| Summary: | 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.
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