Categories and Performance Analysis of Nuclear Cables
There are many kinds and quantities of Nuclear-Grade cables.According to estimates,a million kilowatt class nuclear power unit needs more than 100 types of cables,with a total length of nearly 2 million meters and a value of about 100 million yuan.According to the use,there are five categories:power cable,control cable,measurement cable,communication cable,fire-proof cable(silicon insulated cable).They should not only have the general characteristics of ordinary cables,but also have the characteristics of low smoke,halogen-free,flame retardant,and have specific environmental resistance(such as radiation resistance,LOCA resistance).At present,although there are cable companies 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 the biggest difference between Nuclear-Grade cables and general industrial cables is that the performance requirements of cable materials for nuclear power plants are low smoke,halogen-free,flame-retardant and common low-voltage flame-retardant cables are generally insulated and sheathed with chlorine-containing polymers such as PVC.
The insulation and sheath materials of Nuclear-Grade cables must adopt halogen-free flame-retardant cable materials with low smoke,non-toxicity and non-corrosiveness,such as thermoplastic flame-retardant halogen-free or cross-linked flame-retardant halogen-free materials,in order to meet the special nuclear safety requirements.When halogen-free cables are in fire,the amount of smoke released by combustion is very low,without toxicity and corrosiveness.Its flame retardant components can effectively play the role of flame retardant,and will not make the cables become the channel of flame spread.
Halogen-free cables use halogen-free polymers as binders,which do not produce acid gas when burning,so their toxicity and corrosiveness are much lower than those of ordinary PVC cables.
Flame Retardant Mechanism of Flame Retardant Halogen-Free Flame Retardant Cable
Adding a large amount of fillers such as aluminium hydroxide or magnesium hydroxide into halogen-free polymers can release crystalline water and absorb a large amount of heat when the cables are burned,thus inhibiting the rise of polymer temperature,delaying thermal decomposition and reducing combustion speed.In addition,water vapor produced by dehydration and decomposition can dilute flammable gases and produce flame retardant effect.The flame retardancy of polymers is usually assessed by the oxygen index method,which indicates the lowest oxygen content required for combustion of a sample in a mixture of oxygen and nitrogen.The larger the index,the smaller the flammability,the better the flame retardancy.The general oxygen index(OI)is at least 28 for non-flammability.
When people require cable lines to continue to play a role in the event of fire,and when the cable is installed in high-risk areas due to the need for use,the cable must have fire resistance.In case of fire alarm,this kind of refractory cable can continue to operate safely for a certain period of time,providing electricity for rescue of personnel and equipment.The fire resistance of nuclear power cables varies according to their application requirements,which can be divided into general fire resistance requirements and special fire resistance requirements.The special fire resistance requirements are as follows:1 000%and 5 min combustion test under cable impact condition,5 min continuous impact after flame extinguishing,and high pressure water flushing cable with prescribed pressure.It is required that the cable can be maintained electrified throughout the test process.
After the establishment of IEEE383 cable standard in 1974,the ignition delay accident of nuclear power plant cables occurred every year,which prompted people to attach importance to the strictness of flame retardancy test standard.For the finished cable for nuclear island,halogen-free/low smoke/flame retardant can be achieved,that is to say,the finished cable can pass the bundle combustion test of IEC332-3,the concentration of combustion smoke can meet the technical requirements of IEC1034-2,the burning corrosive gas can meet the recommended value of IEC754-2,and the insulating core can pass the single vertical combustion test prescribed by IPCEAS-19-81.
Nuclear cable materials must have the environmental resistance required by the inherent working environment of nuclear power plants,namely heat resistance,radiation resistance and LOCA resistance.
Because Nuclear-Grade cables often work in high temperature environment,they need to have long-term heat-resistant performance.Polymers that meet the requirements of heat resistance should be selected,and the cable can have a service life of more than 40 years.
In mild and harsh environment,when Nuclear-Grade cables are exposed to a large amount of radiation,the insulation and sheath materials will become brittle and the mechanical properties will deteriorate.Therefore,as insulating and sheathing materials for cables in nuclear power plants,they must have excellent radiation resistance.
Different polymers have different radiation resistance.People usually add anti-irradiation agent in polymer to improve its irradiation resistance.The requirements for L0CA resistance of cables are different for different nuclear power plants.
In summary,besides the performance of ordinary cables,Nuclear-Grade cables should also have halogen-free,low smoke and flame-retardant characteristics,and meet the performance requirements necessary to complete safety functions in normal and accident environments of laying areas,that is,to meet the seismic,normal and Accident-resistant conditions-radiation.Radio,LOCA/HELB Accident-resistant environment,working temperature with power plant design life and other needs.
In addition,the service life of Nuclear-Grade cables,the original design index is more than 40 years,the requirements of the next generation of nuclear power plants reach 60 years.Therefore,whether the original design can meet the requirements of use remains to be further verified by experiments.How to prolong the service life of cable has become an important research topic.
Low smoke halogen-free flame-retardant nuclear power cable is a kind of special cable with high performance and difficult manufacture.Even imported products have exposed many problems in the application process,some even very serious problems.How to deal with these problems is unavoidable.Can low smoke and low halogen flame retardants be considered?
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