Investigations on damage mitigating measures

For two event sequences leading to a complete core meltdown in case of no countermeasures, the efficiency of operator intervention with existing operational- and/or safety-systems to prevent severe core damage has been investigated. The advanced thermohydraulic code DRUFAN-02 was used to analyse two variations of an emergency power case. Exemplarily only interventions concerning the primary system were investigated. By a depressurization of the primary circuit initiated 0.5 to 1,5 hours after start of the sequence, core damage can be prevented or at least delayed sufficiently, to get time for additional countermeasures. Investigations performed to assessed the potential of constructive countermeasures to mitigate the consequences of a core melt accident with containment overpressurization showed that only controlled late containment venting should be investigated in more detail in the future. This measure has different advantages especially with respect to accident management actions. A cost effective realization of such a system seems to be possible. Containment venting is also a messure which is discussed intensely in France and in Sweden and shall be realized there in the near future. Sufficient safe measures to handle the H2-problem in the containment during severe accidents with core melt are still not available. A new measure based on absorption of H2 seems to have a great potential to contribute to the solution of the H2-problem. (orig./HP)