A new analytical model for predicting the radio-thermal oxidation kinetics and the lifetime of electric cable insulation in nuclear power plants. application to silane cross-linked polyethylene

Abstract

The radio-thermal oxidation of silane cross-linked polyethylene (Si-XLPE) was studied in air under different γ dose rates (6.0, 8.5, 77.8, and 400 Gy.h −1 ) at different temperatures (21, 47, and 86 °C). The changes in the physico-chemical and mechanical properties of Si-XLPE throughout its exposure were determined by FTIR spectroscopy, differential scanning calorimetry (DSC), swelling measurements, rheometry in rubbery (DMTA) and in molten states, and uniaxial tensile testing. It was found that oxidation leads to the build-up of a wide variety of carbonyl and hydroxyl products (mostly carboxylic acids and hydroperoxides) and an efficient chain scission process that catastrophically reduces the concentration in elastically active chains and the elongation at break from the early periods of exposure. A new analytical model was derived from the current radio-thermal mechanistic scheme without making the usual assumption of thermal stability of hydroperoxides. After an initial period where the oxidation kinetics occurs with a constant rate, this model allows also predicting the auto-acceleration of the oxidation kinetics when the hydroperoxide concentration reaches a critical value of about 1.6 × 10 −1 mol.L −1 . Choosing this critical value as a structural end-of-life criterion allows a more direct assessment of the lifetime of Si-XLPE in the various radio-thermal environments under study, except at the highest temperature (i.e. 86 °C) where the kinetic model can still be noticeably improved.

A New Kinetic Modeling Approach for Predicting the Lifetime of ATH-Filled Silane Cross-Linked Polyethylene in a Nuclear Environment

“…The main weakness of XLPE is clearly its fairly high sensitivity to oxidation [ 6 , 7 , 8 , 9 , 10 , 11 ]. For this reason, this polymer is generally protected by one or more antioxidants [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ].…”

“…According to the literature, the radio-thermal oxidation of XLPE leads to a predominance of chain scissions over cross-linking, which progressively destroys the elastically active chains of the macromolecular network [ 8 , 10 , 11 ] and can cause the appearance of chemi-crystallization phenomena [ 10 , 11 ]. These two structural changes, i.e., chain scissions and chemi-crystallization, are directly responsible for the sudden and catastrophic embrittlement of XLPE [ 8 , 10 , 11 ]. All these degradation mechanisms have been extensively studied in the literature over the last three decades and are now fairly well understood.…”

“…All these degradation mechanisms have been extensively studied in the literature over the last three decades and are now fairly well understood. Kinetic models derived from these mechanisms are under development and have already started to demonstrate their excellent predictive value [ 11 ]. However, structural end-of-life criteria are still to be determined, in order to non-empirically predict the lifetime of XLPE.…”

“…The effect of three ATH contents (0, 25, and 50 phr) on the changes in the physico-chemical and mechanical properties is carefully and accurately investigated using several complementary analytical methods, in order to identify the structural changes occurring both inside and outside the interfacial region. Then, the role of the ATH fillers is introduced into the radio-thermal oxidation kinetic model previously developed for the unfilled Si-XLPE matrix [ 11 ], in order to predict the changes in elastic and fracture properties and the lifetimes of the composite materials.…”

Enhanced Flame Retardancy in Ethylene–Vinyl Acetate Copolymer/Magnesium Hydroxide/Polycarbosilane Blends

“…Ethylene–vinyl acetate copolymer (EVA) brings much convenience to daily life and industrial manufacturing due to its excellent dielectric properties and weather resistance [ 1 , 2 ]. As an important matrix resin, EVA is widely used in the wire and cable industry.…”