Nuclear reactor which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are "necessary" to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short
Zoé (reactor) 60 aluminum-cased vertical rods containing three tons of uranium oxide pellets, controlled by cadmium rods. The heavy water was purchased from Norsk Hydro. A cooling system was added after the reactor had operated for a time, allowing it to run at a heat release rate of 200 kilowatts. The name Zoé was an acronym, from Zéro de puissance" (zero power, that is, very little capacity to produce electricity, which made it easier and faster to build); Oxyde d'uranium" (uranium oxide), "Eau lourde" (heavy water). The Châtillon site was superseded for later nuclear research by a new site at Saclay. The