Study of oxide and α-Zr(O) growth kinetics in the temperature range 1000–1500 °C during steam oxidation of Indian PHWR cladding

Isothermal steam oxidation of Indian pressurized heavy water reactors (IPHWRs) cladding (Zircaloy-4) has been studied in the temperature range of 1000–1500 °C. Growth kinetics of inherently brittle phases, oxide and oxygen stabilized α-Zr, developed during the high temperature steam oxidation were established. A ductile phase known as prior β-Zr, lying either underneath the α-Zr(O), in case of a single sided oxidation, or in between two layers of α-Zr(O) at the inner and the outer surface of the clad, governs the extent of embrittlement of the clad. Thickness of prior β-Zr is decided by the rate of growth of oxide and oxygen stabilized α-Zr. The kinetic rate constant K_p (cm²/sec) obeyed a parabolic rate equation. The Arrhenius expression of parabolic rate constants for oxide scale and α-Zr(O) layer growth compares well with those established by other investigators. An in-house oxidation model, OXYCON was evaluated against the experimental data of oxide and α-Zr(O) layer thickness. The model under-predicted the oxide layer thickness and over-predicted the α-Zr(O) layer thickness in its existing form. However the prediction of oxide, α-Zr (O) and total oxide+α-Zr(O) improved to a significant extent by using kinetic equations derived in the present study done on the indigenously fabricated Zircaloy-4 cladding. Hence, successively the prediction of the prior β-Zr layer thickness could also be done with further accuracy.