Stephane Portier oral presentation (OA1-Tue2-2-5)
Distribution of lithium and boron in the oxide layer of (Zr-Nb) cladding after irradiation
Paul Scherrer Institut, PSI, 5232 Villigen, Switzerland
Zirconium-based alloys are widely used in nuclear industry mainly as cladding for nuclear fuel in power plants. Due to coolant and irradiation effects an outer oxide layer is formed on the cladding. During the oxide growth process, boron and lithium, present in PWR cooling water for neutron absorption control and cladding corrosion control, are trapped in the oxide layer and diffuse through it.
Two series of samples (Zr-Nb cladding material) have been studied in this work, both coming from the same rod, but at two different elevations. SIMS depth profiles have been performed using the shielded ATOMIKA 4000 SIMS installed in the Hot Lab Division of the Paul Scherrer Institute. Lithium and boron distribution have been investigated with 69Ga+ primary ions through the cladding oxide layer (thickness about 4-6 μm). The crater depths have been measured using an optical interferometer.
The obtained profiles indicate differences in the distribution of lithium and boron as a function of the elevation. In one case, close to the oxide/metal interface, an 800 nm-1000 nm thick sub-layer drastically reducing the lithium and boron diffusion is observed. This sub-layer also shows electrical properties widely different from the rest of the oxide layer. This “protective” layer is absent in the second series.
The difference between the 2 series of samples is attributed to a variation of the O2- ions diffusion through the oxide, which may modify the rate of the oxide growth and the transformation rate from tetragonal zirconia to monoclinic zirconia (t-m transition).
This may also be an indication that the t-m transition is a stepwise change, as supported in literature . This means that the differences between the two series may be attributed to a different “moment of corrosion”.
 A. Garner, J. Hu, A. Harte, P. Frankel, C. Grovenor, S. Lozano-Perez and M. Preuss, Acta Materialia 99 (2015) 259–272.