Reverse Temperature Effect During Radiation Ageing of XLPE Cable Insulation

Abstract

The behaviour of polymer based cable insulation and jacket materials during radiation ageing has been studied extensively over the last 10 years and predictive models have been developed (IEC 1996) for many of the materials used in UK nuclear power stations. Most of these polymeric materials behave in a similar manner, with ageing being a function of the radiation dose rate, total dose and ageing temperature. In general, an increase in ageing temperature results in an increase in the rate of degradation, as would be expected from normal chemical kinetics. However, results from long term ageing tests on cable materials have shown that cross-linked polyethylene (XLPE) insulation materials can show a reverse temperature effect, with the material ageing more rapidly at 20°C than at higher temperatures. This is opposite to the expected behaviour where elevated temperature would normally accelerate degradation. This reverse temperature effect is believed to be dependent on the semi-crystalline nature of XLPE, with significant recrystallisation occurring during radiation ageing at elevated temperature, ‘repairing’ the degradation associated with the irradiation. At lower temperatures, molecular mobility is too low for significant recrystallisation to occur. Such effects are likely to be generic to semi-crystalline polymers.