Analyses of crud deposits on fuel rods in PWRs using Mössbauer spectroscopy

To assess the influence of boron iron compounds deposited on fuel rods on unexpected power shifts in pressurized water reactors, samples of crud scrapes retrieved during refuelling from three reactor cores were analyzed using 57Fe Mössbauer spectroscopy. The fuel crud scrapes were found to contain predominately nickel–iron ferrite NixFe3−xO4 in a composition range 0.6 < x < 0.8, that is with nickel content markedly higher than 0.3 < x < 0.6 found in low boiling duty PWRs. Bonaccordite, Ni2FeBO5, was found only in scrapes from a core that experienced Axial Offset Anomaly of up to −12% in the beginning of examined cycle and as high as −15% in the preceding cycle. The absence of Ni2FeBO5 in fuel crud scrapes from two other cores is in accord with the lack of AOA during examined fuel cycles in these cores.