Abstract FIPMIGR is a computer program for studying migration of fission products in a fuel pin. Migration in a temperature gradient and in a concentration gradient is considered. The geometry is cylindrical with migration only in the radial direction. As an example the diffusion of Ba is calculated and compared with experimental results. The migration of Ba is well described using the diffusion constant for Ba in BaO and a heat of transport of −100 kJ mol −1. The great sensitivity of the theoretical prediction to temperature is clearly demonstrated. Both theory and experiments show that there is a temperature or power above which migration becomes clearly visible. The critical temperature is about 1700 K and the power level in the S176 experiments  was then about 40 kW m −1.