The immediate cause is related to corrosion / erosion of a the cooling pipe of one of the cooling towers. The investigation showed that the leak was due a piercing on a section of ordinary carbon steel pipe in one of the four air coolers units. The boring was caused by ammonium disulfide, formed by the combination of H2S coming from the desulfurization process and the nitrogen coming from the denitrification process in the reactor. As normal practice, an injection of water upstream was continuously provided, to dissolve solid substance and avoid deposits, but a retrofit of the unit five years ago had increased the rate of denitrification in the reactor. An additional degradation mechanism was the erosion derived from the excess of penetration of the weld beads on an elbow of the tube, thereby creating a turbulent flow erosion after a pipe elbow. An examination of the radiographic controls made three months before by the plant inspection service had captured a residual thickness of 1.5 mm on the tube; nevertheless, the radiography operator and service employees did not note this date, due to the difficulties to spot it on the radiographic device. The normal plant procedure in these cases involved replacing the tube when reaching a residual thickness below 2 mm. The uniform nature of the loss of thickness in this part of the tube is more difficult to detect than a punctual loss of thickness on the radiographic films. Similar cases related to corrosion / erosion had already occurred on some of the 92 tubes and 368 elbows belonging to the 2 oldest cooling towers, bringing to replacements without releases.