Power Reactor Flux-Tilt Protection Instrument

An instrument has been developed as part of a flux-tilt protective system for the light-water-cooled, heavy-water-moderated power reactor (CANDU-BLW) at Gentilly, Quebec. This system is independent of the normal over-power protection instrumentation and is designed to prevent overheating of fuel that could result from spatial flux distortions (or tilts). Three identical instruments are used in the system, each comparing currents from ion chambers located at three positions around the reactor. If flux differences become greater than a preset level a trip signal is generated. The reactor is shut down by dumping heavy water if any two instruments generate simultaneous trip signals. Each instrument contains three linear-logarithmic amplifiers, comparators to sense high flux-tilt levels and logic circuits that combine the comparator signals to produce the final output in the form of relay contact closures. A mixed linear-logarithmic signal is developed from the measured ion chamber current to provide a fluxtilt signal that is non-linear with respect to reactor power.