During reactor operation, zirconium (Zr) alloy cladding is continuously oxidized as it gets in contact with fuel, and combines with the fuel to form a firm chemical interaction layer. This affects the thermal conductivity of the fuel gap, the mechanical properties of the cladding, and the mechanical interaction of the fuel cladding.

This work aims to obtain relevant analytical data on the chemical interaction layer between the irradiation Zr-alloy cladding and uranium oxide (UO₂) pellets in a pressurized water reactor (PWR).

First of all, the D13 intact fuel rod with a burnup of 45 GWd·tU^-1 for PWRs in a nuclear power plant was chosen as UO₂ pellets with a pellet enrichment of 4.45 wt%, and M5 Zr-alloy was used as the cladding materials. Then, a series of operations (cutting, pellet separation, inlaying, secondary cutting, inlaying, and polishing of cladding tubes) was conducted in the hot cell. The polished sample was transferred to the lead chamber and the UO₂ fuel pellet was removed using 4 mol?L^-1 of nitric acid solution. The cladding tube was separated from the chemical interaction layer, and a low-speed cutter was used to cut the cladding tube to a width of 2~3 mm in the glove box. Finally, the morphology and structure of the chemical interaction layer were analyzed using metallographic microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and hot cell Raman spectroscopy.

The analysis results show that the gap between the fuel pellet and cladding is approximately 14~19 μm after the fuel running to a burnup of 45 GWd·tU^-1. In the chemical interaction layer, the time sequence of mechanical contact at different locations is different, resulting in discontinuity of the interaction layer. The SEM-EDS results show that the chemical interaction layer is in the shape of "worms" composed of U, Zr, and O to form a mixed phase (U,Zr)O_x compound.

The results of this study indicate that the chemical interaction layer is mainly composed of tetragonal zirconia (t-ZrO₂) and monoclinic zirconia (m-ZrO₂).