The role of a lubricant with highly tenecious and adhesive properties

The nuclear islands of all the French plant units with a pressurised reactor (REP) are fitted with two Diesel power units, each one capable alone of supplying the nuclear island not only if there are no external sources but also if there is a serious nuclear accident. Regular tests on these engines are carried out once per month, at reduced load (approx. 30%) and once per year at full power. For regular tests soft starting is planned but is not yet implemented and connection must be obtained 10 secs. maximum after the start command, which is a severe constraint for large engines. Test frequency is decided on the basis of the following consideration: reliability studies carried out in France and in the United States have shown that the number of accidents at start-up increases if rapid start-ups are too frequent, but increases too if the stoppage period is too long. The optimum is from 23 to 30 days, hence the decision for monthly tests. While this frequency is generally correct for the Diesel unit, it is undeniably unfavourable for lubrication. After a stoppage period of 30 days, the oil film has disappeared from the cylinders, which is very bad especially in the case of a rapid start-up. EDF thinks that this poor lubrication at start-up is an aggravating circumstance in a certain number of accidents which we have observed: segmentation spalling, rupture of lands, liners scratches and even seizure. To overcome this problem EDF decided to specify oils with long tenecious properties: by this we mean oils which, after a one-month stoppage, maintain a film on the cylinder jackets so that there is a good level of lubrication at start-up. In this perspective, SEPTEN has placed laboratory studies and then tests in a single-cylinder PC2-5 engine in the hands of ELF - ELF Research Centre at SOLAIZE, in order to find an oil which is a good lubricant for normal operation, but in addition has tenecious properties of over one month (then, as a safety measure, considered equal to one month)