NDT techniques for strain characterization on zircaloy clad during ballooning under simulated LOCA conditions

Zirconium alloys have been widely used as fuel cladding for most of the water reactors. Considerable attention has been given to study of its behaviour under abnormal reactor conditions such as Loss of Coolant Accident (LOCA). In order to obtain benchmark data on ballooning that could occur in such conditions in PHWR, simulated experiments are planned by heating the clad and pressurizing it internally. A system has been designed and fabricated at Atomic Fuels Division for applying different heating rates and pressures upto 100 bar on zircaloy clad. The objective of the experiments is to collect, amongst other parameters, data on ballooning behaviour and strain rate till burst. On line, in-situ measurement of strain rate accurately to assist fuel design and safety analysis has been a challenging task. Different systems have been employed for this purpose by different laboratories engaged in similar work. An extensive literature survey had revealed that various workers have adopted different techniques to suit their specific requirement which are based on the design codes followed by them. An in-depth study of the problem was carried out and several possible methods were considered. Some of them were evaluated for their possible application in our set up. Wherever required experiments were carried out for this purpose. The paper discusses the in-situ strain rate measurement techniques for Zircaloy clad and problems of each of them. Feasible techniques are further elaborated and results of experiments are described. It also compares the advantages and limitations of X-ray radiography, fluoroscopy, optical films, electronic recordings, laser scanning and Acoustic Emission (AE) methods for strain rate measurement of zircaloy. Data on experiments conducted with AE techniques are analysed. Based on the data collected, possibility of its use for in-service inspection (ISI) of zircaloy components is explored. (author). 3 refs., 4 figs