HEAT-GENERATING RADIONUCLIDE WASTE SIMULATIONS TO ASSESS THE VIABILITY OF SUBSURFACE STORAGE IN THE NEGEV DESERT, ISRAEL

As part of a collaboration between the U.S. National Nuclear Security Administration (NNSA) and the Israel Atomic Energy Commission (IAEC), we are investigating the viability of subsurface nuclear waste disposal in the Negev Desert, Israel. We present flow and transport simulation results on the effect of heat generating nuclear waste in a proposed intermediate depth borehole located between depths of 150-300 m in the vadose zone. Thermal loading caused by radioactive decay leads to changes in saturation driven in part by water vapor transport. Thermally driven phase changes from liquid to vapor cause drying, which can locally modify hydraulic properties and lead to the thermal decomposition of the kerogen present in bituminous carbonate rocks into hydrocarbons. To assess the time- and temperature-dependent changes in proximity to the borehole, we developed a thermal loading model using the Los Alamos multiphase porous flow and transport simulator FEHM. Thermal loading values associated with spent nuclear fuel were collected from literature and rock property measurements (e.g., thermal conductivities) on carbonates provided by Sandia National Laboratory and the Geological Survey of Israel. To reduce computational complexity, heat generation is simulated over 30 years on a two-dimensional radial mesh that preserves three dimensional volumes. The heat generating waste within the borehole segment investigated (0.5 m radius, 25 m length) is assigned an energy boundary condition that diminishes over time in a stepwise function, replicating the time-dependent decay heat of spent nuclear fuel. Peak borehole temperatures are achieved at @10 years. Nearfield temperature gradients show a strong dependence on the thermal conductivity of the surrounding lithologies and the radioactivity of the source. Our simulations include results on two fractured marine lithologies (bituminous marl and chert) where fracture flow has a significant effect on advective transport. We compare simulations using both a generalized dual porosity model and a simpler equivalent continuum model for fracture flow. Compared to the equivalent continuum approach, the dual porosity fracture model results in differences in the saturation and vapor flow fields, with faster dry-out times and greater vapor phase mass transport.