Publications

Nuclear Fuel Cycle (NFC) Technology and Policy Program

Experimental Determination of

Thermal Conductivity of a

Lead-Bismuth, Eutectic-Filled Annulus

D. Carpenter and G. Kohse

MIT-NFC-TR-081 (June 2005)

Abstract

In order to obtain an accurate prediction of the thermal behavior of an annular fuel assembly (see MIT-NFC-PR-048 for a description of the rods), the thermal conduction of the region from the outside of the fuel capsule to the reactor coolant (within the test assembly) must be known. The effective thermal conductivity of this composite structure is dependent on the interaction of the parts via various physical phenomena, and therefore is difficult to infer accurately from the conductivity of the constituent materials. A mock-up of the annular fuel rod containment thimble was created to allow the conductivity of the annular lead bismuth eutectic-filled gap to be measured. An electric rod heater was used to provide temperatures similar to the in-core environment, and conductivity was determined based on thermocouple temperature readings at various points across the gap.

A second series of experiments substituted a steel tube for the aluminum thimble, and used a bucket of stationary water as coolant. The purpose of these changes was to increase the temperature of the eutectic and achieve a larger melted fraction, while at the same time creating a large enough temperature drop across the gap to allow reliable measurements. A third series of experiments refined the setup and were able to produce more precise measurements of the thermal conductivity.

The measured conductivities were between 4 and 8 W/m-K, much lower than the reported conductivity of the lead bismuth at about 10 W/m-K. The difference must be attributed to thermal resistances at the eutectic-aluminum and eutectic-steel interfaces. This, and the inherent difficulty of measuring the interface temperature due to the finite width of the thermocouples and the existence of sharp thermal gradients makes it difficult to further reduce the uncertainty in the measured conductivity.