The design of insulation systems in steam and nuclear radiation environments

The design of a motor insulating system to meet all of the service functions required of the coolant pumping system of a nuclear power plant is a challenge that has not as yet been fully analyzed. Nor has the state-of-the-art for predicting functional performance of motor insulating materials and systems been sufficiently developed to meet the requirements established. First of all, the AEC has specified that this motor, located inside the containment vessel of the reactor, must operate under normal ambient conditions of the enclosure (thermal and radiation aging in air) for a lifetime of forty years. Also, in the event of an accidental breech in the water coolant line the pump will have to be restarted and operate for a period of from one month to one year in an ambient of high-pressure steam, radioactive contaminants, and corrosive decontaminant sprays. Even after the forty years of normal aging it must be capable of performing in this severe environmental exposure. At present, there is no way of comparing insulation durability for such a complex environment. The only system for rating magnet wire insulation now available provides a relative thermal life index, determined at 20,000 hours. Designs of electric motors based upon this rating emphasize thermal life in air. Any change from this basic environment reduces the reliability of the thermal life prediction. In fact, it may be rendered useless. Steam exposures, too, are known to accelerate the decomposition through the mechanism of hydrolysis. Observations reported in this paper will serve to illustrate the seriousness of this effect.