Safety of HTR - state of knowledge and necessary research

World-wide there are many projects to design nuclear reactors for future applications, which offer more safety compared to today's systems. In Germany as an example, the modified atomic power act from 1994 requires a very high safety standard for future plants, the total radioactivity must be retained inside the power plant in all cases of accidents, independently from any probabilities. Evacuation and relocation of people must not be necessary after accidents, no intolerable radiological consequences are allowed to occur outside the nuclear power plants. This type of technology, which fulfills these requirements, can be named 'catastrophe-free nuclear technology'. For the case of a reactor with a thermal power of 300 MW which is explained later on, as an example, the amount of release should be less than of 10-5 of the fission product inventory inside the fuel elements. The safety requirements of this future nuclear reactor technology and the importance to realize 'catastrophe-free' nuclear technology shall be explained additionally. Dependent on the total release of radioactive isotopes with long half-life time (137Cs with 30 years as an example) the contamination and loss of land could be large in case of today's nuclear reactors. This contamination must be avoided. (authors)