Police sources say the accused, identified as 25-year-old Narendra Kumar from the Lakhimpur district in Uttar Pradesh, is being interrogated.
"He came from the sea facing side of Kudankulam. We are also seeking a medical examination of him," police sources said.
The man was first picked up by Central Industrial Security Force (CISF) personnel and later handed over to the police in Tirunelveli district, where the nuclear plant is located.
Two reactors of the Kudankulam power plant are functional with a total of 2000 MW power production capacity.
More details are awaited.