THE DEVELOPMENT OF POROSITY IN THICK ZIRCONIA FILMS

The pore structures of posttransition oxide films on specimens of Zircaloy-4 fuel cladding oxidised in 340-360-degrees-C (613-633 K) water or 400-degrees-C (673 K), 10 MPa steam, have been examined by impedance spectroscopy, scanning electron microscopy, and replica transmission electron microscopy of the oxide surfaces, the oxide-metal interfaces (after dissolution of the metal) and fractured oxide cross sections. The results were compared with measurements of the total oxide thickness derived from weight gains, FTIR interferometry, impedance measurements with mercury contacts, and metallographic sections. The various measurements of total thickness were in good general agreement. Impedance measurements during soaking with aqueous ammonium nitrate solution showed two characteristically different types of behaviour, while the impedance spectra of the soaked oxides also showed a dichotomy of behaviour, but for different groups of specimens. The impedance spectra showed surprising similarities for specimens from similar cladding batches but with very different oxide thickness and morphologies, suggesting that micrometallurgical differences between batches may influence oxide structure irrespective of formation conditions and final thickness. Electron microscopy showed relatively few cracks and pores in the outer oxide films formed on belt-ground surfaces, except for the thick white oxide patches formed in 400-degrees-C steam. Much higher pore densities were seen at the oxide-metal interface than at the surface, in agreement with the higher volume fraction of t-ZrO2 expected at the oxide-metal interface. Relative pore densities at the oxide-metal interface correlated well with the relative oxidation rates during the final oxidation period.