Thermal Evolution near Heat-Generating Nuclear Waste Canisters Disposed in Horizontal Drillholes

Abstract

We consider the disposal of spent nuclear fuel and high-level radioactive waste in horizontal holes drilled into deep, low-permeable geologic formations using directional drilling technology. Residual decay heat emanating from these waste forms leads to temperature increases within the drillhole and the surrounding host rock. The spacing of waste canisters and the configuration of the various barrier components within the horizontal drillhole can be designed such that the maximum temperatures remain below limits that are set for each element of the engineered and natural repository system. We present design calculations that examine the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations. Moreover, we evaluate alternative layouts of a monitoring system to be part of an in situ heater test that helps determine the thermal properties of the as-built repository system. A data-worth analysis is performed to ensure that sufficient information will be collected during the heater test so that subsequent model predictions of the thermal evolution around horizontal deposition holes will reliably estimate the maximum temperatures in the drillhole. The simulations demonstrate that the proposed drillhole disposal strategy can be flexibly designed to ensure dissipation of the heat generated by decaying nuclear waste. Moreover, an in situ heater test can provide the relevant data needed to develop a reliable prediction model of repository performance under as-built conditions.

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…The initial heat output is 100 W; the temperature change, ∆T, shown in Figure 3 can be linearly scaled to different initial heat output values and can be converted to absolute values by adding the ambient temperature of the host rock. Although the maximum temperature (reached after about 3 years) is above the boiling point of water at atmospheric conditions, it is well below the boiling point for water at a depth of 1.0 km below the water table, which is about 310 • C. These simulations are described in detail by Finsterle et al [69]; similar studies for Cs/Sr capsules in water, air and deep vertical boreholes are presented by Arnold et al [47].…”

The State of the Science and Technology in Deep Borehole Disposal of Nuclear Waste

“…The effect of waste canister spacing and barrier configuration on temperature redistribution in a horizontal disposal hole was investigated in one paper [5]. It was shown that the disposal concept can be designed to keep maximum temperatures below critical limits for both engineered and natural components of the repository.…”

“…Waste types, including waste types suitable for deep borehole disposal and the circumstances under which a national waste management programme might wish to consider deep borehole disposal [1], and the status of deep boreholes as a potential option for disposal of high-level waste in Germany [2]; • Engineered barrier performances, including corrosion performances of engineered barrier systems [3] and potential cementing systems suitable for deep borehole disposal [4]; • Performance assessments, including the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations [5], geochemical analyses of deep brines focussed on fluid-rock interactions [6] and post-closure performance assessment calculations for the deep borehole disposal of Cs/Sr capsules [7] and • Safety cases, including deep horizontal drill-holes in sedimentary, metamorphic or igneous rocks [8] and safety case aspects for deep vertical boreholes [9].…”

Corrosion Performance of Engineered Barrier System in Deep Horizontal Drillholes

“…Each Cs/Sr capsule is inserted in a Ni-Cr-Mo alloy canister and sealed. Figure 2 shows a cross-sectional schematic and dimensions of components in a horizontal drillhole for disposal of cesium or strontium capsules [27]. The canister is 0.6 m long with 11.4 cm outer diameter, with a wall thickness of 9.5 mm.…”

“…For deep horizontal drillholes, the evolution of temperature is determined by thermal simulations [27,33]. The evolution of temperature rise above the ambient temperature of the host rock is determined for a given EBS configuration and the heat output from the nuclear waste.…”