Subsurface thermal loading simulations to assess the viability of waste canister disposal in the Negev Desert, Israel

As part of a collaboration between the United States and Israel, we are investigating the viability of subsurface spent nuclear fuel (SNF) disposal in the Negev Desert, Israel. The proposed SNF disposal concept consists of a field of intermediate‐depth boreholes (IDBs) completed to a depth of ∼300 m within the vadose zone. The target geologic repository, the Ghareb Formation, will act as a geologic barrier due to its low permeability and its organic‐rich carbonates with high uranium sorption capacity. SNF will generate heat for thousands of years via radioactive decay and implications of thermal loading must be assessed on the surrounding rock during peak temperatures. Here, we assess deviations of temperature, saturation, and pressure fields due to SNF thermal loading of a single IDB by using the multiphase porous flow and transport simulator, finite element heat and mass transfer code. The kinetic equations to simulate the degradation of organic materials were derived from thermal analysis of Ghareb Formation samples. Since the design of the borehole is not fully realized, including the amount of SNF to be disposed of in each IDB, simulated scenarios including thermal loading, steam and petroleum generation, and the presence/absence of karst features are investigated. Peak temperatures are achieved after 10 years. Near‐field temperature gradients show a strong dependence on the thermal conductivity of surrounding lithologies and the decaying heat source of the simulated waste packages. At temperatures above 265°C, vaporization of pore waters and gas generation from thermal degradation of bituminous marls leads to near lithostatic fluid pressures and probability of hydrofracture.