Ultrasonic system for fuel assembly inspection in pressurized water reactors

Ultrasonic pulses (center frequency at 25 MHz) are incident on the rods of fuel assemblies. The echoes from the tube front wall as well as from the multireflections at the interface between the tube inner wall/gap were collected from a fuel assembly prototype containing intact rods and failed ones (water in the gap between the tube inner wall and the fuel). Detection of failed rods is based on signal analysis of the echo envelopes, employing three methods: exponential envelope estimation, Fisher linear discriminant and neural networks. Around 5,000 US signals were processed and up to 93% of failed rods were correctly identified, for a false alarm detection smaller than 2%.