The interaction between a fuel rod metal clad and the surrounding steam in a nuclear reactor core under hypothetical accident conditions is considered using a thermal balance based model. This enables the calculation of the metal transient temperature and its rate of oxidation, which may possibly lead to ignition and rapid burning. The transient fluid thermal-hydraulic behaviour of the two phase region, as well as the propagation in space and time of its boundaries following a step change in the coolant inlet mass flow rate are solved using a one-dimensional, two-phase homogeneous flow model. The solution scheme consists of first solving the velocity field analytically followed by a numerical solution of the remaining balance equations for the density field. The transient location of the dry-zone region, as well as the other flow primary variables; i.e., vapour quality, and enthalpy are then directly obtained. Numerical results are illustrated based on input data of a partially uncovered AP600 type nuclear reactor core during a bottom-reflooding phase of a loss of coolant accident scenario.

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