Evidence of Ni2FeBO5 and m-ZrO2 precipitates in fuel rod deposits in AOA-affected high boiling duty PWR core

This paper describes the characteristics of corrosion product deposits found in upper regions of high axial offset anomaly (AOA) once-burnt fuel assemblies after Cycle 9 in the Callaway pressurized water reactor (PWR). The ∼100-μm-thick deposits consisted of a new type of highly porous and structured Ni-, Fe-, B-, and Zr-rich material. The analyses showed that deposits contain a large amount (about 50 wt%) of Ni–Fe oxyborate (Ni2FeBO5, mineral name bonaccordite), in the form of matted ∼0.1-μm-thick and ∼10-μm-long, needle-like particles. An especially high density of Ni2FeBO5 needles was found in a 30–40-μm-thick zone on the clad side of the deposits. This compound has not previously been reported as a component of PWR fuel crud. Common fuel crud components such as nickel ferrite and nickel oxide were observed only in small quantities (about 10 wt%). Reference samples of Ni2FeBO5 were obtained by hydrothermal reactions in alkaline aqueous solutions starting from about 400 °C, or by sintering at about 1000 °C. Formation of Ni2FeBO5 has been identified as a new mechanism for boron retention and neutron absorption on PWR fuel. Aggregates of apparently hydrothermally precipitated ∼0.1–0.3-μm-sized particles of monoclinic m-ZrO2(∼30 wt%) were found in the deposits, which is indicative of a dissolution–precipitation process at the cladding surface. This process may be enhanced by a LiOH concentration mechanism in crud, which is a result of both sub-cooled nucleate boiling and 10B(n,α)7Li reactions. Consistently, the isotopic abundance of 10B in Ni2FeBO5 in crud samples was reduced to about 10% of the total boron.