Two houses were razed under the impact of the blast, they said, adding rescue teams rushed to the spot to clear the debris and extricate people believed trapped under it.
DIG of Police Sumith Saran and Superintendent of Police Vijendra Bidari also rushed to the spot.
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. Police suspect that some people involved in criminal cases from Kunthankuli were staying in the two houses.