The bomb blast also demolished two houses.
One of three people seriously injured was identified by police as an anti-nuclear activist and a suspect in the case.
The Kudankulam Nuclear Power Plant, which has inspired years of protests by local residents in Tamil Nadu state, was unaffected by the blast and operating normally, police said on Wednesday.
It is believed the bomb went off accidentally as suspected activists were making several crude explosive devices on Tuesday night in a house 15 kilometres from the plant, police superintendent Vijayendra Bidari said.
Police recovered two unexploded bombs from the site and are seeking an anti-nuclear activist for questioning.
Another anti-nuclear activist was being treated in a hospital for serious injuries from the blast and had yet to be questioned, Bidari said.
Among the six killed were a women and three young children, police said, without saying if they had been in the house or nearby.