IE Information No. 86-37: Degradation of station batteries

While shut down, recent inspections at the Rancho Seco Nuclear Power Plant revealed substantial degradation of all station batteries. The batteries exhibited flaking of the cell plates and flaking of the plate's hook area where the plates connect to the cell posts. Although the licensee performed visual inspections that included examination of the electrolyte level and accumulation of sediment in the bottom of the cell, the severe flaking went unnoticed. The deteriorated cells were of the Plante design and over 15 years old. The batteries were expected to last about 20 years and an inspection by the battery vendor 1 year earlier did not reveal abnormal degradation. Subsequent NRC inspections related to the cell-to-cell spacing and cell-to-rack spacing also revealed that the batteries were not installed in accordance with the battery vendor's seismic qualification recommendations. As a result of the flaking, the battery vendor advised the licensee that the batteries at Rancho Seco should be replaced and the licensee is now in the process of replacing the batteries. On April 16, 1986, North Anna Unit 2 declared three out of four station batteries inoperable. The specific gravities in the pilot cells of the affected batteries were found below the technical specification limit. In two of the batteries, approximately half of the cells had specific gravities that were out of specification. The cause was attributed to the inadequate float voltage. These three batteries had recently been replaced with a different make and model that required a different float voltage. The corrective action included readjusting the float voltage and applying an equalizing charge to the batteries