Due to its favourable properties, in particular, low permeability and swelling capacity, bentonite has been favoured as an engineered-barrier and backfill material for the geological storage of radioactive waste. To ensure its safe long-term performance it is important to understand any changes in these properties when the material is subject to heat-emitting waste. As such, this study investigates the hydraulic response of bentonite under multi-step thermal loading subject to a constant-volume boundary condition, to represent a barrier system used in a crystalline or other hard-rock host rock. The experimental set up allows continuous measurement of the hydraulic and mechanical responses during each phase of the thermal cycle. After the initial hydration of the bentonite, the temperature was raised in 20°C increments from 20 to 80°C followed by a final step to reach 120°C. Each temperature was held constant for at least 7–10 days to allow the hydraulic transients to equilibrate. The data suggest that the permeability of bentonite appears to be sensitive to changes in temperature which may extend beyond those explained by simple changes in water viscosity. However, permeability may be boundary-condition dependent and this should be considered when designing experiments or applying these results to other repository host rocks. Either way, the magnitude of the change in permeability observed in this study is minor and its impact on the hydraulic performance of the barrier is negligible.

The thermally enhanced ball grid array (TEBGA) electronic packaging under thermal cycling and thermal loading was investigated numerically. Two-dimensional finite element analysis by ANSYS was used for calculating the temperature distribution and thermal stress on the symmetric and diagonal cross sections of TEBGA. The thermal failure based on the peel and shear stresses at interfaces of TEBGA took place at the interface between the heat sink and epoxy moulding compound. The Tasi-Hill failure criterion was modified to predict the failure at various interfaces in TEBGA package. The TEBGA geometric parameters, including the thickness of the heat sink, the thickness of the adhesive layer between the heat sink and the die, and the thickness of the reinforcing copper ring, were varied to assess their effects on the failure mode of TEBGA. The results showed that for a TEBGA under thermal cycling, the stress values were reduced for thicker adhesive layers and thinner heat sinks; for a TEBGA under thermal loading, the die-to-ambient thermal resistance of TEBGA decreased for thinner adhesive layers and thicker heat sinks. The slimmer heat sink of extruded plate type can dissipate more heat and can reduce the stress values. Proper choice of geometric parameters of TEBGA package can prevent its failure at interfaces and furthermore, improve the reliability of electronic packaging.