Towards Nondestructive Evaluation of Cross-Linked Polyethylene (XLPE) Insulation Material in Nuclear Power Plant Cables
This talk describes mechanical and electrical properties of cross-linked polyethylene (XLPE) after it is aged both thermally and with gamma radiation. XLPE is commonly used as insulation in nuclear power plant (NPP) cables; as US nuclear power plants reach their initial design life of 40 years, suitable testing methods must be developed to determine the integrity of XLPE insulation. Presently, the end-of-useful life of a cable insulation material is defined when its elongation-at-break (EAB) or breaking strain is reduced to 50 % of its initial value. To develop nondestructive methods of evaluating end-of-useful life, it is useful to seek correlations between electrical properties, some of which are measured nondestructively, and EAB. In this talk, the method and results of elongation tests on tubular XLPE samples extracted from pristine and aged NPP cable, and of dielectric breakdown tests made on similar materials aged from 5 to 25 days will be described. Due to imperfections caused by aging, mechanical and dielectric testing typically yield a wide array of EAB and breakdown voltage values even on the same material. These variations make it difficult to find relationships between the tests, so a procedure for evaluating the quality of their correlation will be presented.