Detection of core barrel motion at WWER-440-type reactors

Abstract

A new method for detecting core barrel motion at WWER-440-type reactors by a pure excore detector arrangement is demonstrated. The method is based on excore neutron noise signals from different azimutal positions and at least one acoustic sensor at the RPV outer surface that measures the sound generated by the leakage flow from the coolant inlet directly to the outlet through the labyrinth seal. If the core barrel vibrates there will be significant coherence between the envelope of the acoustic signal and any neutron noise signal of a chamber positioned at an angle ▪ ǂ 90° distant from the acoustic sensor. A pure core basket motion would not generate coherence. Though only core barrel displacement is mapped in excore neutron detectors the amount of coherence between neutron noise signals depends on the position of the detectors and on the type of the core barrel pendulum motion. As well stochastic amplitudes in different directions as changes of the rotational sense decrease coherence. A mathematical model was developed to identify different types of core barrel motion.