EVALUATION OF LARGE-SCALE TEMPERATURE GRADIENTS TO SUPPORT ASSESSMENT OF CONVECTION AND COLD-TRAP PROCESSES IN HEATED DRIFTS

摘要

This paper provides estimates of large-scale temperature gradientsthat can be used to support modeling of natural convection andcold-trap processes in thermally perturbed drifts. Temperature influencesestimates of the quantity and chemistry of water contactingengineered components (e.g., drip shields and waste packages) in apotential high-level waste repository at Yucca Mountain, Nevada.Temperature gradients drive natural convection and cold-trapprocesses, which could affect the distribution, quantity, and chemistryof liquid-phase water contacting drip shields (waste packages). Theconfluence of high temperatures, liquid-phase water, and high chloridemay lead to corrosion of drip shields and waste packages. Anoverview of the evolution of environmental conditions in thermallyperturbed drifts is presented to provide a context for the estimates oftemperature gradients.A mountain-scale conduction-only model is used to estimatetemperature gradients and temperature differences for a drift locatedat the center of a potential repository. Thermohydrologic models andheat transfer algorithms are used to evaluate the reasonableness ofthe conduction-only results. Estimates of temperature using an in-driftheat transfer algorithm account for the possibility of drift degradationand formation of a rubble pile on the drip shield. Temperature differencesbetween the center and ends of the drift are estimated to rapidlyincrease to 80 °C soon after repository closure. That temperaturedifference decreases over time to 15 °C after 10,000 years. Localtemperature gradients are generally estimated to be less than 0.2°C/m, though the gradients can be much larger near changes in lithologyand near the end of the drift. The estimates of temperature gradientsdo not account for intermediate- and small-scale variations,such as those influenced by differences in heat loads between wastepackages, intra-layer variations in thermal properties of the surroundingfractured rock, or localized drift degradation.