A small stainless-steel, plate-type, water-moderated reactor was subjected to large reactivity insertions in a study of the effectiveness of a fast safety system as compared with the self-shutdown characteristics of the reactor. The effectiveness of the mechanical safety system in controlling a reactor in a runaway condition is shown to be adequately described by the postulation of an effective delay time which can be con¬sidered to be constant for all periods. The delay is comprised of the electronic release time and the time required for the rods to reach an effective position in the core. In the system tested, the short effective delay time of 30 msec prevented reactor damage to periods as short as a few milliseconds.It is also shown that for safety systems with longer effective delay times, the ability of the reactor to cope with large reactivity insertions is determined by the inherent shutdown character¬istics of the reactor and not by the mechanical system.
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