The stringent safety demands for advanced small pebble bed high-temperature reactors (HTRs) are outlined. Main results of German studies on source term estimation are discussed. Core heatup events are no longer dominant for modern fuel, but fission product transport during water ingress accidents (steam cycle plants) and He-circuit depressurizations are relevant, mainly due to remobilization of fission products that were plated out in the course of normal operation or that became dust borne. The following important lack of knowledge was identified: Data on plateout in normal operation are insufficient, as are data on behavior of dust-borne activity in total; better knowledge in these fields is also important for maintenance/repair and design/shielding. For core heatup events, the influence of burnup on temperature-induced fission product release has to be measured for future Pu-containing high burnup fuel; furthermore, transport mechanisms out of the He circuit into the environment require further examination. For water/steam ingress events, mobilization of plated-out fission products by steam or water has to be considered in detail along with steam interaction with kernels of particles with defective coatings. For source terms of depressurization, a more detailed knowledge of flow pattern and shear forces on surfaces is necessary. To improve the knowledge on plateout and dust in normal operation and to generate specimens for experimental remobilization studies, planning/design of plateout/dust examination facilities to be added to HTRs running in the next future reactors [HTR10 and the High-Temperature Engineering Test Reactor (HTTR)] is proposed. For severe air ingress and reactivity accidents, which belong to hypothetical events with frequencies <1 × 10-7 yr-1, behavior of future advanced fuel elements has to be experimentally tested.