Investigation of occurrences of loss of external power supply in Spanish nuclear power plants

The paper reports on an investigation undertaken by the Nuclear Safety Council into occurrences of loss of external power supply in Spanish light water plants between 1981 and 1987. During this period all of the plants investigated (nine), with the exception of one, experienced an event of this type. The causes, effects and duration of each are studied in detail and the conclusion is drawn that the main cause of this type of occurrence is the intrinsic instability of the Spanish national grid. The response of plants to these various failures in external power supply was studied, special attention being paid to the correct operation of safety systems. In addition, the behaviour of other systems not related to safety is discussed, as are modifications and actions taken as a result of accumulated experience with a view to improving the plant response in the event of the failure of the external power supply, and reducing the frequency of such occurrences and their effective duration. The data obtained from this analysis were compared with those obtained in studies performed in other countries and the conclusion was drawn that failures in the external power supply occur slightly more frequently in Spain than in other countries. Similarly, a comparison was drawn between plant response during an actual failure and the response obtained during loss of external power nuclear tests which are carried out in all Spanish plants during the startup (commissioning) programme, in order to ascertain whether the test itself is adequate. (author). 1 fig., 3 tabs