizon. To that end, we have engaged in discussions with previous investigators, reviewed reports, and analyzed archived samples. We have also collected new samples of rock from the ESF, soil profiles from the surface of Yucca Mountain, and opportunistic samples of seep water from inside the south ramp of the ESF. Our sampling strategy in the ESF was to collect new rock samples in a manner that would optimize our chances of finding a 36C1 bomb-pulse signature, if one was present. Therefore the sampling and analytical methodology that yielded prior bomb-pulse ratios was replicated (to the extent possible). Specific geologic features (e.g., faults, cooling joints) and strategic locations (including the Ghost Dance, Sundance, Bow Ridge and Drill Hole Wash faults) were targeted. Moreover, extreme precautions were taken to collect samples, excavating a meter into the tunnel wall in some cases. Experiments were performed measuring trace elements and anions in leachates as a function of time to help guide our leaching conditions. Samples were analyzed for 36C1/C1 ratios, as well as 99Tc and 129I, two other radionuclides that can be associated with the bomb-pulse, in select samples. Finally, a column experiment was conducted mimicking the passage of bomb-pulse 36C1 through Yucca Mountain tuff (Topopah Spring Tuff middle nonlithophysal unit (Tptpmn)). The work faced several obstacles including an extended shutdown of the tunnel. In addition, some of the data collected early in the study were suspect because of unreasonably high 36C1/C1 ratios. Attempts to pinpoint the cause of the seemingly random and spurious results were unsuccessful. After moving to a different laboratory in a separate building and employing new supplies, control was gained over the background and blank results were consistent and acceptably small. Because of the setbacks only half (seven) of the ESF samples, but all of the soil and column-study samples, were analyzed in the favorable laboratory setting before the project ended. Overall, the experience highlights the challenging nature of the work and the high sensitivity of accelerator mass spectrometry (AMS) for 36C1.
Chlorides; Nuclear weapons – Testing; Nevada – Yucca Mountain; Radioactive pollution; Radioactive pollution of water; Radioactive waste repositories
Earth Sciences | Environmental Indicators and Impact Assessment
Smiecinski, A. J.
Bomb-pulse Chlorine-36 at the proposed Yucca Mountain repository horizon: An investigation of previous conflicting results and collection of new data.
Nevada System of Higher Education
Available at: https://digitalscholarship.unlv.edu/yucca_mtn_pubs/67