Verification of the from model for zircaloy oxidation during high temperature transients

Overview of high-temperature fuel behaviour with relevance to CANDU fuel

“…With cooling, the b phase will transform back to the a phase in which the oxygen concentration is significantly different from that of the oxygen-stabilized a and ''prior b". Moreover, under certain conditions, the a layer can also consist of two sublayers (a 1 and a 2 ) [141]. The prior b material can also reveal structural changes where, if enough oxygen is absorbed, ''a incursions" may form with a growth of oxygenenriched a into b.…”

“…As mentioned, diffusion theory has been applied to describe sheathing corrosion for general transient conditions for finite specimens [141][142][143]. These models are quite accurate but difficult to numerically incorporate into the nodal form of large accident analysis codes because they require the numerical solution of partial differential equations representing Fickian diffusion in each layer.…”

“…These partial differential equations are also subject to specified oxygen concentration values at each of the layer boundaries and oxygen conservation relationships as the boundaries move. This moving boundary problem has been solved as the FROM (Full Range Oxidation Model) computer code, which predicts the various corrosion layer thicknesses, transition from two phase to three phase oxidation, and oxygen concentration profiles in the Zircaloy sheath [141]. This latter treatment also incorporates non-equilibrium boundary concentrations that improve the prediction of the oxide layer thickness.…”

“…A variety of structures result in oxidized Zircaloy that depends on the temperature and oxygen concentration [141]. At temperatures below 1144 K, an outer layer of zirconia results adjacent to a layer of alpha Zr (a) (which contains oxygen in solid solution).…”

Steam oxidation of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr and Zircaloy-4 at 900–1200°C

“…Most studies have been focused on the steam oxidation of the as-received cladding state [10][11][12][13][14][15][16], because the steam oxidation of the cladding during a LOCA occurs at high temperatures above 800°C. Practically, however, since a LOCA occurs suddenly during the normal operation of a nuclear power plant, the fuel cladding, which has been oxidized on the outer surface and hydrided in the matrix during the operation period, would undergo a LOCA experience.…”