The Modeling of Fuel Rod Behaviour under RIA Conditions in the Code DYN3D

A description of the fuel rod behaviour and heat transfer model used in the code DYN3D for the simulation of reactivity initiated accidents (RIA) is given. The model comprises a detailed description of the heat transfer in the gas gap between fuel and cladding by conduction, radiation and fuel-cladding contact. The gas gap behaviour is modeled in a mechanistic way taking into account transient changes of the gas gap parameters based on given conditions for the steady state. Thermal, elastic and plastic deformations of fuel and cladding are taken into account. Metal-water reaction of the cladding material in the high temperature region is considered. The cladding-coolant heat transfer regime map covers the region from one-phase liquid convection to dispersed flow with superheated steam.
For the validation of the model, experiments on fuel rod behaviour during RIAs performed in Russian and Japanese pulsed ressearch reactors with shortened probes of fresh fuel rods are calculated. Comparisons between calculated and measured results are shown and discussed. Numerical studies concerning the fuel rod behaviour under RIA conditions in power reactors are reported about. It is demonstrated, that the fuel rod behaviour at high pressure and mass flow rate in power reactors is different from the behaviour at low pressure and stagnant flow conditions in the experiments. It is shown that the mechanisms of fuel rod failure known from the literature can be qualitatively reproduced by the DYN3D fuel rod model.