The lift-off experiment with MOX fuel rod in IFA-610.2. Initial results

The overpressure tests in the series IFA-610 are carried out in order to investigate the integral rod behaviour during cladding lift-off. A high burn-up PWR MOX fuel rod, originally base irradiated in the Gravelines 4 reactor for four cycles, has been re-instrumented with a fuel centreline thermocouple and a clad extensometer and loaded in IFA-610.2. The test was performed under PWR conditions, between July and early September 1998. The rod has been irradiated for 1300 hours and externally pressurised with argon up to 375 bar (+215 bar of rod overpressure). No significant change of the fuel thermal behaviour was observed up to a rod overpressure of +150 bar. Presence of strong PCMI, most likely due to fuel-clad bonding, was clearly shown by the clad elongation data and the noise analysis. This was confirmed by the impossibility to perform gas flow measurements at power. Using the clad elongation data, the solid fission product swelling of the MOX fuel in IFA-610.2 was inferred to a rate of 0.48 percent vol/10 MWd/kgM, which is lower than standard urania fuel. Only a small but significant increase of the normalised temperature was observed under +215 bar of overpressure, suggesting a beginning of clad lift-off. The onset of thermal feedback due to clad lift-off was assessed to be in the range +165 - 180 bar, which is a very high rod pressure (320 bar) for a fuel rod to reach under normal operation. Evidence of the beneficial impact of fuel-clad bonding, in the way that it delays the clad lift-off, was clearly seen (author) (ml)