Types and performance analysis of nuclear grade cables
Types and performance analysis of nuclear grade cables
Nuclear grade cables have a wide variety and quantity.
It is estimated that a million kilowatt nuclear power unit needs more than 100 kinds of cables, with a total length of nearly 200 and a value of about yuan.
If divided by purpose, there are power cables, control cables, measuring cables, communication cables, fire-proof cables (Silicon insulated cable)Such as five categories.
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).
At present, although there are cable enterprises specializing in research and development and production of nuclear-grade cables in China, there are still a few enterprises that can really produce excellent nuclear-grade cables.
The special performance requirements of nuclear grade cables make nuclear grade cables compare with general industrial cables, the biggest difference is that the performance of cable materials used in nuclear power plants requires low smoke halogen-free flame retardant ordinary low-voltage Flame Retardant Cables generally use chlorine-containing polymers such as PVC as insulation and sheath.
The insulation and sheath materials of nuclear grade cables must adopt low smoke, non-toxic and non-corrosive halogen-free flame retardant cable materials, such as thermoplastic flame retardant halogen-free or cross-linked flame retardant halogen-free materials, in order to meet special nuclear safety requirements.
Halogen-free cable in the event of a fire, the amount of smoke released by combustion is very low, without toxicity and corrosiveness. Its flame retardant components can effectively play a flame retardant role and will not make the cable become a channel for flame spread.
Halogen-free cables use halogen-free polymers as base materials, which will not produce acid gases during combustion, thus their toxicity and corrosiveness are much lower than those of ordinary PVC cables.
Flame retardant halogen-free flame retardant cable flame retardant mechanism, adding a large amount of filler such as aluminum hydroxide or magnesium hydroxide to the halogen-free polymer, they release crystal water and absorb a large amount of heat when the cable burns, thereby inhibiting the rise of polymer temperature, delaying thermal decomposition and reducing combustion speed.
In addition, water vapor generated by dehydration and decomposition can dilute combustible gas and produce flame retardant effect.
The flame retardancy of polymers is usually evaluated by the oxygen index method, which indicates the minimum oxygen content required for the sample to burn in the mixture of oxygen and nitrogen. The larger the index, the smaller the flammability and the better the flame retardancy. General oxygen index (OI)At least 28 to have non-combustible characteristics.
Fire resistance, when people require cable lines to continue to play a role in the event of a fire, and when cables are erected on lines in high-risk areas due to the need of use, cables must have fire resistance.
In case of fire, the fire-resistant cable can still continue to operate safely within a certain period of time to provide electric energy for the rescue of personnel and equipment.
The fire resistance of nuclear cable is divided into general fire resistance requirements and special fire resistance requirements according to different use requirements.
The requirements of special fire resistance performance are as follows: Carry out 1000%, 5min combustion test under the condition of cable impact, continue to impact for 5min after the flame is extinguished, and flush the cable with high pressure water with specified pressure at the same time, it is required that the cable can remain energized throughout the test.
After the United States formulated the IEEE383 cable standard in 1974, the nuclear power plant cable fire accident occurred in the year, prompting people to pay attention to the strictness of the flame retardant Test standard.
For the finished cable products for nuclear island, halogen-free/low smoke/flame retardant is required, I . e. the finished cable products can pass IEC332-
3 bunched combustion test, combustion smoke concentration reaches IEC1034-
2 Technical requirements, burning corrosive gases to IEC754-
2 The requirements of the specified recommended value, the insulated wire core passes IPCEAS-19-81 single vertical combustion test specified.
Environmental resistance materials for nuclear grade cables must have the environmental resistance required by the inherent working environment of nuclear power plants, I . e. heat resistance, radiation resistance and LOCA resistance.
Heat resistance because nuclear grade cables often work in high temperature environment, they are required to have long-term heat resistance performance. Polymers with heat resistance to meet the requirements should be selected, and cables can have a service life of more than 40 years.
Radiation resistance in mild environment and harsh environment nuclear grade cables will make insulation and sheath materials brittle and deteriorate mechanical properties when subjected to a large number of rays.
Therefore, as insulation and sheath materials for nuclear power plant cables, they must have excellent radiation resistance.
Different polymers have different radiation resistance.
People usually add anti-radiation agents to polymers to improve their radiation resistance.
Regarding the L0CA resistance of cables, the requirements of different nuclear power plants are also different.
To sum up, nuclear grade cables should not only have the performance of ordinary cables, but also have the characteristics of halogen-free, low smoke and flame retardant, and meet the performance requirements necessary to complete the safety function according to the normal and accident environment of the laying area, that is, to meet the seismic, normal and accident conditions--
The requirements of X-ray irradiation, LOCA/HELB accident environment and working temperature have the service life in the design life of the power station.
In addition, the service life of nuclear grade cable, 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 use requirements, it still needs further experimental verification.
How to prolong the service life of cables has become an important research topic.
Low smoke halogen-free flame retardant nuclear cable is a special cable with high performance level and great manufacturing difficulty. Even imported products have exposed many problems in the application process, some are even very serious problems.
How to deal with and solve these problems cannot be avoided.
Whether it is possible to consider using 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 difficulty of the formula design of the cable material can be reduced, the production process can be easier to realize, the product quality will be more reliable, and it will be safer in practical application.
Nuclear grade cable is a kind of cable with high performance level and difficult manufacturing technology. At present, the production technology is not fully mature and needs further discussion and research.