"The measures regarding Enel Produzione's power station regard the use of ashes in secondary production processes," Enel said in a statement.
State-controlled Enel said it was confident it would be able to prove its production processes were proper and correct.
Seizure of the plant in Cerano in the southeastern Apulia region did not compromise operations at the plant, it added.
The source said prosecutors had also seized 500 million euros (440.28 million pounds)for alleged illicit profits as part of the investigation.
(Reporting by Vincenzo Damiani and Stephen Jewkes, editing by Valentina Za)