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

Stefan Finsterle,James Rector,Joe Payer,Richard Muller,Rod Baltzer

doi:10.3390/en12040596

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.

Highlights

We examine the geologic disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW) using corrosion-resistant canisters placed in deep, sub-horizontal, small-diameter holes drilled in suitable hydrostratigraphic units that safely and securely isolate the waste from the accessible environment
The maximum temperature expected within a horizontal drillhole and the surrounding host formation is an important factor that mainly affects our ability to robustly predict repository performance
These temperatures need to be simulated with acceptably low prediction uncertainty in order to provide a defensible basis for the demonstration that they are below regulatory thermal limits

Summary

Introduction

We examine the geologic disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW) using corrosion-resistant canisters placed in deep, sub-horizontal, small-diameter holes drilled in suitable hydrostratigraphic units that safely and securely isolate the waste from the accessible environment. The horizontal drillhole disposal concept takes advantage of the directional drilling experience gained by the oil and gas industry. A vertical access hole cased with a steel pipe is drilled, preferably at or near the site where the nuclear waste is currently stored in surface facilities. At the kickoff point (slightly above the targeted repository depth), the drillhole gradually curves until it is nearly horizontal, with a slight upward tilt. Canisters containing the waste are lowered into the vertical access hole and pushed into the horizontal disposal section; they are emplaced end-to-end

Results

Discussion

Conclusion