Safety of the Prototype Fast Breeder Reactor ‘MONJU’ was confirmed at the Installation Permission stage by a series of plant behavior analyses which covered anticipated accidents hypothesizing conservative conditions for various safety reviews. In order to quantify the conservativeness of these analytical conditions, the same series of accidents have been simulated based on actual plant performance data measured during the commissioning tests, to predict more realistic plant accident behaviors. The new analytical results show that the safety margin of the maximum mid-wall temperature of fuel-pin clad increases about 100°C compared to the formerly derived value from the safety review. The main reason of this difference is that the safety review assumes the reactivity worth of the control rods in case of plant trips is at the minimum level required for a reactor shutdown. This recent revised margin has been evaluated to be equivalent to about 10% of the potential increase of plant thermal output.

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