IMPACT OF HEAT ON GROUNDWATER FLOW NEAR UNDERGROUND NUCLEAR TESTS

Up to 90% of the energy of an underground nuclear test is released as heat. Approximately one-third of the underground nuclear tests conducted at the Nevada Test Site were detonated below the water table. Numerical simulation using the coupled heat and groundwater flow model NUFT enables evaluation of the impact of test-related heat on groundwater flow over timeframes of days to centuries. For the underground nuclear tests named “Cheshire” and “Almendro,” both detonated in the 1970s in volcanic rocks hundreds of meters below the water table, several temperature logs were obtained at different times from post-test drill-back boreholes penetrating test-altered zones. Measured temperatures near test-generated melt glass zones reached as high as 145 degrees C for Cheshire five months after detonation and over 220 degrees C for Almendro two years after detonation. Additionally, a temperature log obtained from a downgradient monitoring well 11 years after the Cheshire detonation detected groundwater heated about 1 degrees C above background within a permeable zone located 300 m laterally and 400 m above the Cheshire test location. These temperature data proved useful for calibration of thermal groundwater flow simulations, particularly for estimation of the permeability of in-situ rocks and the melt glass zone. Multi-phase flow simulations indicate boiling conditions persisted for days at Cheshire and years at Almendro. Calibrated flow simulations show test-related heat can generate vigorous convection cells and a strong upward component of groundwater flow that can persist from years to decades. Importantly, inclusion of thermal effects on groundwater flow was necessary to produce simulations of radionuclide transport behavior consistent with observations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.