Japanese team develops non-destructive technique for diagnosing ageing cables*28 May 1999
Of Japan’s 51 operating nuclear power plants, 20 have already operated for over 20 years. Thus about 40% of the nation’s nuclear capacity can be said to be ageing. And as nuclear power accounts for about 40% of Japan’s total power, a significant part of the nation’s generating capacity can be considered to be ageing. For this reason, the industry recognises the importance of plant ageing processes to the future operation of its nuclear stations and has done substantial work on ageing issues since the early 1990s.†
Among the concerns of plant ageing is the degradation of cables, as they provide a vital safety function in a nuclear station. In fact, the role of cabling can be compared with that of the neural network of the human body. To ensure their continued good performance over many years of operation, improved maintenance measures are needed.
To tackle this concern, a team from Japan’s utilities and suppliers began development of a non-destructive diagnostic system for ageing cables several years ago. In the course of development, the team conducted studies focusing on the following requirements:
• Identification of ageing indicators.
• Assessment of the applicability of basic techniques available.
• Acquisition of non-destructive diagnostic data.
• Establishment of the correlation between ageing indicators and non-destructive diagnostic data.
As a result, the team found that a non-destructive diagnostic technique that makes use of ultrasonic wave propagation velocity and surface hardness measurements could be applied to cables used in nuclear power plants.
RESULTS OF RESEARCH
Cables gradually become brittle in environments where they are exposed to heat and radiation. In addition, the oxidation caused by the heat and radiation accelerates degradation and ageing of cable insulation.
To determine which characteristics most accurately reflect cable degradation, the possible changes in various parameters over time were calculated by subjecting cables with different insulators to accelerating degradation. Through these tests, the elongation observed at rupture (hereafter referred to simply as “elongation”) proved to be the most suitable indicator for cable aging.
The research progressed as follows:
• First, the team surveyed the trends in developing non-destructive ageing diagnostic techniques for cables, identifying a total of 3700 relevant techniques, categorised as electric, chemical, physico-chemical, or physical methods. A total of 19 basic techniques were identified.
These basic techniques were then further examined in terms of availability of basic data, non-destructive properties and manageability of instruments, to eventually identify nine basic techniques that seemed applicable to non-destructive diagnosis for cable ageing.
• Next, these nine techniques were examined for their suitability using several ageing cables as samples. We found that, along with “elongation”, “ultrasonic wave propagation velocity” and “surface hardness” produced quite good correlations as ageing indicators, and that these would serve successfully for non-destructively diagnosing cable aging.
• To gather more information, the team worked in co-operation with Japanese cable manufacturers that had been developing non-destructive diagnostic techniques for cable ageing. Using the diagnostic techniques developed by different suppliers, a large store of data were collected (from approximately 8000 tests) for use in non-destructive diagnosis of ageing cables.
These evaluations revealed that “ultrasonic propagation velocity” and “surface hardness” correlate quite well with “elongation” as indicators for degradation. “Ultrasonic wave propagation velocity” in particular shows promise for use in the non-destructive diagnosis of EP (ethylene propylene) rubber, a material used widely for safety system cables in nuclear power plants.
Figure 1 below was created from the team’s extensive collection of data. The plot reveals the correlation between “elongation” and “ultrasonic wave propagation velocity” in flame-retardant EP rubber. The data shown represents only part of the observations justifying the correlation.
THE DIAGNOSTIC TECHNIQUE
The team has now developed a new non-destructive diagnostic technique using ultrasonic wave propagation velocity which makes use of the changes in physical properties of the materials as they degrade.
Generally, when rubber and plastic materials are exposed to heat and radiation they degrade, which involves changes in their physical properties, such as their thermal expansion coefficients. In theory, the propagation velocity of ultrasonic waves travelling inside these materials will also change along with the changes in the physical properties. Therefore, this could be used to monitor degradation.
Unfortunately, this is not so straight forward with cables. When ultrasonic waves are injected vertically into a material, their velocity depends on the thickness of the material and the time taken to travel back and forth. However, it is difficult to accurately measure the thickness of insulators and sheaths.
To solve this problem, two types of ultrasonic probes, one for transmitting and another for receiving, are used to detect ultrasonic waves injected in the direction of the cable axis (see Fig 2).
First, the propagation time of an ultrasonic wave (T1) for the distance (L1) between the probes is measured. Next, the propagation time of an ultrasonic wave (T2) for covering the distance (L2) between the probes is measured. The propagation velocity of ultrasonic waves is then calculated from the difference in the travel distances (L2-L1) and propagation times (T2-T1).
The next figure shows the structure of an ultrasonic diagnostic system for ageing cables. The system is composed of a signal processor section and an instrument section. The instrument section of a prototype ultrasonic system designed for ageing cables is shown in the following picture.
Following these extensive investigations, the team concluded that a diagnostic system using the “ultrasonic wave propagation velocity” technique allows pinpoint diagnosis of areas subject to severe environmental stress, without causing any adverse effects on the cables.