Pin-Scale Code Development

A mechanistic, smeared fuel cracking model for UO2 has been implemented in BISON and tested with simulations of IFA-432 Rod 1, an experiment conducted in the Halden reactor. (“Smeared” refers to the fact that cracks are represented in aggregate, rather than as discrete, individual cracks.) Failure to account for fuel cracking can result in temperature predictions that are off by as much as 200°C at beginning-of-life. Excellent agreement between prediction and measurement is obtained when an empirical correlation for fuel relocation is used (as in the NRC’s FRAPCON), which was expected, since the empirical correlation was fit to data that included this experiment; this result is important, however, in that it indicated that BISON predictions are at least as good as those of FRAPCON for the same empirical models but without the need to calibrate the model to experimental data. [INL]

A model for calculating the radial power profile in a fuel pin, local burnup, and local concentrations of important fission products was also completed in BISON. This model is critical for making accurate fuel temperature predictions in thermal reactor fuels of all types, since a failure to account for the flux depression inside a fuel pin can amount to over-prediction of temperature by several hundred degrees. Knowledge of local concentrations of important fission products is vital input for modeling many fuel behavior models, especially swelling and gas release. [INL]