Effects of Secondary Phase Particle Dissolution on the In-Reactor Performance of BWR Cladding

Abstract

Although the complete mechanisms are not yet fully understood, it is well established that the secondary phase particle (SPP) size distribution and chemical composition have a crucial affects in the reactor corrosion rate and hydrogen uptake in boiling water reactor (BWR) cladding. To further study these effects, different selected fuel rod assemblies with standard LK3 cladding materials that have been irradiated in KernKraftwerk Leibstadt (KKL) for three, five, six, seven, and nine annual cycles were investigated using transmission electron microscopy (TEM). TEM analysis of the samples showed that the average size of SPPs increases as the small SPPs are dissolved during irradiation. After three cycles the Fe-Cr bearing SPPs have been identified as completely amorphous over the whole range of examined samples and the Fe-Ni bearing SPPs remained crystalline. The EDX analyses of several Fe-Ni bearing precipitates show that the Fe/Ni ratio stays more or less constant for the irradiated material at 1.5 to 1.6 and drops to about 1 after fast fluence of 17.9 × 1021 n/cm2 (>1 MeV) for nine cycles sample. Results from this study confirms that the increased oxide thickness, the higher hydrogen content, and the accelerated growth of the rods at rod average burnup of 78 MWd/kgU goes along with a change in the appearance of SPPs by TEM.