A model to predict the failure of zircaloy-4 fuel sheathing during postulated loca conditions

Abstract A failure model has been developed and used to predict the burst temperature and burst strain for Zircaloy-4 fuel sheathing in inert or steam atmospheres in the 900 to 1600 K range. The model assumes that the deformation of the thin-walled tube is controlled by steady-state creep and that there is a relationship between the tangential stress and the temperature at the instant of failure. For any temperature and pressure sequence, the steady-state creep rate is numerically integrated to obtain the tangential strain and stress as a function of time. The steady-state creep rate of oxidized Zircaloy is calculated using a homologous temperature concept in which it is assumed that fuel sheaths with the same homologous temperature have the same properties. Failure is predicted when the tangential stress reaches the value given by the burst-stress/burst-temperature correlation. The model also provides for a circumferential variation of temperature by approximating the thin-walled tube as a membrane. Both local and average stresses and strains can be calculated. The model has been used to assess the effect of some important variables such as anisotropy, heating rate, oxidation and circumferential temperature variation on sheath failure. Predictions using the model have been compared with data obtained in inert and steam atmospheres, and with burst strain data for a known circumferential temperature variation. The model accurately predicts the results of the experiments.