H. Arlt's scientific contributions

Geological repositories designed to isolate high-level radioactive waste need natural and engineered barriers that prevent or slow the release of radioactive elements into the accessible environment to acceptable regulatory limits. Under the U.S. Nuclear Regulatory Commission's (NRC's) regulations, a barrier is any material, structure, or feature that prevents or substantially reduces the rate of movement of water or radionuclides from the repository to the accessible environment. In 1982, Congress passed the Nuclear Waste Policy Act which directed the NRC to include multiple barriers in regulating geologic disposal of high-level radioactive waste. Accordingly, as provided in 10 CFR Part 63, the NRC's regulations for Yucca Mountain require a repository to include multiple barriers to ensure the system is robust and not wholly dependent on any single barrier. Any potential license application to construct a repository at Yucca Mountain must identify the multiple barriers (both natural and engineered), describe the capabilities of each barrier, and provide the technical bases for the capabilities of the barriers. The NRC believes that understanding the capability of the repository's component barriers improves understanding of the overall system. The objective of this paper is to discuss potential natural barriers of the geosphere at Yucca Mountain and describe the NRC regulatory requirements for such barriers. To better understand the natural barriers of the geosphere, it helps to divide the barriers into groups of features and their associated processes. Natural barriers, i.e., barriers not constructed by man, ideally include processes that delay the transport of radionuclides from reaching the accessible environment or limit the amount of water that can seep from a ground surface to the depth of an underground repository. Natural barriers at Yucca Mountain may include: topographic influences on precipitation runoff; soil and plants influences on evaporation and transpiration; effect of surface bedrock characteristics on infiltration; influences of unsaturated zone rocks above the repository on quantity and characteristics of downward flowing water; and the effects of the repository tunnel wall geometry and rock characteristics on seepage into the tunnels. Unsaturated zone rocks below the repository may influence water and radionuclide migration into either fractures or the rock matrix, where processes such as matrix diffusion and sorption can retard radionuclide movement. Properties of different saturated zone rock units may slow the radionuclide flow rate while structural features within the saturated zone rocks (faults, heterogeneities) control water flow rate and direction. The saturated zone alluvium may reduce the water velocity while radionuclides sorption onto the alluvium can further delay radionuclides from reaching the accessible environment.

The Topopah Spring Tuff is the host rock for a proposed repository for high-level nuclear waste. Underground tunnels and alcoves in this tuff that have been sealed from ventilation provide potentially useful data on natural moisture conditions and can help address the question of whether significant amounts of percolating groundwater drip into tunnels under present-day conditions. Given the low infiltration rates in the region, natural seepage and dripping in the sealed tunnels would provide evidence of focused flow within fracture networks that could be used to help calibrate seepage models for present-day conditions. These observations can then be used to estimate seepage fluxes during future, wetter climates. In 1999 the Department of Energy (DOE) sealed a nearly 1-km long tunnel bored near the proposed repository area. Four bulkheads isolate four sections of this tunnel, commonly called the Cross Drift, to allow a return to natural, ambient moisture conditions. Alcove 7, which crosses the Ghost Dance Fault, is a niche that has also been sealed with a bulkhead. Observations made in the sealed tunnels under unventilated conditions help to ensure that moisture observations will be little affected by the rapid drying effects of ventilation. Evidence of humid conditions has been seen during such unventilated entries, including small puddles apparently produced by condensation dripping. DOE is attempting to systematically collect drips in sample bottles and in plastic sheets so that chemical analyses can be used to identify sources of the water (i.e., natural seepage, condensation, or a mixture). To date two locations of possible natural seepage have been observed: one in Alcove 7 and the other in a sealed section of the Cross Drift. Both of these drip zones occur outside the proposed repository footprint. DOE is continuing work in the sealed drifts to address agreements with NRC. Hydrologic data from the sealed tunnels provide a reference point for DOE's performance assessments of deep percolation and seepage. The NRC staff believes that long-term empirical observations in sealed tunnels could be an element of a performance confirmation plan for Yucca Mountain.

To evaluate the suitability of Yucca Mountain, Nevada, as a potential nuclear waste repository, the U.S. Department of Energy (DOE) conducts total-system performance assessment analyses. The saturated zone flow and transport system is one component of the natural barriers to radionuclide transport. To include saturated zone flow and transport in total-system performance assessment analyses, DOE abstracted flow paths from their site-scale saturated zone flow model. The U.S. Nuclear Regulatory Commission (NRC) staff, with assistance from the Center for Nuclear Waste Regulatory Analyses (CNWRA) staff, are responsible for reviewing the DOE saturated zone process model and total-system performance assessment analyses abstraction to assure that the DOE approach is justified by available data, that data and modeling uncertainties are appropriately considered, and that reasonable alternative conceptual models are considered. Review of the DOE approach and the development of an independent total-system performance assessment analyses abstraction necessitates an in-depth understanding of saturated zone hydrogeology at Yucca Mountain and a means to independently evaluate model and data uncertainties and potentially important alternative conceptual models for saturated zone flow. To this end, the CNWRA staff are developing a three-dimensional groundwater flow model of the Yucca Mountain, Nevada, region. The foundation of this flow model is the CNWRA hydrogeologic framework model, which was also developed independently from the DOE model. The insights gained through such independent model development are useful for a risk-informed review of DOE models. The CNWRA flow model can be used as a tool to evaluate the potential effects of various data and model uncertainties on saturated zone flow paths. Those evaluations can then be used for comparison with the level of uncertainty considered in the DOE performance assessments resulting from factors such as groundwater specific discharge (flux) and flow path lengths through various material types. In addition, the 3D groundwater flow model provides a means for evaluating the sensitivity of simulated flow paths, groundwater travel time, and capture zones and drawdowns of pumping wells, to the underlying interpretation of geologic structures and hydrogeological features. The CNWRA flow model provides the NRC and CNWRA staffs with a tool to help resolve key technical issues pertaining to radionuclide transport via groundwater pathways in the saturated zone.