Creep failure in high temperature steels occurs via the formation of isolated grain boundary cavities followed by their coalescence, ultimately leading to the formation of a catastrophic crack. Traditionally metallurgists have employed alloying strategies aimed to postpone the formation of such pores. However, recently we have demonstrated that Fe-Cu, Fe-Au, Fe-Mo and Fe-W alloys of carefully selected compositions can show extended creep lifetime which was attributed to the filling (i.e. healing) of the creep cavities rather than to the prevention of their formation. To demonstrate and quantify the pore filling process, we used synchrotron X-ray nano-tomography to resolve the size distribution and co-location of cavities and precipitates in such self-healing alloys with a resolution down to 25 nm per voxel. A quantitative classification of the object shape statistics and of the distribution of their spacing provides an effective method to identify isolated and linked cavities. The determined filling ratio of each individual cavity was mapped and shows different filling paths for isolated and linked cavities. Tomographic data of samples loaded for selected fractions of their expected lifetime demonstrate that creep cavities nucleate continuously and that precipitates form continuously inside these cavities. The filling kinetics of cavities by precipitation is captured by a model that takes into account the nucleation, growth and linkage of cavities as well as the formation of precipitates within such cavities.