Kinetics of hydrogen desorption from Zircaloy-4: Experimental and Modelling

Abstract : Under normal operating conditions in PWRs, the external surface of the zirconium alloys cladding is oxidized, process during which the alloy absorbs a fraction of the hydrogen produced by water reduction. During the transport of spent fuel, hydrogen may desorb from the cladding, which implies safety concerns. The aim of the present study is to identify and quantify the rate-limiting step in the desorption process of hydrogen initially present in the alloys. To better understand the desorption process, the Thermal Desorption Spectrometry (TDS) was extensively used in association with X-ray Photoelectron Spectroscopy analyses. The TDS results were analyzed thanks to simulations by finite elements (F.E.) using the Cast3M code and the optimization of the kinetic constants of hydrogen desorption has been performed with URANIE tool developed at CEA. Results showed the hydrogen desorption kinetics from the metal is limited by the surface molecular recombination. Arrhenius-type temperature dependence of rate constants allowed to simulate with a good agreement experimental data. The optimized activation energy and the pre-exponential factor for desorption processes were in the range of 290 ± 10 kJ.mol-1 and exp(23 ± 1) m.s-1 respectively.