BUCHANAN, N.Y. (CBSNewYork) — A transformer fire that shut down Indian Point nuclear power plant in May was caused by a failure of insulation, according to an internal investigation.

Entergy Corp., which runs the plant, said the failure lead to the transformer in Unit 3 to short-circuit and catch fire on May 9.

The investigation also confirmed that water and foam used to put the fire out as well as non-nuclear fluid from the transformer overflowed the containment system’s capacity.

“We have been working closely with independent engineers, and with federal and state agencies, to address issues surrounding the May 9 transformer failure, and corrective actions are well underway,” said Bill Mohl, president of Entergy Wholesale Commodities, the Entergy business unit that owns Indian Point. “These actions reinforce our commitment to environmental responsibility and transparency, as well as the continued safe, secure and reliable operation of Indian Point.”

More: Full Investigative Report From Entergy

Entergy said the transformer has since been replaced, Unit 3 has returned to service, “and no remediation was warranted at most locations of reported sheens from fluids that reached the Hudson River.”

Officials and environmental groups expressed concerns following the fire, after a sheen could be seen in parts of the Hudson River near the nuclear plant.

Oil sheen seen in the Hudson River near Indian Point nuclear plant following a transformer fire on May 11, 2015 (Credit: Riverkeeper)

Oil sheen seen in the Hudson River near Indian Point nuclear plant following a transformer fire on May 11, 2015 (Credit: Riverkeeper)

According to the investigation, the transformer’s dielectric fluid — a clear mineral oil that acts as an electrical insulator and coolant — mixed with water and foam from firefighters battling the blaze, and flowed from a moat into a storm drain leading to the plant’s discharge canal and ultimately the Hudson River.

In response, officials placed sorbent and hard environmental protective booms in and outside the discharge canal and removed the fluid, Entergy said.

Indian Point Nuclear Power Plant

Booms deployed into the Hudson River after a fire at Indian Point Nuclear Power Plant caused an oil overflow (credit: CBS2)

About 3,000 gallons of dielectric fluid reached the river, the U.S. Coast Guard estimated.

Entergy said its contractors inspected about 25 sheens on the Hudson River and of those, 19 did not require remediation.

“Entergy and its independent engineers are reviewing the environmental response to the May 9 incident, and have confirmed that all actions taken to date, including fluid release notifications and calls for response assistance, were consistent with applicable regulations and appropriately implemented,” the corporation said Tuesday.

The Indian Point Energy Center supplies electricity for homes, businesses and public facilities in New York City and Westchester County.