Source term estimation for small-sized HTRs: Status and further needs, extracted from German safety analyses

The stringent safety, demands for advanced small pebble bed high-temperatur e reactors (HTRs) are outlined. Main results of German studies on source te rm 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 d ue to remobilization of fission products that were plated out in the course of normal operation or that became dust borne. The following important lac k of knowledge was identified: Data on plateout in normal operation are ins ufficient, as are data on behavior of dust-borne activity, in total; better knowledge in these fields is also important for maintenance/repair and des ign/shielding. For core heatup events, the influence of burnup on temperatu re-induced fission product release has to be measured for future Pu-contain ing high burnup fuel; furthermore, transport mechanisms out of the He circu it into the environment require further examination. For water/steam ingres s events, mobilization of plated-out fission products by steam or water has to be considered in detail along with steam interaction with kernels of pa rticles with defective coatings. For source terms of depressurization, a mo re detailed knowledge of flow pattern and shear forces on surfaces is neces sary. To improve the knowledge on plateout and dust in normal operation and to generate specimens for experimental remobilization studies, planning/de sign 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 accident s, which belong to hypothetical events with frequencies <1 x 10(-7) yr(-1), behavior of future advanced fuel elements has to be experimentally tested.