Simulating the single tank scram system of a BWR

As an alternative to the accumulator tank system, KWU has developed a scram system with single tanks for which a reliability analysis had to be carried out. It was not possible to apply the usual analytical techniques because, as a result of the steady-state behavior of the system, the dynamic nuclear physics conditions for systems failure could not be fixed. The study outlined in this report is an example of a method which allows to prove the technically safe function of a system even if 'failure of the system' as the undesired event cannot be pinpointed at all, or not meaningfully, for nuclear physics reasons, so that the failure probability of the system cannot be ascertained. However, this is based on the precondition that, in the sense of a risk strategy, controllability must not be assessed or proved for incidents and systems defects whose probabilities of occurrence are in a range which allows them to be safely neglected. (orig.)