Basic risk analyses for high-temperature reactors

For medium-sized HTRs (e.g. HTR-500) of current design failure of active systems for decay heat removal, resulting in core heatup, clearly dominates the risk and leads to the largest releases of radioactive nuclides into the environment. For small-sized HTRs (e.g. HTR-Module) temperature-induced releases from the fuel are insignificantly low for all types of accident; plate-out activities on the steam generator surfaces remobilized in the course of water ingress accidents can be regarded as the main contribution to the comparatively small source term. The largest releases are so low for all HTR concepts that early health effects can be ruled out in any case, including no evacuation. For small HTR plants even late cancer effects need practically not to be expected. A comparison with licensed released values has shown that the applicable current requirements are met by all HTR concepts examined. However, small HTRs especially offer an additional potential for compliance with more stringent safety requirements, 'taking the fear out of hypothetical accidents', by limiting maximum releases. Incidentally, the classically defined 'risk' to the population from both plants is generally very low. (orig./HP)