THE CHEMICAL-STATE OF DEFECTIVE URANIUM-PLUTONIUM OXIDE FUEL-PINS IRRADIATED IN SODIUM COOLED REACTORS

Steady-state irradiation experiments were conducted in the sodium loop of the Siloe reactor on artificially failed mixed oxide pins that had been pre-irradiated in fast reactors up to 11.5% burnup. The formatio n of the predominant reaction product Na-3(U,Pu)O-4 starts on the fuel surface and is terminated when a lower O/(U + Pu) threshold of the fu el is attained. The axial extent of the reaction product depends on th e size of the initial cladding defect. The occurrence of secondary cra cks is possible. Na(U,Pu)O-3 forms at higher fuel temperatures. The ex istence of Na3U1-xPuxO4 is shown in pre-irradiated blanket pins after artificial defect formation. Caesium in the oxocompounds is reduced to the metallic state and is dissolved in the coolant. Evidence of a ver y low chemical potential of oxygen in defective fuel pins is sustained by the occurrence of actinide-platinum metal phases formed by coupled reduction of hypostoichiometric fuel with epsilon-(Mo,Tc,Ru,Rh,Pd) pr ecipitates. Continued operation of defective pins is not hazardous by easy precautions. (C) 1997 Elsevier Science B.V.