The importance of the temperature gradient for the processes occurring during high-temperature oxidation of the cladding, its mechanical deformation and oxidation of the melt

Unlike isothermal conditions in some single effect tests, prototypical reactor conditions and the bundle tests simulating them always exhibit both axial and radial temperature gradients. Such gradients are crucial for a number of effects observed in accident conditions at high temperatures. This presentation describes three such effects: (1) Redistribution of oxygen in cladding under steam starvation conditions; (2) Growth of ceramic precipitates in a metal melt during its oxidation; (3) Dependence of cladding deformation on radial temperature gradient.
In the first case, the heat flow from the pellet to the outer surface of the cladding creates a temperature gradient across the outer cladding oxide layer. In areas of steam starvation, this leads to the formation of metal precipitates in the oxide layer already at temperature (at T≳1200 °C), and not only during the cooling stage.
The second case involves the release of molten cladding metal (through the degrading oxide cladding layer) into the interrod space and its oxidation in the steam. Due to the pronounced exothermic oxidation reaction, the temperature at the surface of the resulting molten pool is higher than in its bulk. ... mehrThis drives the formation of ceramic precipitates within the melt already at temperature, leading to its complete transition to the ceramic phase.
The third case concerns the formation of asymmetric cladding ballooning during a design basis accident. Fuel pellets can maintain their geometric shape up to burnups of 30 MWd/kgU, and a gap remains between the pellet and the cladding. Several pellets are displaced from their axisymmetrical arrangement and touch the cladding at a specific point, forming a hot spot there. As a result, the cladding begins to plastically deform at T>600 °C, primarily near this hot spot. As a result, the circumferential strain of the cladding can remain below 40%, significantly lower than in single rod tests with direct current heating of the cladding.