Safety related studies on the accident behaviour of the HTR-500

Safety related studies on the accident behaviour of the HTR-500 comprised a total of 8 subsections on HTR safety and served the purpose of making an initial statement about the risk to be expected from this plant concept. The major emphasis of these studies was on accidents with failure of the forced core cooling, so-called core heat-up accidents, because they determine the risk of an HTR of this power size. A mean frequency of approx. 10-4/a was determined for core heat-up accidents; however, in more than 90% of these cases the reactor pressure vessel only opens momentarily. Only in 1per mille of the cases the integrity of the pressure vessel liner is lost by additional failure of the liner cooling system and radioactivity is released into the environment without passing the filters. By failure of the forced cooling system the maximum core temperatures rise to 17000C in the pressurized reactor and up to 25500C in a depressurized reactor. In either case, however, only a few per cent of the core volume are affected. Only in the second case the mean core temperature slightly exceeds the value of 16000C, the beginning of increased failure of the fuel particle coating. Core cooling can be restored within 10 h without thermally overloading the cooling systems. (orig./GL)