Abstract:On the basis of the literature on the aging management of I&C cables in the containment of nuclear power plants, the composition and aging mechanism of I&C cables are introduced, and the environmental identification, condition monitoring and life assessment of I&C cables are described, hoping to help carry out this work in China.
Instrumentation and control; Cable; Aging; containment; Nuclear power plants
Keywords:InstrumentationandControl; Cable; Ageing; Containment; NPP
With the increase in the number of nuclear power plants and the extension of operating hours, the aging effect of nuclear power plant equipment has attracted more and more attention, and how to implement effective management of the aging of nuclear power plants, the safety and reliability of in-service nuclear power plants has aroused serious concern from the IAEA and the world's nuclear power powers, and extensive work has been carried out. As one of the important components of nuclear power plant safety, the aging assessment and management of instruments and control cables in the containment have also been deeply studied, and more research results have been obtained. The IAEA and major nuclear energy agencies have published a number of thematic reports [1]-[4].
China's Qinshan and Daya Bay nuclear power plants have been put into operation for more than 10 years, although the operation time is not very long, but it has faced the aging problem of the I&C cable in the containment, and with the increase of service time, this problem will be even worse. At present, there is no systematic research on the aging assessment and life management of I&C cables in containment in China, and on the basis of relevant literature, this paper introduces the content of aging management of I&C cables in nuclear power plants in containment, in order to help carry out this work.
1. Instrument and control cable and its use environment
Nuclear power plants contain thousands of kilometers of cables of different types and specifications, which constitute medium-voltage power circuits, low-voltage power circuits, control circuits, instrumentation circuits, grounding circuits, etc., and Table 1 shows the distribution of various circuits in dual-unit nuclear power plants
1.1 The purpose and composition of the I&C cable
An instrument cable is a low-voltage, low-capacity cable that connects a wide variety of transmitters, sensors, and transmits digital or analog signals; The control cable is also low-voltage and low-capacity, and is used to control the operating mechanism of switches, pumps, valves, etc., and the control loop of relays and contactors.
The main parts that constitute the I&C cable are: conductor, insulating material, shield, sheath, filler between multi-core conductors, and external bandage. The so-called aging of cables refers to the aging of organic materials in the cable structure. Although the padding and external wrapping are also organic, they do not have a significant effect on cable aging, so the focus of research is on insulation materials and sheaths.
The composition of the insulator and sheath used in the cable is a polymeric material synthesized by some additives and fillers, and in nuclear power plants, vinyl, propylene-based synthetic rubber, glass fiber, and cables with chlorosulfonated polyethylene, polyethylene and polyethylene imide as insulating materials are used in the instrumentation and control circuit.
1.2 The working environment of the I&C cable
The internal I&C cable of the containment is placed in different use environments, and the important influencing factor is the natural environment, mainly the influence of temperature, humidity and nuclear irradiation in the presence of oxygen, and the values of temperature, humidity and nuclear irradiation should be obtained from the design documents.
Under normal operation, the containment is not affected by humidity. The effects of irradiation can be obtained from the relevant technical data, and the cumulative value of irradiation in the containment is 3×107rad over a 40-year period under normal operation. I&C cables in containment are generally not affected by vibration, and unless required, aging due to vibration is not considered [5].
2. The aging mechanism of the cable
In the field environment, the polymer materials such as the insulation and sheath of the cable undergo various slow, irreversible chemical and physical changes over time, and these changes are the aging process of the cable. From a macroscopic point of view, it is manifested as a decrease in the elongation of the material, that is, the tensile strength of the material is weakened; The hardness or compressive modulus of the sheath material increases; an increase in the density of the material; Changes in electrical properties (e.g., increased dielectric losses).
The aging mechanism of cables can be divided into chemical aging mechanisms that affect the molecular structure and physical aging mechanisms that affect the composition of material mixtures.
2.1 Chemical aging mechanism
(1) Polymer chain breakage: A polymer chain is broken into two or more new chains, generally alkoxy or peroxide chains, resulting in changes in the properties of substances.
(2) Cross-linking reaction: cross-linking occurs in the structure of the covalent bond between two adjacent polymers, which reduces the active ingredients of the original substance.
(3) Oxidation reaction: This is a chain reaction of free radicals, at the beginning of the oxidation reaction, under the influence of temperature and irradiation, due to the breaking of covalent bonds, reactive substances, that is, free radicals, oxidation reaction not only leads to chain breakage, but also generates cross-linking, which depends on the molecular movement of each stage in the process of oxidation chain reaction, which varies with the different additives in the polymer.
(4) Oxygen diffusion control process: when the initial rate of free radicals in the polymeric material is greater than the rate of dissolved oxygen diffusion, the aging speed is controlled by oxygen diffusion.
(5) Synergistic effect: This effect will occur when the combined effect of each environmental factor is greater than the sum of its individual effects, such as for polymers, both heating and irradiation.
2.2 Physical aging mechanism
(1) Plasticizer evaporation: the plasticizer on the surface of the material volatilizes into the surrounding air, and the void left by it is filled by the plasticizer diffused from the core of the material to the surface.
(2) Plasticizer migration: In multi-layer cables using plasticizer materials, plasticizers migrate between different material layers until the plasticizer in each layer of material reaches an equilibrium state.