Inverse modelling of thermal conductivity from temperature measurements at the Prototype Repository, Äspö HRL

Abstract

The Prototype Repository, at the Äspö HRL (Hard Rock Laboratory), is a demonstration project for the deposition of spent nuclear fuel, and provides a full-scale reference for testing predictive models relating to a spent nuclear fuel repository, both its individual components as well as the complete system. The final layout involves six deposition holes, four in an inner section and two in an outer, each fitted with an electrically heated canister. The access tunnel is backfilled with a mixture of bentonite and crushed rock. In 2001, the inner section was completed and monitoring of the heating process started. Temperature measurements in the rock mass are performed at 37 different points. In this paper, the measured thermal response in the surrounding rock is analysed by inverse modelling of the thermal conductivity of the rock mass. A three-dimensional finite difference model of the prototype repository (canisters, buffers, tunnel, etc.) is used to calculate the transient temperature increase due to the heat generation in the canisters. The value of a homogeneous rock thermal conductivity is chosen to obtain the best fit with measured data for each of the 37 temperature sensor points. The evaluation period for the fitting procedure is varied in order to study sensitivity to different time-scales. Measurements of thermal properties have been conducted within the prototype repository prior to the full-scale test. The thermal properties were predicted based on both field and laboratory measurements. These predictions are verified by comparison with thermal conductivity values calculated through inverse modelling.