The re-irradiation of MIMAS-MOX fuel in IFA 629.1

Two MIMAS-MOX fuel rods previously irradiated in the St Laurent Bl reactor to 25 MWoV kgMOX have been re instrumented with centre-line thermocouples and re-irradiated in the IFA 629.1 general ramp assembly. One rod was also equipped with a clad extensometer and the second with a bellows pressure transducer. The re-irradiation sequence consisted of a stepped ramp to power to establish the fission gas release threshold followed by a steady state at powers in excess of 30 kW/m. The fission gas release threshold is comparable to that of standard urania but post-inter linkage releases are currently in excess of 25 percent. The thermal performance indicates that the fuel thermal conductivity is degraded but following the large fission gas releases, the normalised fuel centre temperatures increased before starting to decline at a linear rate with burnup. The clad elongations during uprates are driven by fuel-clad contact and controlled by thermal expansion of the fuel in the pellet land regions. Subsequent relaxation of the cladding strains are possibly associated with enhanced irradiation creep in the MOX fuel and comparisons indicate an enhancement factor of eight over UO2. This is possibly the origin of the good PCI resistance of MOX fuel. Enhanced irradiation creep of the MOX fuel still appears to dominate the clad elongation during the steady state and evidence of expected steady state fission product swelling is currently absent (author) (ml)