The thermal conductivity of sintered stoichiometric UO2 -pellets was determined in the temperature range 500 - 2200°C and at different burn-up levels from 0 to 0.8 x 1020 fiss/cm3. The conductivity was evaluated from temperature measurements at the centre and two off-centre positions in the UO2-pellets. The power was determined by thermocouples measuring temperature differences in the heat conduction path out from the UO2-pellets. These thermocouples were power-calibrated by means of an electric furnace. The rig design makes it possible to replace the centrally positioned thermocouple (during reactor shut-down) in the event of thermocouple failure or to eliminate irradiation-induced changes in its emf-characteristic. Furthermore, it is possible to replace the gases in the fuel capsule in order to investigate the influence of fission gases on the fuel to cladding heat transfer. The result of the experiment showed: - Good agreement with literature data on thermal conductivity of UO2, early in the irradiation (burn up ∼ 1019 fiss/cm3). - A reduction in the thermal conductivity, with burn-up, at temperatures
below 1000°C. This reduction caused a 9% increase of centre fuel temperature at a linear heat rating of 350 W/cm, after 0.8x1020 fiss/cm3 . - No influence of bum-up on the thermal conductivity above 1000 °C.
No influence of fission gases on the fuel to cladding heat transfer (initial
gap = 0.57% of fuel diameter). With regard to the rig design, the system for exchange of the central thermocouple seems to be limited to a maximum fuel temperature of 1400°C.
|Place of Publication||Roskilde, Denmark|
|Publisher||Risø National Laboratory|
|Number of pages||35|
|Publication status||Published - 1972|
|Series||Denmark. Forskningscenter Risoe. Risoe-R|
- Risø report 269