Review of nuclear power plant safety cable aging studies with recommendations for improved approaches and for future work.

Abstract

Many U. S. nuclear power plants are approaching 40 years of age and there is a desire to extend their life for up to 100 total years. Safety-related cables were originally qualified for nuclear power plant applications based on IEEE Standards that were published in 1974. The qualifications involved procedures to simulate 40 years of life under ambient power plant aging conditions followed by simulated loss of coolant accident (LOCA). Over the past 35 years or so, substantial efforts were devoted to determining whether the aging assumptions allowed by the original IEEE Standards could be improved upon. These studies led to better accelerated aging methods so that more confident 40-year lifetime predictions became available.Since there is now a desire to potentially extend the life of nuclear power plants way beyond the original 40 year life, there is an interest in reviewing and critiquing the current state-of-the-art in simulating cable aging. These are two of the goals of this report where the discussion is concentrated on the progress made over the past 15 years or so and highlights the most thorough and careful published studies. An additional goal of the report is to suggest work that might prove helpful in answering some of the questions and dealing with some of the issues that still remain with respect to simulating the aging and predicting the lifetimes of safety-related cable materials. EXECUTIVE SUMMARYMany U. S. nuclear power plants are approaching 40 years of age and there is a desire to extend their life for up to 100 total years. Safety-related cables were originally qualified for nuclear power plant applications based on IEEE Standards that were published in 1974. The qualifications involved procedures to simulate 40 years of life under ambient power plant aging conditions followed by simulated loss of coolant accident (LOCA). Over the past 35 years or so, substantial efforts were devoted to determining whether the aging assumptions allowed by the original IEEE Standards could be improved upon. These studies led to better accelerated aging methods so that more confident 40-year lifetime predictions became available.In particular, three of the aging assumptions used were found to be overly simplistic. Thermal aging was typically carried out at a few high temperatures, then they were modeled with an Arrhenius function to derive an Arrhenius activation energy E a and finally the results were extrapolated to ambient conditions assuming no change in E a . Recent studies show that E a often drops at low temperatures, reducing the extrapolated lifetime. For radiation aging, an equal damage, equal dose assumption (e.g., no dose rate effects) was used. Careful studies have now shown that dose rate effects that reduce extrapolated material lifetime are both common and expected and come from several different mechanisms. Finally, for simulating combined radiation plus thermal environments, sequential aging (usually thermal aging followed by room temperature radiation aging) was allowed whereas rece...

Kinetic degradation model for insulating materials used on nuclear power plant safety cables

“…This result is simply due to the fact that the TED values are constant, but it is still important from a practical viewpoint. Namely, this proportional relationship and its slope are called the chemical dose-rate effect and the thermal line, respectively [16]. Where the DED value begins to become inclined, it represents quasi "equal dose equal damage" behavior [28,29].…”

“…Equation (16) shows that a T can be estimated to understand the temperature effect on the chemical reaction and the TED values by taking 1/T -1/T ref as a temperature parameter, even in this situation.…”

“…Various degradation phenomena have been investigated for polymeric insulating materials exposed to both heat and radiation [14][15][16]. Containment is determined in terms of these two degradation factors, and the average temperature and radiation dose rate are approximately 50 °C and ~10 -3 Gy/h, respectively [14].…”

Position of long-term prediction model in aging management of nuclear power plant safety cables

“…As shown in Table I, various radio-oxidation effects have been clarified and the findings have been reflected to type test procedures. Temperature effects are still assumed to fit to Arrhenius law, but recently, some results recommends to consider its non-linear phenomena called 'non-Arrhenius behaviors' [5], [26]. This requires to bend the extrapolation line in Arrhenius plots to predict long-term aging conservative than the traditional way.…”

“…This is important not only for the model sophistication, but also for the development of CBQ. As for the acceleration aging, so-called 'wear-out approach' has been proposed recently [26]. This applies additional artificial aging on deposit or used cables to highlight their aging, and their aging status in service are estimated by back calculation.…”