This paper presents a case study of the safety instrumentation for a class of intensive heat-transfer systems that must be protected against severe loss of cooling while preventing premature intervention for momentary loss of cooling. A safety experiment conducted in a test nuclear reactor to simulate abnormal conditions in a prototypic Liquid-Metal-Cooled Fast Breeder Reactor core subassembly is used to illustrate the surveillance technique. The protective action arising from a decision based on loss-of-cooling severity is referred to as controlled intervention. The decision-making process is based on real-time interpretation of measured signals by hard-wire analog methods; the test reactor’s plant-protection system requires a simple and reliable sensing device. The output of a moving-time-average filter (whose input is the coolant-flow-rate measurement signal) is found to be capable of discriminating between insignificant and significant flow disruption and it is therefore used to trigger the decision logic to scram the reactor automatically.

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