The lift-off experiment IFA-610.5 with UO2 fuel rod: evaluation of in-pile results

The rod overpressure test IFA-610.5 has been performed in order to investigate integral rod behaviour during cladding lift-off. A high burnup PWR rod was re-instrumented and equipped with a fuel centre thermocouple and a cladding elongation sensor. The test was performed in PWR coolant conditions and the rod was over-pressurised with argon through external gas lines up to +180 bar (around 340 bar in absolute pressure). The test was started in August 2001 and finished in January 2002. The rod experienced 2697 hours of operation time. The fuel temperature increase rate at 15 kW/m was estimated as 12.1 degrees Celsius / 1000 hours for an overpressure of +177 bar. The result is consistent with previous data from IFA- 610.1, 610.2 and 610.4. In order to clarify the temperature influence on the creep rate, coolant temperature was raised by 10 degrees Celsius in the second half of the experiment, and a temperature increase rate of 8.7 degrees Celsius / 1000 hours was obtained for +171 bar overpressure. However, it is difficult to assess the temperature influence, because the data scatter is larger than the expected change made by the higher coolant temperature condition. The equivalent hydraulic diameter was estimated from gas flow measurements. The results showed the lift-off development with operation time, which is consistent with the observation of the fuel temperature increase. The obtained hydraulic diameter seems somewhat bigger than the gap used in calculations by the FTEMP3 code and than the gap suggested by strong PCMI observed in cladding elongation behaviour in power ramps. The difference probably comes from an assumption of a perfect annular gap in the gas flow measurement. (Author)