1. Field of the Invention
The present invention relates to a radiation-proof sheath material and a radiation-proof cable. In particular, it relates to a radiation-proof sheath material and a radiation-proof cable, which are excellent in flame retardancy and radiation resistance and which are used in nuclear power stations.
2. Description of the Related Art
Electric wires and cables used in nuclear power stations, such as boiling water reactors (BWRs), or pressurized water reactors (PWRs), are exposed to heat and radiations during normal running of each nuclear reactor operated in specified running conditions, and also in the event of loss-of-coolant accidents, flames, etc. Accordingly, in case of these accidents, electric wires and cables used in nuclear power stations are required to have high flame retardancy and radiation resistance.
Used as conventional electric wire and cable sheath materials are polychloreprene rubber (CR), chlorosulfonated polyethylene (CSM), etc. Also known as a sheath material is chlorinated polyethylene (refer to JP-A-8-64039, for example). Further known is a composition in which 3-50 parts by wt of lead silicate obtained by melting, cooling and milling a mixture of lead oxide and silicon dioxide is combined with 100 parts by wt of chlorinated polyethylene as a stabilizer, in which the chlorine content is 15% by wt to 30% by wt, and in which the remaining crystal by differential scanning calorimetry is 5 cal/g to 15 cal/g (refer to JP-A-4-216843, for example).
The CR sheath materials axe used mainly for BWRs (heat resistance required: 121° C.×7 days, radiation resistance required: 760 kGy), while the CSM sheath materials are used mainly for PWRs (heat resistance required: 140° C.×9 days, radiation resistance required: 2 MGy). Also, the sheath material disclosed by JP-A-8-64039 can be used as weld cables, conductive cables, and conductive cords, which require thermal and mechanical properties. Further, the composition using chlorinated polyethylene disclosed by JP-A-4-216843 can be used in wiring materials for electrical apparatuses, electronic apparatuses of each kind, etc., which require flame retardancy.
Also, because as an evaluation method for cable covering materials, it is difficult to simultaneously apply heat and radiations to the cable covering materials because of requiring a special apparatus, the evaluation method uses sequential ageing (irradiation after thermal ageing), Also, there is considered reversed sequential ageing (thermal ageing after irradiation).
Refer to JP-A-8-64039 and JP-A-4-216843, for example.
However, the CR sheath materials are poorer in heat resistance than the CSM sheath materials, and therefore cannot be used for PWRs. Also, the present inventors have found that the reversed sequential ageing evaluation results of heat resistance and radiation resistance of the CR and CSM sheath materials show poorer properties compared to the sequential ageing evaluation thereof. Further, the chlorinated polyethylene disclosed by JP-A-8-64039 and JP-A-4-216843 cannot be used for BWRs or PWRs because of being not at all suitable for use requiring radiation resistance.