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 ͑SPP͒ size distribution and chemical composition have a crucial affects in the reactor corrosion rate and hydrogen uptake in boiling water reactor ͑BWR͒ cladding. To further study these effects, different selected fuel rod assemblies with standard LK3 cladding materials that have been irradiated in KernKraftwerk Leibstadt ͑KKL͒ for three, five, six, seven, and nine annual cycles were investigated using transmission electron microscopy ͑TEM͒. 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 17.9ϫ10 21 n/cm 2 ͑Ͼ1 MeV͒ 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.

The Effect of Iron on Dislocation Evolution in Model and Commercial Zirconium Alloys

“…Away from an SPP, where the local strain is lower and IIP is able to occur, there is a delay in c-loop formation as there are more sites for vacancy annihilation. Likewise, Fe segregation to dislocation loops [16,26,32] may be expected due to a simple strain field effect [17]. In the present work, an effect similar to that proposed for oxide dispersion strengthened (ODS) steels is observed by which widespread nanoprecipitates, ~6-10nm diameter, result in a decrease in observable damage structures and a reduced effect on mechanical properties after irradiation in comparison to non-ODS steels [60][61][62].…”

“…It is also difficult to accurately investigate the role of hydrides, as they can form both during irradiation and during sample preparation. Neutron-irradiated Zircaloy-2 TEM foils were supplied by Westinghouse and Studsvik Nuclear, taken from BWR cladding material (280-330ºC) [32]. These samples had been irradiated to a fluence of between 8.7 and 14.7 x 10 25 n m -2 for E > 1MeV.…”

The Convolutional Multiple Whole Profile (CMWP) Fitting Method, a Global Optimization Procedure for Microstructure Determination

“…Taking into account the small size and wide size distribution of the strained volumes of small loops, the satellite profiles, I SAT hkl (s + ∆s), were modeled as size profiles according to Equation (8), where ∆s is the shift of the satellite peaks relative to the main hkl reflection. In References [64][65][66], it was shown that although the strained volumes are coherent with the matrix, the intensities scattered by the strained volumes are incoherent; therefore, the intensities of the main and the satellite peaks are additive. The satellite intensities are usually a few percent of the intensities of the main peaks; therefore, the optimization of the main diffraction pattern and the satellite peaks can be done in separate steps.…”

“…In References [64][65][66], it was shown that although the strained volumes are coherent with the matrix, the intensities scattered by the strained volumes are incoherent; therefore, the intensities of the main and the satellite peaks are additive. The satellite intensities are usually a few percent of the intensities of the main peaks; therefore, the optimization of the main diffraction pattern and the satellite peaks can be done in separate steps.…”

Forescattered electron imaging of nanoparticles in scanning electron microscopy

“…In the Zr-Nb system, most of the SPPs are small β-Nb particles (diameter <150 nm) homogeneously distributed inside the grains [3], and these alloys have shown better corrosion resistance than Zircaloy-4 in which most of the SPPs are Zr-Cr-Fe type [4]. In service as a fuel cladding alloy, radiation effects like irradiation-induced elemental segregation, amorphization and dissolution can change the size and distribution of different kinds of SPPs, and further affect the in-reactor corrosion performance [5]. Precise determination of the size and number density of nano-particles in nuclear cladding materials before and after radiation damage is therefore crucial for the development of reliable mechanistic models for predicting the safe lifetime of Zr alloys in-reactor.…”