Flame retardant mechanism of flame retardant halogen-free flame retardant cable
Adding a large amount of filler such as aluminum hydroxide or magnesium hydroxide to the halogen-free polymer, they release crystal water when the cable is burned, and absorb a large amount of heat, thereby inhibiting the temperature rise of the polymer, delaying thermal decomposition, and reducing the burning speed. . In addition, the water vapor generated by dehydration decomposition can dilute the flammable gas to produce a flame retardant effect. The flame retardancy of a polymer, usually assessed by the oxygen index method, represents the minimum oxygen content required for the sample to burn in a mixture of oxygen and nitrogen. The larger the index, the smaller the flammability and the more flame retardant. it is good. Generally, the oxygen index (OI) is at least 28 to have non-combustible properties.
Fire resistance
When a cable line is required to continue to function in the event of a fire, and the cable must be erected on a high-risk area due to the need for 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 varies according to the requirements of use, and is divided into general fire resistance performance requirements and special fire performance requirements. The special fire performance requirements are: 1000%, 5min burning test under cable impact condition, and the impact is continued for 5min after the flame is extinguished. At the same time, the high-pressure water is used to punch the cable with the specified pressure, and the cable can be kept energized throughout the test.
After the United States developed the IEEE383 cable standard in 1974, the annual nuclear power plant cable fire and flame accident occurred, prompting people to pay attention to the stringency of the flame retardant test standard. For the nuclear island cable finished product, halogen-free / low-smoke / flame retardant, that is, the cable finished product can pass the IEC332-3 bundle burning test, the combustion smoke concentration meets the technical requirements of IEC1034-2, and the combustion corrosive gas reaches IEC754-2 The specified recommended value requires the insulated core to pass the single vertical burning test specified in IPCEAS-19-81.
Environmental resistance
The material for nuclear grade cables must have the environmental resistance required for the inherent working environment of the nuclear power plant, namely heat resistance, radiation resistance, and LOCA resistance.
Heat resistance
Since nuclear grade cables are often operated in a high temperature environment, they are required to have long-term heat-resistant performance, and a polymer having heat resistance is required, and the cable has a service life of more than 40 years.
Radiation resistance
To alleviate the environment, when the harsh environment nuclear grade cable is subjected to a large amount of radiation, the insulation and sheath materials become brittle and the mechanical properties deteriorate. Therefore, as an insulating and sheathing material for nuclear power plant cables, it is necessary to have excellent radiation resistance.
A variety of different polymers, which have different radiation resistance. Anti-irradiation agents are often added to the polymer to improve its radiation resistance. Regarding the L0CA resistance of the cable, the requirements of different nuclear power plants are also different.
In summary, in addition to the performance of ordinary cables, nuclear grade cables must have halogen-free, low-smoke, and flame-retardant properties, and meet the safety functions according to their normal and accidental environment in the laying area. The required performance requirements, that is, the need to meet the requirements of earthquake resistance, normal and accidental conditions - radiation exposure, LOCA/HELB accident-resistant conditions, and service life at the operating temperature.
In addition, the service life of nuclear-grade cables is 40 years or more, and the requirements for next-generation nuclear power plants are 60 years. Therefore, whether the original design can meet the requirements for use remains to be further verified by experiments. How to extend the service life of cables has become an important research topic.
The low-smoke halogen-free flame-retardant nuclear cable is a special cable with high performance level and difficult manufacturing. Even imported products have exposed many problems in application, and some are even serious problems. How to deal with these problems can not be avoided. Whether it is possible to consider the use of low-smoke and low-halogen flame retardant materials to produce special cables for nuclear power plants is an idea that cable industry workers can explore. In doing so, the design difficulty of the cable material can be reduced, the production process can be easily realized, the product quality can be more reliable, and the practical application can be more secure.
Nuclear-grade cables are cable types with high performance levels and difficult manufacturing techniques. The current production technology is not fully mature and needs to be further explored.