A laser boring technique has been used to simulate failed coatings of fuel particles. The cesium fractional release from irradiated failed particles is measured in time intervals during an annealing treatment at 1000 and 1200°C, respectively. In addition, the distribution of cesium along kernel and coating is measured by destructive gamma spectrometry. The fractional release versus time data are quantitatively recalculated by superposing diffusion from kernel and buffer layer using a two-zone assembly in spherical geometry. The derived kernel diffusion constants agree reasonably well with corresponding data from in-pile measurements.