Microscopic void characterization for assessing aging of electric cable insulation used in nuclear power stations

In the last 20 years no new nuclear power plants have been ordered in the US and many are being decommissioned. As a result, over half of the nuclear utility owners are expected to seek to extend the license term of their operating plants under approaches allowed by Title 10 Code of Federal Regulations Part 54 (the License Renewal Rule). An essential element of this process is demonstrating the effects of aging of electric cable insulation "will be adequately managed to assure its intended function will be maintained over the license renewal term" typically 20 years. However, a reasonable, reliable method of assessing remaining life of electric cable insulation has continued to elude the nuclear industry. Building on the research efforts of Laurent and Mayoux and others, this paper discusses the development of a promising approach for assessing electric cable life using void characterization by microscopic techniques. Analysis of research to date has shown that void size and density are not only an indication of degree of aging but proximity to a limiting parameter value may be correlated to a level of remaining life for a given set of environmental and operation imposed stresses. Additional complexity is introduced by the nuclear plant operating environment. Not only is normal aging multi-stressor (e.g., thermal, radiation, and others) in nature, but in many station applications a degree of margin for "accelerated aging" during a design basis event must also be considered.