Those killed in the blast in a colony in Idinthakarai coastal village, included a woman and three children, all aged below five years, police said, adding preliminary probe indicated that country-made crude bombs were used.
A senior official of the Department of Atomic Energy said the plant, which was about two km from the blast site, is running fine and is safe.
Police said it was probing all aspects and did not want to speculate on who could be behind the explosion.
DIG of Police Sumith Saran and Superintendent of Police Vijendra Bidari also rushed to the spot.
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People of Idinthakarai, the hub of protests against KNPP, backed by People's Movement Against Nuclear Energy, have been agitating for more than two years, demanding its closure.
Unit-1 had attained criticality on July 13 this year following protests against the project by anti-nuclear activists in areas around the complex, citing safety reasons.
Police had raided Kunthankuli village near Idinthakarai in 2012 and early this year and seized some country bombs from some huts.