Nuclear power plant cables refer to the materials used for insulation and sheathing in nuclear power plant cables, including various types such as plastics and rubbers. The types of cables are determined by the cable type. There are two classification methods for nuclear power plant cables: one is by function, including measurement cables, communication cables, instrument cables, fire-resistant cables (silicone-insulated cables), etc.; the other is by safety level, with the safety level of cables used in nuclear power plants belonging to the IE class, which should also have a service life of over 40 years. The IE-class nuclear power plant cables are further divided into K1, K2, and K3, each with three safety levels. The development process of nuclear power plant cable products is essentially an update of materials, with the selection of raw materials for cable types, as shown in Table 1. From Table 1, it can be seen that nuclear power plant cables mainly use the following types.

Selection of cable components for nuclear power plants:

Cross-linked polyethylene

Cross-linked PolyethyleneInked polyethylene, or XLPE, is a high polymer formed by treating linear polyethylene with appropriate methods to create a reticulated or macromolecular structure. It boasts excellent heat resistance (softening point of 200%), electrical insulation, low-temperature resistance, and chemical resistance, along with good radiation resistance, and is used as an insulation material for cables.

EVA elastomer

Ethylene-vinyl acetate copolymer (EVA) is a copolymer of ethylene and vinyl acetate. It boasts excellent radiation resistance and chemical resistance properties, and is used as cable sheath. However, it requires the addition of a significant amount of flame retardant to achieve flame retardancy.

Silicone rubber

Silicone rubber or silicone compounds are elastomers with a saturated silicon-oxygen backbone structure, known for their high chemical stability, excellent resistance to thermal aging, ozone, radiation, and high-pressure steam, as well as superior electrical insulation properties. They are suitable for use as insulating materials. Typically, nuclear-grade cables use ethylene propylene rubber (EPR) for insulation (some also employ double insulation, such as inner EPR and outer EVA), and cross-linked EVA rubber for sheathing. The reason is that rubber materials are less prone to deformation under high-temperature and high-pressure tests, ensuring the cable's normal structure and offering greater safety compared to plastic materials. The insulation and sheathing for nuclear-grade cables within containment vessels are primarily made of thermoplastic flame-retardant halogen-free or cross-linked flame-retardant halogen-free materials, such as using XLPE for insulation and low-smoke halogen-free polyolefins for sheathing. Some also use EPR for insulation and cross-linked EVA for sheathing.

Cable characteristics

Low smoke performance

Insulation and jacket materials for nuclear power plant cables must be flame-retardant, low-smoke, non-toxic, and non-corrosive halogen-free cables, such as thermosetting flame-retardant halogen-free or cross-linked flame-retardant halogen-free materials, to meet the special nuclear safety requirements. Halogen-free cables emit very low smoke when on fire, are non-toxic and non-corrosive, and their flame-retardant components effectively prevent combustion, preventing the cable from becoming a conduit for flame spread. The main technical characteristics of halogen-free flame-retardant cables include: (1) Total smoke accumulation of the cable for nuclear power plants, Dm < 150; (2) Non-toxic and non-corrosive, meaning the cable does not release HCl and CO upon combustion; (3) Flame retardancy, which is typically evaluated by the oxygen index (OI), generally OI ≥ 28.

Environmental resistance

The materials for nuclear power plant cables must be environmentally resistant, i.e., heat-resistant, radiation-resistant, and LOCA-resistant.

(1) Heat Resistance: Due to the fact that nuclear power plant cables often operate in high-temperature environments, they are classified as high-temperature cables. Consequently, they must possess long-term heat resistance for continuous use, and the selected polymers must allow the cables to have a lifespan of over 40 years.

(2) Radiation Resistance (Mild Environment, Severe Environment) Cables used in nuclear power plants can become brittle in their insulation and sheath materials when exposed to a high dose of radiation, leading to a decline in mechanical properties. Therefore, insulation and sheath materials for nuclear power plant cables must possess excellent radiation resistance. Different types of high polymers exhibit varying degrees of radiation resistance. People typically add radiation stabilizers to high polymers to enhance their radiation resistance.

(3) In nuclear power plants with LOCA-resistant designs, Loss of Cooling Accident (LOCA) and High Energy Line Break (HELB) incidents are commonly referred to as LOCA. During a LOCA/HELB event, cables are subjected to the impact of high-temperature, high-pressure steam and the action of corrosive chemicals, as well as higher doses of radiation than during normal operation. Therefore, nuclear power plant cables should be LOCA-resistant.