Thermal-Hydrologic Modeling for the Disposal of Spent Nuclear Fuel in a Generic Repository in Crystalline Host Rock

Disposal of commercial spent nuclear fuel in a generic geologic repository in crystalline host rock is studied. The analysis supports repository thermal design and overall barrier capability. Commercial spent nuclear fuels contain significant thermal output which may result in higher temperatures and vapor migration in the near field. This could impact repository stability and enhance radionuclide transport to the accessible environment. The aim of this study is to quantify the effect of higher thermal output fuels on fluid flow and heat transfer in the engineered barrier system. In this first study the host rock is represented by average material properties. Future studies will include representation of the fractured rock using discrete fracture network and fracture continuum models.

For the modeling work the geologic repository was assumed to be in crystalline rock at 500 m depth. The study looked at a portion of the repository layout assuming symmetry conditions. This allows a detailed thermal analysis of the disposal area and its surroundings. A highly refined mesh was utilized with refinements near heat sources and at intersections of the different materials. The modeling work includes sensitivity analysis using a range of parameter values for properties of components of the engineered barrier system (i.e. buffer, disturbed rock zone and the host rock). The simulations also looked at the effect of surface storage times to reduce thermal output of the nuclear waste. Results of the different modeling cases are presented and include temperature and fluid flow distributions in the near field at different simulation times. Time history results are also presented at specified locations.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. SAND2018-8427 A.