Flame retardant halogen-free flame retardant cable flame retardant mechanism
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Flame-retardant flame-retardant cable Flame-retardant mechanism Flame-retardant characteristics When people require that cable lines continue to function in the event of a fire, and when the cable is placed on a high-risk area line due to its use, the cable must be fire-resistant. . In the event of a fire, such fire-resistant cables can continue to operate safely for a certain period of time, providing electrical energy for the rescue of personnel and equipment. The fire resistance of nuclear cables is different according to the requirements of use. It is divided into general fire resistance requirements and special fire resistance requirements. The special fire resistance requirements are as follows: 1000%, 5min combustion test under the cable impact condition. After the flame is extinguished, it will continue to impact for 5min. At the same time, the high pressure water rushed cable with the specified pressure requires that the cable can continue to be energized during the entire test.
After the United States established the IEEE 383 cable standard in 1974, an accident of fire and extension of the cable in the nuclear power plant occurred in the year, prompting people to pay attention to the strictness of the flame retardancy test standard. For finished products of nuclear island cables, halogen-free/low-smoke/flame-retardant is achieved, which means that the finished cable product can pass the IEC332-3 bunched-combustion test, the combustion fume concentration meets the technical requirements of IEC1034-2, and the combustion corrosive gas reaches IEC754-2. The requirements of the specified recommendations require that insulated cores pass the single vertical burn test specified in IPCEAS-19-81.
Environmental resistance Nuclear power plant cable materials must have the environmental resistance required by the inherent working environment of the nuclear power plant, ie, heat resistance, radiation resistance, and LOCA resistance.
Heat resistance Because nuclear power plant cables often work in high temperature environments, they are required to have long-term heat-resistant serviceability. Polymers that meet the requirements of heat resistance should be used, and cables can have a service life of more than 40 years.
Irradiation-responsive environment, harsh environment When a large amount of radiation is applied to a cable used in a nuclear power station, the insulation and sheathing materials become brittle and the mechanical properties deteriorate. Therefore, as the insulating and sheathing material for a nuclear power plant cable, it must have excellent radiation resistance.
Various different polymers have different radiation resistance. People usually add anti-radiation agent in the polymer to improve its radiation resistance. Regarding the cable's resistance to L0CA, the requirements of different nuclear power plants are also different.
In summary, in addition to the performance of ordinary cables, the cables of nuclear power plants must have the characteristics of halogen-free, low-smoke, and flame-retardant, and meet the safety functions according to their normal and accidental environment in the laying area. The necessary performance requirements, that is, to meet the needs of seismic, normal and accident conditions - radiation, LOCA/HELB accident conditions of the environment, the working temperature of the plant with the service life of the design life.
In addition, the service life of nuclear cables, the original design index is more than 40 years, the next generation of nuclear power plant requirements to reach 60 years, therefore, whether the original design can meet the requirements of the application, has yet to be further experimental verification. How to extend the service life of cables has become an important research topic.
The cables used in nuclear power plants are of various types and quantities. According to estimates, a million kilowatt-class nuclear power unit requires more than 100 types of cables, and the total length is nearly 2 million meters. The value is about 100 million yuan. If divided by purpose, there are five categories: power cables, control cables, measurement cables, communication cables, and fire-resistant cables (silicon insulated cables). They should not only have the general characteristics of ordinary cables, but also have low-smoke, halogen-free, flame-retardant, and other characteristics, and have specific environmental resistance (such as radiation resistance, LOCA resistance). Although there are currently domestic cable companies that specialize in R&D and production of nuclear grade cables, there are still a few companies that can truly produce excellent nuclear grade cables.
The requirements for the special performance of nuclear cables make the nuclear grade cables the most different from those of general industrial cables. The biggest difference lies in the performance requirements of the cable materials used in nuclear power plants. Low-smoke and halogen-free flame retardant ordinary low-voltage flame-retardant cables are generally composed of PVC and other chlorine-containing polymers. Insulation and sheathing.
The insulating and sheathing material of the cables used in nuclear power plants must be made of low-smoke, non-toxic, non-corrosive halogen-free flame retardant cable materials, such as thermoplastic flame-retardant, halogen-free or cross-linked flame-retardant and halogen-free materials, to meet special requirements. Nuclear safety requirements. In the event of a fire, halogen-free cables emit low amounts of fumes without toxicity and corrosiveness. The flame-retardant components of the halogen-free cables can effectively function as flame-retardant materials, and will not make the cable a flame propagation path.
Halogen-free cables use halogen-free polymers as the base material and do not generate acid gases when they are burned. Therefore, their toxicity and corrosivity are much lower than ordinary PVC cables.