Vibration measurements on PFBR fuel subassemblies

Abstract

Prototype Fast Breeder Reactor (PFBR) is a 500 MWe liquid metal cooled fast breeder reactor, which is in the final stage of construction. The core of PFBR consists of 1,758 subassemblies supported at the bottom on the grid plate sleeves. Liquid sodium is used as the coolant and flow through the maximum rated fuel subassembly is 36 kg/s. The coolant flows axially from the bottom of the subassembly to top and it is in highly turbulent regime. This turbulent flow can excite flow-induced vibration of fuel subassembly which can cause failure of the fuel pin clad tubes from fatigue, wear and vibration induced fretting. Excessive vibration of fuel subassembly can also results in reactivity noise, fatigue or rattling. Flow induced vibration studies of dummy fuel subassemblies in water were conducted in subassembly test facility and the design was qualified for PFBR. However it is planned to measure the amplitude and frequency of vibration during pre-commissioning tests of PFBR. Measurements are planned during the isothermal run of PFBR at 200 °C with dummy subassemblies loaded in the core. Since measurement has to be carried out in high temperature sodium environment, conventional contact type sensors such as accelerometers, strain gages etc. cannot be employed for vibration measurement. Non-contact measurement technique using ultrasound waves was planned to be developed for vibration measurement. Extensive experiments were carried out in various test facilities and ultrasonic vibration measurement technique was established and demonstrated. Based on the experimental results, a device named SONAR was designed and developed for PFBR. The SONAR device is equipped with ultrasonic sensors, which focuses on subassembly crown region, and is capable of movement in Z-axis (up and down) and in Theta-axis (rotation). The movement of the subassembly is detected from the train of ultrasonic pulses and echoes from the target subassembly. Time signal and frequency spectra of vibration are extracted from the ultrasonic signals using signal processing technique implemented in LabVIEW platform. This paper discusses the details of the FIV measurements on PFBR fuel subassemblies, details of ultrasonic technique and SONAR device, its testing and results and conclusion.