PNNL

Abstract

This report summarizes research relevant to cable aging knowledge gaps identified in NUREG/CR-7153, “Expanded Materials Degradation Assessment (EMDA) Volume 5: Aging of Cables and Cable Systems” (EMDA Vol. 5), performed since the EMDA Vol. 5 was published in 2014. It begins with a discussion of the status of cables in long-term operation of U.S. nuclear power plants (NPPs) and the most common cables found in NPP containment. Next, the major polymer cable insulations, and the mechanisms of concern for degradation of those polymers in service are reviewed. A description of the environmental qualification (EQ) process historically used for safety-related cables is provided as is a review of the potential concerns with that process highlighted in the EMDA Vol. 5. Research addressing each of these is then reviewed. Finally, three potential strategies to support continued safe operation of aging cables are proposed: advanced condition-based verification, targeted material aging studies, and predictive simulation. The most important properties of cable insulation are dielectric strength and mechanical durability. Mechanisms of deterioration of dielectric and mechanical properties are presented herein, which vary between crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR)-type insulations based on differences in their molecular structure and product formulation. Reviewing the degradation mechanisms of thermal and radiation aging provides a unified and systematic knowledge base to explain anomalous phenomena identified as knowledge gaps. Degradation due to moisture and affected dielectric properties is a distinct area of interest from thermal-radiation aging. Electric Power Research Institute (EPRI) guidelines on Tan Delta testing and acceptance criteria were statistically examined and confirmed as a primary tool for condition monitoring of medium voltage (MV) cables in wet or submerged environments. Environmental service condition data collected by EPRI concluded that the actual temperatures and integrated total dose are lower than design values [40 ~ 50 °C, 50 Mrad (500 kGy)]. Knowledge gaps identified in the EMDA Vol. 5 represent concerns that the assumptions made in 40-year environmental qualification of cables may be weak, that the pre-aging of cables prior to loss of coolant accident (LOCA) testing may have represented less than 40-year equivalence, and that consequently the EQ process may not be conservative and thereby overpredict cable useful lifetime. While cable failures are historically few in the first 40-60 years of plant operation, the concern is that lack of conservatism in the 40-year qualification is a more serious issue in licensing up to 80 years. Subsequent research by the U.S. Department of Energy (DOE) and others in the years following publication of the EMDA Vol. 5, as reviewed herein, found that the Arrhenius and equal dose/equal damage assumptions of thermal and radiation aging behavior on which the historical qualification process was based hold for some relevant cable materials, accelerated aging conditions, and performance metrics and do not hold for others. While the pre-aging process appears to be not conservative in some cases, it appeared to be conservative in others. Three approaches to increase confidence in the continued reliable performance of existing nuclear electrical cables are: 1) advance comprehensive cable condition monitoring programs using existing and newly developed tools to inform decisions to repair, replace, or retain aged cables, 2) pursue additional aging studies and the characterization of harvested materials for greater understanding of nuclear cable insulation degradation in the nuclear plant environment, and 3) utilize modeling and simulation to predict cable performance from material measures. Use of a testing-based approach is anticipated to be the most promising strategy for future cable aging management in light water reactors.