Lifetime Prediction of Polyethylene Cable Insulators in Nuclear Plants

During the last two decades, many efforts have been made, in our laboratory, in order to build a non-empirical model for lifetime prediction of polymeric materials subjected to γ radiation in collaboration with different industrial partners (COGEMA, CEA, EDF). This talk is focused on the problem of radio-thermo-oxidation of polyethylene electrical cable insulators used in nuclear plants. It has been shown, in our previous papers, that the experimental lifetime values tF, compiled from the literature for unstabilised polyethylene at ambient temperature, are situated on a master curve when they are plotted versus dose rate I. The curve can be decomposed into three distinct domains, each one corresponding to a different kinetic regime: the 'pure radio' (I > 5 10-1 Gy.s-1), the 'radio-thermo' (2 10-3 < I < 5 10-1 Gy.s-1) and the 'pure thermo-oxidation' (I < 2 10-3 Gy.s-1). In each domain, a simplified analytical expression can be determined for the lifetime as a function of the dose rate. The aim of this work is to predict the entire shape of the master curve with a single but, above all, non empirical kinetic model. The model is derived from a radical chain oxidation mechanism in which two initiations compete: the thermal decomposition of hydroperoxydes and the polymer radiolysis. It is solved using adequate numerical tools, without any simplifying assumptions, and gives access to the spatial distribution (in the sample thickness) of oxidation products and their evolution against time. The kinetic parameters are determined, using an optimization procedure, by adjusting experimental data obtained on thin polymer films subjected to 'pure thermo-oxidation' and 'pure radio-oxidation' in oxygen excess. It is shown that, whatever the dose rate I may be, there is a satisfying agreement between theory and experiment, as well for the kinetic curves as for the thickness distribution curves. As an example, the model predicts accurately the thickness of oxidized layer TOL and, setting a critical hydroperoxyde concentration (corresponding to the onset of the reaction autoacceleration) as endlife criterion, typically [POOH]C ∼ 0,15 mol.l-1, the lifetime tF