Investigation confirmed that the plant systems are not affected, said officials.
Government-run Nuclear Power Corp of India Ltd (NPCIL), which runs nuclear reactors across the country, said today it had identified malware in one of its computers last month but its plant systems were unaffected.
"The investigation revealed that the infected PC belonged to a user who was connected in the internet network used for administrative purposes. This is isolated from the critical internal network," the utility said in a statement.
On Tuesday, the NPCIL rejected media reports that its Russian-built reactors in Kudunkulam, Tamil Nadu, had suffered a cyber attack.
Some cyber security experts tweeted on Tuesday that NPCIL's computers were potentially compromised.
One of the experts, Pukhraj Singh who works independently, said he had notified the government on September 4 that there had been an incident at the Kudankulam nuclear power plant.
NPCIL said the government's CERT-In department, which is responsible for emergency response, had notified about the malware on September 4 and that it was immediately investigated by the department of atomic energy specialists.
"The networks are being continuously monitored. Investigation also confirms that the plant systems are not affected."
NPCIL, which designs, constructs and runs nuclear power reactors in the country, currently operates 22 commercial nuclear power reactors with an installed capacity of 6,780 megawatts, according to its website.
The government has previously attempted to tighten rules for businesses entering its power transmission sector, making stringent checks on both power and telecoms equipment to guard against malware.