Among the dead were a woman and three children, all aged below five, while two persons were injured. Police said the victims' bodies were charred beyond recognition.
Security has been tightened around the nuclear plant. The entire Idinthakarai village has been taken under police control, police said.
Meanwhile, Uthayakumar denied any role in the explosion.
“The people of Idinthakarai or members of (anti-nuclear) struggle committee have nothing to do with this explosion. We will cooperate with police and other government officials,” he said.
PMANE has been spearheading protests for more than two years against the Indo-Russian joint venture in Tamil Nadu's Tirunelveli district demanding its closure, citing safety concerns following the Fukushima disaster in Japan in 2011.
Unit-1 had attained criticality on July 13 this year.
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