Mechanical strength degradation of crosslinked polyethylene insulation by radiation and thermal stress. Effect of antioxidant concentration

In Japan, the integrity of the cables installed in nuclear power plants is confirmed by the environmental qualification test in accordance with 'Recommended Practice of IEEJ', which is based on IEEE Std. 323 - 1974 and 383 - 1974. The project of 'Assessment of Cable Aging for Nuclear Power Plants' was carried out from FY2002 to FY2008 by Japan Nuclear Energy Safety Organization to establish the highly reliable cable evaluation method, which is based on the studies on the suitable accelerated aging tests, the appropriately assumed environmental condition and the integrity judgment method considering operating condition and aging in actual plants. The test materials for the cable included XLPE, EPR, silicone rubber (SIR), and more. As a result, 'Guide for Cable Environmental Qualification Test for Nuclear Power Plant' was issued in July 2009. In order to clarify the deterioration mechanism under thermal and radiation environment, specimens made of XLPE insulating material for typical cable insulation have been aged in acceleration conditions with elevated temperature and high dose radiation condition. To simulate long term aging, the antioxidant concentration has been changed. The oxidation rate and antioxidant concentration of the specimen have been measured as well as the mechanical breakdown strength of the specimens. We found the clear relationship between the mechanical strength and material property change as a step toward the sophistication of the evaluation method with accelerated aging test. (author)