Modelling and Simulation of Reactor Fuel Cladding under Loss-of-Coolant Accident Conditions

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We present a unified model for calculation of zirconium alloy fuel cladding rupture during a postulated loss-of-coolant accident in light water reactors. The model treats the Zr alloy solid-to-solid phase transformation kinetics, cladding creep deformation, oxidation, and rupture as functions of temperature and time in an integrated fashion during the transient. The fuel cladding material considered here is Zircaloy-4, for which material property data (model parameters) are taken from the literature. We have modelled and simulated single-rod transient burst tests in which the rod internal pressure and the heating rate were kept constant during each test. The results are compared with experimental data on cladding rupture strain, temperature, and pressure. The agreement between computations and measurements in general is satisfactory. The effects of heating rate and rod internal pressure on the rupture strain are evaluated on the basis of systematic parameter variations of these quantities. In the α-phase of Zr, the burst strain decreases with increasing heating rate, whereas in the two-phase coexistence (α+β) domain and β-phase, the situation is more complex. Also, the mechanism for creep deformation in the (α+β) domain is not well understood; hence, its mechanistic constitutive relation is presently unknown.